AN5177 Datasheet, PDF (1/5 Pages) Dynex Semiconductor – Improved Gate Drive For GTO Series Connections

AN5177 Application Note
AN5177
Improved Gate Drive For GTO Series Connections
Replaces September 2000 version, AN5177-3.0
Application Note
AN5177-3.1 July 2002
Using an improved gate drive to ease GTO series connection
problems.
INTRODUCTION
There are problems encountered with dynamic voltage sharing
of series connected GTOs both at turn-on and turn-off. This
application note will deal with the problems associated with turn-
off and a further note will address turn-on problems.
Switching high voltages using rectifier diodes or thyristors is
routinely done using devices connected in series. With resistor
and capacitor networks connected, 100 devices or more can be
wired in series.
The basic problem of series connection is to ensure good voltage
sharing between devices under both static and dynamic
conditions. By using devices selected within defined limits of
leakage current and reverse recovery charge, together with
correctly sized resistors and capacitors reliable operation is
assured.
Unfortunately, series connection of GTOs involves more
restraints. For traditional applications such as rail traction where
high turn-off currents are important series connection of GTOs
has not usually been cost effective. A higher voltage GTO has
been the preferred solution. However, this application note
shows that even standard design GTOs when used with improved
performance gate drive units can be successfully connected in
series for certain applications.
In this note, a specific type of application is considered where
reverse blocking GTOs are used in series in a current source
inverter. In this application, the GTOs are not usually required to
turn-off near to their I limits.
TCM
Fig. 1 GTOs in series with snubber networks
voltage source inverters turn-off by anode current reversal is not
usually relevant.
THE SERIES CONNECTION PROBLEM
Three modes of operation need to be considered:
1. At GTO turn-on.
2. At GTO turn-off by gate commutation. This is the conventional
mode for most GTO applications.
3. At GTO turn-off by anode current reversal i.e. natural
commutation. This is similar to diode reverse recovery.
All these operating modes apply to current source inverters. For
TURN-ON
Figure 1 shows two GTOs in series with snubber networks fitted.
For simplicity, static sharing parallel resistors are not shown.
There will always be a time difference, ∆T, between the GTOs
turning on. This time difference is the combination of differences
in td and tr of the GTOs, together with the gate drive units
propagation time variations.
Figure 2 shows the waveforms at turn-on.
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