5.9 Unexpected Shoot-through with dVDS/dt

Unwanted MOSFET turn-ON, caused by C_GD × dV_DS/dt (see Figure 5-9) is often the cause of unexplained shoot through in the halfbridge circuit. Depending on the ratio of the C_GS/C_GD, when the dV_DS/dt across low side MOSFET (Q₂) occurs (i.e when high side MOSFET turns ON), there can be a voltage applied to the gate of the Q₂ MOSFET, turning ON Q₂, and causing shoot through. In effect, a gate bouncing occurs causing a ringing on the VS line and the power ground.

Figure 5-9. Unexpected Shoot Through With dV_DS/dt

Considering Figure 5-9

$I_{G D} = C_{G D} \times {d V}_{D S} / d t$

I_GD will flow towards the resistive load (and small inductive due to parasitics) of the gate driver and the C_GS of the MOSFET. Hence, this unwanted condition may be minimized by looking at the Ciss/Cres in the MOSFET datasheet (Ciss/Cres gives an indication of C_GS/ C_GD); having a Ciss/Cres as large as possible will minimize this phenomenon. Also, an external capacitor can be added to the gate-source of the MOSFET (for example 1 nF), which will increase C_GS/C_GD.