Switching losses
&
Snubber circuit
Ideal switch vs practical switch
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
Switching losses
Since voltage and current do not change instantaneously,
there will be power losses during switching – ‘switching
losses’
Switching Power Loss is proportional to:
• switching frequency
• turn-on and turn-off times
Switching losses
iT
turn-off
turn-on
Io
turn-off
turn-on
Vd
vT
The profile must contained within
the safe operating area (SOA) of the
device
Switching losses
The profile becomes worse if the stray inductance is
considered during the switching
vsw = Vd - Ls
di
dt
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 circuit
Turn-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 circuit
Turn-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 circuit
Turn-on snubber circuit operation
•
The stress is reduced by reducing the switch voltage
when the switch current increases.
Snubber circuit
Turn-on snubber circuit operation
•
The stress is reduced by reducing the switch voltage
when the switch current increases.