AN-990 Datasheet, PDF (11/17 Pages) International Rectifier – Application Note

It is important to remember that switching energy changes significantly with temperature and all calculations
should be carried out with numbers that are appropriate for the operating temperature.
Turn-on and turn-off losses can be calculated with the same two-step technique described in the previous
section, complemented by the following additional corrections:
1) The energy loss thus obtained must be scaled with voltage. Data sheet measurements have been
taken with a defined supply voltage that may or may not be the same as the supply voltage of the
specific application.
2) Similarly, the gate drive resistance of the test circuit in the data sheet may be different from what is
used in the actual application. Recent data sheets have a figure that correlates switching energy
with gate drive impedance.
3) Switching energy must be multiplied by frequency to obtain power losses.
The turn-on transient of a clamped inductive load is complicated by the reverse recovery of the anti-parallel
diode of the complementary device. When the IGBT turns on it will take over the entire load current plus the
reverse recovery current of the diode that was carrying the load current when the IGBT turns on. Switching
energy data in recent data sheets include the “diode induced” losses.
Earlier data sheets used a different test circuit with an “ideal diode”. This test circuit measured turn-on energy
without the diode induced losses. It follows that, to obtain the total turn-on losses two components have to be
calculated separately and added together.
Figure 7 shows a typical turn-on waveform. Notice how the reverse recovery of the diode increases the turn-
on energy in two ways:
 it adds to the turn-on current when the collector voltage is still close to the supply voltage
 it delays the collapse of the gate voltage.
www.irf.com
August 2012
AN-990
11