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In general, high frequency operation allows the use of small-sized passive components in switch mode power supplies (SMPS), while it causes switching losses to increase in a hard switching mode. To reduce switching losses at high switching frequencies, many soft switching techniques have been developed. Among them, load resonant techniques and zero voltage transition techniques are widely used.
Load resonant techniques use a resonant feature of capacitors and inductors during whole switching period, which causes the switching frequency to be variable depending on the input voltage and load current. The change of the switching frequency, i.e., pulse frequency modulation (PFM) makes it difficult for designers to design an SMPS including input filters. Since there is no output inductor for filtering, the clamped voltage across output-rectifying diodes allows designers to select low voltage rating diodes. However, the absence of the output inductor burdens the output capacitors when the load current increases so that the load resonant techniques are not suitable for applications with high output current and low output voltage. On the other hand, the zero voltage transition techniques use a resonant feature between parasitic components during only the moment of turn-on and/or turn-off transitions in the switches. One of the advantages of these techniques is to use the parasitic components such as the leakage inductance of the main transformer and the output capacitance of the switches. So there is no need to add more external components to achieve soft switching. In addition, these techniques take pulse width modulation (PWM) up with fixed switching frequency. Therefore, these techniques are easier to understand, analyze, and design than load resonant techniques.
Due to its simple configuration and zero voltage-switching (ZVS) characteristic, an asymmetric PWM half-bridge converter is one of the most popular topologies using the zero voltage transition technique. Not only that, the ripple component of the output current becomes small enough to be handled by an appropriate output capacitor due to an output inductor compared with the load resonant topologies such as LLC converters. Being easy to analyze and design and having an output inductor, it is generally used for the applications with high output current and low output voltage e.g. PC power supplies and servers. To handle the output current more, a synchronous rectifier in the secondary side is widely used since the conduction losses can be obtained as ohmic losses instead of diode losses. It is much easier to implement the driver for the synchronous rectifier for an asymmetric PWM half-bridge converter than an LLC converter. In addition, a current doubler is a popular solution to increase the utilization of the main transformer when the output current is high.
This article describes the general features of the asymmetric PWM half-bridge converters with current doubler and synchronous rectifier. In addition, one example with some experimental results is shown using a power switch for asymmetric-controlled topologies.
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