NCD5703A, NCD5703B, NCD5703C
www.onsemi.com
16
Figure 23. Current Path without Miller Clamp
Protection
Figure 24. Current Path with Miller Clamp Protection
Desaturation Protection (DESAT)
This feature monitors the collector−emitter voltage of the
IGBT in the turned−on state. When the IGBT is fully turned
on, it operates in a saturation region. Its collector−emitter
voltage (called saturation voltage) is usually low, well below
3 V for most modern IGBTs. It could indicate an overcurrent
or similar stress event on the IGBT if the collector−emitter
voltage rises above the saturation voltage, after the IGBT is
fully turned on. Therefore the DESAT protection circuit
compares the collector−emitter voltage with a voltage level
V
DESAT−THR
to check if the IGBT didn’t leave the saturation
region. It will activate FLT output and shut down driver
output (thus turn−off the IGBT), if the saturation voltage
rises above the V
DESAT−THR
. This protection works on
every turn−on phase of the IGBT switching period.
At the beginning of turning−on of the IGBT, the
collector−emitter voltage is much higher than the saturation
voltage level which is present after the IGBT is fully turned
on. It takes almost 1 ms between the start of the IGBT turn−on
and the moment when the collector−emitter voltage falls to
the saturation level. Therefore the comparison is delayed by
a configurable time period (blanking time) to prevent false
triggering of DESAT protection before the IGBT
collector−emitter voltage falls below the saturation level.
Blanking time is set by the value of the capacitor C
BLANK
.
The exact principle of operation of DESAT protection is
described with reference to Figure 25.
At the turned−off output state of the driver, the DESAT pin
is shorted to ground via the discharging transistor (Q
DIS
).
Therefore, the inverting input holds the comparator output
at low level.
At the turned−on output state of the driver, the current
I
DESAT−CHG
from current source starts to flow to the
blanking capacitor C
BLANK
, connected to DESAT pin.
Appropriate value of this capacitor has to be selected to
ensure that the DESAT pin voltage does not rise above the
threshold level V
DESAT−THR
before the IGBT fully turns on.
The blanking time is given by following expression.
According to this expression, a 47 pF C
BLANK
will provide
a blanking time of (47p *6.5/0.25m =) 1.22 ms.
t
BLANK
+ C
BLANK
@
V
DESAT−THR
I
DESAT−CHG
After the IGBT is fully turned−on, the I
DESAT−CHG
flows
through the DESAT pin to the series resistor R
S−DESAT
and
through the high voltage diode and then through the
collector and IGBT to the emitter. Care must be taken to
select the resistor R
S−DESAT
value so that the sum of the
saturation voltage, drop on the HV diode and drop on the
R
S−DESAT
caused by current I
DESAT−CHG
flowing from
DESAT source current is smaller than the DESAT threshold
voltage. Following expression can be used:
V
DESAT−THR
u
R
S−DESAT
@ I
DESAT−CHG
) V
F_HV diode
) V
CESAT_IGBT
Important part for DESAT protection to work properly is
the high voltage diode. It must be rated for at least same
voltage as the low side IGBT. The safety margin is
application dependent.
The typical waveforms for IGBT overcurrent condition
are outlined in Figure 26.
