Power MOSFET Tutorial - Part 2

Static electrical characteristics

V_(BR)DSS — Drain-source breakdown voltage
V_(BR)DSS (sometimes called BVDSS) is the drain-source voltage at which no more than the specified drain current will flow at the specified temperature and with zero gate-source voltage. This tracks the actual avalanche breakdown voltage.

As shown in Figure 8, V_(BR)DSS has a positive temperature coefficient. A MOSFET can block more voltage when hot than when cold. In fact, when cold, V_(BR)DSS is less than the maximum V_DSS rating, which is specified at 25°C. For the example shown in Figure 8, at -50°C, V_(BR)DSS is about 90% of the 25°C maximum VDSS rating.

Figure 8 Normalized Breakdown Voltage versus Temperature

V_GS(th) — Gate threshold voltage
This is the gate-source voltage at which drain current begins to flow, or stops flowing when switching off the MOSFET. Test conditions (drain current, drain-source voltage, and junction temperature) are also specified. All MOS gated devices exhibit variation in threshold voltage between devices, which is normal. Therefore, a range in VGS(th) is specified, with the minimum and maximum representing the edges of the VGS(th) distribution. As discussed previously under Temperature Effects, VGS(th) has a negative temperature coefficient, meaning that as the die heats up, the MOSFET will turn on at a lower gate-source voltage.

R_DS(on) — ON resistance
This is the drain-source resistance at a specified drain current (usually half the ID current) and gate-source voltage (usually 10 Volts), and at 25°C unless otherwise specified.

I_DSS — Zero gate voltage drain current
This is the drain-source leakage current at a specified drain-source voltage when the gate-source voltage is zero. Since leakage current increases with temperature, I_DSS is specified both at room temperature and hot. Leakage power loss is I_DSS times drain-source voltage and is usually negligible.

I_GSS — Gate-source leakage current
This is the leakage current that flows through the gate terminal at a specified gate-source voltage.