Functional description TD350E
8/18 DocID018539 Rev 3
5 Functional description
5.1 Input
The input is compatible with optocouplers or pulse transformers. The input is triggered by
the signal edge and allows the use of a small-sized, low cost pulse transformer. Input is
active low (output is high when input is low) to ease the use of the optocoupler. When driven
by a pulse transformer, the input pulse (positive and negative) width must be larger than the
minimum pulse width t
onmin
.
5.2 Voltage reference
A voltage reference is used to create accurate timing for the two-level turn-off with external
resistor and capacitor.
5.3 Desaturation protection
Desaturation protection ensures the protection of the IGBT in the event of overcurrent.
When the DESAT voltage goes higher than 7 V, the output is driven low (with 2-level turn-off,
if applicable). The FAULT output is activated. The FAULT state is exited at the next falling
edge of IN input.
A programmable blanking time is used to allow enough time for IGBT saturation. Blanking
time is provided by an internal current source and external capacitor.
DESAT input can also be used with an external comparator for overcurrent or
overtemperature detection.
5.4 Active Miller clamp
A Miller clamp allows the control of the Miller current during a high dV/dt situation and can
eliminate the need for a negative supply voltage.
During turn-off, the gate voltage is monitored and the clamp output is activated when gate
voltage goes below 2 V (relative to GND). The clamp voltage is VL+3 V max. for a Miller
current up to 500 mA. The clamp is disabled when the IN input is triggered again.
5.5 Two-level turn-off
The two-level turn-off is used to increase the reliability of the application.
During turn-off, gate voltage can be reduced to a programmable level in order to reduce the
IGBT current (in the event of overcurrent). This action prevents both dangerous overvoltage
across the IGBT and RBSOA problems, especially at short-circuit turn-off.
The two-level turn-off (T
a
) delay is programmable through an external resistor and capacitor
for accurate timing (refer to Equation 1).
Turn-off delay (T
a
) is also used to delay the input signal to prevent distortion of input pulse
width.
