Basic characteristics of power MOSFET

Basic characteristics of power MOSFET

Power Metal Oxide Semicon-ductor Field Effect Transistor is referred to as Power MOSFET. It is a unipolar voltage control device. It has self-shutdown capability, high input impedance, low drive power, fast switching speed, working frequency up to 1MHz, no secondary breakdown problem, and wide safe working area. However, its voltage and current capacity are relatively small. At present, the withstand voltage of general power MOSFET product design is below 1000V, so it is widely used in high-frequency medium and small power power electronic devices.

Basic characteristics

(1) Static characteristics

The static characteristics of power MOSFET mainly refer to the transfer characteristics and drain volt-ampere characteristics of the MOSFET.

The transfer characteristic is the relationship curve between the drain current ID of the power MOSFET and the gate-source voltage UGS under a certain drain-source voltage UDS, as shown in Figure 1a. This characteristic reflects the ability of the gate-source voltage UGS of the power MOSFET to control the drain current ID. It can be seen from the figure that when the ID is large, the relationship between ID and UGS is approximately linear, and the slope of the curve is defined as the transconductance Gfs of the MOSFET, namely

Basic characteristics of power MOSFET
(1-1)

The transconductance Gfs represents the amplification capability of the MOSFET, and the unit is Siemens (S). It can be seen from Figure 1a that only when UCS>UGS(th), the device is turned on, and UGS(th) is called the turn-on voltage. MOSFET is a voltage-controlled device with extremely high input impedance and very small input current.

Figure 1b is the drain volt-ampere characteristic of the MOSFET, that is, the output characteristic. It can be seen from the figure that the output characteristics include three areas: non-saturated area Ⅰ (corresponding to the saturated area of GTR), saturated area Ⅱ (corresponding to the enlarged area of GTR), cut-off area Ⅲ (corresponding to the cut-off area of GTR), Avalanche area Ⅳ. The concept of saturation and non-saturation here is different from GTR. Saturation means that the drain current is almost unchanged when the drain-source voltage increases, and non-saturation means that the drain current increases correspondingly when the drain-source voltage increases. The power MOSFET works in the on-off state, that is, it switches back and forth between the cut-off region and the non-saturated region.

Basic characteristics of power MOSFET
Figure 1 – Transfer characteristics and volt-ampere characteristics of power MOSFET

Due to the structure of the power MOSFET itself, between its drain and source, there are parasitic diodes connected to the MOSFET in anti-parallel between the P region, the Ndrift region and the N+ region. The parasitic diode and the MOSFET form an inseparable whole, so that the device is turned on when a reverse voltage is applied between the drain and the source. Therefore, if it is necessary to withstand the reverse voltage when in use, a fast diode should be connected in series in the MOSFET circuit.

(2) Dynamic characteristics

The power MOSFET is an approximately ideal switch with high gain and extremely fast switching speed. This is because it is a unipolar device, which relies on majority carriers to conduct electricity, does not have the storage effect of minority carriers, and the storage time associated with the turn-off time is greatly reduced. Its turn-on and turn-off are only affected by the inter-electrode capacitance and are related to the charging and discharging of the inter-electrode capacitance.

Now use the circuit shown in Figure 2a to test the switching characteristics of the power MOSFET. In the figure, up is a rectangular pulse voltage signal source (see Figure 2b for the waveform), RS is the internal resistance of the signal source, RG is the gate resistance, RL is the drain load resistance, and RF is the source resistance.

Basic characteristics of power MOSFET
Figure 2 – Switching process of power MOSFET

The switching waveform of the power MOSFET is shown in Figure 2b. The turn-on time ton of the power MOSFET can be defined as the sum of the turn-on delay time td(on) and the current rise time tr, namely

                 ton = td(on) + tr                 (1-2)

The turn-off delay time td(off) and the current fall time tf are defined as the turn-off time toff of the MOSFET, namely

                 toff = td(off) + tf                 (1-3)

Generally, the switching time of MOSFET is between 10 and 100 ns, and its operating frequency can reach more than 100 kHz, which is the highest among major power electronic devices. The switching time of bipolar devices is calculated in microseconds, even reaching tens of microseconds. In addition, although the power MOSFET is a field-controlled device, almost no input current is required in the static state, but the input capacitor needs to be charged and discharged during the switching process, and a certain driving power is still required. The higher the switching frequency, the greater the drive power required.