Other dynamic elements in MOSFET

Other dynamic elements

Equivalent circuit of a power MOSFET, including the dynamic elements (capacitors, inductors), the parasitic resistors, the body diode.

[edit]

Packaging inductances

To operate, the MOSFET must be connected to the external circuit, most of the time using wire bonding (although alternative techniques are investigated). These connection exhibit a parasitic inductance, which is in no way specific to the MOSFET technology, but has important effects because of its high commutation speed. Parasitic inductances tend to maintain their current constant and generate overvoltage during the transistor turn off, resulting in increasing commutation losses.

A parasitic inductance can be associated with each terminal of the MOSFET. They have different effects:

• the gate inductance has little influence (assuming it is lower than some hundreds of nanohenrys), because the current gradients on the gate are relatively slow. In some cases, however, the gate inductance and the input capacitance of the transistor can constitute anoscillator. This must be avoided as it results in very high commutation losses (up to the destruction of the device). On a typical design, parasitic inductances are kept low enough to prevent this phenomenon;
• the drain inductance tends to reduce the drain voltage when the MOSFET turns on, so it reduces turn on losses. However, as it creates an overvoltage during turn-off, it increases turn-off losses;
• the source parasitic inductance has the same behaviour as the drain inductance, plus a feedback effect that makes commutation last longer, thus increasing commutation losses.
  • at the beginning of a fast turn-on, due to the source inductance, the voltage at the source (on the die) will be able to jump up as well as the gate voltage; the internal V_GS voltage will remain low for a longer time, therefore delaying turn-on.
  • at the beginning of a fast turn-off, as current through the source inductance decreases sharply, the resulting voltage across it goes negative (with respect to the lead outside the package) raising the internal V_GS voltage, keeping the MOSFET on, and therefore delaying turn-off.