Digitally controlled point of load converter with very fast transient response

Abstract

This paper presents a new Digital Self-Oscillating Modulator (DiSOM) that allows the duty cycle to be changed instantly. The DiSOM modulator is shown to have variable switching that is a function of the duty cycle. Compared to a more traditional digital PWM modulator based on a counter and comparator the DiSOM modulator allows the sampling frequency of the output voltage control loop to be higher than the switching frequency of the power converter, typically a DC/DC converter. The features of the DiSOM modulator makes it possible to design a digitally controlled DC/DC converter with linear voltage mode control and very fast transient response. The DiSOM modulator is combined with a digital PID compensator algorithm is implemented in a hybrid CPLD/FPGA and is used to control a synchronous Buck converter, which is used in typical Point of Load applications. The computational time is only three clock cycles from the time the A/D converter result is read by the control algorithm to the time the duty cycle command is updated. A typical POL converter has been built and the experimental results show that the transient response of the converter is very fast. The output voltage overshoot is only 2.5% of the nominal output voltage when a load step of 50% -100% of nominal output current is applied to the converter. The settling time is approximately 8 PWM cycles.

Sliding-Mode Control of a Dc/Dc Postfilter for Ripple Reduction and Efficiency Improvement in POL Applications

“…Moreover, the inductor current of the BuckPS first stage, that is, the Buck converter, and the output current are given in (10). Contrasting such results with the BuckS characteristics given in (2), it is recognized that the BuckPS requires three inductors instead of one, but such devices must support the half of the current imposed on the BuckS inductor:…”

“…This controller scheme requires an A/D converter, which increases the system cost in comparison with analog implementations. In the same way, [10] proposes a selfoscillating digital modulator to change instantaneously the dutycycle of the PWM signal driving the converter switch. Thus, it is possible to achieve a sampling frequency of the output voltage, required in the control loop, higher than the switching frequency of the power converter.…”

Digitally controlled envelope tracking power supply for an RF power amplifier

“…where D is the duty cycle, n is the number of bits used to represent the Ref input, f clock is the clock frequency of the integrator and Window is the hysteresis window [9]. Another important feature of the DiSOM is that the switching output is fed back to the integrator whereby any noise due to clock frequency quantization is integrated.…”

“…The internal resolution of the digital PID compensator is 24 bits and the ADC decoding schemes has 85 values, resulting in a total memory allocation for the lookup tables of 765 bytes. A detailed description of a similar lookup table based PID compensator can be found in [9].…”