Design of a Simple Feed-Forward Controller for DC-DC Buck Converter

Design of a Simple Feed-Forward Controller for DC-DC Buck Converter Shu Wu, Yasunori Kobori, Zachary Nosker, Murong Li, Feng Zhao, Li Quan, Qiulin Zhu, Nobukazu Takai, Haruo Kobayashi Gunma University, Japan Tetsuji Yamaguchi, Eiji Shikata, Tsuyoshi Kaneko, AKM Technology Corporation Kimio Ueda Asahi Kasei Microdevices 2013/11/16 1

Outline Research Objective Proposed Feed-forward Control Method Capacitor Charge Balance Proposed Feed-forward Controller Simulation Results SISO Buck Converter Simulation SIDO Buck Converter Simulation Conclusion 2013/11/16 2

Outline Research Objective Proposed Feed-forward Control Method Capacitor Charge Balance Proposed Feed-forward Controller Simulation Results SISO Buck Converter Simulation SIDO Buck Converter Simulation Conclusion 2013/11/16 3

Background Dual Power Supply Circuit (DC-DC converter) Single Inductor + + Conventional approach SIDO Converters Reduce number of inductors Reduce cost Reduce volume SIDO: Single Inductor Dual Output 2013/11/16 4

Cross-Regulation SIDO buck converter with feedback control Essential problem: transient response 2013/11/16 5

Conventional Method Feed-back control Control Delay Feed-forward + Feed-back control Accurate control variables modulation are required digital non-linear feed-forward control Complicated for SIDO converter Not cost-effective 2013/11/16 6

Principle Research Objective Charge balance of output capacitor Advantage Simple Fast transient response Cross-regulation improvement for SIDO buck converter 2013/11/16 7

Outline Research Objective Proposed Feed-forward Control Method Capacitor Charge Balance Proposed Feed-forward Controller Simulation Results SISO Buck Converter Simulation SIDO Buck Converter Simulation Conclusion 2013/11/16 8

Charge Balance (1) SISO buck converter Charge by input and inductor Discharge to output 2013/11/16 9

Charge Balance (2) I I L I o I o Steady State I C Charge=Discharge 0 T s I C dt = 0 0 T s I c t Load is changed I L can t step change I o Capacitor more discharge discharge I C Charge Discharge 2013/11/16 10

Charge Balance (3) S 0 decides how much power is supplied SIDO buck converter with exclusive control S 2 decides which converter is served 2013/11/16 11 Timing chart

Charge Balance (4) I I o1 + I o2 Server for converter1 Server for converter2 Steady State A=B = C=D I o1 T s I o1 I c1 I c2 This balance is useless for feed-forward control I o2 A D C B t Approximate Balance I o2 T s I C1 + I C2 dt 0 I o1 I o1 0 2013/11/16 12

System Block Diagram S 0 L I c V o C load PWM Comp Sawtooth Generator - + V con Feed-forward controller V error V con + + V Feed-back controller 1 period integrator N-period integrator Error Amplifier Amp - + V ref Capacitor current sensor 2013/11/16 13

Integrate the current of output capacitor (1) Integrator Circuit 2013/11/16 14

Integrate the current of output capacitor (2) Integrator Timing Chart CLK CLK2 CLK1 V 1 DC bias t 3 2 T s t V I_peak = 1 R 3 C I T s V i_c t dt 0 2013/11/16 15

Control Variable Compensation (1) N-period integrator V I_1 V I_2 V I_nT V I_n V I_1 V I_2 V I_n N-period integrator Timing chart 2013/11/16 16

Control Variable Compensation (2) V I_nT ---output of N-period integrator V con ---single period integration control variable 2013/11/16 V ---constant compensation 17

Peak voltage of sawtooth--v P_S (V) Saw-tooth Generator 25 20 15 Sawtooth 10 5 0 0.5 1 1.5 2 2.5 3 3.5 4 4.5 5 5.5 6 6.5 7 7.5 8 Control variable-v con (V) Sawtooth Generator 2013/11/16 18

Outline Research Objective Proposed Feed-forward Control Method Capacitor Charge Balance Proposed Feed-forward Controller Simulation Results SISO Buck Converter Simulation SIDO Buck Converter Simulation Conclusion 2013/11/16 19

Parameters and Phase Compensation Converter Parameters V in Input voltage 12V V out Output voltage SISO 6V L Inductor 20μH C Output capacitor 500μF ESR Equivalent Series Resistance 5mΩ f switch Switching frequency 500kHz SIDO 6V, 4V Open-loop Bode Plot Phase Compensation Open-loop LC filter Phase compensation Phase Margin 45 f cross = 20KHz 2013/11/16 20

SISO buck converter(1) Without feed-forward With feed-forward Inductor current Output voltage I o : 0.5A 1A Output under-shoot: Decrease 11mV 5mV Response time: Decrease 500μ s 8 μ s (1) load current (2) inductor current (FB) (3) inductor current FF (4) output voltage (FB) (5) output voltage (6) PWM signal (7) saw-tooth signal 2013/11/16 21

SISO buck converter(2) Without feed-forward With feed-forward Inductor current I o : 0.5A 1.5A Output voltage (1) load current (2) inductor current (FB) (3) inductor current FF (4) output voltage (FB) (5) output voltage (6) PWM signal (7) saw-tooth signal 2013/11/16 22

SIDO buck converter(1) I o1 : 0.5A 0.96A, I o2 : 0.5A Without feed-forward With feed-forward 6V 4V Feed-forward With Without Self-regulation -- 25mV Cross-regulation -- 30mV (1) load current of sub-converter1 (2) load current of sub-converter2 2013/11/16 23

SIDO buck converter(2) I o1 : 0.5A, I o2 : 0.5A 1.85A Without feed-forward With feed-forward 6V 4V Feed-forward With Without Self-regulation 5mV 35mV Cross-regulation 10mV 40mV (1) load current of sub-converter1 (2) load current of sub-converter2 2013/11/16 24

Outline Research Objective Proposed Feed-forward Control Method Capacitor Charge Balance Proposed Feed-forward Controller Simulation Results SISO Buck Converter Simulation SIDO Buck Converter Simulation Conclusion 2013/11/16 25

Conclusion Proposed new feed-forward controller Simple: Only output capacitor current is detected Digital nonlinear calculation not required Applicable to SIDO converter Cost-effective Available: Transient response is Significantly improved Cross-regulation of SIDO converter is improved 2013/11/16 26

The End Thanks for your attention 2013/11/16 27

Presented by 呉澍 (Wu Shu) 2013/11/16 28

Q&A 2013/11/16 29

Question and Answer 1 The simulation result in page 21 and 22, there is oscillation in output voltage after transient response. Is it a right result? No, it is not a good result. This oscillation is caused by qualitatively multi-period integration compensation. But feed-forward require accurate control variable modulation. This problem will be improved in further.

Question and Answer 2 Normally when we talk about the voltage control mode and the current control mode of switching power supplies, they belong to feedback control scheme. What is the different between your proposed method and current control mode? Current control mode is feedback control. Inductor current is detected and compared with error signal. By this comparison, duty cycle is regulated. While our proposed method use capacitor current to regulate sawtooth signal. When transient response happen, we don t need wait until error signal is changed, then regulate duty cycle.