International Journal of Modern Research in Engineering & Management (IJMREM) ||Volume|| 1||Issue|| 6 ||Pages|| 01-08 || June 2018|| ISSN: 2581-4540
Digital Current Mode Controller for Buck Converter 1,
Ahsan Hanif, 2, Prof. Dr. Abdul Sattar Larik, 3, Dr. Anwar A. Sahito 1,
Student M.E. (Electrical Power) IICT, Mehran UET Jamshoro; 2, Professor, Electrical Engg. Mehran UET, Jamshoro 3, Associate Professor, Electrical Engg. Mehran UET, Jamshoro
--------------------------------------------------------ABSTRACT-------------------------------------------------Power electronics applications are widely used in different fields of engineering like computer, Telecommunication, electrical power and Mechanical), one of the most useful power electronics converters is DC-DC buck converter. Owing to its numerous applications, its performance needs to be improved through a suitable controller. In this Paper, A digital current mode controller is proposed and implemented for Buck converter. Proposed current mode control technique is simulated in MATLAB/SIMULINK and results are validated through hardware implementation. Both simulation and experimental analysis show effectiveness of the proposed controller.
INDEX TERMS: Buck converter, Digital current mode controller, Steady-State response, Atmega16 Microcontroller ----------------------------------------------------------------------------------------------------------------------------- ---------Date of Submission: Date, 30 May 2018 Date of Accepted: 04 June 2018 -------------------------------------------------------------------------------------------------------------------------------------- -
I.
INTRODUCTION
Today the most interesting topic of study in technical profession is the power electronics components configurations which are called as the power converter. Power electronics converters are of four types namely AC to AC (Cyclo-converters), DC to DC (Choppers), DC to AC (Inverters) and AC to DC (Rectifiers). These converters are widely used in variety of applications [1, 2].Power converters can be configured in different ways, whether these are isolated or non-isolated. Isolation is provided by means of a nonphysical connection between the converter and source such as transformer. In power converters the power switches are used (normally MOSFETS, IGBTs) which have a sensitive terminal, hence require an opto isolator to provide the external high frequency signal [1, 3]. Switch mode controllers for power electronic converters operate according to the switching pulses on power switches at high frequencies. The controlling opportunity and the reduced size are obtained at higher frequency operation thus overall cost is reduced. Real time switching in the power converter produces some nonlinearity. Thus, the converter becomes nonlinear itself, these nonlinearities are considered in designing a controller. DC-DC converter operates in the absence of a closed loop controller (open loop), and the voltage regulation cannot be achieved for very long time. Moreover, high overshoot and settling time must be maintained for the controller designing, (Raymond B. Ridley 1991) presented the continuous-time model for current mode control. One of the earliest control models which had proved to be very accurate at half of the switching frequency. But there were many problems with designing and implementing such a model due to the complicated mathematical modeling [1]. Some of the last decades, Digital controllers are designed to enhance the performance of the digital signal processors (DSPs), microcontrollers, and field-programmable gate arrays (FPGAs) has found some extensive applications in motor-drive controllers and high-voltage and high current power electronics. In developing these kinds of control designing, the low switching frequencies, e.g. at tens of kilohertz are examined, therefore some restriction must be in consideration when dealing with these types of controllers. Continuous research has leaded the digital controller technology to the development of highly integrated, least cost and improved performance models. In these types of controller, such as these are integrated on Very LargeScale Integration(VLSI) Chip IC, there are some conditional blocks within those, Advance Analog to Digital Converter’s(ADCs); Digital I/Os; flash ROM; CPU for programming, communication, diagnostics, power management, etc, because these blocks are extensively used for most of the digital applications. Digital controller not only provides the tight output voltage regulation but better performance than the analog controllers [2, 3]. Work done in [2] uses adaptive digital current mode controller for DC-DC converter which was somewhat unrealistic when it comes to the implementation in a hardware model. Digital PWM based controller based on time domain analysis was developed and tested in [5], which shows good response but complicated circuitry was a major disadvantage of the proposed controller.
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Digital Current Mode Controller for Buck… Cascaded controller is designed and proposed in [6] and implemented through analog op-amps circuits. Performance of the controller was based on critical damping condition and thus shows no overshoot but need more settling time.
II.
BUCK CONVERTER
The buck converter performs the operation of stepping down the output voltage from the high input voltage; an analogous to the step-down transformer in AC systems. It is the simplest converter in DC-DC power converters designed with the help of static switch (e.g. MOSFET), diode, inductor and capacitor. The buck converter has two modes of conduction called as Continuous Conduction mode (CCM) and Discontinuous Conduction Mode (DCM). Operation of buck converter depends upon switching of MOSFET. When MOSFET is conducting, inductor current is increasing thus having a positive going ramp. When MOSFET is in OFF condition, inductor current decreases thus having a negative going ramp. In CCM inductor current start a positive going ramp without reaching to zero while in DCM, it reaches to zero and remains there for a time period and then starts positive going ramp [4]. Figure 1 shows simple circuit diagram of the buck converter using a switch where switching signal will be applied.
Figure 1: Circuit Diagram of Buck Converter According to the mathematical model, the duty cycle is given as in Eq. (1)
D=
ton Vo = t Vin
(1)
The power switch conduct current to load initially through inductor (which stores energy), capacitor (which reduces the ripples in output voltage), and the diode is reverse biased, then according to the Kirchhoff’s current and voltage laws the mode 1 can be represented by Eq 2.
iL. 0 .= vo 1 C
1 1 L iL + L V 1 vo in − 0 RC −
(2)
The mode 2 can be stated that the power switch will not conduct when the inductor is fully charged; the inductor current starts discharges through the capacitor and diode which is represented by Eq 3
i 0 .= vo 1 C . L
1 L iL 0 + V 1 vo 0 in − RC −
(3)
Overall operation of the buck converter for the two modes can be analyzed by multiplying the right-hand sides of Eq 2 and Eq 3 with the addition of D and (1-D) Respectively, Eq 4
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Digital Current Mode Controller for Buck…
iL. 0 .= vo 1 C
1 D L iL + L V 1 vo in − 0 RC −
(4)
The simulation diagram of the buck converter is shown in Figure 2
Figure 2: State space model of buck converter
III. SWITCHING CONTROLLERS Switching controller can be operated in many ways by means of different topologies in the feedback loop for the DC-DC converters [5]. Types and orders of different controllers vary with the application requirements. Commonly used switching controllers are voltage mode control, current mode control, sliding mode control and V2 mode control. Each of them has some advantages and disadvantages over the others [3] and the designer have to choose according to applications requirements second order nonlinear controller is used in [4] to improve the transient Response of DC-DC converter. Controllers are used to produce an inter relationship between the inputs and outputs of the converter for maintaining the regulation at the output. In Voltage mode controller, the feedback signals is taken from the output of the converter in the form of voltage and sends to the input of PWM after modifying and compare it with the reference signal. An error signal is generated for the output of the voltage amplifier and this signal fed as input of the PWM, the other input is fixed with the constant ramp amplitude provided from the internal clock. Internal clock pushes the output signal of voltage controller to the power stage through a switch driver. Switching signal is generated through comparator. Duty cycle of the switch varies to regulate output voltage [5]. In case of a current mode controller implementation, the error signal need to be generated through a voltage comparator by comparing the feedback voltage with a reference signal at the input of it, and this error signal appear as the input of the PWM similarly as in voltage mode controller. In current mode control, the second input is taken from inductor current, which is in the form of ramp. The second loop comparator generates directly signal for the switch. This inclusion of second current loop increases control action. Hence these converters are also called cascaded controllers.
IV.
SYSTEM DESCRIPTION
In this research work, current mode controller is designed for Buck converter. As discussed earlier there are two loops in current mode controller; an outer voltage loop and the inner current loop. Voltage signal for outer voltage control loop is taken from the output capacitor and the current signal from the inductor in the power stage. An AVR atmega16 microcontroller is used for these purposes; the ADC in the microcontroller takes the signal and delivers to the power switch after the modification. Beside the Hardware implementation the model was successfully tested using the MATLAB/SIMULINK. The Simulink model of current mode controller for buck converter is shown in fig.3, The parameters of the converter are: Vs=35V, Vref=10V, f=350kHz, R= 10ohms, L=68uH, C=100uF, the actual output signal is compared against the reference voltage and the resulting error signal is transformed into the controlled PWM signal by means of current compensator. The analog signal from the voltage error amplifier is processed by the discrete time integral compensator in current loop, the PWM generated is fed to the power MOSFET switch in buck converter. Simulation results for inductor current and output voltage are observed at different parameter values of the component blocks used in the model.
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Digital Current Mode Controller for Buck‌
Figure 3: Simulation Model of Current Mode Controller for Buck Converter The Hardware model shown in figure 4 contains digital implementation of current mode controller for buck converter. Microcontroller (atmega16) is used for implementation of the control strategy and generation of required PWM signal for switching of the MOSFET in buck converter. An Opt coupler is used as a gate driver for power MOSFET.
Figure 4: Hardware Model
V.
RESULTS AND DISCUSSIONS
Developed simulation and hardware models of the buck converter with digital current mode control scheme are analyzed under varying operating conditions. Figures 5 and 6 show inductor current and output voltage response for initial transient at start of the buck converter with proposed digital current mode controller. There is an Overshoot of 0.75A, which is then settled after 20ms and gained the stable state at 1A. The Output Voltage at has an Overshoot of 2V and settling time of 20ms.
Figure 5: Simulation Diagram of Inductor Current
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Digital Current Mode Controller for Buck‌
Figure 6: Simulation Diagram of Output Voltage Figures 7 and 8 show output voltage and inductor current response for supply variation from 35 to 50V. The Output voltage at has an undershoot of 2V and settling time of 15ms. No overshoot is observed for inductor current which settles down in 15 ms.
Figure 7: Output Voltage When Vin is changed from 35V to 50V
Figure 8: Inductor Current When Vin is Changed from 35V to 50V Figures 9 and 10 show output voltage and inductor current response for supply variation from 35 to 20V. The Output voltage at has an undershoot of 2V and settling time of 15ms. No overshoot is observed for inductor current which settles down in 15 ms.
Figure 9: Simulation of Output Voltage at Vin= 35V to 20V
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Digital Current Mode Controller for Buck…
Figure 10: Simulation of Inductor Current at Vin= 35V to 20V Figures 11 and 12 show output voltage and inductor current response for load variation from 10Ω to 8Ω. The Output voltage has an undershoot of 2V and settling time of 15ms. No overshoot is observed for inductor current which settles down in 15 ms.
Figure 11: Output voltage when load varies from 10Ω to 8Ω
Figure 12: Inductor current when load varies from 10Ω to 8Ω Figures 13 and 14 show output voltage and inductor current response for load variation from 10Ω to 14Ω. The Output voltage has an undershoot of 0.5V and settling time of 15ms.
Figure 13: Output voltage when load varies from 10Ω to 14Ω
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Digital Current Mode Controller for Buck…
Figure 14: Inductor Current when load varies from 10Ω to 14Ω Hardware prototype of buck converter with digital current mode control is tested in laboratory. Now the hardware results are presented to explain the comparison between simulation and Hardware model of the proposed Current mode controller, as in figure 14 shows that the input voltage of the buck converter is kept to 20V(Vin=20V) and the output voltage was noticed as 10V.
Figure 15: Shows that the Vin =20V and Vout=10V
Figure 16: Output voltage when the input Voltage is increased suddenly If the input voltage at the input of the buck converter suddenly increases from 20V to 24V, then the output Voltage would be as shown in figure 15 which is 10V and the Corresponding Duty Cycle would be 40%.
Figure 17: The Output Voltage when the Input Voltage is decreased suddenly
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Digital Current Mode Controller for Buck… In Hardware model certain more readings at different line variations were taken which is just as shown in figure 19, as the input Voltage is getting increase from 20V to 35V the duty cycle is decreasing to 28%.
VI.
CONCLUSIONS
This paper has explained a different innovative method to apply the Digital Current mode controller for Buck converter using Simulink and Hardware model (based on microcontroller atmega16). It is seen that the proportional and integral gains of Voltage loop (PI controller) can be selected with respect to the input error signal value to significantly improved the steady state response of the DC-DC converter. The implementation involves MATLAB/Simulink based digital compensator which is well suited for practicing high-frequency power switching controllers. The digital Current mode controller is designed for buck converter and its steady state characteristics are experimentally tested which were very much authentic. Simulation and experimental results are shown for 35V-to-10V point-of-load Buck converter to show the improved steady state response.
VII. ACKNOWLEDGEMNTS Authors are thankful to Mehran University of Engineering & Technology Jamshoro for providing necessary resources for carrying this research work.
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Ahsan Hanif, et al. “Digital Current Mode Controller for Buck Converter.” International Journal of Modern Research In Engineering & Management (IJMREM), vol. 1, no. 6, 10 June 2018, pp. 01–8., www.ijmrem.com. www.ijmrem.com
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