What is in this article?:
- P-Channel Power MOSFETs Approach N-Channel Performance
- MATCHING P-CHANNEL AND N-CHANNEL MOSFETS
Using the latest generation of trench and polar power MOSFET technologies, both trench and polar P-channel power MOSFETs have been developed that retain all the features of comparable N-channel power MOSFETs, including very fast switching, voltage control, ease of paralleling and excellent temperature stability. Intended for applications that require the convenience of reverse polarity operation, they have an N-type body region that provides lower resistivity in the body region and good avalanche characteristics, because the parasitic PNP transistor is less prone to turn-on. Compared with N-channel power MOSFETs having similar design features, P-channel power MOSFETs have better forward-bias safe operating area (FBSOA) and are practically immune to single-event burnout phenomena. The most important advantage of P-channel power MOSFETs is the simplified gate driving technique in the high-side (HS) switch position.
The source voltage of a P-channel device is stationary when the device operates as an HS switch. Conversely, the source voltage of an N-channel device used as an HS switch varies between the low side (LS) and the HS of the dc bus voltage. Thus, to drive an N-channel device, an isolated gate driver or a pulse transformer must be used. The driver requires another power supply, while the transformer can sometimes produce incorrect operations. But in many cases, the LS gate driver can drive the P-channel HS switch with a simple level shifting circuit. Doing this simplifies the circuit and often reduces the overall cost. The main disadvantage of a P-channel device is its relatively high RDS(ON) compared with an N-channel device. This means the cost-effective solutions with P-channel power MOSFETs require optimization of devices toward reduced RDS(ON).
We have developed two families of P-channel Power MOSFETs (Polar™ and TrenchP™), covering VDS range of -50 V to -600 V and ID25 range of -10 A to -170 A. Both families offer best-in-class performance in industry-standard power packages and the proprietary ISOPLUS family packages. Fig. 1 repeats the symbols for the MOSFET types.
Driving a P-channel MOSFET is simpler and more cost-effective than driving an N-channel MOSFET as an HS switch.Fig. 2 shows one example of a gate driving circuit for an HS P-channel power MOSFET. This is much simpler and more cost-effective than driving N-channel MOSFETs. In this circuit, Dz, Rz and Ch were added to the typical gate driving circuit for an N-channel power MOSFET. Capacitor “Ch” holds dc voltage between the higher and lower gate drive circuits, so it must be much larger than the input capacitance of the P-channel MOSFET. Dz keeps the gate to source voltage in the range of -Zener voltage to 0.
The product of Ch and Rz determines the speed of the dc voltage adjustment across Ch. If it's too small, there will be a large current, which can damage the gate drive IC or Dz. If it's too big, the P-channel MOSFET will switch on too slowly due to the slower rise time of the gate pulse amplitude and can damage the MOSFET. Rh2 and Rl2 are resistors for controlling MOSFET turn-off speed. (Rh1 + Rh2) and (Rl1 + Rl2) are resistors for controlling MOSFET turn-on speed. It's often better to have slower turn-on speed than turn-off speed.
In many cases, both P-channel and N-channel MOSFETs can be driven by a single gate drive IC, as shown in Fig. 3. This is the most cost-effective and simplest gate driving method of half-bridge. To avoid cross conduction, dead time is to be provided by the difference of turn-on and turn-off speed. If dead time is too short, there is a chance of too much heat generation and risk of MOSFET failure. If dead time is too long, the output voltage of the bridge circuit may be reduced. Fig. 4 shows the dead times in a single gate drive IC case. With this circuit, at the beginning of the turn-on period of each MOSFET, the gate source voltage is not enough to fully turn on the MOSFET, and it will result in some additional power loss. Therefore, this circuit may not be suitable for hard switching applications. However, for some zero-voltage switching (ZVS) applications, in which MOSFETs are turned on while the opposite MOSFET operates in diode mode, this circuit can be cost-effective. 
Commonly used in automotive applications, almost all loads are connected between switches and body ground. All switches in automotive applications are located at the positive side. To drive the positive side N-channel Power MOSFET at a very low frequency, pulse transformer or bootstrap techniques cannot be used. Fig. 5 shows the circuit for providing a gate voltage higher than the dc-link voltage. When the square-wave generator output is at ground, diode Dc charges the charge pump capacitor, Cp. When the square-wave generator output is at the positive dc-link voltage, diode Dd discharges Cp. The charge is transferred to Cd, which is the power source of the HS gate drive circuit.
As shown in Fig. 6, the P-channel MOSFET greatly simplifies the overall circuit of Fig. 5. Generally, the simpler circuit is more reliable. Although the P-channel MOSFET has higher A*R DS(ON) than that of the N-channel MOSFET, in many cases, this simple circuit makes the larger expensive P-channel MOSFET the most cost-effective solution.