Fly-back Converter
fall 2012
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Basic Topology of a Fly-back
Converter
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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.
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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 flyback converter is around 65%-75%.
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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).
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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
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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.
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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 8
flux mode of operation.
Mode-2 Equivalent Circuit
The primary and secondary windings of the flyback transformer don’t carry current
simultaneously
The fly-back transformer works differently from a
normal transformer.
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Continuous Conduction Mode
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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
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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
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Discontinuous Conduction Mode
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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.
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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)
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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.
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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
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A Practical Fly-Back Converter
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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
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Answer: c
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Answer: a
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Answer: d
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Answer: c
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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
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An Example from National Semiconductor
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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
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