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PWM controllers & resonant controllers – Getting started

General purpose PWM controller

Pulse Width Modulation (PWM) Controllers are control ICs for switch mode power supplies which regulate the target parameter by modulating the Duty Cycle (pulse width) of the switch drive at a (user settable) fixed frequency. These devices support multiple topology (Multi-topology) configurations which include Boost, Buck, Buck-boost, Push-pull, Half-bridge, Full-bridge, Flyback, Forward, and Resonant LLC capability with current and voltage mode feedback control options used in various power supply architectures for a broad range of end equipment.


The Flyback topology is a popular choice for single and multiple output designs at power levels of 150 Watts or less. Derived from the Boost-buck topology, it uses the transformer for both isolation and as the storage inductor, reducing component count and costs. With high peak currents, the topology is typically not used in high output current designs.



The Forward topology is derived from the Buck topology and uses a transformer for input-output ground isolation. The popularity of the Forward topology is primarily its simplicity, performance, and efficiency at power levels up to 200 Watts. The Forward topology is a good choice for high output current designs. The Active Clamp Forward topology is a further improvement of the Forward topology which uses an additional switch and a clamp capacitor to recycle and reset the transformer.



The Push-pull topology is a Forward design with two primary windings to utilize the transformer core more efficiently. Push-pull topologies can scale to higher output power than Flyback and Forward designs, but the design is more complicated to carefully control MOSFET switching to avoid shoot through and to reduce the inherently high switching stress.



Derived from the Forward design, the Half-bridge topology can be scaled to high power levels. The power stage switching stress is lower than a Push-pull topology and well suited to high-line. However, similar to the Push-pull, careful management of the MOSFET switching dead-time is required to avoid shoot-through current.


Resonant LLC

An increasingly popular Half-bridge topology with zero-voltage switching on the primary side using resonant techniques to reduce switching losses, EMI and offer very good efficiency. The Resonant LLC scales to high power output levels. Complexity, efficient operation in light load conditions and system cost are the traditional challenges associated with this topology.


The Full-bridge topology is used for high power applications, with output power typically in the kW range. Four MOSFETs are switched so that the current through the transformer primary reverses every alternate half cycle so it operates in two quadrants of the B-H curve. This efficient use of the transformer results in greater power density than in a Forward configuration. A further improvement of the Full-bridge topology is Phase-shifted full-bridge control which achieves zero-voltage switching to reducing switching losses, reduce EMI and increasing overall system efficiency.