FAN7392 Fairchild Semiconductor, FAN7392 Datasheet - Page 14

• Manufacturer Part Number: FAN7392
• Description: The FAN7392 is a monolithic high- and low-side gate drive IC, that can drive high-speed MOSFETs and IGBTs that operate up to +600V
• Manufacturer: Fairchild Semiconductor

FAN7392 Rev. 1.0.3
© 2009 Fairchild Semiconductor Corporation
Application Information
Negative V
The bootstrap circuit has the advantage of being simple
and low cost, but has some limitations. The biggest diffi-
culty with this circuit is the negative voltage present at
the emitter of the high-side switching device when high-
side switch is turned-off in half-bridge application.
If the high-side switch, Q1, turns-off while the load cur-
rent is flowing to an inductive load, a current commuta-
tion occurs from high-side switch, Q1, to the diode, D2,
in parallel with the low-side switch of the same inverter
leg. Then the negative voltage present at the emitter of
the high-side switching device, just before the freewheel-
ing diode, D2, starts clamping, causes load current to
suddenly flow to the low-side freewheeling diode, D2, as
shown in Figure 34.
This negative voltage can be trouble for the gate driver’s
output stage, there is the possibility to develop an over-
voltage condition of the bootstrap capacitor, input signal
missing and latch-up problems because it directly affects
the source V
35. This undershoot voltage is called “negative V
sient”.
Figure 35. V
Figure 34. Half-Bridge Application Circuits
GND
V
Q1
GND
S
S
S
DC+ Bus
pin of the gate driver, as shown in Figure
Transient
S
Waveforms During Q1 Turn-Off
Q1
V
Q2
S
D1
D2
i
i
LOAD
freewheeling
Load
Freewheeling
S
tran-
14
Figure 36 and Figure 37 show the commutation of the
load current between high-side switch, Q1, and low-side
freewheelling diode, D3, in same inverter leg. The para-
sitic inductances in the inverter circuit from the die wire
bonding to the PCB tracks are jumped together in L
L
low-side switch, Q4, are turned on, the V
below DC+ voltage by the voltage drops associated with
the power switch and the parasitic inductances of the cir-
cuit due to load current is flows from Q1 and Q4, as
shown in Figure 36. When the high-side switch, Q1, is
turned off and Q4, remained turned on, the load current
to flows the low-side freewheeling diode, D3, due to the
inductive load connected to VS1 as shown in Figure 37.
The current flows from ground (which is connected to the
COM pin of the gate driver) to the load and the negative
voltage present at the emitter of the high-side switching
device.
In this case, the COM pin of the gate driver is at a higher
potential than the V
ated with freewheeling diode, D3, and parasitic ele-
ments, L
E
for each IGBT. When the high-side switch, Q1, and
Figure 37. Q1 Turn-Off and D3 Conducting
C3
and L
Figure 36. Q1 and Q4 Turn-On
DC+ Bus
DC+ Bus
E3
S
.
L
Q1
L
V
Q3
L
L
Q1
V
Q3
L
L
L
L
C1
E1
S1
C3
pin due to the voltage drops associ-
E3
C1
E1
S1
C3
E3
V
V
V
V
LC1
LE1
D1
D3
LC3
LE3
D1
D3
i
i
LOAD
freewheeling
i
i
LOAD
freewheeling
Load
Load
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D2
D4
D2
D4
S1
V
V
V
V
LC4
LE4
LC4
LE4
node is
L
L
V
L
L
Q2
Q4
L
L
V
L
L
C2
Q2
Q4
E2
S2
C4
E4
C2
E2
S2
C4
E4
C
and