The flyback regulator circuit shown below can be used as a step-up or step-down circuit.

The flyback regulator

Flyback Regulator Waveforms

In a forward (or buck) regulator power is continuously supplied to the outlet\filter capacitor. In a flyback regulator, however, energy is pumped in a cyclic manner. The filter capacitor therefore has to be of a higher value.

Flyback regulators can operate in one of two modes: Continuous or Discontinuous. In Continuous mode, a large value of inductor is used such that the current in the inductor never falls to zero. In Discontinous mode, the current in the inductor falls to zero before the switch closes. Usually the circuit is designed such that at worst case conditions (max output current, min input voltage) the current only falls to zero for an instant, ie as soon as the diode stops conducting the switch is closed.

For a flyback regulator, the peak current is given by:

Ipk = (Vin x Ton)/L

where:

Ipk = Peak current - Amps

Vin = Input voltage - Volts

Ton = switch conduction time - Seconds

L = Inductance - Henries

The conduction time of the diode (which may or may not be the same as the off time of the switch) is given by:

Tdon = (Ipk x L) / Vout

where:

Ipk = Peak current - Amps

Vout = Output voltage - Volts

Tdon = Diode conduction time - Seconds

L = Inductance - Henries

The output power from a flyback regulator is given by:

Pout = Vout x Iout = 0.5 x L x Ipk^2 x f

where:

Iout = Average output current - Amps

Vout = Average output voltage - Volts

Tdon = Diode conduction time - Seconds

L = Inductance - Henries

Ipk = Peak current - Amps

f = Frequency of operation - Hertz

Iout is also the average current through the diode (since all output current must flow through D:

Iout = (Ipk / 2) x (Tdon x f)

where:

Iout = Average output current - Amps

Ipk = Peak current - Amps

Tdon = Diode conduction time - Seconds

f = Frequency of operation - Hertz

The reverse polarity output voltage is given by:

Vout = (Pout x Rl)^0.5

or:

Vout = Ipk x ((L x f x Rl)/2)^0.5

where:

Vout = Average output voltage - Volts

Pout = Average Output power - Watts

Rl = Load Resistance - Ohms

Ipk = Peak current - Amps

L = Inductance - Henries

f = Frequency of operation - Hertz

The output voltage of the circuit can be regulated by operating the circuit at a fixed frequency and varying the transistor duty cycle. However, because of the pumping action, the output voltage sags while the transistor switch is on and rises when the transistor is off. This makes the circuit difficult to control in a fixed frequency manner. A better approach to controlling the flyback converter when operating in the discontinuous mode is to have a fixed peak current in the inductor and hence fixed anode conduction time. The transistor switch 'on' time can then be varied inversely to any changes in the output voltage. This gives rise to the circuit having variable frequency of operation.

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