Characteristics of Power MOSFETs

For a MOSFET, unlike thyristors, the division of characteristics into on-state and off-state is conditional, since this power semiconductor device can function in active mode. However, as a rule, in power converter technology, power MOSFETs are also used in the switch mode, so the mentioned subdivision of characteristics makes practical sense.

The current-voltage characteristic in the off-state corresponds to the current-voltage characteristic of the reverse-biased drain p-n junction. It is usually characterized by the maximum allowable drain-source voltage V_ds and the corresponding leakage current I_ds. Conditions for measuring these characteristics are some specified often negative gate voltage, and I_ds measurements that are usually carried out for the maximum permissible temperature.

The current-voltage characteristic in the on-state characterizes the value of the resistance in the on-state r_on at a specified value of the gate voltage and the range of changes in the drain current. The dependence of the drain current at a constant collector-emitter voltage on the gate voltage is described by the transient characteristic of the MOSFET. The most complete picture both in the field of operating currents and limiting the drain current is given by the current-voltage characteristic at different gate voltages. The limiting characteristic of the on-state is the maximum permissible peak drain current I_dmax.

The gate characteristics are usually given as the maximum allowable gate-source voltage V_gs and the corresponding maximum allowable gate-source leakage current I_ge. The value of the gate-source threshold voltage for the specified values of the drain current and the drain-source voltage V_GS(th) is also included. The gate-source and gate-drain capacitance values are also standardized.

Due to the Miller effect, the gate capacitance of the MOSFET structure strongly depends on the gate-source voltage, especially during the transition of the structure from a non-conducting to a conducting state and vice versa. Therefore, the volt-coulomb characteristic of the gate is important. It is the dependence of the gate charge Q_g on the gate-source voltage under specified conditions in the drain-source circuit. Knowing this dependence, it is easy to calculate what charge in the gate circuit will be required to switch the device.

The turn-on dynamics is characterized by the turn-on time t_on. It is the time interval from the moment the voltage pulse is applied to the gate until the drain current increases or the drain-source voltage decreases to a specified value. The turn-on time is divided into two phases – the delay time t_don and the rise time t_r. The boundary between these two components of the turn-on time is determined by the beginning of the drain current rise or the beginning of the drain-source voltage drop to the specified values.

The energy losses in the transient process of the power MOSFET turn-on are characterized by the switching energy E_on. It is the integral of the power loss during the turn-on time.

It should be noted that the turn-on process of the MOSFET is physically very fast and, since it is a device with unipolar conductivity, it is not accompanied by significant power losses.

However, it is often slowed down by introducing some resistance into the gate circuit, slowing down the process of its recharge. At the same time, the rise phase increases in duration, and the MOSFET limits the rise of the drain current, i.e., forms a current rise edge. The value of the E_on in this mode increases significantly, however, the smooth switching on of the device allows for the safe reverse recovery of the freewheeling diode working in conjunction with the MOSFET in autonomous voltage inverters with a bridge circuit.

The turn-off dynamics are characterized by the turn-off time t_off. It is the time interval from the moment the gate voltage begins to decrease until the drain current drops to a specified value.

The turn-off time is divided into two phases – the delay time t_s, and the decay time of the collector current t_f. The boundary between these two components of the turn-off time is determined by the beginning of the drain current drop to a specified value. The power loss in the transient processes of turning on and off the MOSFET, as a rule, is much lower than those of power semiconductors with bipolar conductivity, which allows you to significantly increase the operating frequency range for this type of power semiconductor device.

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