Choosing the Right AC-Input Power-Supply Topology

New European Union initiatives are driving the use of energy-saving topologies in power supplies. Quasi-resonant, resonant and soft-switching topologies are replacing the older hard-switching topologies such as forward and flyback.

One common challenge is to decide which topology is optimal for a given application. The following topologies will be considered: LLC resonant topology, asymmetric half-bridge topology and quasi-resonant flyback topology. Fairchild offers solutions for these designs starting from 1W all the way up to 450W.

 

 

The quasi-resonant flyback topology has successfully been used in power supplies up to 250W. It has been used in applications such as televisions since the late 1990s. This topology requires one power MOSFET and one wound component (the flyback transformer), so it is a low-cost technology. At higher power levels, the large transformer size and correspondingly large leakage inductance increase the power-supply size, cost and losses. Based on these observations and comparing with the other topologies discussed, a quasi-resonant converter can be used for power ratings up to 100W or 150W.

Fairchild Semiconductor’s FAN6300 quasi-resonant controller is ideal for these higher power applications. For lower power applications, extra integration can be achieved by combining the MOSFET and controller into one package. An example is the FSQ series of FPS™ power switches, which are suitable for power supplies from 1W to around 70W.

In the range of 100W to 500W, two different topologies come under consideration. The output voltage is the determining factor governing which will work best.

For output voltages below 24V, the asymmetric half-bridge topology is recommended. Above this output level, the voltage rating of the output diodes are much higher than with the LLC resonant converter, causing unnecessary losses. If a synchronous output stage is used, the voltage rating on the output MOSFETs would be high, resulting in extra system cost. Fairchild Semiconductor’s FSFA2100 asymmetric half-bridge converter contains the two MOSFETs and the controller required for the primary side of such a converter. The efficiency of the 360W power supply in Fairchild application note AN-4153 is more than 93%.

 

 

For output voltages above 24V, and for power ratings in the region of 1kW, the LLC resonant converter is a good choice. For output voltages below 24V, the ripple-current rating on the output capacitor increases, resulting in extra systemcost as a larger number of output capacitors, ormore expensive high ripple-current devices, are needed tomeet the higher ripple current requirement. Synchronous rectification, also, is not so simple in this topology.

 

 

Fairchild Semiconductor’s FSFR2100 LLC resonant converters contain the two MOSFETs and the controller required for the primary side of an LLC converter. The efficiency of this 450W PFC+LLC solution from Fairchild’s Global Power Resource^SM centre is more than 90%. The efficiency of the 200W solution is 93%. Efficiency levels of up to 95%-96% are possible when using synchronous rectification. A new controller-only part is planned to support designs up to 1kW.

 

Conclusion

As well as evaluating various topologies, in the past designers have also had to overcome challenges such as ensuring stable operation, noise immunity, and correct timing of resonant switching to achieve maximum efficiency. As more designs turn to resonant switching, dedicated controller ICs such as the FAN6300, FSFA2100 or FSFR2100 mentioned in this design note solve these issues and combine with guidance on topology to help improve performance and shorten time tomarket.

 

Fairchild Semiconductor / Design Note