# Resonant Asymmetrical Half-Bridge Flyback Converter
## 1 Introduction  
The flyback converter is widely used in the low to medium output power range because of the advantages of fewer circuit components and simple control. Compared with the LLC converter, the flyback converter requires fewer components, but the switching voltage stress of the flyback converter is higher than that of the LLC converter. The voltage stress of the main switch of the flyback converter is the input voltage plus the reflection voltage and spike voltage. The snubber circuit is added to switches of the flyback converter to reduce the voltage surge. However, the voltage stress of an LLC converter is only equal to the input voltage, so low-voltage components can be used. The advantage of flyback converter over LLC converter is the wide input voltage and the simple control method, so that no pre-stage voltage regulator is required. Although the research in [1,2] provides the input voltage range with wide variation range of switching frequency, it also results in efficiency reduction and EMI problems. The multi-level approaches proposed in [3, 4, 5, 6, 7, 8, 9, 10, 11] make LLC converters extend the operating voltage range, but still have the problems of the number of components, complex structure, and difficult control, so as to make it difficult to balance cost and performance. Based on the reasons above, the flyback-based topologies are still attractive solutions.  
To increase conversion efficiency, more technologies have been added to the conventional flyback converter, such as quasi-resonance (QR) [12, 13, 14, 15] or active clamping [16, 17, 18, 19] to achieve zero-voltage switching (ZVS) and reduce switching loss. Synchronous rectification (SR) [20, 21] can also be applied to reduce the conduction loss and further improve the conversion efficiency. The difference in the number of components between the active clamp flyback (ACF) converter and the LLC converter is that the ACF converter needs only a single secondary SR switch, which is the cost advantage of the ACF converter.  
The asymmetric half-bridge (AHB) flyback converter has been proposed in previous studies [20, 21, 22, 23, 24, 25, 26, 27, 28, 29], whose main advantage is that the number of components is the same as that of ACF converter, and it also provides the same voltage stress on the switchingelements as the LLC converter with ZVS turn-on. Most studies of AHB flyback converters adopt pulse width modulation (PWM) control.  
However, previous researches in [20; 21; 22; 23; 24; 25; 26; 27] suggest that PWM should not be used because \(T_{\mathit{off}}\) of PWM mode varies with the input voltage and load condition to affect the ZVS operation range. In [28], the AHB flyback converter is unstable with the PWM control method. The article [20; 30] proposes a resonant AHB flyback, which provides a ZVS function on its switches. However, the efficiency at light load and dynamic response are not mentioned.  
The resonant high step-down converter [31], shown in Figure 1, is proposed with constant off-time (COT) and pulse frequency modulation (PFM), which is verified with a wide range of ZVS operations. In this paper, a resonant AHB flyback converter with COT control is proposed as shown in Figure 2 to improve the ZVS operation. A prototype with a specification input of 48 V to output 2.5 V/8 A is made to verify that the concept is feasible.