Shoot-through suppression: the diodes make the difference

March 07, 2016 // By John Dunn, EDN
When two switching devices are placed in series to serve as a hi-low switching pair (think of half-bridge inverters, of full-bridge inverters and the like), one insidious phenomenon that must be guarded against is simultaneous conduction of both devices, however brief, during the switching cycles.

 

Consider the SPICE simulation immediately below where two ideal controlled switches are keyed 180° out of phase with each other from square wave signals that are rounded by a series resistance and a shunt capacitor. The series resistance represents the sum of the source impedance of a driving IC plus a series resistor in our circuit board plus a channel resistance within the switching device while the capacitor represents an input gate capacitance. (Yes, I know that input capacitance doesn't just look like a capacitor, that the capacitance will vary radically versus gate input voltage, but we can ignore that fact for purposes of this discussion.)

 

(right-click/view image for more detail )

 

Even in this simplified model, we can see that there are spikes of current "shooting" through the two switches from the +5V rail (an arbitrary choice of voltage on my part) to ground. These pulses represent the switching devices' repeated attempts to short circuit the rail voltage to ground and they are bad news for all sorts of obvious reasons.

 

Have you ever wondered why diodes are used in gate drive circuits and what those diodes are doing in cases like this?

 

When two diodes are added across the gate drive resistors in the directions as shown, they make the transitions of gate voltage from high to low faster than the transitions going from low to high. As a result, each switch tends to turn off faster than the other switch can turn on. This helps guard against the two devices being on at the same time which, as we said before, is a bad thing.

 

 

In this model with the diodes added in, the shoot-through current pulses have gone away and that is a good thing.