PowerPoint Presentation

Switching losses & Snubber circuitIDEALPRACTICALBlock arbitrary large forward and reverse voltages with zero current flow when off

Finite blocking voltage with small leakage current during OFFInstantaneous 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 dropNegligibly small power required from control source to trigger the switch

Some devices required appreciable power to be triggered Ideal switch vs practical switchIn 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 efficiencyIdeal switch vs practical switchSwitching Power Loss is proportional to: switching frequency turn-on and turn-off timesSwitching lossesSince voltage and current do not change instantaneously, there will be power losses during switching switching losses

Switching lossesThe profile must contained within the safe operating area (SOA) of the deviceiTvTIoVdturn-offturn-onturn-offturn-on

Switching lossesThe profile becomes worse if the stray inductance is considered during the switching

Switching losses

Snubber circuitIn order to reduce the stress and over voltage of the device, a snubber circuit is used to alter the v-i profile of the deviceThere are various type of snubbers: we will only consider the turn-off and turn-on snubbers

Turn-off snubberTurn-on snubberSnubber circuitTurn-off snubber circuit operation

The switch current can fall immediately since there is a path provided by the capacitor CsThe capacitor voltages increase and stop conducting when Cs=Vd at which diode Df start to conductSnubber circuitTurn-off snubber circuit operation

The switch current can fall immediately since there is a path provided by the capacitor CsThe 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 operationThe stress is reduced by reducing the switch voltage when the switch current increases.