TLTO technique, while
shows TLTO in use. The prevention of intense
oscillations in the gate voltage and collector-emitter voltage can be clearly seen. More
important, however, is that the resulting over-voltage has been reduced. In this
example, a peak voltage of 1125V occurs i
. In th
on theother hand, the voltage is only 733V (in each casewith aDC-bus voltageof 400V
and using a400A/1.2kV IGBT).
It is not possible to calculate the best alignment of the TLTO voltage and time, so these
have to be discovered by trial and error. Values of around 11V and 1µs to start with
have provedbeneficial inpractice.
As the TLTO is integrated in the driver circuit, it cannot be turned on or off at will. It
plays a part in every switching sequence, regardless of whether an over current or short
circuit needs to be turned off. The additional losses this causeswhen the IGBT is turned
off are, however, not important and can be ignored in practice. In general, this core
principle of the TLTO has advantages when compared to the soft shutdown principle
described below, as it inhibits a possible simultaneous short circuit and normal (PWM
control) turning off. With the soft shutdown principle, this would lead to an immediate
Example of an IGBT driver IC with integrated TLTO function (TD350 from
Although using a TLTO limits the switching over-voltages of an over current or
short circuit turn-off, it is advisable, although not essential, to implement Active
as an additional safety layer.
6.7.7 Soft shutdown
If the driver detects a short circuit, e.g. by means of the U
monitoring, the soft
shutdown function does not switch the IGBT hard to 0V or a corresponding negative
gate voltage. Instead, the gate turn-off voltage is supplied by a comparatively high
impedance stage that delays the discharging of the gate. In its simplest form, this is a
(comparatively) high impedance resistor, through which the gate-emitter capacity is
discharged. The IGBT is shut down softly. As soon as a lower voltage of e.g. 2V is
reached, the high impedance stage or resistor is short circuited with a low impedance
stage, ensuring rapid, complete dischargeof the gate-emitter capacity.