The Buck-Boost Regulator

The very popular flyback converter (see section 5(a)) is not actually derived solely from the boost. The flyback only delivers stored inductor energy during the switch off-time. The boost, however, also delivers energy from the input. The flyback is actually based on a combined topology of the previous two, called the buck-boost or non isolated flyback regulator. 

When the switch is on, the diode is reverse biased and the input is connected across the inductor, which stores energy as previously explained. At turn-off, the inductor voltage reverses and the stored energy is then passed to the capacitor and load through the forward biased rectifier diode.

The waveforms are similar to the boost except that the transistor switch now has to support the sum of Vin and Vo across it. Clearly, both the input and output currents must be discontinuous. There is also a polarity inversion, the output voltage generated is negative with respect to the input. Close inspection reveals that the continuous mode dc transfer function is as shown below:-

Vo/ Vi = D /1 – D

Observation shows that the value of the switch duty ratio, D can be selected such that the output voltage can either be higher or lower than the input voltage. This gives the converter the flexibility to either step up or step down the supply.

This regulator also suffers from the same continuous mode control problems as the boost, and discontinuous mode is usually favoured.

Since both input and output currents are pulsating, low ripple levels are very difficult to achieve using the buck-boost. Very large output filter capacitors are needed, typically up to 8 times that of a buck regulator. The transistor switch also needs to be able to conduct the high peak current, as well as supporting the higher summed voltage. Theflyback regulator (buck-boost) topology places the most stress on the transistor. The rectifier diode also has to carry high peak currents and so the r.m.s conduction losses will be higher than those of the buck.