The off-line flyback converter topology is one of the most common switched-mode power supplies around. Pretty much any “wall-wart” charger these days will be based around a flyback converter due to their relative simplicity and low cost. Let’s take a look at how they work.
Figure 1 shows the converter part of a typical off-line flyback converter. I have omitted any details of the control chip and voltage feedback, as well as the input mains rectifier and any associated protection circuitry. I have also left off the EMI-suppression components which should be present in a good quality supply.
The key thing to keep in mind in analyzing a flyback converter is that the “transformer” in the circuit is in fact two coupled inductors. True transformer action, in which a changing current in the primary winding induces a counteracting current in the secondary, never occurs. Only one winding can conduct at any given time as we shall see.
Let’s look at what happens when the MOSFET is turned on. The left-hand side of Figure 2 shows a simplified version of what happens. The input voltage VIN is connected across the primary winding. The voltage across the secondary winding will be VIN / N where N:1 is the ratio of primary to secondary turns on the transformer. Note that the dot on the secondary winding shows that it is connected in the opposite polarity to the primary. This induced voltage will be negative, reverse biasing the rectifier diode.
The current in the primary therefore begins to rise linearly from t0 as shown in Figure 3. This rise is dictated by the input voltage and the primary winding inductance. During this phase energy is being stored in the transformer’s magnetic field.
At time t1 the control chip turns the MOSFET off. As we all know the current in a conductor cannot change instantaneously. The just switched-off current that was flowing out of the non-dotted primary terminal finds another path through the non-dotted terminal of the secondary winding. The circuit now looks like the right-hand side of Figure 2, with the MOSFET off and the diode forward-biased. Because of the turns ratio, this current will of course be N times that in the primary. This current flows via the diode into the filter capacitor and load.
Figure 3 shows that the secondary current ramps down linearly at a rate determined this time by the output voltage and the secondary inductance. During this time the secondary voltage is (approximately) VOUT and the primary winding will see an induced voltage of VOUT with a negative polarity. This means the MOSFET will see a voltage of VIN + NVOUT. The energy stored in the core is therefore dumped into the output capacitor and load.
Note that the secondary current reaches zero at time t2 before the MOSFET begins to conduct again. This means this Flyback converter operates in discontinuous current mode. (DCM) It is possible to design a flyback converter to operate in continuous current mode (CCM), where the current in the transformer never falls to zero. The design challenge is to ensure you operate always in one mode or another, as getting the control loop stable in a converter that transitions from one mode to another is quite difficult. Most low power converters operate in DCM, since there is no minimum load requirement as there is for CCM.
Before we finish, I want to point out one (of the many) real-world effects that the designer needs to be aware of.
The coupling between the two windings is never perfect and there is some primary leakage inductance caused by some of the flux produced by the primary winding not being completely couple by the secondary. The energy in this small leakage inductor has to go somewhere when the MOSFET is turned off and will cause a high voltage spike on the primary terminal connected to the MOSFET. The solution is a snubber network we have thus far ignored in Figure 1. The snubber diode provides a path to dump the leakage current into the snubber capacitor, which is discharged by the parallel resistor.
Horowitz, Paul, and Winfield Hill. The Art of Electronics. Third edition, 11th printing, with Corrections. Cambridge New York, NY: Cambridge University Press, 2017.
“A Guide to Flyback Transformers | Coilcraft.” Accessed March 2, 2021. https://www.coilcraft.com