DIGITAL CONTROL OF POWER CONVERTERS 4 Advanced controllers
Autotuning
Autotuning Techniques for Digitally-Controlled Point-of-Load Converters with Wide Range of Capacitive Loads Shirazi, M. Zane, R. Maksimovic, D. Corradini, L. Mattavelli, P. APEC 2007 Design challenges in PoL Converters Number, types and values of load decoupling capacitors often unknown to the designer Component tolerances Temperature variations or aging Equivalent C and ESR can vary orders of magnitude Digital control of Power Converters 3
Autotuning Techniques for Digitally-Controlled Point-of-Load Converters with Wide Range of Capacitive Loads Shirazi, M. Zane, R. Maksimovic, D. Corradini, L. Mattavelli, P. APEC 2007 Digital control of Power Converters 4
Self-tuning digital PID controller Objective of the tuning algorithm: determine the PID parameters (K,z1,z2) to maximize closed-loop bandwidth while meeting stability margins and dynamic performance specifications Digital control of Power Converters 5
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Theoretical basics The dc-dc converter can be viewed as a linear, time invariant system impulse response output voltage duty cycle noise and perturbations Input-output cross correlation A Modified Cross-Correlation Method for System Identification of Power Converters with Digital Control Botao Miao, R.egan Zane, Dragan Digital Maksimovic control of Power Converters 9
Theoretical basics II if u(k) is white noise So, the cross correlation is the impulse response Digital control of Power Converters 10
Theoretical basics III How to generate white noise PSRBS (Pseudo-Ramdom Binary Sequence) The data length for one period of an n-bit maximum length PRBS is given by M = 2 n - 1, and the signal itself has only two possible values:+/- e Digital control of Power Converters 11
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Limit cycle based autotuning typical limit cycle oscillation Limit cycle oscillations are caused by non-linear quantization effects in ACD and DPWM. When the resolution of the DPWM is low (compared to ADC)the quantized output cannot result in zero error Digital control of Power Converters 21
Limit cycle based autotuning typical limit cycle oscillation Not desirable in steady-state Contain valuable information Characterized by A max, A min, T LC In a digitally controlled power supply they depend on the inductance, capacitance and load (they depend on the digital controller and the input voltage) Only three parameters can be determined In this case only the T LC and the peak to peak amplitude will be used to calculate the C0 and the load frequency amplitude Digital control of Power Converters 22
Limit cycle based autotuning Auto-tuner block diagram Not desirable in steady-state Contain valuable information Characterized by A max, A min, T LC In a digitally controlled power supply they depend on the inductance, capacitance and load (they depend on the digital controller and the input voltage) Only three parameters can be determined In this case only the T LC and the peak to peak amplitude will be used to calculate the C0 and the load Digital control of Power Converters 23
Limit cycle based autotuning When LCO is steadily excited the total loop has a gain of 1 and a phase shift of 180 degrees N DPWM (A LC, ε) describes the gain of the DPWM to obtain the gain of the DPWM we use describing functions [53] [53] H. Peng, D. Maksimovi c, A. Prodi c, and E. Alarcon, Modeling of quantization effects in digitally controlled DC- DC converters, in Proc. IEEE Power Electronics Specialist Conference, 2004, pp. 4312 4318. Digital control of Power Converters 24
Limit cycle based autotuning Digital control of Power Converters 25
The power stage of the system is: 12V-to-5V, 10W buck converter Fs 200 khz. All functional blocks of the digital controller but DPWM are realized using an Analog Devices ADMC-401 DSP Board. An Altera 10K FPGA system is used for the DPWM DPWM resolution steady-state 8 bits Identification 4 bits Pre-stored PID coefficients are placed in three 30 word 10-bit look-up tables 200 mv/div-ac Digital control of Power Converters 26
PEF (Predictor Error Filter) based Autotuning Digital control of Power Converters 27
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