Fly-Buck Converter

Used for: Generating an isolated auxiliary voltage from a non-isolated buck converter


  • Said to be simpler than an isolated flyback converter for the same power
  • Can use one of many standard buck controller ICs for low cost and high performance
  • Isolated output is quite well regulated under all conditions


  • Only makes sense if the main non-isolated output can be used in the application. Otherwise the minimum load required is just wasted power.


Control IC, C1, C3, and primary of T2 form a normal buck converter

Observe that when the lower switch is conducting in the IC, transformer primary start (dot) is always about 0V and transformer primary finish is always at Vout. So if a winding is added and phased so that D1 conducts when the lower buck switch is on, during this interval, the Vaux winding is nominally constant at Vout x Nsec/Npri. The diode is effectively peak rectifying the waveform so the auxiliary DC output is (Vout x Nsec/Npri) - Vdiode.

The auxiliary output is deriving its energy from what is stored in the inductor at the end of the buck converter on-interval which is 0.5LI2 where I is the sum of the inductor magnetising current at the interval end and the load current on the main output. This limits what power can be taken from the auxiliary output each cycle.

If the load on the main output is very low, the buck converter will enter discontinuous mode and the on-period will reduce. This limits energy stored in the inductor for the auxiliary output. Ultimately, the current available for the auxiliary output is what is transformed through the coupled inductor = Iout (main) + magnetising current so if both are low, the aux output will not maintain its voltage. The circuit is most useful then when there is a load on the main output and the auxiliary output power is less than the main output power.

The buck controller can be fixed frequency or variable, in constant-on, -off or hysteretic modes.

If the supply input is close to the main output voltage, the duty cycle is high and the off period short. This can lead to high peak currents in the auxiliary output filter so a buck-boost can be used instead. The main output is then a negative voltage with respect to the input return but this may not matter in say, a battery powered application. The auxiliary output is of course isolated, so it can be referenced to any voltage.

The two windings should be tightly coupled, otherwise leakage inductance will cause the auxiliary output voltage to be delayed and the current to rise resonantly. The leakage inductance will also cause voltage spikes.

The fly-buck approach in forward converters

The same approach can be taken to generate a semi-regulated output from an isolated forward converter.  Compared with multiple main transformer windings and multiples rectifier/filter stages, this is a very cost effective way to generate an isolated auxiliary rail.