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Microcontroller-based Critical Mode Control with Improved Zero-Current-Detection for MHz Totem-pole PFC

Fig. 1. Waveform with the improved CRM control method.
In order to implement critical conduction mode (CRM) control for a GaN-based MHz totem-pole PFC with a commercial low-cost microcontroller (MCU), a novel but very simple zero-current-detection (ZCD) method is proposed to solve a signal processing delay related issue, which is significant at MHz high frequency.

Previously, the inductor current positive-to-negative zero-crossing is sensed to trigger SR turn-off. However, a signal processing delay in the MCU and gate driver makes the SR turn off later, which causes a significant unwanted negative current at MHz high frequency. The unwanted negative current leads to an increase of current ripple, conduction loss and EMI filter size.

In this paper, an inductor current positive-to-negative zero-crossing is used for triggering control switch turn-off. Ideally, the interval between the zero-crossing instant and the control switch turn-off instant is the on-time, T_on, which means after zero-crossing is sensed, delay by T_on, and then turn off control switch. Considering the signal processing delay, since the on-time is typically greater than this delay, compensation is easily made by subtracting this delay from T_on to get the actual time delay from the sensed zero-crossing instant to control switch turn-off instant, which the MCU needs to know, as shown in Fig. 1.

Also for the MCU, the actual time delay for SR turn-off and for control switch turn-on are shown as TSR_delay and Tdelay in Fig. 1.

With this improved ZCD method, the inductor current waveform in half line-cycle is shown in Fig. 2. The unwanted negative current, and its impact on conduction loss and on EMI filter size are eliminated.

Fig. 2. Line-cycle inductor current simulation waveform (a) without improved ZCD (b) with improved ZCD.
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