Switching losses &

Snubber circuit

IDEAL PRACTICAL

Block arbitrary large forward and reverse voltages with zero current flow when off

Finite blocking voltage with small leakage current during OFF

Instantaneous switching from one state to the other

Does not instantaneously switch; there is turn-on and turn-off time

Conduct arbitrary large current with zero voltage drop when on

Finite current conduction capability with small forward voltage drop

Negligibly small power required from control source to trigger the switch

Some devices required appreciable power to be triggered

Ideal switch vs practical switch

• In this course, we use ideal switch to describe the power electronic circuits operations – occasionally practical switches will be mentioned.

• PE circuit design need to use actual device model in order to analyze power dissipation and efficiency

Ideal switch vs practical switch

Switching Power Loss is proportional to:• switching frequency• turn-on and turn-off times

Switching losses

Since voltage and current do not change instantaneously, there will be power losses during switching – ‘switching losses’

Switching losses

The profile must contained within the safe operating area (SOA) of the device

iT

vT

Io

Vd

turn-offturn-on

turn-off

turn-on

Switching losses

The profile becomes worse if the stray inductance is considered during the switching

Switching losses

Snubber circuit• In order to reduce the stress and over voltage of the device, a snubber circuit

is used to alter the v-i profile of the device• There are various type of snubbers: we will only consider the turn-off and

turn-on snubbers

Turn-off snubber Turn-on snubber

Snubber circuitTurn-off snubber circuit operation

• The switch current can fall immediately since there is a path provided by the capacitor Cs

• The capacitor voltages increase and stop conducting when Cs=Vd at which diode Df start to conduct

Snubber circuitTurn-off snubber circuit operation

• The switch current can fall immediately since there is a path provided by the capacitor Cs

• The capacitor voltages increase and stop conducting when Cs=Vd at which diode Df start to conduct

Snubber circuitTurn-on snubber circuit operation

• The stress is reduced by reducing the switch voltage when the switch current increases.

Snubber circuitTurn-on snubber circuit operation

• The stress is reduced by reducing the switch voltage when the switch current increases.