Flyback
Switch Mode Regulator Calculator
See our other Electronics
Calculators.
Most power supply tutorials assume that you create
the transformer or inductor from scratch. This of course is costly,
and time consuming. A better approach is to chose off the shelf magnetic
components first, and then design the other components.
In this design calculator we assume that we have an
appropriate transformer, and then figure out the other appropriate
parameters for designing the rest of the circuit.
Transformers are specified in several ways.
Datasheets nearly always tell you the inductance of the primary, and of
course the turns ratio. The other critical parameter is the
saturation current, of either the primary or the secondary. If the
current is specified for the secondary, then this can be reflected back
and computed for the primary by using the turns ratio.
Sometimes datasheets only specify the Volt-Time
product. This is the maximum amount of time that the transformer can
have a constant voltage applied to it. For example if the Volt-Time
product for a transformer is 200Vusec, then this tells us that the
transformer can only handle 20 volts for 10us before it saturates, or
alternatively 10V for 20us. This value together with the duty cycle
sets the minimum frequency of operation of the regulator.
The Volt-Time product isn't always given, instead
inductance and saturation current are given. It turns out that these
two values are equivalent:
VT=Isat*L.
In the calculator you can specify the transformer
with either VT or the primary's saturation current.
The secondary voltage should be higher than the
target voltage, otherwise the target output voltage will be unrealizable.
Vin*N*Duty(max)> Vout
Neglecting the voltage across the diode and
transistor the Duty cycle can be expressed as:
Duty= Vout(Vin*N)
Knowing the Duty cycle and the maximum on time we
can now compute the minimum frequency of operation. If the frequency is
below F(min) then the inductor will saturate, and the transistor switch
will overheat and be destroyed from excessive current through the
transformer primary. Novice engineers will sometimes make the
mistake of lowering the frequency of operation when the transistor fails,
or is overheating. Raising the frequency will resolve saturation
problems.
F(min)= Duty/Ton(max)
Frequency for a given duty cycle can not be lower
than Fmin to avoid saturation. However, you can decrease duty cycle,
and decrease F proportionally such that Ton(max) is never
exceeded.
The switching regulator
can be chosen, by the frequency of operation, and range of duty
cycle. Nearly any boost regulator will work as a flyback regulator,
if it operates an the appropriate frequency and duty cycle range.
To compute the power transfer we first compute the
energy stored in the inductor for each cycle.
E(cycle)= L*Isat2/2
The Volt Time parameter of equivalently the
I(sat)Lpri products are a direct indicator of the power transfer
capability of the transformer. The higher these products the more
power can be passed.
Now knowing the energy per cycle we can compute the
power by merely multiplying it by the number of cycles per second, or
frequency.
P(max)= E(cycle)*f= F&L*Isat2/2
In practice you wont want to drive the transformer
all the way to saturation, and so P(max) will be less to give some error
margin.
The available output current will be equal to P(max)
divided by the output voltage, if there is a single output
winding.
Iout1= P(max)/Vout1
For an example flyback circuit see: Flyback
SMPS
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