AN1944

Desaturation protection feature

13/21

6

Desaturation protection feature

The desaturation function provides a protection against over-current events. Voltage across

the IGBT is monitored, and the IGBT is turned off if the voltage threshold is reached. A

blanking time is made of an internal 250µA current source and an external capacitor. The

high voltage diode blocks the high voltage during IGBT off state (a standard 1kV or more

diode is usable); the 1k

Ω (approx.) resistor filters parasitic spikes and also protects the

DESAT input (see Figure 14).

During operation, the DESAT capacitor is discharged when TD350 output is low (IGBT off).

When the IGBT is turned on, the DESAT capacitor starts charging and desaturation

Equation 2

Equation 3

When a desaturation event occurs, the fault output is pulled down and TD350 outputs are

low (IGBT off) until the IN input signal is released (high level), then activated again (low

level).

Figure 15 shows a desaturation fault at 150A on a typical 25A module.

Figure 14.

Application schematic for DESAT feature

Note that during half-bridge commutation, the DESAT pin can experience a voltage peak. It

can depend proportionally to the parasitic capacitante (Cj) of the desaturation diode, to the

voltage value of the DC bus and in inverse proportion to the value of the capacitance placed

on the DESAT pin and to the value of the resistor in series with the desaturation diode. The

voltage peak on the DESAT pin must not exceed the absolute maximum rating indicated in

the TD350 datasheet.

t

B

7.2 V

[]

Cdesat

250

µA

[]

----------------------

•

=

t

[]

0.03

Cdesat pF

[]

•

=

TD350

14

DESAT

7,2V

1K

100pF

1k V

diode

Control

Block

8

GND

3

FAULT

VH

10K

250µA

Vc e