Flyback converter

“A POWER CONVERTER IS AN ELECTRICAL OR ELECTRO-
MECHANICAL DEVICE FOR CONVERTING ELECTRICAL ENERGY.”
FLYBACK CONVERTER
21-Feb-18QUEST | NAWABSHAH | PAKISTAN 1
HASSAN KHALID UPPAL
DR. G.M. BHUTTO
15-EL-40
Name:
Teacher:
Roll #:

• The flyback converter is a simple switch
mode converter used in both AC/DC and
DC/DC conversion with galvanic isolation
between the input and any outputs.
• The flyback converter is a buck-
boost converter with the inductor split to
form a transformer, so that the voltage
ratios are multiplied with an additional
advantage of isolation.
21-Feb-18QUEST | NAWABSHAH | PAKISTAN 15-EL-40 2
FLYBACK CONVERTER

• Flyback converters can be used to generate
DC output from either in AC or DC input
source.
• Flyback converters are designed in such a
way that the power from the input can
transfer to the output during the off-time
of the primary switch.
• They are generally used in a low to mid
power range. [100 W]
FLYBACK CONVERTER

Low Parts Count:
 Input capacitor
 Primary switch (MOSFET)
 Couple Inductor (FLYBACK TRANSFORMER)
 Output rectifier
 Output capacitor
FLYBACK CONVERTER

These components are
use to create a Flyback
converter.
FLYBACK CONVERTER

Transformer is used for input and
output isolation.
Careful design of the turns ratio
between primary and secondary
that enable the output to be
higher or lower to the input.
FLYBACK CONVERTER

• A Flyback converter can support multiple
outputs by adding more windings to the
transformer.
• The Flyback converter uses the single
magnetic of a common reference of a
transformer actually behave as the coupled
inductor, this transformer combines the
functions of energy stored, energy
transferred and isolation. So the need of
separate LC filter on each output is
laminated. This may reduces the overall
cost of flyback converter.
FLYBACK CONVERTER

The flyback converter is a power supply
topology that uses mutually coupled inductor,
to store energy when current passes through
and releasing the energy when the power is
removed. The flyback converters are similar
to the booster converters in architecture and
performance. However, the primary winding
of the transformer replaces inductor while
the secondary provides the output. In the
flyback configuration, the primary and
secondary windings are utilized as two
separate inductors.
What is a Flyback Converter?

When the MOSFET turns
on, energy filled in input
source that stored in the
transformer, Specifically
the gap in the transformer.
OPERATION PRINCIPLE OF FLYBACK CONVERTER

When enough energy
stored in the transformer
the MOSFET switch is
turned off and diode now
connects, and the energy
from transformer now
delivered to the load.

It means FLYBACK
transformer transfer
energy to the load when
diode is conducting.

It is important to know
that the diode and
MOSFET should not
conduct at the same
time.

When the MOSFET turns on:
The input voltage is applied to
transformer primary side.
The primary side current
ramps up, during this time the
secondary side diode is
reverse biased.

When the MOSFET turns on:
The voltage applied to the diode
is equal to output voltage plus the
reflected input voltage.
The output capacitor supply slow
current during the on time

When the MOSFET turns off:
The current in the transformer
transfer to secondary and close to
the diode that is now forward
biased.
The secondary side current ramps
down as the transformer core
demagnetizes.

When the MOSFET turns off:
Now primary side is considered as
open circuit.
The voltage applied to the
MOSFET is equal to
“input voltage plus reflected
output voltage”

When the power stage is designed in
such a ways to allow the transformer
to completely demagnetize during
each switching cycle.
The simplest form of a DCM
flyback is designed with a fixed
switching
frequency and modulates the peak
current to support the load demands.
OPERATING IN DISCONTINUOUS CONDUCTION MODE

At the start of the switch
period, the on-time begins
and the primary side current
ramps up from zero.

At the end of the on-time, the primary
current of collapses back to zero and
current flows to the secondary
windings.
It begins at its peak proportional to the
turns ratio and ramps down to zero,
completely demagnetizing the
transformer during every switching
cycle.

After the demagnetizing time, there is a delay
before the primary side switch turns on again
to start the next switching cycle. This delay is
referred to as dead time.
During this portion of the switching period,
neither the diode nor the MOSFET is
conducting.
This dead-time, where the transformer is
completely demagnetized and no current is
being conducted, is why this operating mode
is known as discontinuous or DCM.

During this dead time, a resonant ring is
generated by the interaction between the
primary inductance of the transformer and
the parasitic capacitance at the switch
node.
A converter in deep discontinuous mode
can have a dead-time long enough for the
resonant ringing to dampen completely, at
which point the drain to source voltage will
have settled to be equal to the input
voltage.

A flyback is operating in DCM
when the power stage is designed
in such a way as to allow the
transformer to completely
demagnetized during each
switching cycle.

At the start of switch period, the in-time
begins and the primary side current ramps up
from zero. At the end of the no-time, the
primary current collapses back to zero, and
current flows to the secondary windings. It
begins at its peak proportional to the turns
ratio and ramps down to zero, completely
demagnetizing the transformer during every
switching cycle.

After the demagnetizing time, there is
a delay before the primary side switch
turns on a gain to start the next
switching cycle. This delay is referred
to as dead-time or resonant time.
During this portion of the switching
period, neither the diode nor the
MOSFET is conducting.

This dead-time, where the transformer
is completely demagnetized and no
current is being conducted, is why this
operating mode is called discontinuous
(DCM).
During this dead-time, a resonant ring is
generated by the interaction between
the primary inductance of the
transformer and the parasitic
capacitance at the switch node.

A converter in deep discontinuous mode
can have a dead-time long enough for the
resonant ringing to dampen completely, at
which point the drain to source voltage will
have settled to be equal to the input
voltage.
The high frequency ringing when the
MOSFET is turned off is the result of the
resonance formed between the leakage
inductance and the switch node parasitic
capacitance.

Because the leakage inductance is
much smaller than the primary
inductance, this ringing will be at a
much frequency than the resonant ring
during the dead-time.
The high-frequency ringing will add to
the drain to source voltage stress, and
it should be included in the voltage
range of the MOSFET.

For a fixed input voltage, increasing load demand
results in longer in-time allowing the peak current
to rise higher.
The amplitude of the peak current is modulated
and converter is operating in the AM or
Amplitude Modulation range.
The secondary side peak current will
proportionally rise to a higher peak when the
diode begins to conduct.
The dead-time portion of the switching period
will decrease to maintain the constant switching
frequency.

The advantages of DCM flyback are that
there are no reverse recovery losses in the
output rectifier, because it’s able to ramp
down to zero amps during every switching
cycle.
 The primary inductance is lowest out of all
the flyback, which may result in a smaller
transformer.
 A DCM flyback is inherently more stable,
because it doesn’t have a right-half-plane
to zero in its transfer function.
ADVANTAGES OF DISCONTINUOUS CONDUCTION MODE

DCM flyback do have the disadvantage of
very large ripple currents, which may
require large EMI filters.
Fixed frequency DCM flyback have higher
losses because they can turn off the switch
when the drain to source voltage may be
relatively high.
It can be ringing higher than the input
voltage at the moment of turn-off, this
could contribute a considerable hit to
efficiency, as the switching losses are
proportional to the square of this voltage.
DISADVANTAGES OF DISCONTINUOUS CONDUCTION MODE

Valley switching is a specialized
form of discontinuous conduction
mode and requires a controller
that is specifically designed to
detect when the resonant ring
during the dead-time is at a low
point.
VALLEY SWITCHING

Before turning the MOSFET on
to start the next switching
cycle, minimizing switching
losses.
VALLEY SWITCHING

In order to maintain the required average
output power, the controller will modulate
the switching frequency by skipping one or
more valleys from one cycle to the next.
Controllers that modulate the frequency to
meet the average load demand every cycle
are operating in FM(frequency modulation)
mode.
This is sometimes also called frequency fold
back because as the load demand is
decreased the switching frequency is also
decreased or folded back.
VALLEY SWITCHING

Valley switching can occur at any
resonant valley during the dead-time
as long it’s large enough for the control
to detect.
The switching waveforms may appear
to dither as the controller adjusts it’s
dead-time in its search for the nearest
valley.
VALLEY SWITCHING

Valley switch flyback have all the
advantages of traditional DCM flyback
with the added bonus of lower switching
losses de to consistently turning the
MOSFET off when the drain to source
voltage is at a low value.
This also helps to reduce the turn-on
current spike at the current sense resistor.
The dithering produced by valley skipping
helps to reduce EMI.
ADVANTAGES OF VALLEY SWITCHING

Unfortunately valley skipping will
result in higher output voltage ripple.
Also valley switching is ineffective if
the converter is operating in deep
discontinuous mode as the controller
will not have enough of a signal to
detect a valley.
DISADVANTAGES OF VALLEY SWITCHING

Quasi-resonant mode or QR may
be referred to as critical condition
mode or transition mode.
Quasi-resonant operation is a
specific valley switching operating
mode of DCM where the
switching occurs on the very first
and deepest resonant valley.
QUASI-RESONANT MODE

QR delivers the maximum amount of power
by adjusting both the peak current and the
switching frequency to turn the MOSFET on
at the first resonant valley for minimal losses.
QR controllers operates in AM and FM mode
at the same time to meet the demands of
energy transfer.
QR controllers will decrease the switching
frequency as the load increases. This is just
the opposite of frequency fold back
mentioned earlier.
QUASI-RESONANT MODE

Most Valley switching controllers can operate
in Quasi-resonant mode but only at the
specific operating point of maximum load and
minimum input voltage when designed
accordingly.
This limited QR range of operation is due to
the control method used in the valley
switching controllers, where only the
frequency is modulated.
Not both the frequency and the peak current
like in dedicated QR controllers.
QUASI-RESONANT MODE

QR mode converters switch at the
lowest drain to source voltage, they
achieve the lowest possible switching
losses and have high efficiency over
the entire operating range.
This is a soft switching converter only
small EMI filters are needed.
ADVANTAGES OF QUASI-RESONANT MODE

QR converters are difficult to
compensate due to the wide peak
current and switching frequency
ranges.
Considerable phase margin is
required to maintain stability over
the entire operating range.
DISADVANTAGES OF QUASI-RESONANT MODE

CCM refers to
CONTINUOUS CONDUCTION MODE.
A continuous current is always flowing
in the transformer during each
switching cycle.
CONTINOUS CONDUCTION MODE

When the MOSFET is turned on the
primary current ramps up. But it
doesn’t start from zero amps as in
DCM.
In CCM the current ramps from an
offset that is due to residual energy
that is continuously maintained in the
transformer.

When the switches turned off energy is
transferred across the secondary and the
transformer demagnetizes resulting in the
secondary side current ramping down.
But it does not ramp all the way to zero amps.
Residual energy is maintained in the
transformer. The next switching cycle begins
before the current is completely depleted.
As shown, the current waveform on both the
primary and the secondary is trapezoidal in
shape. This is some times referred to as a
ramp on a step.

Note that there is no dead-time in
CCM.
Current is always being conducted
somewhere in the transformer but also
note that despite continuously
conducting current the MOSFET and
the diode do not conduct at the same
time.

As the load demand decreases the
store of residual energy the step
portion of the wave form decreases.

CCM flyback transformers are designed
based upon the ripple current or ramp
portion of the wave form which is
considerably less than the ripple seen
in DCM flyback.
Controllers specifically designed for
Valley switching will not operate in
CCM as there is no resonant ring
available and the transformer is not
allowed to fully demagnetize.

Lower peak currents means smaller filter co
mponents.
The advantages of CCM are the small ripple
and RMS current which result in lower
capacitor losses.
These lower current also help lower
conduction and turn-off losses when
compared to DCM flyback.
Lower peak current means smaller filter
components.
ADVANTAGES OF CONTINOUS CONDUCTION MODE

The most noted disadvantage of CCM
flyback is the presence of a right-half-plane
zero in the power stage transfer function,
This limits the bandwidth of the control
loop and will impact the converter’s
dynamic response.
Also CCM flyback require a larger
inductance which may require a large
magnetic component.
DISADVANTAGES OF CONTINOUS CONDUCTION MODE

The operating point just on the cusp of DCM
and CCM is referred to as the boundary
condition.
BOUNDARY CONDITIONS

This is the
operating point where the MOSFET is
turned on at the precise moment when
the transformer has demagnetized
so there is no resonant ring. But at the
same time, there is no energy stored
in the transformer. There is no step or
residual current stored.
BOUNDARY CONDITIONS

Converters that can operate in
both CCM,
as shown:
BOUNDARY CONDITIONS
Next
slides

Will transfer through the boundary
condition when it passes
to DCM operating.
BOUNDARY CONDITIONS

CCM converters will pass through
into DCM at very light loads.
BOUNDARY CONDITIONS

Because CCM has a more limited bandwidth,
converters that allow passing from DCM to
CCM should be
compensated for CCM. Converters that
require resonant valley detection to switch do
not allow CCM, and so will
not operate at the boundary condition.
BOUNDARY CONDITIONS

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QUESTI
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THANKS!

Flyback converter

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