Application Note 91

VISHAY SEMICONDUCTORS
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Optocouplers and Solid-State Relays
Application Note 91
IGBT/MOSFET Gate Drive Optocoupler
INTRODUCTION TO IGBT
The Insulated Gate Bipolar transistor (IGBT) is a cross
between a MOSFET (metal oxide semiconductor field effect
transistor) and a BJT (bipolar junction transistor) since it
combines the positive aspects of MOSFETs and BJTs.
voltage drive, which means that it is possible to have a less
complex circuit with lower power consumption compared to
a BJT. IGBTs are used for high current, high voltage
applications when switching speed is important (table 1).
The IGBT has the fast switching capability of the MOSFET
and is capable of handling the high current values typical of
a BJT. In addition, IGBTs have a lower on-state voltage drop
and are capable of blocking higher voltages.
TABLE 1- IGBTS VS. MOSFETS
IGBTS
POWER MOSFETS
TYPE
VOLTAGE DRIVEN
VOLTAGE DRIVEN
The IGBT, as a first approximation, can be modeled as a
PNP transistor driven by a power MOSFET, as shown in
figure 1.
Current density per
voltage drop
Very high, small
trade-off with
switching speed
High at low voltage
low at high voltage
Switchung losses
Low to medium
Very low
C
IGBT SWITCHING BEHAVIOR
One of the important performance features of any switching
device is the switching (turn-on and turn-off) characteristic,
since significant power losses are incurred during these
switching states.
When driving inductive loads, the device under goes higher
stress. Hence, it makes sense to study the turn-on and
turn-of time of the IGBT/MOSFET when driving inductive
loads.
G
E
Fig. 1 - Simplified IGBT Equivalent Circuit
The IGBT’s internal input capacitance (CGE) and Miller
capacitance (CGC) impacts the IGBT turn-on behavior. But
the CGC effect is very small and negligible. Figure 3
illustrates the parasitic IGBT capacitances.
C
Figure 2 shows the most common symbols used for IGBT.
C
C
CGC
G
CCE
G
E
E
CGC = feedback or Miller
capacitance
CGE = input capacitance
CCE = output capacitance
CGE
Fig. 2 - IGBT Symbols
The equivalent circuit for the input of IGBT is the same as a
MOSFET and is purely capacitive. This allows the use of a
Rev. 1.3, 24-Oct-11
E
Fig. 3 - IGBT Parasitic Capacitances
Document Number: 81227
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APPLICATION NOTE
G
Application Note 91
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Vishay Semiconductors
IGBT/MOSFET Gate Drive Optocoupler
Most datasheets specify the voltage-dependent low-signal
capacitance of IGBT/MOSFETs in the off-state (table 2).
C
IC
TABLE 2 - IGBT AND MOSFET CAPACITANCE
CAPACITANCE
Input
IGBTS
POWER MOSFETS
Ciss = CGE + CGC
Ciss = CGS + CGD
Crss = CGC
Crss = CGD
Coss = CGC + CCE
Coss = CGD + CDS
Reverse transfer
Output
CCG
Rgate
dV/dt
IC = CCG x dV/dt
CGE
The turn-on behavior of the IGBT is identical to the power
MOSFET, since the IGBT acts as a MOSFET during most of
the turn-on interval. When a gate signal is applied, the gate
emitter voltage of the IGBT rises from zero to VGE(TH), as
shown in figure 4. This voltage rise is due to the gate
resistance (Rgate) and the CGE.
QGE
VGE
QCG
E
Fig. 5 - Rgate Effect on dV/dt
The turn-off behavior of the IGBT, as shown in figure 6, has
a dual characteristic of both power MOSFET and BJT
devices.
QCG
VGE(TH)
QGE
VGE
VGE = 0 V
VCE
VGE/TH)
VGE = 0 V
IC
I(t)
V(t)
VCE(sat)
IC = 0 A
ti-rise
VCE
IC
tv-fall
V(t)
19892
Fig. 4 - IGBT Turn-On Sequence
APPLICATION NOTE
The turn-on time is a function of the output impedance of the
drive circuit and the applied gate voltage. Hence, it is
possible to control the turn-on speed of the device by
choosing an appropriate value of gate resistance (Rgate).
In other words, by varying the Rgate it is possible to vary the
time constant of the parasitic net equal to Rgate x (CGE+CCG)
and then dV/dt. Therefore, the Rgate value strongly impacts
the power losses, since its variation also affects the dV/dt
slopes as illustrated in figure 5.
I(t)
IC-tail
VCE
IC = 0 A
trise
tfall
Fig. 6 - IGBT Turn-Off Sequence
At turn-off, the gate voltage begins to decrease until it
reaches the value when the Miller effect occurs; during this
time the VCE voltage increases changing the output
characteristics with constant IC.
Next, the Miller effect and the VGE voltage remain constant
because of modulation of the collector gate capacitance,
which is due to VCE voltage rapidly increasing to its
maximum value. During this time, the collector current
begins to fall quickly, and continues with a “tail” which is due
to recombination of minority carriers in the substrate.
The faster (and first) part of the IC current is due to the
turn-off of the MOSFET portion of the IGBT structure. The
IC-tail, which is due to the turn-off the BJT portion of the IGBT
structure, causes the major part of the switching losses.
Rev. 1.3, 24-Oct-11
Document Number: 81227
2
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Application Note 91
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Vishay Semiconductors
IGBT/MOSFET Gate Drive Optocoupler
IGBT POWER DISSIPATION
C
The maximum switching frequency of an IGBT is limited by
its power dissipation during switching. The junction
temperature (Tj) during normal operation depends on the
amount of power dissipated from the device and the
efficiency of the heat sink.
Tj = TC + Ptot × θ JC
Where:
θSA
θJC
θCS
Ptot
TC
ΔT
Ptot
−θ JC − θ CS
(2)
= heat sink to ambient thermal resistance
= junction-to-case thermal resistance
= case to heat sink thermal resistance
= total power dissipation
= case temperature
When an efficient heat sink is used, TC will be lowered. This
means that the difference between junction-to-case
temperature will be greater. Hence, a higher amount of
power can be dissipated.
Ptot is the maximum continuous power dissipated by the
device for a given case temperature. The maximum power
dissipation is related to permissible case temperature rise
and junction-to-case thermal resistance.
Ptot =
TJ − TC
θ JC
APPLICATION NOTE
Fig. 7 - FWD is used with IGBT to Adjust Switching Speed
The reverse-recovery and turn-on characteristics of the
FWD can be controlled to a certain extent by adjusting the
speed of the IGBT. In the event of a diode becoming too
snappy in an application, the IGBT turn-on can be slowed
down, hence reducing the value of dV/dt applied to the
diode and so reducing the diode losses. However, this is at
the expense of increasing the IGBT losses. An alternative
method of reducing the FWD losses in a bridge
configuration is to turn on the IGBT with a reduced VGE. This
limits the peak reverse recovery current, Irr, of the FWD in
the opposite side of the arm, according to the IGBTs'
forward output characteristic.
As illustrated in figures 4 and 6, there is a finite time interval,
during both turn-on and turn-off of the IGBT, where finite
VCE and IC coexist. CISS, COSS, and CRSS affect the turn-on
and turn-off times as well as turn-on and turn-off delay times
and are responsible for some energy losses.
Average IGBT power losses during both turn-on and turn-off
can be computed as follows:
ESW = ESW ( on ) + ESW ( off )
ESW =
∫V
(5)
(t) × I C (t ) dt
(6)
ESW ( on) = ∫ i-rise VCE × I C (t ) × dt + ∫ v-fall VCE (t ) × I C × dt
(7)
1
× VCE × I C × (ti-rise + tv-fall )
2
(8)
CE
(4)
IGBT SWITCHING POWER LOSSES
When turning an IGBT on or off, the level of VGE and Rgate
affects the switching losses of the device. The effect of
increasing VGE or reducing Rgate is to reduce the delay time,
rise time, and fall times of the device and hence to reduce
the switching losses. Reducing the level of VGE or increasing
Rgate results in increased switching losses but can reduce
electromagnetic interference (EMI). Other factors affecting
the switching losses include the anti parallel diode (FWD),
circuit inductance, snubbers, device junction temperature,
operating voltage, and current. The use of FWD is illustrated
in figure 7.
Rev. 1.3, 24-Oct-11
E
(3)
The main factor is to determine the Ptot of IGBT is the
VCE(SAT) level, which is dependant on junction temperature,
collector current ,and gate emitter voltage.
Ptot = I CE ( ave) × VCE (sat)
FWD
(1)
To select a heat sink, which keeps the junction temperature
at or below a given temperature, the following equation can
be used:
θ SA =
Rgate
ESW ( on ) =
ESW ( off ) = ∫ v-rise VCE (t ) × I C × dt + ∫ i-fall VCE × I C (t ) × dt (9)
Document Number: 81227
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Application Note 91
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Vishay Semiconductors
IGBT/MOSFET Gate Drive Optocoupler
1
ESW ( off ) = × VCE × ( I C × tv-rise + I C-tail × ti-fall )
2
(10)
1
E SW = ×VCE × I C × (ton + toff )
2
(11)
Where:
ESW = switching energy loss
ton = ti-rise + tv-fall = turn-on time
toff = tv-rise + ti-fall = turn-off time
PSW = f SW × ESW
(12)
MOSFET drive techniques can be used where the off biasing
needs to be stronger. The positive gate drive should be such
that the full saturation is guaranteed and short-circuit
current is limited. A negative voltage bias is used to improve
the IGBT immunity to collector emitter dV/dt injected noise
and to reduce turn-off losses. Figure 8 shows a simplified
IGBT gate driver.
It is good practice to connect some back-to-back zener
diodes directly across the gate emitter terminals of the
IGBT. This prevents damage from over-voltage on the
collector (by limiting the level of VGE). This is because when
a short circuit appears while the device is already
conducting the voltage and collector current rise very
quickly. The rapidly rising dV/dt coupled with the Miller
capacitance (CGC) can increase the effective VGE seen by
the IGBT, further increasing the short circuit current level.
Where:
PSW = switching power loss
fSW = switching frequency
VCC
Power dissipated during turn-on is calculated as follows:
PSW ( on ) =
1
× VCE × I C × f SW × ton
2
Rgate
(13)
VG
Figure 6 indicates that the power dissipated during turn-off
is due to two factors.
VEE
19894-1
The first such factor is the speed at which the collector
voltage reaches its maximum value. The second factor is the
duration of the tail of the collector current. The collector tail
current is due to the recombination of the minority carriers
that cannot be extracted from the base of the PNP BJT
section that is already open. The length of this “tail”
depends on the lifetime of these carriers and causes the
major part of the switching losses.
Hence, the turn-off power losses can be approximated as
follows:
APPLICATION NOTE
1
PSW (Off ) ≈ × VCE × f SW × ( I C × tv-rise+ I C −tail × t i-fall )
2
(14)
The turn-off switching losses of the MOSFET portion of the
IGBT structure are negligible. This is because the time that
the MOSFET portion are responsible for the IGBT turn off is
only a very small fraction of ti-fall time and much shorter time
than that of the BJT portion.
IGBT GATE DRIVER IC POWER LOSSES
IGBTs are voltage controlled devices and require a gate
voltage to establish collector emitter conduction. Due to the
large input gate emitter capacitance (CGE) of IGBTs,
Rev. 1.3, 24-Oct-11
Fig. 8 - Simplified IGBT Gate Driver
The value of the gate resistance (Rgate) has a significant
impact on the dynamic performance of the IGBTs. A smaller
Rgate charges and discharges the IGBT input capacitance
faster, which reduces the switching time and hence the
switching losses and provides immunity to dV/dt turn on.
But, a small Rgate can cause oscillation between the IGBT
input capacitance and parasitic lead inductance.
The minimum peak current capability of the gate drive
power source and the average power required to drive an
IGBT is as follows:
I gate ( peak ) = ±
Δ VGE
Rgate
(15)
Where:
Δ VGE = VGE ( on ) + VGE ( off )
(16)
When determining the gate drive requirements for the
switching IGBT, the key specification to look for is the gate
charge. The main reason for looking at gate charge rather
Document Number: 81227
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Application Note 91
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Vishay Semiconductors
IGBT/MOSFET Gate Drive Optocoupler
than gate capacitance is the Miller effect. The Miller
capacitance (CGC) effects on gate drive of IGBT are
characterized in the gate charge value. Figure 3 is
representative of parasitic gate capacitances.
The charging process for the gate of an IGBT is shown in
figure 9.
Where:
fSW = switching frequency
Hence, the power dissipated for the output of the IGBT
driver IC is:
2
POutput = C gate × VGE × f SW
(20)
The amount of power dissipated in the IGBT driver IC
emitter is:
VGE (V)
QGE
PEmitter = I F × VF × D
QGC
Where:
D = maximum LED duty cycle
IF = LED forward current
VF = LED forward voltage
Qgate (C)
Fig. 9 - Total IGBT Gate Charge Waveform During Switching
First, the CGE is charged (the CGC is also being charged, but
the amount of charge is very low and negligible). Once the
CGE is charged up to the gate threshold voltage (VGE(TH)), the
device begins to turn on and the current ramps up to the full
value of current in the circuit. Once the full current is
reached, the VCE voltage begins to collapse and the gate
voltage becomes flat due to CGC being charged and the
collector voltage falling off. After the collector voltage has
fallen to its final level, the CGE and CGC are charged to the
gate drive voltage.
To better understand gate charge, it can be shown with the
following equations.
Qgate = VGE × C gate
(21)
(17)
The amount of power dissipated in the IGBT driver internal
circuitry is:
PInternal = ICC × (VCC - VEE )
(22)
Where:
ICC = Supply current, output open
The total gate driver IC power losses are:
Pgate −driver (tot ) = POutput + PEmitter + PInternal
(23)
In many applications, the gate drive circuitry needs to be
isolated from the control circuit to provide level shifting and
improve noise immunity and safety. This is what the Vishay
IGBT driver provides by means of optical isolation.
Qgate
(18)
VGE
NC
Where:
Qgate = total gate charge
Cgate = total gate capacitance
VGE = driver’s supply voltage
A
Rev. 1.3, 24-Oct-11
Vout
0.1 µF
VO
Rgate
C
This means that the charging and discharging the IGBT gate
can be seen as the charging and discharging a capacitor.
1
2
Pgate = × C gate ×VGE × f SW
2
VCC
Shield
APPLICATION NOTE
C gate =
LOAD
+ VDC
(19)
VO
NC
VEE
- VDC
Fig. 10 - Simplified Vishay IGBT Driver Circuit
Document Number: 81227
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Application Note 91
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Vishay Semiconductors
IGBT/MOSFET Gate Drive Optocoupler
IGBT GATE DRIVE REQUIREMENTS AND
FEATURES
Minimum Output Current
A very important requirement for an IGBT gate driver
optocoupler is to supply the minimum output or gate current
(IOL or Igate) to switch the IGBT to the low impedance state.
This is illustrated in figure 11.
Where:
tSW = switching time
IGE = current in to CGE
IGC = current in to CGC
Gate Resistor (Rgate) Value
Rgate will need to be selected such that the maximum peak
output current rating of the gate driver optocoupler (IOL(peak))
is not exceeded.
VCC
VCC
CGC
Igate = IGC + IGE
Rgate
CGE
VEE
Fig. 11 - IGBT Gate Current
Fig. 12 - IGBT Gate Resistor (Rgate)
The IOL is specified when output voltage is low, i.e. when the
gate drive optocoupler is charging the IGBT gate.
Hence, the load draws the highest output current. The
required IOL or Igate to switch the IGBT can be calculated by
using the gate capacitances of the IGBT.
The following equation can be used to calculate the
appropriate Rgate value:
VGE / GC =
1
CGE / GC
VGE / GC =
APPLICATION NOTE
VEE
1
CGE / GC
I GE / GC =
For
I gate =
× ∫ I GE / GC (t )dt
(24)
× I GE / GC × t SW
(25)
VCC −VEE −VOL
I OL ( peack )
(29)
Where:
VOL = low-level output voltage of the gate driver optocoupler
UVLO
VGE / GC × CGE / GC
t SW
(26)
I gate = I GE + I GC
(27)
VGE × CGE VGC × CGC
+
t SW
t SW
(28)
Rev. 1.3, 24-Oct-11
Rgate =
The minimum acceptable gate drive voltage for an IGBT is
important because falling below this value will result in
switching from on state to a highly dissipative linear mode.
Hence, the Vishay IGBT drivers have under-voltage lock-out
(UVLO) to ensure that gate drive is removed for low drive
condition. This will prevent the IGBT from entering the linear
conductive mode.
Document Number: 81227
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Application Note 91
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Vishay Semiconductors
IGBT/MOSFET Gate Drive Optocoupler
LED DRIVE CONSIDERATIONS
LED On-State (CMH)
As shown in figure 13, Vishay IGBT/MOSFET driver IC has
a transparent faraday shield on the driver IC, represented by
the dashed line between the emitter (input LED) and
detector.
A high-CMTR LED drive circuit must keep the LED on during
common mode transients. This is achieved by overdriving
the LED current beyond the input threshold so that it is not
pulled below the threshold during a transient.
LED Off-State (CML)
A
VO
C
VO
A high-CMTR LED drive circuit needs to keep the LED off
(VF < VF(OFF)) during common mode transients. As long as
the low-state voltage developed across the logic gate
(driving the LED) is less than VF(OFF), the LED will remain off
and no common-mode failure will occur. The circuit shown
in figure 15 is recommended for high-CMTR performance.
+ VDC
Load
VCC
Shield
NC
VCC
VDD
NC
A
Shield
VEE
NC
Fig. 13 - Vishay IGBT Driver
NC
Shield
This shield diverts the capacitively coupled current away
from the sensitive IC circuitry. Although the shield improves
common mode transient response (CMTR) performance, it
does not eliminate the capacitive coupling between the LED
and VCC and VEE output pins. These capacitances are
shown in figure 14.
VO
NC
VEE
- VDC
Fig. 15 - Recommended LED Drive Circuit
for High CMTR Performance
IGBT/MOSFET DRIVER DESIGN
CONSIDERATIONS
VO
When designing and building driver circuits for an
IGBT/MOSFET, the following will need to be taken in to
consideration to prevent unwanted voltage spikes,
oscillation or ringing, and false turn-on.
CESU
A
Rgate
VCC
CSOU
8
C
Vout
0.1 µF
VO
1. Layout
C
CESL
VO
2. Power supply by-passing
3. Mismatch of driver to the driven IGBT/MOSFET
APPLICATION NOTE
NC
CSOL
VEE
Fig. 14 - IGBT Driver Parasitic Capacitance
This capacitive coupling causes perturbations in the LED
current during common-mode transients and becomes the
major source of CMTR failures for a shielded optocoupler.
The main design objective of a high-CMTR LED drive circuit
becomes keeping the LED in the proper state (on or off)
during common-mode transients.
The following methods can be used to ensure the LED is in
the desired (on or off) state.
Rev. 1.3, 24-Oct-11
To ensure a robust and problem free IGBT/MOSFET driver,
designers are advised to pay close attention to what is
recommended in the next few paragraphs.
Layout
A very crucial point is proper grounding. A very
low-impedance path for current return to ground avoiding
loops is a good design practice. The three paths for
returning current to ground are between:
1. Driver and the logic driving it
2. Driver and its own power supply
3. Driver and the source/emitter of the IGBT being driven
Document Number: 81227
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Application Note 91
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Vishay Semiconductors
IGBT/MOSFET Gate Drive Optocoupler
All these paths should be very short in length to reduce
inductance. Also, these paths should be as wide as possible
to reduce resistance. In addition, these ground paths need to
be kept separate to avoid returning ground current from the
load to affect the logic line. It is very important to note that all
ground points in the circuit should return to the same physical
point to avoid generating differential ground potentials.
Since all IGBT/MOSFET driver ICs have some losses, it is
necessary to calculate the power dissipated in the driver for
a worst-case condition. The total power dissipated in the
IGBT driver IC (as described previously) is:
Power Supply By-Passing
Where:
Pgate −driver (tot ) = POutput + PEmitter + PInternal
Since turning an IGBT/MOSFET on and off amounts to
charging and discharging large capacitive loads, the peak
charge current need to be within the capability of drive
circuit. At the same time the driver will have to draw this
current from its power supply in a short period of time. This
means that using of proper by-pass capacitors for the
power supply becomes very important. A pair of by-pass
capacitors of at least 10 times the load capacitance with
complementary impedance, used in parallel and very close
to the VCC pin, can take care of this issue. These by-pass
capacitors should have the lowest possible equivalent
series resistance (ESR) and equivalent series inductance
(ESL) and the capacitor lead lengths should be as short as
possible.
2
(23)
POutput = C gate × VGE × f SW
(20)
PEmitter = I F × VF × D
(21)
PInternal = ICC × (VCC - VEE )
(22)
Since ambient temperature in the vicinity of the
IGBT/MOSFET driver will have an effect on the actual power
dissipation capability of the driver, the maximum allowable
power dissipation at this temperature will need to be
derated accordingly (in comparison to room temperature).
The selected IGBT/MOSFET driver can only be used if the
maximum allowable power dissipation at this temperature is
within the capability of this IGBT/MOSFET driver.
Mismatch of Driver to the Driven IGBT/MOSFET
VISHAY IGBT DRIVER APPLICATION EXAMPLES
Some applications for the IGBT/MOSFET drivers are:
+ High Voltage
DC
VCC1
VIN
Floating supply
NC
A
Control
input
Shield
Open
collector
RIN
0.1 µF
VO
Rgate
6
VO
C
GND 1
3-Phase
AC
NC
VCC4
RIN
NC
Open
collector
A
Control
input
Shield
APPLICATION NOTE
VIN
0.1 µF
VO
Rgate
GND 1
C
- High Voltage
DC
VO
NC
Fig. 16 - 3-Phase Motor Drive Application
Note
• The value for RIN is dependent upon VIN, the desired LED input current (IF), and input forward voltage (VF).
Rev. 1.3, 24-Oct-11
Document Number: 81227
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THIS DOCUMENT IS SUBJECT TO CHANGE WITHOUT NOTICE. THE PRODUCTS DESCRIBED HEREIN AND THIS DOCUMENT
ARE SUBJECT TO SPECIFIC DISCLAIMERS, SET FORTH AT www.vishay.com/doc?91000
Application Note 91
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Vishay Semiconductors
IGBT/MOSFET Gate Drive Optocoupler
+ High Voltage
DC
VCC1
VIN
RIN
+
0.1 µF
_
NC
Shield
Open
collector
A
Control
input
VO
VO
C
GND
Rgate
3-Phase
AC
NC
VEE1
- High Voltage
DC
Fig. 17 - Negative IGBT Gate Drive Application
+ VDC
VCC
VIN
NC
A
Control
input
Shield
RIN
Open
collector
+
High Voltage
DC
-
0.1 µF
VO
Rgate
C
GND
VO
NC
- VDC
APPLICATION NOTE
Fig. 18 - Boost Converter Application
Rev. 1.3, 24-Oct-11
Document Number: 81227
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