### Design the MOSFET RCD Snubber Circuit

When the power MOSFET is turned off, there is a high

voltage spike on the drain due to the transformer leakage

inductance. This excessive voltage on the MOSFET may

lead to an avalanche breakdown and eventually failure of the

FPS. Therefore, it is necessary to use an additional network

to clamp the voltage.

The RCD snubber circuit and MOSFET drain voltage

waveform are shown in Figure 10 and 11, respectively. The

RCD snubber network absorbs the current in the leakage

inductance by turning on the snubber diode (Dsn) once the

MOSFET drain voltage exceeds the voltage of node X as

depicted in Figure 10. In the analysis of snubber network, it

is assumed that the snubber capacitor is large enough that its

voltage does not change significantly during one switching

cycle. The snubber capacitor used should be ceramic or a

material that offers low ESR. Electrolytic or tantalum

capacitors are unacceptable due to these reason

Circuit diagram of the snubber network

The first step in designing the snubber circuit is to determine

the snubber capacitor voltage at the minimum input voltage

and full load condition (Vsn). Once Vsn is determined, the

power dissipated in the snubber network at the minimum

input voltage and full load condition is obtained as

the snubber capacitor voltage at the minimum input voltage

and full load condition (Vsn). Once Vsn is determined, the

power dissipated in the snubber network at the minimum

input voltage and full load condition is obtained as

where Ids-peak is specified in equation (8), fs is the FPS

switching frequency, Llk is the leakage inductance, Vsn is the

snubber capacitor voltage at the minimum input voltage and

full load condition, VRO is the reflected output voltage and

Rsn is the snubber resistor. Vsn should be larger than VRO

and it is typical to set Vsn to be 2~2.5 times VRO. Too small a

Vsn results in a severe loss in the snubber network as shown

in equation (23). The leakage inductance is measured at the

switching frequency on the primary winding with all other

windings shorted.

Then, the snubber resistor with proper rated wattage should

be chosen based on the power loss. The maximum ripple of

the snubber capacitor voltage is obtained as

switching frequency, Llk is the leakage inductance, Vsn is the

snubber capacitor voltage at the minimum input voltage and

full load condition, VRO is the reflected output voltage and

Rsn is the snubber resistor. Vsn should be larger than VRO

and it is typical to set Vsn to be 2~2.5 times VRO. Too small a

Vsn results in a severe loss in the snubber network as shown

in equation (23). The leakage inductance is measured at the

switching frequency on the primary winding with all other

windings shorted.

Then, the snubber resistor with proper rated wattage should

be chosen based on the power loss. The maximum ripple of

the snubber capacitor voltage is obtained as

where fs is the FPS switching frequency. In general, 5~10%

ripple of the selected capacitor voltage is reasonable.

The snubber capacitor voltage (Vsn) of equation (26) is for

the minimum input voltage and full load condition. When

the converter is designed to operate in CCM under this

condition, the peak drain current together with the snubber

capacitor voltage decrease as the input voltage increases as

shown in Figure 11. The peak drain current at the maximum

input voltage and full load condition (Ids2 peak) is obtained as

ripple of the selected capacitor voltage is reasonable.

The snubber capacitor voltage (Vsn) of equation (26) is for

the minimum input voltage and full load condition. When

the converter is designed to operate in CCM under this

condition, the peak drain current together with the snubber

capacitor voltage decrease as the input voltage increases as

shown in Figure 11. The peak drain current at the maximum

input voltage and full load condition (Ids2 peak) is obtained as

where Pin, and Lm are specified in equations (1) and (6),

respectively and fs is the FPS switching frequency.

The snubber capacitor voltage under maximum input voltage

and full load condition is obtained as

respectively and fs is the FPS switching frequency.

The snubber capacitor voltage under maximum input voltage

and full load condition is obtained as

where fs is the FPS switching frequency, Llk is the primary

side leakage inductance, VRO is the reflected output voltage

and Rsn is the snubber resistor.

side leakage inductance, VRO is the reflected output voltage

and Rsn is the snubber resistor.

From equation (26), the maximum voltage stress on the

internal MOSFET is given by

internal MOSFET is given by

where VDC max is specified in equation (3). Check if Vds

max is below 85% of the rated voltage of the

MOSFET (BVdss) as shown in Figure 12. The voltage rating

of the snubber diode should be higher than BVdss. Usually,

an ultra fast diode with 1A current rating is used for the

snubber network.

In the snubber design in this section, neither the lossy

discharge of the inductor nor stray capacitance is considered.

In the actual converter, the loss in the snubber network is

MOSFET (BVdss) as shown in Figure 12. The voltage rating

of the snubber diode should be higher than BVdss. Usually,

an ultra fast diode with 1A current rating is used for the

snubber network.

In the snubber design in this section, neither the lossy

discharge of the inductor nor stray capacitance is considered.

In the actual converter, the loss in the snubber network is

Less than the designed value due to this effects

Labels: RCD Snubber

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