Power Developer: November 2014 by EEWeb Magazines

November 2014

Power Integrations

The ULTIMATE Flyback

FluxLink

速

Power Conversion Technology Interview with Douglas Bailey VP of Marketing at Power Integrations

New

HIGHLY INTEGRATED Power Module GaN

TRANSISTORS

Poised for Revolution

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TECH The ULTIMATE The Ultimate Flyback

Power Flyback Developer

which combines the simplicity of primary-side regulation solutions with the performance benefits of secondary-side control.

mechanism—most commonly an optocoupler. This, of course, adds to the complexity of the circuit design as well as the BOM cost. As with all components, there are high-end optocouplers, but they tend to be expensive. Low-cost optocouplers, which are used in many chargers to keep the eventual product cost low, can suffer from aging, temperature drift, and varying current gain, factors that will compromise stability and even reliability in some cases.

Many successful PSR-based designs do exist with adequate operation to regulate output voltage and current under the transient load conditions of the particular application. However, the main challenge of all PSR solutions is that it is only possible to see what is happening on the output after switching the primary-side transistor. Effectively, every time the transistor switches Another approach is to use capacitive coupling you get a glimpse of the power supply output techniques. Capacitors themselves are inexpensive, load conditions. However, high but they are also difficult to energy-efficiency requirements integrate. High-voltage capacitive A new approach demand that the switching coupling on a single die is expensive Mike Matthews ower electronics is a $90 billion-per-year By market combines the simplicity to build in, especially when it is frequency is reduced at light VP Product Development s we discussed in the first part of our series on of primary-side regulation necessary to meet the typical 6kV loads; therefore these glimpses Power Integrations become less frequent, inherently high-potential isolation required ompact modeling concerns for Si-based power solutions with the compromising the ability of A challenge facing manufacturers of performance benefits of during testing for AC/DC power ectronics), and as figure 1 suggests, silicon gets the power supply to respond supplies. But perhaps the most secondary-side control. smart mobile devices, set-top boxes, to fast transient loads. So the serious challenge that is raised by dine at the exclusive power electronics table networking equipment, and computer system is always playing catch the use of capacitive feedback is nly because it was able to buy its way there. up, inevitably leading to compromises in the coping with electrostatic system discharge (ESD) peripherals is to develop low-cost, application and specifications of PSR solutions. pulses. In many modern consumer electronic efficient adapters and chargers that specifications, such pulses can exceed +/-15kV here’s no denying the cost advantages that meet ever-more-demanding energy A further disadvantage with PSR controllers is and are applied directly at the output of the power Figure 1. licon possesses. The biggest contributing factor For that they infer what is happening on the power supply, giving rise to capacitive currents through consumption regulations. supply output from waveforms on the primary the isolation barrier that can damage control A comparison of gallium nitride with silicon ehind the higher costs of GaN simplicity manufacturing is and cost reasons, flyback bias winding of the transformer, rather than circuitry. Also, common mode effects due to voltage across various performance metrics. afer yields. Silicon manufacturing technology designs using primary-side regulation directly measuring output voltage and current. fluctuations can cause problems that require extra This means that transformer manufacturing circuitry and associated design and BOM costs. would normally FluxLink: as matured enough to enable (PSR) masstechniques production InnoSwitch-CH highly integrated switcher ICs – the performance of tolerances, along with primary-clamp circuit • Low component count be favoured. However, secondarywafers with up to an 18-inch diameter,secondary-side control with the simplicity of primary-side control. whereas design, become factors that must be allowed The third approach used to implement SSR of • Ease of manufacturability side6-inch regulation (SSR) provides more for during development and mass production. a power supply is to use a pulse transformer. InnoSwitch-CH ICs combine primary FET, primary and secondary aN wafers are still fabricated on wafers. • Performance (CV, CC and no-load efficiency Magnetic coupling is extensively used in that complies with global standards)Transformers are infamous for manufacturing accurate control and is less sensitive to controllers with synchronous rectification FET drivers, lossless ubstrate options for GaN manufacturing range variances, complicating the management of highhigh-end communications products, but • Integrated (primary-side FET, primary controller, production tolerances in the transformer current sensing and secondary-to-primary FluxLink™ safety-isolated secondary controller with SR FET, lossless volume production with PSR solutions, ultimately has, until now, been prohibitively expensive om silicon or sapphire substrates (cheap but current sensing, and FluxLink communication) communications link. This dramatically reduces component count and other external components. impacting cost effectiveness if yields suffer. for low-cost charger and adapters. rge lattice mismatch and very large coefficients Meets all efficiency standards: and eliminates the need for bulky and slow optocouplers for smart

FluxLink

™

CONTENTS

Power Conversion Technology

he State of GaN

CONTENTS

dvantage TECH REPORT

The Ultimate Flyback FluxLink™ Power Conversion Technology

InnoSwitch™-CH — a revolution in switch-mode flyback power conversion

thermal expansion) to silicon carbide mobile chargers. With highly accurate secondary-side control and efficient SR, the InnoSwitch-CH family outperforms any competing iC) substrates (low lattice mismatches but solution. ohibitive costs). That said, even though market ices today are in silicon’s favor, improvements Find out efficiency as well as power-bill savings accrued more about igbt drivers y deploying GaN-based electronics will erode licon cost advantages within the next few years.

TECH SERIES

• European Union Code of Conduct (V5/T2) • US Department of Energy standards (DoE 6) • CQC China 5000 meter altitude requirement

PI Corporate advert US Letter Oct 14 final.indd 1

gure 2 paints a snapshot of the various pplications where GaN high-electron mobility ansistors (HEMT) are being used today. GaN EMTs already find use in low or medium-power onsumer UPS—systems that readily convert ored energy in batteries from DC to AC power the case of a power supply failure from the id. In addition, due to their excellent switching sses and bidirectional current flow, GaN is ommercially deployed in power factor correction FC) units and point-of-load regulators in lecom systems (where linear or DC/DC

alley-based ectronic e powerated circuits

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GaN Transistors Poised for Revolution

at www.power.com

4 12

27/10/2014 09:54

Interview with Douglas Bailey INDUSTRY INTERVIEW Silicon gets to dine at the exclusive power electronics VPTech of Marketing The Advantage at table only because it was able to buy its way there! Interview Douglas Bailey, Power Integrations Powerwith Integrations

Figure 2. GaN-based applications across voltages and currents, in varying stages of conception.

EEWeb spoke with Douglas Bailey, PRODUCT WATCH MIC45212at Power Module vice president Micrel’s of marketing Power Simplified Design, Exceptional Performance Integrations, about the current state of the power industry, the company’s highly effective MOSFET technology, and challenges in its development. Bailey also discussed ways to build a small power supply, limitations of those methods, and how the company is working to solve power-efficiency challenges.

23 30

Power Developer

The ULTIMATE The Ultimate Flyback FluxLink™

Flyback

Power Conversion Technology By Mike Matthews VP Product Development Power Integrations A challenge facing manufacturers of smart mobile devices, set-top boxes, networking equipment, and computer peripherals is to develop low-cost, efficient adapters and chargers that

InnoSwitch™-CH — a revolution consumption regulations. For in switch-mode flyback power simplicity and cost reasons, flyback conversion designs using primary-side regulation meet ever-more-demanding energy

(PSR) techniques would normally InnoSwitch-CH highly integrated switcher ICs – the performance of be favoured.secondary-side control with the simplicity of primary-side control. However, secondaryside regulation (SSR) provides more InnoSwitch-CH ICs combine primary FET, primary and secondary accurate control and is less sensitive to controllers with synchronous rectification FET drivers, lossless production tolerances in the transformer current sensing and secondary-to-primary FluxLink™ safety-isolated communications link. This dramatically reduces component count and other external components. and eliminates the need for bulky and slow optocouplers for smart mobile chargers. With highly accurate secondary-side control and efficient SR, the InnoSwitch-CH family outperforms any competing

solution.

FluxLink: • Low component count • Ease of manufacturability • Performance (CV, CC and no that complies with global sta • Integrated (primary-side FET secondary controller with SR current sensing, and FluxLin

Meets all efficiency standards • European Union Code of Co • US Department of Energy sta • CQC China 5000 meter altitu

TECH REPORT

ntil recently, designers faced this ageold choice: cost or performance. Now, a new approach has been developed which combines the simplicity of primary-side regulation solutions with the performance benefits of secondary-side control.

So, given the challenges with PSR solutions, what are the options with traditional secondary-side regulation? SSR requires an isolated feedback mechanismâ&#x20AC;&#x201D;most commonly an optocoupler. This, of course, adds to the complexity of the circuit design as well as the BOM cost. As with all components, there are high-end optocouplers, but they tend to be expensive. Low-cost optocouplers, which are used in many chargers to keep the eventual product cost low, can suffer from aging, temperature drift, and varying current gain, factors that will compromise stability and even reliability in some cases.

onduct (V5/T2) andards (DoE 6) ude requirement

Power Developer Leading power integrated circuit (IC) company, Power Integrations took a long, hard look at the problem and with its new InnoSwitch™ family of highly integrated switcher ICs has come up with a digital magnetic communications function—termed FluxLink™—within the IC package at virtually no extra cost. Effectively, a magnetic coupling between the primary and secondary side is created without the need for high permeability magnetic cores, using only the standard bill of materials for the manufacture of the IC package (figure 1).

Figure 1. InnoSwitch switcher ICs with FluxLInk technology: a magnetic coupling between the primary and secondary side is created without the need for high permeability magnetic cores.

Full internal galvanic isolation—exceeding that used in most optocouplers—is achieved, meeting UL, TÜV and all other global safety standards, while external pin-to-pin creepage of over 9.5 mm is achieved with a custom surface-mount package designed for this application. Furthermore, by occupying the space on the PCB normally reserved for the primary to secondary isolation region, the InnoSwitch IC essentially takes no PCB area. In fact, the package and pin-out are designed so that the most convenient location in most layouts is directly underneath the power transformer, making compact layout very simple for space savings and PCB-cost reduction. The design allows for simple resistor-divider direct sensing of the power supply output voltage while the power supply output current measurement is fully integrated inside the package, eliminating external current-sense circuitry altogether. Secondary sensing brings several other benefits. As well as eliminating the often-unreliable optocoupler, it enables a simple transformer to be specified since the circuit will not be sensitive to the bias winding location or transformer inductance tolerances. Switching frequency jitter effectively spreads the EMI spectrum, enabling designs using only standard magnetic-wire primary and triple insulated wire (TIW) secondary windings without the need for copper shields (see figure 2).

SECONDARY BIAS PRIMARY CANCELLATION

Figure 2. Switching frequency jitter enables designs using only standard magneticwire primary and TIW secondary windings without the need for copper shields.

TECH REPORT But perhaps the most significant benefit of InnoSwitch ICs is the provision of simple and rugged synchronous rectification, resulting in high efficiency without the usually expected cost penalty. Synchronous rectification (SR) improves efficiency by replacing lossy diodes with power MOSFETs on the output of the power supply. The voltage drop of a standard diode is typically between 0.7V and 1.7V, but even high-efficiency Schottky diodes will typically exhibit a voltage drop of 0.4 to 0.5V, which in a 5V system, such as a USB charger, represents a 10 percent loss in the output stage.

However, anyone who has designed a flyback with SR will be aware that timing is key. Simultaneous conduction of primary transistor and SR FET creates an effective short-circuit condition across the primary transformer winding which usually leads to primary transistor damage. On the other hand, a delay in turning on the SR FET once the primary transistor has turned off compromises efficiency. In traditional SR solutions, the need for a separate secondary-side controller to drive the SR FET also adds cost and complexity to the circuit, which is why SR has sometimes had the reputation of being an expensive luxury.

By contrast, MOSFETs can be specified with an on-resistance as low as 10 mΩ. Therefore in a typical charger design, the voltage drop might be 50mV, representing a loss of only 1 percent, which is ten times less than with Schottkys. The latest SR power MOSFETs are even 20 to 40 percent cheaper than Schottkys, so SR seems to be the obvious approach for flyback topology power supplies.

This is all set to change. With InnoSwitch ICs, the FluxLink element introduces precise cycle-bycycle, digital communication controlling both the primary transistor and secondary SR FET switch timing. For the first time, users therefore have a truly foolproof SR solution where the complete operation is integrated in a single IC rather than having to wrestle with the independent operation of separate primary and secondary controllers normally required in SR solutions with optocoupled SSR or PSR power supplies. In addition, the instantaneous communication afforded by FluxLink technology allows the secondary controller to determine the optimum turnon and turn-off times of the SR FET across the entire load range, whether the power supply is operating in discontinuous mode, continuous mode, and even under fault conditions.

Figure 3. FluxLink provides extremely fast transient response.

Power Developer This optimized SR function allows Innoswitch ICs to easily comply with even the most stringent, future efficiency standards such as the California Energy Commission, European Union Code of Conduct Tier 2, and DoE6. A further benefit of the instantaneous FluxLink communication is extremely fast-transient response. As can be seen in figure 3 (previous page), if an event happens on the output, the primary will receive a signal to turn on within a single switching cycle period (<10 Îźsecs), virtually eliminating output voltage undershoot, even for 0-100 percent load transients. This allows output capacitor values to be reduced compared to PSR solutions where the slow response to transients typically requires large capacitors to meet the transient energy requirements. InnoSwitch power-supply ICs include the highvoltage power MOSFET, primary- and secondary-side controllers, FluxLink feedback link, and an integrated synchronous rectifier (SR) controller within a single, safety-rated, 16-pin eSOPâ&#x201E;˘ surface-mount package. Devices feature highly accurate CV and CC control (+/-3% and +/- 5% respectively) as well as low ripple. Operating efficiency is typically better than 84 percent in a 5V output 10 watt power supply at

full load (as high as 88 percent in higher output voltage designs and even higher under mediumload conditions), and no-load consumption is below 10mW. InnoSwitch ICs start up using bias current drawn from a high-voltage current source connected to the DRAIN pin, eliminating the need for external start-up components; an external bias winding reduces no-load and increases system efficiency during normal operation. The ICs also include comprehensive system-level safety features such as output over-voltage protection, overload power limiting, hysteretic thermal protection, and frequency jitter to reduce EMI. As shown in figure 4, a typical 2.5 A, 5V mobile device charger can be achieved using just 30 components, roughly 33 percent fewer than equivalent performance solutions. And as smart mobile devices become larger, they will require higher currents for fast charging. Where previously the idea of 5V/4 A chargers would have raised eyebrows, now such devices are starting to appear. InnoSwitch ICs facilitate highly efficient, cost-effective charger designs up to 25W and are designed to be compatible with emerging rapidcharge technologies, easily justifying the claim to be the most effective and efficient means of implementing flyback power supply designs.

Figure 4. A typical 2.5 A, 5V mobile device charger can be achieved using just 30 components, roughly 33 percent fewer than equivalent performance solutions.

The Ultimate Flyback

InnoSwitch™-CH — a revolution in switch-mode flyback power conversion InnoSwitch-CH highly integrated switcher ICs – the performance of secondary-side control with the simplicity of primary-side control. InnoSwitch-CH ICs combine primary FET, primary and secondary controllers with synchronous rectification FET drivers, lossless current sensing and secondary-to-primary FluxLink™ safety-isolated communications link. This dramatically reduces component count and eliminates the need for bulky and slow optocouplers for smart mobile chargers. With highly accurate secondary-side control and efficient SR, the InnoSwitch-CH family outperforms any competing solution.

Find out more about

igbt drivers

FluxLink: • Low component count • Ease of manufacturability • Performance (CV, CC and no-load efficiency that complies with global standards) • Integrated (primary-side FET, primary controller, secondary controller with SR FET, lossless current sensing, and FluxLink communication) Meets all efficiency standards: • European Union Code of Conduct (V5/T2) • US Department of Energy standards (DoE 6) • CQC China 5000 meter altitude requirement

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Power Developer

GaN

TRANSISTORS Poised for Revolution Compact Modeling for Gallium Nitride Power MOSFETs

n the power electronics community, compound semiconductors such as

By Sachin Seth and Arif Sonnet Texas Instruments

gallium nitride (GaN) are drawing more

attention as they try to displace silicondoing the heavy lifting for the past 30 years. It’s not hard to see why—with an impressively, wide band-gap, superior

COMPACT MODELING CONCERNS for Silicon-based Power MOSFETs Power electronics, at first brush, does not have the glamorous appeal of slick and quick digital electronics—microprocessors, microcontrollers, etc.—that we’ve come to associate with the nanotechnology revolution in the past two to three decades. Evaluated solely on the basis of process nodes and manufacturing technology, power transistors lag their digital counterparts by several years. And while cutting-edge processors allow you to talk, text, and play “Flappy Bird” all at the same time, most power electronics are still thought to fulfill one function only—a glorified ON/OFF switch! All of that holds water only at the first brush. Dig a little deeper, and it becomes immediately apparent why power semiconductors have been attracting so much attention recently. To put things in perspective, the market size for microprocessors in 2012 was roughly $90 billion. This figure includes the revenue from chip sales across all segments—PC, embedded, servers, etc. Impressive, right? On the other hand, the estimated market size for power electronics is, yes, $90 billion! In fact, both technologists and policy-makers have identified power electronics as an investment that could enable a functioning low-carbon economy. Thus, newer applications, such as inverters in solar panel installation that convert DC to AC or batterycharge management in electric vehicles, are supplementing steady demand from traditional sources such as aeronautical and consumer applications. This has ensured that the market size for power electronics remains substantial.

than silicon), and a far superior thermal conductivity than silicon, GaN becomes a logical choice for any high-speed and highpower application1. Without a compound semiconductor-based uninterrupted power supply (UPS) system, a screaming fast silicon-based microprocessor is useless.

Power electronics are employed in broadly two engineering applications: power distribution and power conversion. While the former is still a bastion of mechanical relays and circuit breakers, it is the latter where semiconductors have made the largest dent. Power transistors in commercial manufacturing today can service demands for applications beyond 1000V! The market is wide open, with various process technologies like Silicon (Si), Gallium Nitride (GaN) and Silicon Carbide (SiC) battling it out to establish supremacy and gain market share. Figure 1 outlines the relative costs of these technologies as well as the broad voltage ranges of their usability. Out of all these contenders, only silicon technologies are mature enough to be reliably mass-produced at a very low cost. GaN and SiC, both wider band-gap materials that possess better breakdown and high-frequency characteristics than Si, are very expensive substrates to fabricate with a much lesser yieldper-wafer than Si. Thus, Si-based power devices (FETs) that are compatible with standard CMOS manufacturing are driving the power electronics

Silicon Carbide

Source

• Highest performance advantage (breakdown/temp) • Most expensive

N+

Gate

Drain

N+

LEFF

(a) Regular CMOS

P Well P+ Sub

• Expensive epitaxial manufacturing • Better DC/RF performance than Si

Silicon • Low cost • Best yield • Reliable

Gate

Source

N+ Axis of symmetry

LEFF

800 V

Voltage Figure 1: Cost of manufacturing vs. usable voltage ranges for contending semiconductor materials vying for the power electronics market share.

Drain

LOV

P Well

N+ N Well

(b) DEMOS

P+ Sub Body + Source

200 V

breakdown fields (ten times larger

Power Transiators

Gallium Nitride

Cost

based power devices, which have been

P+ N+ LEFF P Well

Gate

Drain

LOV

N+ N Well

P+ Sub

Figure 2: Comparison of the simplified crosssections of CMOS, DEMOS5 and LDMOS6 transistors

industry single-handedly, with cost, yield and reliability advantages. A majority of Si-based power electronics rely on two types of transistors that function as its workhorse: the Drain Extended MOSFET (DEMOS) and the Lateral Double-Diffused MOSFET (LDMOS). Mask reuse within standard CMOS manufacturing ensures DEMOS and LDMOS transistors can be fabricated “on the side” without additional costly processing steps. In a particular process node, while the scaled CMOS components are used in digital and highperformance analog circuit design (products in critical timing or signal paths, etc.), DEMOS and LDMOS transistors find extensive usage in switching power converter applications. These applications include switched DC-DC converters that convert sources of direct current from one voltage level to another, where the key benchmarks are maximizing power conversion efficiency and minimizing switching losses. For comparison, highly simplified cross-sections of n-channel CMOS, DEMOS and LDMOS transistors are shown in Figure 2. At their simplest, power FETs such as DEMOS and LDMOS can be thought of as a conventional MOSFET (both n- or p-channel) with an extended drain region under the polysilicon gate. In DEMOS devices, the NWELL creates the lightly doped drain extension region, which allows for greater depletion on the NWELL side and a shorter channel length. The DEMOS channel region looks exactly like a CMOS channel, but is usually longer for greater process tolerances and punch-through resistance due to the higher operating voltages. In LDMOS devices, the body (p+) and source (n+) are shorted together via a process known as double-diffusion (hence the name!). LDMOS devices, which really are types of specialized DEMOS devices, are optimized for a shorter channel region to give them superb linear resistance and higher gains compared to DEMOS. While DEMOS usage tends toward high-voltage, low-current applications, LDMOS can be used in high-voltage, highcurrent applications. Both transistors, however, do not make good high-speed analog and digital circuit components due to extended geometry features.

To read the first article in this series, click on the image above.

TECH REPORT

The State of GaN Power electronics is a $90 billion-per-year market (as we discussed in the first part of our series on compact modeling concerns for Si-based power electronics), and as figure 1 suggests, silicon gets to dine at the exclusive power electronics table only because it was able to buy its way there. There’s no denying the cost advantages that silicon possesses. The biggest contributing factor behind the higher costs of GaN manufacturing is wafer yields. Silicon manufacturing technology has matured enough to enable mass production of wafers with up to an 18-inch diameter, whereas GaN wafers are still fabricated on 6-inch wafers. Substrate options for GaN manufacturing range from silicon or sapphire substrates (cheap but large lattice mismatch and very large coefficients of thermal expansion) to silicon carbide (SiC) substrates (low lattice mismatches but prohibitive costs). That said, even though market prices today are in silicon’s favor, improvements in efficiency as well as power-bill savings accrued by deploying GaN-based electronics will erode silicon cost advantages within the next few years. Figure 2 paints a snapshot of the various applications where GaN high-electron mobility transistors (HEMT) are being used today. GaN HEMTs already find use in low or medium-power consumer UPS—systems that readily convert stored energy in batteries from DC to AC power in the case of a power supply failure from the grid. In addition, due to their excellent switching losses and bidirectional current flow, GaN is commercially deployed in power factor correction (PFC) units and point-of-load regulators in telecom systems (where linear or DC/DC

Figure 1. A comparison of gallium nitride with silicon across various performance metrics.

Figure 2. GaN-based applications across voltages and currents, in varying stages of conception.

Silicon gets to dine at the exclusive power electronics table only because it was able to buy its way there!

Power Developer regulators need to be placed as close as possible to their point of use). GaN-based electric vehicle charging systems (think Tesla), high-power UPS systems for industrial applications, motor drives, and photovoltaic inverters are already in the prototyping and development stages. The key takeaway is that GaN devices are poised to revolutionize and miniaturize applications where silicon can’t even compete due to inherently poor performance metrics (figure 1), which will in turn subsidize GaN cost of manufacturing and make its pricing competitive with silicon. Having GaN deployed ubiquitously in electrical systems in the near future will be analogous to getting a Ferrarilike performance while paying for a Honda Civic.

The GaN Heterostructure Figure 3 compares the construction of a regular n-channel MOSFET to a GaN HEMT. The n-FET has a polysilicon gate as well as a natively grown oxide (Si02)—both cheap and easy fabrication steps. Normally on AlGaN/ GaN HEMTs use the thin sheet of electrons known as the 2-dimensional electron gas (2DEG) that forms at the AlGaN-GaN interface as the channel. Due to lack of ionized impurity

scattering in a 2DEG-based conduction channel, this transport mechanism allows a much higher mobility of carriers than in bulk GaN, SiC, or Si. GaN and AlGaN both possess Wurtzite (hexagonal) crystal structure. The Wurtzite crystal structure is known to be noncentrosymmetric (meaning, it lacks inversion symmetry). The result is the spontaneous polarization (PSP) of the III-V compound semiconductor material, as also shown in figure 4. In addition, applying stress to GaN distorts the hexagonal crystal structure further (for instance, when an AlGaN layer is pseudomorphically grown on GaN), causing further polarization. This is denoted by PPE (piezoelectric polarization) in figure 4, wherein internal crystallographic mechanical stress leads to an internal electric field. Donor-like surface traps serve as a source of electrons in 2DEG, which neutralizes the net positive AlGaN polarization charges. Thus, the internal polarization and resultant 2DEG (coupled with high breakdown in AlGaN) is exploited to form very fast transistors with large breakdown voltages—a win-win scenario.

Figure 3. Comparison of the simplified cross-sections of (a) regular CMOS and (b) GaN HEMT transistors.

Figure 4. Polarization established due to mechanical stresses, leading to formation of 2DEG.

TECH REPORT Another really interesting property exhibited by GaN crystals is inverse piezoelectric effect3. In this phenomenon, applied terminal voltages create strong internal electric fields that end up giving rise to mechanical stress within the device. Since GaN HEMTs are used in highvoltage applications, a high-drain voltage VD (upwards of 100V) applied to the HEMT will create high electric fields on the drain-side edge of the gate. Inverse piezoelectric effect generates local stresses near the drain, leading to defect formation4. These defects become a reliability concern. To reduce the effect of high electric fields generated due to a high VD, field plates (FP) have to be employed5 (as seen in figure 5); doing so allows device engineers to control electric fields within the device and to diminish the inverse piezoelectric effect, as well as other high e-field driven failure mechanisms. However, nothing in transistor engineering comes without a price. The field plates themselves add to compact modeling challenges as will be seen in the next section.

require a negative voltage on the gate electrode to turn the HEMT off (depletion mode). However, most concerns discussed in this article apply to “normally-off” devices as well. Since power conversion circuits are easier to design when transistors are “normally-off,” various companies have now developed enhancement-mode GaN (eGaN) devices that can be manufactured in the same facility as silicon ICs.6

Compact-Modeling Concerns for GaN HEMTs

We have discussed HEMTs that are “normallyon” (on even when gate-voltage VG=0), which

One of the biggest compact modeling challenges for GaN HEMTs is dynamic current-collapse, where the on-state current is temporarily reduced following off-state stress. Off-state stressing is done by applying a large negative gate-voltage VG to the GaN HEMT to ensure no conduction channel exists due to 2DEG, while drain is held high (usually at rail voltage VDD) and the source is grounded (VS=0)—a condition that the GaN HEMTs often find themselves under when used in DC/DC converter circuits. Qualitatively, current-collapse is show in figure 6. Pre-stress ID -VDS curves are shown in blue, with a much smaller on-resistance

Figure 5. A detailed GaN HEMT cross-section with two Field Plates to improve breakdown, referenced from [5].

Figure 6. Dynamic current-collapse in GaN HEMTs (empirical behavior only).

Power Developer (VDS/ID) Ron1. Immediately following off-state stress, the on-resistance increases to Ron2 and the ID-VDS curves degrade. This effect is also known as dynamic-Ron, where the on-state resistance shows memory effects, i.e. it depends on bias history of the GaN HEMT. Currentcollapse makes even simple DC modeling a challenge for GaN HEMTs, the effects of which can spill over into modeling high-frequency figures of merit like f T, fMAX, and noise. While reams of research literature have been published to explain the mechanisms behind GaN HEMT current-collapse, the general consensus is that there are three responsible major effects at play. First, in the off-state, electrons from a negatively charged gate are injected into the donor-like surface trap states next to the gate (also shown in figure 4). Once the device turns on again, the trapped electrons now act like a negatively-biased gate. This depletes the 2DEG underneath the trapped electrons, increasing Ron. A few seconds later, the trapped electrons are “de-trapped,” 2DEG is restored, and all is well in the GaN universe7. The second major cause of currentcollapse is buffer trapping—electrons are trapped in the GaN substrate in the off-state8. Once the device turns on again, these trapped electrons in the substrate partially deplete the 2DEG above it, which again increases the Ron. A few minutes later, the trapped electrons are “de-trapped,” 2DEG is restored, and all is well again in the GaN universe.

The third major cause of performance degradation is inverse piezoelectric effect, as described in the previous section. To mitigate that effect, field plates are employed. The addition of a dual-FP structure (which combines a conventional FP and a source-terminated FP) is used to improve current collapse, breakdown characteristics, and gain characteristics, as seen in figure 5. The first FP (connected at the gate electrode) reduces the magnitude of the peak electric field at the drain side of the gate, thereby improving the device breakdown characteristics9. In addition, as this FP layer can control the depletion layer at the surface, it effectively suppresses the currentcollapse behavior caused by the surface trapped charges10. The second FP (which is connected to the source) does not modulate the surfacedepletion layer, though it reduces the charge injection to the surface traps. Consequently it works to relax the electric field for the high breakdown region thereby shielding the electric field between the first FP and the drain. As a result of this decreased electric field, the capacitance between the drain and gate (CGD) decreases shown in figure 7. A reduced CGD naturally increases the gain characteristics of the GaN HEMTs, along with improved linearity and stability. However, it must be noted that the shape of these capacitance curves is highly dependent on FP design and may require using specific (nonstandard) behavioral models to properly predict capacitance behavior. This tends to throw another wrench in the works when it comes to making standard, compact models for GaN HEMTS.

Trapped electrons are “de-trapped,” 2DEG is restored, and all is well again in the GaN universe.

TECH REPORT

Figure 7. Capacitance behavior as a function of VDS, referenced from [5].

Compact Models for GaN Devices As GaN process and fabrication technology matures, the research focus shifts towards development of standardized compact models for GaN HEMTs to be used by circuit designers. Unlike the BSIM compact model for MOSFETs or MEXTRAM for BJTs, standardized industrywide compact models for GaN HEMTs do not exist. This is due to the development of accurate compact models with numerical simplicity for the GaN HEMTs, whose unusual electrical behaviors notwithstanding, have proven to be exceptionally challenging. Because of structural differences between silicon MOSFETs and GaN HEMTs, their respective device simulation methodologies are different. For instance, in addition to conventional transistor material properties which dictate electrical behavior (i.e. band gap, electron affinity, permittivity, carrier mobility, and impact-ionization parameters), other GaNspecific device issues need to be addressed as well to make high-fidelity compact models. These additional features include calculation of lattice temperature necessary for high-

One modeling methodology is quite fast but sophomoricâ&#x20AC;&#x201D; extrapolation outside of measured data range is not accurate, and correct scaling to other device geometries is fraught with gross inaccuracies.

power simulation, tunneling at the contacts and interfaces due to Schottky gate11, polarization effect due to fixed charge and built-in charges12, hydrodynamic model as a replacement of typical drift-diffusion model13, impact of 2DEG on the carrier mobility formulation14, to mention a few. To achieve the full performance potential of GaN HEMTs, accurate and fast circuit simulations are required. The accuracy and speed of such simulations depend heavily on the type of compact model used to mimic HEMT electrical behavior. A very rudimentary type of compact model available for GaN HEMTs is a table-based model, which uses all the measured device data to be stored in large look-up tables15. The circuit simulator computes the simulation result by selecting measured data at any given test condition from a look-up table. This modeling methodology is quite fast but sophomoricâ&#x20AC;&#x201D; extrapolation outside of measured data range is not accurate, and correct scaling to other device geometries is fraught with gross inaccuracies. This seriously undermines the ability to simulate a circuit under whichever test conditions a circuit designer may fancy.

Power Developer

Figure 8. Equivalent circuit of Angelov model from [17].

Another type of compact modeling approach for GaN HEMTs is the empirical modeling approach, where the model is not based in semiconductor physics and instead relies on arbitrary mathematical functions to capture the electrical behavior. Such a widely-used (however, nonstandard) compact model for GaN HEMTs is the Angelov model16. The Angelov model describes the drain current as a product of two factors where the first one is dependent only on the drain voltage and the second only on the gate voltage:

The model uses a hyperbolic tangent (tanh) function to describe the current whose first derivative has the same signature bell-shaped characteristics as found in the measured transconductance data, . A good representation of the voltage dependencies and its derivatives can be obtained from tanh function, which is also available in commercial simulator.

Figure 8 shows an equivalent circuit of Angelov model. While many of the elements in the circuit shown are used to capture the parasitics at different device nodes, the core model equations govern currents Id, Ig, and capacitances Cgs, Cds, Cgd. 17 Over time, this model has evolved to a point where it can now capture temperature dependencies, dispersion effects, and soft breakdown capabilities18. Since this model predicts the unusual currentâ&#x20AC;&#x201D;collapse behavior native to GaN HEMTs with high fidelity, it can also faithfully predict high-frequency and other nonlinear device phenomena. However, since the foundations of the Angelov model are not rooted in device physics, it is nonextrapolatoryâ&#x20AC;&#x201D; any changes to the device dimensions or process technology will force a refit of the model to new data. This is not suited to rapid product turn-around times and slows shipping the final IC. In addition, it makes the model nonscalable with regard to geometry, which is

TECH REPORT an important criterion expected of compact models. In spite of all these limitations, the Angelov model remains the most widely used compact model in the industry. As discussed in the previous section, the formation of a two-dimensional electron gas (2-DEG) is the core of the GaN HEMT device operation. A physics-based analytical expression for free carrier density ns is therefore a primary requirement in the development of a compact model for these devices. The challenge in accurately extracting ns stems from its complicated dependency on the applied controlling voltage. A third type of compact model for GaN HEMTsâ&#x20AC;&#x201D;the physics-based modelsâ&#x20AC;&#x201D;aims to do just that. This model formulation is based on analytical, self-consistent equations derived from modeling physical device phenomena. This type of model starts with calculating the 2DEG charge density19, which is then extended for validity in the subthreshold region as well. Calculation of ns can be obtained from the equation below where E0 and E1 are the two lowest subbands, Ef is the Fermi level and Vth is the thermal voltage.

After defining different physical aspects to model a unified form of total free carrier density equation valid for low and high applied voltages can be formulated as: 19

These charge density equations are used as the foundations of calculating drain current model, to which important effects such as carrier velocity saturation, channel-length modulation, shortchannel and self-heating effects are added. Recently, a new physics-based model (known as the MIT virtual source model) implemented in Verilog-A and validated against state-of-the-art GaN devices has been proposed20. The MIT VS model, based on the concept of virtual source carrier transport and extended to the driftdiffusion regime, incorporates full scalability from the linear to saturation regimes and includes self-heating effects also. Thus, while physics-based models for GaN HEMTs are still very much a work-in-progress, they seem to be the most promising as they have physically meaningful parameters, unparalleled scaling behavior in terms of device dimensions, and temperature dependencies all accounted for.

While physics-based models for GaN HEMTs are still very much a work-in-progress, they seem to be the most promising.

Power Developer Summary

A comparison of existing compact models for GaN HEMTs is recorded in the table below21. All models have substantial numbers of parameters. Larger parameter count means longer parameter extraction times, which is undesirable from a business perspective. All of these models are empirical models and the newer ones often incorporate the best features of existing models, for example, the Auriga model is very similar to the Angelov model, however with geometric dimension scalability added to it.

In spite of cost disadvantages, GaN HEMTs are poised to take over the power electronics market because of their superior physical properties. Analog circuit designers increasingly prefer GaN HEMTs over silicon MOSFETs in CAD simulation environments to design power management ICs. This necessitates a detailed scrutiny of compact models for GaN HEMTs to understand their respective strengths and limitations, which will ensure a first-pass design success. In this article, we compared the GaN HEMT with the ubiquitous silicon MOSFET, exploring the unique challenges encountered specifically while making compact models for GaN HEMTs. Unlike silicon, where the compact models are mature, compact modeling for GaN HEMTs is a hot field of research right now with many players vying for a piece of the ultimate prizeâ&#x20AC;&#x201D;to have their version established as the standard, industrywide compact model for GaN HEMTs.

The jury is still out for a standardized industrywide GaN HEMT compact model. The Compact Model Council, a body comprising of all major semiconductor companies, is currently in the process of choosing a standard GaN HEMT compact model. At stake lies capturing the computer aided design (CAD) intellectual property for a market that is, by many estimates, already worth $90billion annually.

Approx. Number of Parameters

Electrothermal (Rth-Cth) Model

Geometry Scalability Built-In

Originally Built For

Curtice3

GaAs MESFET

Motorola Electrothermal

Yes

LD MOSFET

CMC (Curtice Modelithics)

Yes

LD MOSFET

BSIMSO13

191

Yes

SOI MOSFET

CFET

Yes

HEMT

EEHMET

Yes

HEMT

Angelov

Yes

HEMT/MESFET

Angelov GaN

Yes

HEMT

Auriga

100

Yes

HEMT

FET Models

Table 1. Comparison of existing empirical compact models for GaN HEMTs.

TECH REPORT Having GaN deployed ubiquitously in electrical systems in the near future will be analogous to getting a Ferrari-like performance while paying for a Honda Civic.

REFERENCES 1. “The Toughest Transistor Yet,” Lester Eastman and Umesh K. Mishra, IEEE Spectrum, vol. 39, Issue 5, 2002. 2. Totem pole bridgeless PFC using GaN transistors”, http://www.gansystems.com/_ uploads/ whitepapers/425686_App%20 Note%20-%20Totem%20pole%20bridgeless%20 PFC%20using%20GaN%20transistors.pdf 3. “GaN HEMT Reliability,” Jesus A. del Alamo et al., Microelectronics Reliability, Vol. 49, Issue 11, 2009. 4. “Mechanisms for Electrical Degradation of GaN High-Electron Mobility Transistors,” J. Joh et al, IEEE Electron Devices Meeting, Dec. 2006 5. “Novel AlGaN/GaN Dual-Field-Plate FET with High Gain, Increased Linearity and Stability,” Y. Ando et al, IEEE International Electron Devices Meeting, pp. 576-579, 2005. 6. http://epc-co.com/epc/Products/eGaNFETs.aspx 7. “The Impact of Surface States on the DC and RF Characteristics of AlGaN/GaN HFETs,” R. Vetury et al, IEEE Transactions on Electron Devices, vol. 43, no. 8. 2001. 8. “Buffer Design to Minimize Current Collapse in GaN/ AlGaN HFETs,” M. J. Uren et al, IEEE Transactions on Electron Devices, vol. 59, no. 12, 2012. 9. “A 28 V over 300 W GaAs heterojunction FET with dual field modulating plates for W-CDMA base stations,” K. Ishikura et al, IMS 2005 Digest. 10. “10-W/mm AlGaN-GaN HFET with a field modulating plate,” Y. Ando et al, IEEE Electron Device Letters, vol. 24, pp. 289-291, 2003. 11. “On reverse gate leakage current of GaN high electron mobility transistors on silicon substrate”, L. Xia et al, Applied Physics Letters, 102 (11), 113510 (2013).

13. “Drift-diffusion and hydrodynamic modeling of current collapse in GaN HEMTs for RF power application”, S. Faramehr et al, Semiconductor Science Technology, 29 025007, 2014. 14. “Electron mobility in two-dimensional electron gas in AIGaN/GaN heterostructures and in bulk GaN”, M. Shur et al, Journal of Electronic Materials, V. 25, Issue 5, pp 777-785, 1996. 15. “Large-Signal Model for AlGaN/GaN HEMTs Accurately Predicts Trapping- and Self-HeatingInduced Dispersion and Intermodulation Distortion,” A. Jarndal et al, IEEE Transaction 16. “A new empirical nonlinear model for HEMT and MESFET devices,” I. Angelov et al, IEEE Transactions on Microwave Theory and Techniques, vol. 40, no. 12, pp. 2258-2266, 1992. 17. “A compact transport and charge model for GaN-based high electron mobility transistors for RF applications”, U. Radhakrishna et al., MS Thesis, MIT, 2013 18. “Extensions of the Chalmers nonlinear HEMT and MESFET model,” I. Angelov et al, IEEE Transactions on Microwave Theory and Techniques, vol. 44, no. 10, 1996. 19. “A Physics-Based Analytical Model for 2DEG Charge Density in AlGaN/GaN HEMT Devices”, S. Khandelwal et al, IEEE Transactions on Electron Device, vol. 58, no. 10, 2011. 20. http://www-mtl.mit.edu/wpmu/ar2013/mitvirtual-source-ganfet-high-voltage-mvsg-hv-model-aphysics-based-compact-model-for-hv-gan-hemts/ 21. “Modeling GaN: Powerful but Challenging,” L. Dunleavy et al, IEEE Microwave Magazine, vol. 11, no. 6, pp. 82-96, 2010.

12. “Polarization effects, surface states, and the source of electrons in AlGaN/GaN heterostructure field effect transistors”, J. P. Ibbetson et al, Applied Physics Letters 77 (2), 250 (2000).

TECH REPORT

Power Integrations

The

TECH

Advantage

Power Integrations is a Silicon Valley-based supplier of high-performance electronic

Interview with Douglas Bailey VP of Marketing at Power Integrations

components used in high-voltage powerconversion systems. Their integrated circuits and diodes enable compact, energy-efficient AC/DC power supplies for a vast range of electronic products including mobile devices, TVs, PCs, appliances, smart utility

EEWeb spoke with Douglas Bailey, vice president of marketing at Power Integrations, about the current state

meters, and LED lights. The companyâ&#x20AC;&#x2122;s insulated-gate bipolar transistor drivers enhance the efficiency and reliability of high-power applications such as industrial motor drives, solar and wind energy

of the power industry, the companyâ&#x20AC;&#x2122;s highly effective MOSFET technology, and challenges in its development. Bailey also discussed ways to build a small power supply, limitations of those methods,

systems, electric vehicles, and high-voltage DC transmission.

and how the company is working to solve power-efficiency challenges.

Power Developer What is the current state of the power industry, and what are some of the latest innovations? Many engineers view their system power supply as a somewhat necessary evil. Historically, the power supply usually did not enhance the value of a system—if the power supply unit produced volts and amps and did not blow up, then it was fine. In the modern world, energy efficiency and effective uses of power have forced engineers to pay much more attention to it. And in many applications, the power supply does in fact enhance the value of a product to consumers, with the extreme end of the value proposition being solid-state lighting. In LED lighting, the customer experience is dictated by the quality of the power supply. That is also becoming true with devices such as smart phones in which the size of the power supply has become a branded item—Apple “cube chargers,” for example. Nowadays, many cell phone makers specify chargers that are small and inconspicuous, leveraging ordinary USB cables. Energy efficiency and compactness requirements make it very interesting from an engineering perspective. The field is much more challenging now, especially because of the heightened consumer awareness of power consumption.

Power Integrations’ specialty is in the high-voltage and high-power section of the industry. What are some of the company’s differentiating factors? Every company that is successful has to have an unfair advantage over its competitors. Power Integrations’ unfair advantage is the process technology that we have been developing and improving over 20 years, which allows us to connect a high-voltage MOSFET with a 15V controller on the same piece of silicon. Power MOSFETs switching high voltages at high currents through inductive loads generate voltage spikes and noise, which creates a challenging environment in terms of accuracy for an analog controller. That’s our fundamental technical advantage—we make a cost-effective and rugged MOSFET that can share a single die with a very accurate and highly specialized controller.

What are some of the challenges in developing your MOSFET technology? Noise is definitely one of the biggest challenges. Imagine a single piece of silicon with a very high current being switched in the same piece of silicon as the controller. Voltage spikes, substrate currents, and high-frequency switching noise are the kinds of challenges that need to be overcome in addition to interference from

TECH REPORT the FET with the controller and making sure the controller controls the FET properly. But this also provides opportunities. For example, the MOSFET and controller are closely coupled, and we gain a system advantage by tapping some of the energy out of the MOSFET to run the controller. We don’t need a separate power supply for it. This saves external components as well as energy during start up. We can also measure the MOSFET very accurately which enhances reliability.

What types of customers find your products attractive? Our products are often used in brand-name goods rather than low-end products. When you are selling a dishwasher or a microwave oven or something which has a high capital cost or high installation-servicing cost, you want to ensure that it lasts for a very long time. Our ability to manage the switch and prevent failures makes it appealing for these applications. Companies that are trying to develop a brand or a brand reputation for quality, even in low-power applications like a cell phone charger, don’t want customers to have a bad out-of-box experience in which they plug in the charger and smoke comes out of the device. Our products are designed and built to survive the harsh environment of the AC line, and our customers appreciate that.

What are the current ways to build a power supply, and how are they limited? There are two current ways to build a small power supply. One way is to use an optocoupler to measure the output and feed that back to the primary-side switch. The optocoupler is required because there must be a safety isolation barrier to protect the user from the high-voltage AC line. The low-voltage DC output may not be connected to the high-voltage side, or constructed in a way that it might become connected due to a simple fault of the power supply. Information about the output voltage and current is transmitted back across this isolation barrier using an optocoupler. Our TinySwitch™ and TOPSwitch™ product lines use this method. People really love using these products because they have been reliable performers over the years. Another method for building a small power supply is to employ a recent technology that uses primaryside regulation (PSR). This replaces the optocoupler with a winding on the transformer that’s used to sense what is happening on the secondary side. The primary side is where all of the intelligence is, and there is no measurement hardware on the secondary side. What you are trying to do is figure out the current flow in the winding and derive from this when the output diode is conducting or is not

“We make a cost-effective and rugged MOSFET that can share a single die with a very accurate and highly specialized controller.”

Power Developer conducting on the secondary side. If you know when the diode is conducting or not conducting, you can use algorithms to interpret what to do to manage the switcher’s power delivery. You know the diode is only going to conduct forward when it has higher voltage on the anode than the cathode side. In that respect, you can measure those currents and determine what is going on. It’s not as accurate because it is not a direct measurement, and it has a couple of other deficiencies. One of the main deficiencies is that you can’t see what is going on in the secondary side without sending a pulse. It’s like sonar—you send a ping or pulse in order to figure out what’s going on. The downside of sending a pulse is that you also send energy. The problem with sending energy occurs if you don’t need any such as during noload. In this case, the voltage will increase on the output capacitor. This is called peak charging, which ratchets up the output voltage of this device, even if you are not pulling any power from it. The answer, of course, is that you have to waste power. In no-load mode with a PSR circuit, you have to send pulses often enough to see what is going on in case of change. Those pulses of energy

need to be burned in a “dummy” load resistor to avoid voltage rise. A PSR is challenged to perform well in both transient response, from zero load to full load, and in no-load energy consumption.

Efficiency is so important to consumers in the market today. What is Power Integrations doing to solve the efficiency challenges that the industry is facing? The primary-side control technique we just discussed is inexpensive, and we have some great products, such as the LinkSwitch™ product line, that are primary-side controllers. Their challenge is to balance transient response and no-load; you can’t do both. We set up the devices to favor excellent no-load performance because battery charging applications are generally tolerant of transient response. If you wanted to achieve a very fast transient response, you had to use a TinySwitch or a TOPSwitch—until now. Power Integrations has created a new product that we are calling InnoSwitch™. This product uses a magnetic couple that is built into the package. This magnetic couple bridges the primary and secondary switches. It does all of

“InnoSwitch combines the entire active part of the power supply in the same package.”

TECH REPORT the sensing and measurement accurately on the secondary side and feeds that back to the primary side using the digital, high-frequency link we’re calling “FluxLink™.” InnoSwitch combines the entire active part of the power supply in the same package. FluxLink provides instant transient response, and because it is low power and digitally implemented, it also has very good no-load performance. As a further enhancement of efficiency, this product replaces the output diode with a MOSFET and controls that MOSFET so you don’t suffer diode losses on the secondary side. This feature is called synchronous rectification, and it allows you to save efficiency because you are not suffering from a diode drop. InnoSwitch is our newest contribution to power-supply efficiency enhancement.

In terms of real-world data, what kind of efficiency gains do you see compared to primary-side regulation and secondary-side regulation? With a primary-side controller, there is always a struggle to meet the new Department of Energy specifications for a small adapter. For example, let’s take a 5V, 2 A adapter, which is standard for something like a smart phone. There are new rules coming into effect in 2016, set up by the Department of Energy and a group called the Code of Conduct in Europe that require a very highefficiency rating. Our new technique exceeds those levels by 3 percent, which is a difference of between 79 and 83 percent efficiency in that particular application. With the primary-side technique, it would be a challenge to meet these specifications, whereas with the new InnoSwitch, it’s not.

Power Developer

Micrelâ&#x20AC;&#x2122;s MIC45212 Power Module

PRODUCT WATCH

Micrelâ&#x20AC;&#x2122;s

Highly-Integrated

Power Module Simplified design and exceptional performance Micrelâ&#x20AC;&#x2122;s highly-integrated power modules simplify the power design process while offering exceptional performance. Their MIC45212, 14-amp variant from a line of integrated, medium voltage DC-to-DC power modules, provides a DC-to-DC conversion solution that improves time to market and reduces total design size and complexity.

Power Developer Hardware Micrel MIC45212 Control Module

2 Power out

Control/ status pins

3 Voltage setting

Power in

PRODUCT WATCH

Specs The MIC45212 module is 12mm square by 4mm tall, which is 36 percent less volume than competing products at 15x15x4mm. The module integrates the inductor, PWM controller, power MOSFETs, and passives into the package. This integration reduces the total application size, simplifies the design and PCB layout, and improves reliability. Integrating the passives in the package also allows the module to achieve better EMI performance by reducing the parasitic capacitance and inductance that is unavoidable when placing these passives on the PCB. By integrating the passives, with this setup, Micrel is able to effectively reduce the AC loop size when compared to a traditional regulator with passives routed on the PCB. MIC45212 modules have been shown to pass the more stringent CISPR 22, Class B specification, simplifying PCB layout and reducing time to market.

Watch Video To watch a video overview and demonstration of Micrelâ&#x20AC;&#x2122;s MIC45212 module, click the image below:

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