There are two ways of doing this just by connecting a

Info iconThis preview shows page 1. Sign up to view the full content.

View Full Document Right Arrow Icon
This is the end of the preview. Sign up to access the rest of the document.

Unformatted text preview: ronment for implementing soft turn-off. There are two ways of doing this. Just by connecting a resistor of appropriate value from Gate to source/emitter, the CGS gets discharged through this resistor and, depending on the value of the connected resistor, soft turn-off of any duration can be achieved. Another way, as shown in the Fig. (2), is to use a signal MOSFET Q1 to pull down the Gate, when short circuit is detected. The resistor in series with this signal MOSFET determines the time duration of this soft turn-off. Soft turn-off helps protect IGBT/MOSFET from any voltage transients generated due to L*dIC/dt (or L*dID/ dt) that could otherwise bring about avalanche breakdown. The PC board layout and Bill of Materials for this circuit are shown in Fig. (3). For an IGBT, Desat detection (Desat = Desaturation of forward voltage drop) is a method used for short circuit/overload protection. When short circuit/overload occurs, the forward voltage drop of the IGBT (V CE) rises to disproportionately high values. One must ignore the initial turn-on rise in VCE, when output from Driver has still not risen to high enough value. Nevertheless, when V CE rises to a level of, say, 7 Volts, in presence of sufficient Gate Drive voltage, it means the collector current IC has risen to a disproportionately high value, signalling overload. When a voltage level higher than 6.5 Volts is detected, Gate signal can be softly turned off, resulting into soft turn-off of the IGBT. Fig. (12) shows how Desat feature can be wired into a total Driver Circuit, using also other features, such as Opto-isolation and –ve turn-off bias. 3. HIGH SIDE DRIVING TECHNIQUES 3.1 EMPLOYING CHARGE-PUMP BOOTSTRAP METHODS AND For driving the upper MOSFET/IGBT in a phase leg employed in a bridge topology, a buck converter or a 2-transitor forward converter, low side drivers cannot be used directly. This is because the source/emitter of upper MOSFET/IGBT is not sitting at ground potential. Fig. (5) shows how a charge pump creates a higher Vcc to be used for the...
View Full Document

This note was uploaded on 01/15/2014 for the course ECE 624 taught by Professor Staff during the Winter '08 term at Ohio State.

Ask a homework question - tutors are online