Abstract:
Insulated Gate Bipolar Transistors (IGBTs) are popular power electronic
switching devices with several advantages. However, they have been known to
fail in the field when subjected to significant variations in power dissipation –
known as power cycling. In the work presented here, a novel alternating-current
(AC) power cycling test method for IGBTs together with their free-wheeling
diodes is proposed and verified.
A review of previous work revealed that the parameter that most affects IGBT
lifetime under power cycling conditions is the variation in its junction-case
temperature difference. Through simulation, the behaviour of a
conventional single phase inverter (H-bridge) using simple pulse width
modulation (PWM) control was quantified, and the effect of switching frequency
and load power factor was studied.
Results of the simulations and literature review were used to develop design
criteria for a new AC test circuit. The new AC test circuit (a modified version of
the conventional H-bridge) was then designed and its performance compared to
the criteria and to the simulation results of the conventional circuit. The circuit
was then built and its performance was validated. The circuit complied with the
performance criteria, in particular the desired variation in 7jc, to an adequate
degree of accuracy.
The proposed test circuit is novel for several reasons. The stresses on devices used
in a conventional H-bridge using a high power factor inductive load are
reproduced using a low power factor inductive load, considerably reducing the
energy cost of running such a test. IGBT switching losses are not actively
reduced, as is normal practice, but instead are actively increased to generate the
required losses. Free-wheeling diodes are also tested, but do not have significant
switching losses, as the nature of the test circuit dictates that these be transferred
to the IGBTs.
The main drawback of the proposed test circuit is that a larger number of devices
are needed; however, this tradeoff is necessary to obtain the energy cost savings
provided by this circuit.
