The Buck converter

The Buck converter

The forward converter family which includes the push-pull and bridge types, are all based on the buck converter, shown in Fig. 2. Its operation is straightforward. When switch TR1 is turned on, the input voltage is applied to inductor L1 and power is delivered to the output. Inductor current also builds up according to Faraday’s law shown below:-

V = L dI/ dt

When the switch is turned off, the voltage across the inductor reverses and freewheel diode D1 becomes forward biased. This allows the energy stored in the inductor to be delivered to the output. This continuous current is then smoothed by output capacitor Co. 

The LC filter has an averaging effect on the applied pulsating input, producing a smooth dc output voltage and current, with very small ripple components superimposed. The average voltage/sec across the inductor over a complete switching cycle must equal zero in the steady state. (The same applies to all of the regulators that will be discussed.)

Neglecting circuit losses, the average voltage at the input side of the inductor is VinD, while Vo is the output side voltage. Thus, in the steady state, for the average voltage across the inductor to be zero, the basic dc equation of the buck is simply

Vo/ Vi = D

D is the transistor switch duty cycle, defined as the conduction time divided by one switching period, usually expressed in the form shown below:-

D = ton T ; where T = ton + t off

Thus, the buck is a stepdown type, where the output voltage is always lower than the input. (Since D never reaches one.) Output voltage regulation is provided by varying the duty cycle of the switch. The LC arrangement provides very effective filtering of the inductor current. Hence, the buck and its derivatives all have very low output ripple characteristics. The buck is normally always operated in continuous mode ( inductor current never falls to zero) where peak currents are lower, and the smoothing capacitor requirements are smaller. There are no major control problems with the continuous mode buck.