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) -
The auxiliary output is deriving its energy from what is stored in the inductor at the end of the buck converter on-
If the load on the main output is very low, the buck converter will enter discontinuous mode and the on-
The buck controller can be fixed frequency or variable, in constant-
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-
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.
buck approach in forward converters
The same approach can be taken to generate a semi-