Fly-back Converter fall 2012
Fly-back Converter • Fly-back converter is the most commonly used SMPS circuit • Low output power applications • The output voltage needs to be isolated from the input main supply • The output power may vary from few watts to less than 100 watts.
Fly-back Converter • The circuit can offer single or multiple isolated output voltages • Operate over wide range of input voltage variation. • In respect of energy-efficiency, fly-back power supplies are inferior to many other SMPS circuits but its simple topology and low cost makes it popular in low output power range. Typical efficiency of a fly-back converter is around 65%-75%.
Principle of Operation Mode-1 When switch ‘S’ is on, the primary winding of the transformer gets connected to the input supply with its dotted end connected to the positive side. At this time the diode ‘D’ connected in series with the secondary winding gets reverse biased due to the induced voltage in the secondary (dotted end potential being higher).
Mode-1 Equivalent Circuit Mode 1: Switch is ON; Diode is OFF; At the end of Mode-1, energy stored in the primary winding is
Principle of Operation Mode-2 Mode 2: Switch is OFF, Diode is ON When Switch turns off, the current in the primary winding drops suddenly, the voltage across the primary winding reverses. The diode becomes forward biased.
The secondary winding, while charging the output capacitor (and feeding the load), starts transferring energy from the magnetic field of the fly back transformer to the output in electrical form. If the off period of the switch is kept large, the secondary current gets sufficient time to decay to zero and magnetic field energy is completely transferred to the output capacitor and load. Flux linked by the windings remain zero until the next turn-on of the switch, and the circuit is under discontinuous flux mode of operation. Alternately, if the off period of the switch is small, the next turn on takes place before the secondary current decays to zero. The circuit is then under continuous flux mode of operation.
Mode-2 Equivalent Circuit The primary and secondary windings of the fly-back transformer don’t carry current simultaneously The fly-back transformer works differently from a normal transformer.
Principle of Operation Mode-3 After complete transfer of the magnetic field energy to the output, the secondary winding emf as well as current fall to zero and the diode in series with the winding stops conducting. The output capacitor however continues to supply uninterrupted voltage to the load. This part of the circuit operation has been referred to as Mode-3 of the circuit operation Discontinuous Conduction Mode
Mode-3 Equivalent Circuit During discontinuous mode, MOSFET is OFF; Diode is OFF. The output capacitor continues to supply uninterrupted voltage to the load. Discontinuous Conduction Mode
CCM vs DCM Flyback Waveform A smaller transformer can be used in DCM operation There are pros and cons of DCM operation. DCM advantages: smaller transformer, better stability, lower RFI, etc.
Circuit Equations under CCM The primary winding current rises from IP to Io in T time. IP – Io = (Edc / Lpri) T Under steady state Energy to the primary winding during each ON transition Edc x 0.5x(Ip+ Io) T Output energy in each cycle VoIloadT Edc x 0.5x(Ip+ Io) T = VoIloadT The mean(dc) voltage across primary and secondary windings must be zero Switch is ON, primary winding voltage equals input voltage. Switch is OFF, the reflected secondary voltage across the primary winding. Edc = (N1/N2)Vo(1-) Required ratings for switch Vswitch=Edc + (N1/N2)Vo Required ratings for diode Vdiode=Vo+Edc(N1/N2)
Circuit Equations under DCM At the end of Mode-1, the magnetic field energy rises to (1/2)LpriIp2, The entire energy is transferred to the output at the end of Mode-2 assuming loss-less operation. The output power Po = (1/2)LpriIp2fswitch Under DCM we have Edc ≤ (N1/N2)Vo(1-) Average voltage across windings over a switching cycle is still zero. The inequality sign is due to the fact that during part of OFF period of switch (1-)T, the winding voltages are zero. The expression for Vswitch and Vdiode still holds.
A Practical Fly-Back Converter Due to the non-ideal coupling between the primary and secondary windings when the primary side switch is turned off some energy is trapped in the leakage inductance of the winding. The energy associated with the leakage flux needs to be dissipated in an external circuit (known as snubber). Unless this energy finds a path to dissipate, there will be a large voltage spike across the windings which may destroy the circuit. The snubber circuit consists of a fast recovery diode in series with a parallel combination of a snubber capacitor and a resistor
Snubber In order that snubber capacitor does not take away any energy stored in the mutual flux of the windings, the minimum steady state snubber capacitor voltage should be greater than the reflected secondary voltage on the primary side, Vc > Vo x N1/N2 by keeping RC time constant of the snubber >> switching time period. For initial powering up of the circuit the control power is drawn from the input supply through a resistor Rs
When the switch is ON, energy transferred to the primary inductance is W = (1/2) Lpri ipp where ipp is the peak primary current Power to the load is Po = W/T = (1/2) Lpri ipp2 /T where T is the switching period ipp = Vin ton / Lpri Po = Vo2 /RL Vo2 /RL = (1/2) Lpri (Vin2 ton2 / Lpri2 ) (1/T) Vo = Vin ton (RLfs /2Lpri)1/2
For low output power applications, a clamp zener or a transient suppressor can be used as shown on the flyback application of the LM3488 datasheet. A typical snubber circuit is a resistance and a capacitor connected in series between the input voltage and the drain of the mosfe