3 Direct Peak Current Mode Control

Qiu et al. [1] have introduced a direct current control technique for LLC resonant converters, which focuses on regulating the peak current flowing through the resonant tank. This method provides precise control over the converter’s output.

In this approach, the resonant tank current is continuously monitored using a current transformer. The transformer’s secondary winding delivers a scaled current signal to a peak detector and hold circuit (see Figure 4-2), which captures and retains the highest current value during each switching cycle.

The output from the peak detector is then routed to a comparator. The comparator’s non-inverting input is connected to a Digital-to-Analog Converter (DAC) that supplies a reference voltage.

When the output of the peak detector matches the DAC reference voltage, the comparator output goes high. This high signal serves as a capture event for the resonant mode control logic, immediately truncating the high-side switch. The high-side on-time is then applied to the low-side switch, ensuring symmetrical operation and effective cycle-by-cycle current regulation.

Furthermore, an additional signal is used to reset the peak detector and hold circuit by discharging the hold capacitor, preparing the system for the next measurement cycle.

The timing and behavior of these signals, including the current waveform and control pulses, are illustrated below. Here, the peak hold capacitor is being discharged by the PWM1L signal.