DCM Flyback

10

Design Note DN 2013-01

V1.0 January 2013

5B).

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.

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:

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.

Figure 4: Flyback Primary Clamp Circuit

Figure 5: Clamp VDS Voltage

Figure 5: Example of transformer winding scheme (A) using margin tape (B) using triple insulated wire