We review
switching losses involving diode recovery charge, transistor switching with
clamped inductive load, device capacitance and leakage, package, and stray
inductances, and the efficiency versus switching frequency curve.
The switching characteristics
of the diode during turn-off time is shown in Fig. 1.
Figure 1.
Turn-Off Characteristics of Power Diode: a) Variation of the Current if ; b) Variation of the Voltage Drop vf ; c) Variation of the Power Loss
If we apply a reverse voltage
across the diode, current through the diode comes to zero value, and the diode
continues to conduct in the opposite direction because of the presence of
stored charges in the depletion layer and the p or n layer. The diode current
flows for a reverse recovery time trr. This is the time between when the
instant forward diode current becomes zero and the instant reverse recovery
current decays to 25 % of its reverse maximum value.
Time Ta: Charges stored in the depletion layer
removed.
Time Tb: Charges from the semiconductor layer
is removed.
Total recovery time:
TRR=Ta+TbTRR=Ta+Tb
Peak reverse
current:
IRM=TadidtIRM=Tadidt
[Equation 1]
Reverse-recovery charge, QRRC, is the amount of charge that flow
through the diode when diode changes its state from forward conduction mode to
reverse blocking mode. This is given by the area enclosed below the x- axis by
the path of current.
Thus,
QRRC≅12IRMxTRRQRRC≅12IRMxTRR
⇒IRM=2QRRCTRR⇒IRM=2QRRCTRR
[Equation 2]
From Equations 1 and 2, we
get,
TRRTa=2QRRCdidtTRRTa=2QRRCdidt
[Equation 3]
Usually,
Tb<<Ta.Tb<<Ta.
Thus,
TRR≈TaTRR≈Ta
[Equation 4]
From Equations 3 and 4, we
get,
TRR≅(2QRRCdidt)−−−−−−−−√TRR≅(2QRRCdidt)
[Equation 5]
From Equations 5 and 2, we
get,
IRM=(2QRRCdidt)−−−−−−−−−√IRM=(2QRRCdidt)
[Equation 6]
From a practical point of
view, one is more concerned about the TRR and IRM which
are clearly dependent on the stored charge QRRC and reverse applied
didtdidt
from Equations 5 and 6.
Power BJT
The delay in time during turn
on or turn off is due to the time taken by the minority carrier to reach
suitable density point. Its switching characteristics are specified in relation
to external load circuit and the waveform for the base current. The clamped
inductive switching circuit with flat base drive is shown in Fig.2.
Figure 2.
Circuit for Switching Power BJT
The key point for the power
BJT is the storage time and switching losses. Switching losses occurs during
turning on and turning off time and they dominate for the fast-switching
operations. Waveforms showing change in the voltage and current during
transition for resistive and inductive load is shown in Fig. 3.
Figure 3.
Transition of the Voltage and Current during Turning-Off for (a) Inductive Load
(b) Resistive Load
Complete inductive load
switching characteristics for the power BJT is shown in Fig. 3
A base current should be high
enough to ensure that the BJT turns quickly for the fast-switching operations.
It should be maintained at a certain level after turning it on to keep it in
the saturation state so as to minimize the conduction losses. As in case of
power diode, reverse recovery time is dependent on the stored charge and the
slope of the current.
Turn-on time for power BJT
can be reduced by increasing the rate of the rise of the base current.
Figure 4. Power
BJT Switching Characteristics for the Inductive Load
MOSFET
If the load current is
constant over the switching interval of small duration and also due to
inductive load; it can be considered as a constant-current source.
The switching circuit with
the clamped inductive circuit in the case of the power MOSFET is shown in Fig.
5. The gate is driven by an ideal step-voltage source. Also, diode is taken to
be ideal and thus not showing any reverse recovery characteristics.
Figure 5. Switching Circuit for MOSFET with Clamped Inductive Load
As the gate voltage is
applied, it starts to rise towards VGG having time constant = RG (CGS + CGD)
Figure 6.
Variation of CGS and VGD with VDS
The gate source capacitance
of a MOSFET is the greatest among all the internal capacitances of MOSFET. The
gate-to-drain transfer capacitance of a MOSFET has a large value in the ohmic region and then decreases to small value in the
active region. The above graph clearly shows the variation of this capacitance
with VDS. The switching characteristics for
power MOSFET during turn-on and turn-off time are shown in Fig. 7 and Fig.8
respectively.
Figure 7.
Turn-On Characteristics for Power MOSFET
Figure 8.
Turn-Off Characteristics for Power MOSFET
IGBT
Switching circuit for IGBT
with clamped inductive load is shown in Fig.9.
Turn-on and turn-off
characteristics for the clamped inductive load is shown in Fig.10 and Fig.11.
IGBT passes through active region during turning on and turning off time. Let
the time constant of the inductive load i.e.
T=LRT=LR
be large enough such that load current
is assumed constant during steady-state condition.
Figure 9.
Switching Circuit for IGBT with Clamped Inductive Load
Figure 10. IGBT
Switching Characteristics during Turn-On
During the turning on of an
IGBT, the rate of fall of its voltage slows down toward the end since the
output of p-n-p transistor follows its active region more slowly compared to
the MOSFET.
Figure 11. IGBT
Switching Characteristics during Turn-Off
During the turning off of the
IGBT, a current tail may be created due to stored excess charges. When the IGBT
is turned off, the gate-to-emitter voltage is kept at a negative value to avoid
the latch up.
Device Capacitance: It is the internal capacitance
that comes in parallel with the switch such as IGBT and MOSFET as shown in Fig.
5 and Fig. 9 when the switch comes in turning-on state. The energy stored is
vanished during the turning on transition. Total energy stored in the capacitor
is given by,
Ec=∑i=0n12CiVi2Ec=∑i=0n12CiVi2
Stray Inductances: These are inductances that appear
effectively in series with switch when it starts to become open circuit. This
stored energy is vanished during the turn-off transition.
EL=∑i=0n12LiIi2EL=∑i=0n12LiIi2
Besides this losses during
the turn-on and turn-off transitions, there may be switching losses due to
diode stored charge, parasitic capacitances, parasitic inductances and ringing.
Power is proportional to the
energy stored multiplied by the frequency. Efficiency of a converter decreases
with the increase of the switching frequency. But, there exist a certain
critical value of a frequency which the efficiency decreases drastically.
Figure 12.
Efficiency vs Switching Frequency Curve