Critical-Mode-Based Soft-Switching Modulation for High-Frequency ThreePhase Bi-Directional ACDC Converters
Abstract: In this paper, a novel critical-conduction-mode (CRM)-based modulation is proposed for three-phase bi-directional AC/DC converters. With this modulation, the switching frequency variation range shrinks, zero-voltage-switch (ZVS) soft switching is achieved and the switching related loss is reduced, which is especially beneficial for systems operating above hundreds of kHz high switching frequency with wide-band-gap (WBG) power semiconductor devices to achieve both high power density and high efficiency. A 25 kW SiC-based high-frequency three-phase bi-directional AC/DC converter prototype is designed to achieve a power density of 127 W/in3, which is at least five times higher than commercial products. All the control functions are digitally implemented with one low-cost microcontroller (MCU), and the aforementioned benefits are experimentally verified on this prototype under both inverter mode and rectifier mode operations. With the proposed soft-switching modulation, the tested peak efficiency is close to 99.0% for this prototype even at above 300 kHz high switching frequency operation. Existing system:
It is found the switching frequency variation range is wider as the modulation index goes higher. Although the time-variant switching frequency over a line cycle is an intrinsic feature in CRM AC/DC converters, a wide switching frequency variation range brings large switching related loss and difficulty in control. Additionally, these three-phase four-wire topologies cannot work if the AC line-toneutral voltage is higher than half of the DC voltage (M > 0.866). In sum, the above three-phase four-wire topologies to decouple three phases have limitations under high modulation index conditions. Proposed system: Therefore, at high switching frequency operation, zero-voltage-switch (ZVS) turnon is preferred since the dominant turn-on loss can be totally eliminated, while the turn-off loss is small. To achieve ZVS soft switching turn-on, critical conduction mode (CRM) is preferred because it is simple while not adding any physical complexity to the system. Compared with CCM hard switching, although the inductor current ripple increases at CRM operation and there is an increase in turnoff loss and conduction loss, the total device loss can still be reduced. Therefore, CRM is the preferred operation mode for SiC-based systems to achieve high efficiency at high switching frequency operation. In [16-18], high-frequency CRM control combined with average current regulation is proposed to achieve ZVS soft switching. Advantages: electrical energy is becoming more and more important for human life all over the world, and three-phase bi-directional ac/dc converters are widely used in grid-tied applications, such as the photovoltaic (pv) inverters in solar energy systems for the harvest of electrical energy and the interface with power grids, and the chargers in electric vehicle (ev) charging systems for storage and utilization of electrical energy from power grids. the efficiency of those systems is very high. for the commercial products of string pv inverter systems and of ev charging systems Disadvantages:
In three-phase AC/DC systems, however, only two phases are independent, and therefore simple independent CRM control cannot be achieved for all three phases at the same time, which is the main challenge of CRM control in three-phase AC/DC converters. To overcome this, one method is to decouple three phases by using split capacitors at the DC side and connecting the DC side midpoint to the AC side neutral point, which is reported in the literature, based on two-level H-bridge structure , and three-level T-type structure [20], respectively. In this way, these two structures become three-phase four-wire systems. Both of them allow bi-directional operation. Modules: Discontinuous Pulse Width Modulation (DPWM) for Decoupled Control among Three Phases : From the literature, three-phase three-wire systems have commonly used discontinuous pulse width modulation (DPWM). With DPWM, at any instant during the whole line cycle, one phase is clamped to the positive or negative DC bus, while the other two phases operate at high-frequency pulse width modulation (PWM). Under unity power factor condition, the most common DPWM clamping option is to apply clamping to the phase with the highest amplitude of AC reference current (Iref) to reduce switching loss. The polarity of the AC reference current determines whether to clamp to the positive or negative DC bus. This clamping option over a whole line cycle is shown in Fig. 1. (θ = ωt is the line-cycle phase angle with the unit of degree; different θ values indicate different line-cycle operating points). Taking the first 60 ̊ line-cycle interval as an example, phase B is clamped to the negative DC bus, which is denoted as “B N”. Synchronous rectifier : provides extra conduction time for the synchronous rectifier (SR) switch (SA2, SC2 in inverter mode while SA1, SC1 in rectifier mode) after the inductor current zero crossing occurs to provide some amount of negative inductor current when necessary, and thus ZVS turn-on of the control switch (SA1, SC1 in inverter mode while SA2, SC2 in rectifier mode) is achievable. The constant-slope saw tooth
signal Se is reset to zero once every switching cycle after the inductor current zero crossing is sensed by ZCD and extra off-time is applied by the off-time extender. Meanwhile, the control signal VCtrl is generated from the average current loop compensator HC. The intersections between Se and VCtrl determine the switching instants of the control switch, and when VCtrl is higher than Se, the control switch is during ON state. The gate signal of the SR switch (VGS,SR) is complementary to that of the control switch (VGS,Ctrl) when ignoring the dead-time. Phase A and phase C are controlled independently according to the above control concept of CRM with average current regulation. Synchronization of Switching Frequency (Fs sync) for Limiting Switching Frequency Range : According to the previous analysis, one simple method for limiting the switching frequency variation range and for eliminating the oscillation is synchronizing the switching frequencies in the two phases operating at high frequency PWM. For a better understanding, consider the first 30 ̊ line-cycle interval as an example. According to Fig. 3 (a), switching frequency in phase A is higher than switching frequency in phase C. Therefore, switching frequency in phase A needs to be synchronized to that in phase C. The way to implement this concept of synchronization of switching frequency (Fs sync) is to change the operation mode in phase A from CRM to discontinuous conduction mode (DCM), while phase C still operates at CRM. The turn-on instant in phase C is determined by the inductor current zero crossing point in phase C, while the turn-on instant in phase A are determined by the information from phase C in every switching cycle to keep switching frequencies in these two phases synchronized. Operation Analysis Of The Proposed Modulation : Another important aspect of the performance with the proposed modulation is whether ZVS is achieved during a whole line cycle. Take the first 30 ̊ line-cycle interval as an example where phase A operates at DCM, phase C operates at CRM, and phase B is clamped to the negative DC bus. Considering the device output capacitance (Coss), for both phase A and phase C, in each switching cycle after inductor current zero crossing happened, LC resonance between the converter-side inductor (hereinafter “inductor” unless otherwise specified) and the device output
capacitance occurs, which brings a small amount of negative inductor current and helps to discharge the control switch output capacitance (and thus the control switch drain-source voltage is reduced). However, before the control switches are turned on in phase A and in phase C, inductor currents have already touched zero and LC resonance occurs in both of these two phases, which makes the equivalent circuit become a 4th-order LC resonance circuit at this instant and therefore very complicated to derive analytical expressions of device drain-source voltage.