ICE3BR0365 Datasheet, PDF (10/16 Pages) Infineon Technologies AG – Design Guide for Off-line Fixed Frequency DCM Flyback Converter

DCM Flyback
5B).
Design Note DN 2013-01
V1.0 January 2013
Figure 5: Example of transformer winding scheme (A) using margin tape (B) using triple insulated wire
Using triple insulated wire (reinforced insulation) on the secondary is easier and more preffered way of
meeting theis safety requirement. Take note that in fitting triple insulated wire on the choosen core/bobbin, the
outside diameter is thicker than the same gauge normal magnet wire.
STEP 9:
Design the primary clamp circuit: During turn off, a high voltage spike due to the transormer's
leakage inductance appears on MOSFET. This excessive voltage spike on the MOSFET may lead to an avalanche
breakdown and eventually failure of the MOSFET. A clamping circuit placed across the primary winding helps to
limit the voltage spike caused by this leakage inductance to a safe value.
There are two types of clamping circuit that can be used as shown in Figure 4. These are the RCD clamp and
Diode-Zener clamp. The easiest way is to used a Zener clamp ciruit which consist of a diode and high voltage zener
or TVS (transient voltage suppressor) diode. The Zener
diode effectively clips the voltage spike until the leakage
energy is totally dissipated in the Zener diode. The
advantage of using this circuit is that it will only clamps
whenever the combined VR and Vspike is greater than
it's breakdown voltage. At low line and lighter loads
where the spike is relatively low, the Zener may not
clamp at all, therefore there is no power dissipated in the
clamp.
Choose the Zener/TVS diode rating to be twice the
reflected voltage VR. The diode should be ultra
fast type with voltage rating greater than the maximum DC link voltage.
Figure 4: Flyback Primary Clamp Circuit
The RCD type not only clamps the voltage level but slows down the MOSFET
dv/dt. We can used the RCD clamp if passing EMI compliance is an issue
without it. The resistor element is crucial in limiting the maximum voltage spike. A
lower Rclamp will helps lower Vspike but increases the power dissipation. On the
other hand, a higher Rclamp value lower the power dissipation but allows higher
Vspike.
Setting VR= Vspike Rclamp can be determined by:
Figure 5: Clamp VDS Voltage
Where Lleak is the leakage inductance of the transformer, which can be determined through measurement by
shorting the secondary windings. If this is not known, assume Lleak around 2-4% of the primary inductance.
The capacitor Cclamp needs to be large enough to limit the voltage rise while absorbing the leakage energy.
Cclamp value may range 100pF- 4.7nF. Rclamp will discharge the capacitor back to the initial value of the switching
cycle.
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