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Wide-input, High-temperature GaN Active-clamp Flyback Converter with Continuous Current Operation Mode

Fig. 1. Dual-output active-clamp flyback converter.
Compared with traditional flyback topology, active-clamp flyback (ACF) topology is appealing to universities and industry due to its zero-voltage switching mechanism, relative simplicity com-pared with other topologies used in low power applications, and lower switch voltage stress. Fig. 1 shows the schematic circuit of the ACF. This topology is a variant of the flyback converter with the additional active-clamp circuit consisting of an auxiliary switch S2. The clamping capacitor Cr collects the leakage energy stored in the transformer and subsequently lowers the turn-off volt-age stress across the power switch. Zero-Voltage Switching (ZVS) is achieved by using the transformer leakage inductance and the Cr. Two methods have been proposed to achieve ZVS. One method runs in the critical current mode (CRM); the other runs in the continuous current mode (CCM). The former operates with bidirectional magnetizing current, which means that the transformer magnetizing current ripple current is allowed to become negative for a portion of each switching cycle. The Zero-Current Switching (ZCS) of the output rectifiers is satisfied when the secondary-side current Is reaches zero before the end of the switching cycle. In the CCM op-eration, the magnetizing current never reaches zero, and there is no ZCS in the output rectifiers. The main waveforms of the CRM and the CCM are shown in Fig. 2. The CRM is preferred when the loss in diodes are more dominating compared with the loss in the primary switches. In the case when the losses in the primary switches are dominating, it is more desirable to design the ACF running in CCM operation to restrict the transformer ripple current. The selection between the CRM and the CCM becomes difficult when accompanied with a wide input voltage range, as both the losses of the switches and diodes vary with the change in the input voltage, leading to the difficulty in addressing the optimized point. In this paper, the evaluation of a wide-input voltage, constant-frequency, soft-switching, ac-tive-clamp flyback converter is presented. The basic principle of the two operations is analyzed, and a design procedure of the ACF with a wide input voltage range is developed. Experimental results are then presented, illustrating the converter function and verifying the presented analysis.
Fig. 2. Main waveforms of (a) the CRM, and (b) the CCM in steady state.