FAN7392_10 Datasheet, PDF(14/18 Page) Fairchild Semiconductor – High-Current, High- and Low-Side, Gate-Drive IC

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FAN7392_10 Datasheet, PDF (14/18 Pages) Fairchild Semiconductor – High-Current, High- and Low-Side, Gate-Drive IC

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Application Information

Negative VS Transient

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.

DC+ Bus

iLOAD

ifreewheeling

Load

Figure 34. Half-Bridge Application Circuits

This negative voltage can be trouble for the gate driverâs

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 VS pin of the gate driver, as shown in Figure

35. This undershoot voltage is called ânegative VS tran-

sientâ.

GND

Freewheeling

Figure 35. VS Waveforms During Q1 Turn-Off

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 LC and

LE for each IGBT. When the high-side switch, Q1, and

low-side switch, Q4, are turned on, the VS1 node is

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 VS pin due to the voltage drops associ-

ated with freewheeling diode, D3, and parasitic ele-

ments, LC3 and LE3.

DC+ Bus

LC1

VLC1

LE1

VLE1

VS1

LC3

iLOAD

ifreewheeling

Load

LC2

LE2

VS2

VLC4 LC4

LE3

VLE4 LE4

Figure 36. Q1 and Q4 Turn-On

DC+ Bus

LC1

iLOAD

ifreewheeling

LE1

VS1

Load

LC3

VLC3

LE3

VLE3

LC2

LE2

VS2

VLC4 LC4

VLE4 LE4

Figure 37. Q1 Turn-Off and D3 Conducting

FAN7392 Rev. 1.0.3

www.fairchildsemi.com

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