When to use Quasi-resonant fly back design

The advantages of quasi-resonant operation in flyback converters are well known. Among these advantages include higher efficiency, lower noise emissions and lower stress in the switching device over fixed frequency, hard switching converters. Lower noise emissions are particularly useful in video applications where keeping the picture clean of spurious switching transients is vital.

However, quasi resonant flyback operation may not be the best solution in all applications. For example, many of today’s set-top boxes are increasing in power particularly lower output voltages with higher output currents. With lower output voltages the flyback voltage, in quasi resonant designs, appearing across the primary winding during FET off time is less. This results in more charge in the parasitic capacity that needs to be discharged when the FET turns back on. This will increase the current spike at FET turn on which increases noise and stress in the device. Moreover, since a quasi-resonant flyback operates inherently in discontinuous conduction mode the RMS currents in the output capacitors can be exceedingly high. This results in either more output capacitors or higher quality capacitors or both. Either way, the cost of the power supply goes up.

In cases like this it may be more beneficial to consider a different topology. With lower output voltages particularly with high output currents a forward topology may be more appropriate. This is because a forward converter will inherently have significantly less ripple current in the output capacitors for the same output current. The number and size of the output capacitors can be reduced resulting in less cost. The resulting savings can, in many cases, offset the necessary output inductor.

As an example, a reference design for a high power (130 W) set-top box was created using the forward topology. The design, based around the FS7M0880, has two outputs, 7 V @ 1.6 A and 12 V @ 10 A. A dual dc/dc converter was then used to step the 12 V down to 3.3 V @ 9.85 A and 5 V @ 7.25 A. The forward converter also included synchronous rectification for increased efficiency and a single, coupled inductor for both outputs to reduce cost. Resulting efficiencies are 88-89%. This appears to be a cost effective alternative to a quasi resonant design in this case since each output required only one output capacitor.