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Design Article

# Power MOSFET Tutorial - Part 2

## 12/6/2006 1:38 PM EST

Power MOSFET Tutorial Part 1

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 VDSS 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

VGS(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.

RDS(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.

IDSS — 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, IDSS is specified both at room temperature and hot. Leakage power loss is IDSS times drain-source voltage and is usually negligible.

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