The working principle, main parameters and safe working area of power

The working principle, main parameters and safe working area of power MOSFET

The structure and working principle of power MOSFET

There are many types of power MOSFETs, which can be divided into P-channel and N-channel according to the conduction channel. When the gate voltage is zero, there is a conductive channel between the drain and the source, which is called depletion type; for N (P) channel devices, when the gate voltage is greater than (less than) zero, the conductive channel is called enhanced. The power MOSFET mainly adopts an N-channel enhancement type structure. Figure 1a shows a cross-sectional view of a cell in the N-channel enhancement mode VD MOS; the graphic symbol of the power MOSFET is shown in Figure 1b. The power MOSFET has 3 pins: drain D, source S and gate G.

The working principle, main parameters and safe working area of power MOSFET
Figure 1 – Power MOSFET structure

For the N-channel enhancement mode power MOSFET, when the gate-source is applied with a forward voltage (UGS>0), the internal channel of the MOSFE appears, forming a drain-to-source current, and the device is turned on; conversely, when a reverse voltage (UGS<0) is applied to the gate-source, the channel disappears and the device is turned off.

Main parameters of power MOSFET

In addition to the aforementioned transconductance Gfs, turn-on voltage UGS(th), and various time parameters in the switching process, the power MOS FET also has the following main parameters:

(1) On-state resistance Ron

On-state resistance Ron refers to the DC resistance between the drain and the source when the power MOSFET enters the saturation region from the unsaturated region under a certain gate-source voltage UGS. The on-state resistance Ron has a positive temperature coefficient, which is beneficial to the current sharing when the devices are connected in parallel.

(2) Drain breakdown voltage UDS

Drain breakdown voltage The highest voltage that UDS power MOSFET can withstand, which is nominally a parameter of power MOSFET voltage rating. UDS is usually selected to be 2 to 3 times the actual working voltage.

(3) Drain DC current ID and drain pulse current amplitude IDM

The drain DC current ID and the drain skin impulse current amplitude IDM are the parameters of the nominal power MOSFET current rating. These two current parameters are limited by the operating temperature of the device.

(4) Gate-source voltage UGS

The insulating layer between the gate and the source is very thin. If the gate-source voltage is too high, it will cause breakdown of the insulating layer, and its limit value is ±20V.

(5) Interelectrode capacitance

Between the 3 electrodes of the MOSFET, the inter-electrode capacitances CGS, CGD, and CDS are parasitic. Generally, manufacturers provide input capacitance Ciss, common-source output capacitance Coss and reverse transfer capacitance Crss when the drain-source is short-circuited. The relationship between them is:

Ciss = CGS + CGD

Crss = CGD

Coss = CDS + CGD

The aforementioned input capacitors can be replaced by Ciss, and these capacitors are non-linear.

Safe working area of power MOSFET

The safe operating area of the power MOSFET is shown in Figure 2. It is surrounded by 4 boundary limits: the drain-source on-state resistance (I), the maximum allowable drain current (Ⅱ), the withstand voltage between the drain and the source (Ⅲ) and the maximum power dissipation (Ⅳ) determine the safe operating area of the power MOSFET. Generally speaking, the power MOSFET does not have the problem of secondary breakdown, but in actual use, you should still pay attention to leaving an appropriate margin. Figure 2 also marks the safe working area in three cases of direct current (DC) and pulse width of 10ms and 1ms respectively.

The working principle, main parameters and safe working area of power MOSFET
Figure 2 – Power MOSFET safe working area