P-Channel Power MOSFETs Approach N-Channel Performance

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.^[1] 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.^[2] The most important advantage of P-channel power MOSFETs is the simplified gate driving technique in the high-side (HS) switch position.^[3]

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 R_DS(ON) compared with an N-channel device. This means the cost-effective solutions with P-channel power MOSFETs require optimization of devices toward reduced R_DS(ON).

We have developed two families of P-channel Power MOSFETs (Polar^™ and TrenchP^™), covering V_DS range of -50 V to -600 V and I_D25 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.

GATE DRIVING

Driving a P-channel MOSFET is simpler and more cost-effective than driving an N-channel MOSFET as an HS switch.^[5]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.^[4]

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. ^[4]

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.^[4]

MATCHING P-CHANNEL AND N-CHANNEL MOSFETS

It is impossible to produce a P-channel power MOSFET that has the same electrical characteristics as an N-channel power MOSFET. Because the mobility of carriers in an N-channel power MOSFET is about 2.5 times to 3 times higher, for the same RDS(ON) value, the P-channel power MOSFET size must be about 2.5 times to 3 times that of an N-channel power MOSFET. Because of the larger area, the P-channel device will have lower thermal resistance and higher current rating. Its dynamic performance (capacitance, gate charge, etc.) would be affected proportionally by the chip area.

In low-frequency switching applications in which conduction loss is dominant, the P-channel MOSFET should have a current rating similar to that of the N-channel MOSFET. If two MOSFETs have the same current rating, then their junction temperatures can be thought to be similar at the same case temperature and the same current. In this case, the P-channel MOSFET chip area is 1.5 ~ 1.8 times that of the N-channel MOSFET chip area.

In high-frequency switching applications in which switching loss is dominant, the P-channel MOSFET should have similar total gate charge to that of the N-channel MOSFET. If two MOSFET have the same gate charge and are driven in a similar way, then their switching losses are similar. In this case, the P-channel MOSFET has similar chip area and the current rating is lower than that of N-channel MOSFET.

For operation in linear mode, we need to match P-channel and N-channel devices with similar FBSOA characteristics in the real operating area. Frequently, this means the same rated Pd, but attention needs to be paid to the ability of the device to operate in this mode.⁸ In real applications, the suitable P-channel must be carefully selected in between the same current rating and the same gate charge. Applications requiring the same RDS(ON) are very rare.

APPLICATION EXAMPLES

Audio amplifier is perhaps the most important application of P-channel MOSFETs. Fig. 7 shows a class AB audio amplifier circuit, which has a complementary power MOSFET output stage, a differential input stage and a biasing circuit for the output stage. It offers performance improvements over the equivalent bipolar output stage and allows a reduction in the complexity of the driver circuit.

The input stage has a PNP differential comparator that receives input signal through R1 and C1 and the negative feedback of the output stage to the base of Q2 through the resistor R6. The comparator drives the transistor, Q4, which drives the output stage. Components R6 and R5 determine the feedback loop gain as β = R5/R5+R6). R2 determines the bias current at the input stage and is typically 2mA. R4 and C3 create a filter that provides extra power supply ripple suppression.

VBE multiplier consisting of R7, R8, R9, C5 and Q3 provides a bias voltage, Vb, between the gates of transistor Q5 and Q6. The capacitor C5 holds the voltage. If Vbe of Q3 is ~0.6 V, R9 ~10 K and R7 ~ 100 K, then the value of the bias voltage would be about Vb ~10 × Vbe ~ 6 V. The purpose of this voltage is to bias the gates of Q5 and Q6, keeping them in a slightly ON state that results a quiescent current flowing through in the output stage. The quiescent current reduces the zero crossing distortion associated with the output stage. The small capacitors C2 and C4 make the entire circuit stable.[6]

The output stage comprises N- and P-channel power MOSFETs (Q5 and Q6) connected in series between the high-voltage (+V_DD) and low-voltage (-V_DD) terminals. The sources of Q5 and Q6 are connected to the OUTPUT terminal, which delivers an output signal to the LOAD (speaker). The output stage is a source follower circuit with gain close to 1 (but <1.0), which is almost an ideal voltage source. Its output voltage is practically insensitive to the output current.^[6]

Both MOSFETs in a Class AB amplifier require extended FBSOA as they operate in linear mode. Power dissipation would be high because of linear operation.

Linear voltage regulators are widely used to supply power to electronic devices. They have a variety of configurations for many different applications. One application example is shown in Fig. 8. The resistive divider (R3 and R4) monitors the output voltage and provides a voltage feedback (VFB) to the positive (+) terminal of the op-amp (U1). The negative (-) terminal of the op-amp receives a reference voltage (V _Ref) from a Zener diode (ZD1). The op-amp provides a control voltage to the regulating transistor (Q1), a P-channel power MOSFET. Because the voltage drop across the P-channel MOSFET can be lowered nearly zero, this circuit has a wide input voltage range.

The power dissipation in the device (Q1) used in the linear voltage regulator is high because it's the function of the difference between input and output voltage and the output current. The P-channel power MOSFET operates in the linear mode and requires an extended FBSOA characteristic, which is offered by both families of IXYS P-channel power MOSFETs.

Fig. 9 shows a battery charging and discharging system for Li-ion (Li+) cells. One MOSFET enables the charging of the battery pack while the other MOSFET enables the discharging. When both MOSFETs are off, the cells are isolated from the external environment to protect the battery. At the beginning of the charging cycle, a constant current can be implemented and the MOSFET will be operated in the linear region. When the battery cell reaches a predefined voltage level, the system voltage loop will begin to reduce the charging current in order to maintain the desired voltage level, hence the constant voltage-mode operation. ^[5]

REFERENCES

1. Erickson, R. W. and Maksimovis, D. “Fundamentals of Power Electronics,” University of Colorado, Second Edition.

2. Dodge, J. “Reduced Circuit Zapping from Cosmic Radiation,” Micro Semi, September 2007, http://powerelectronics.com/power_semiconductors/power_mosfets/circuit-zapping-cosmic-radiation-0907/.

3. “How P-Channel MOSFETs Can Simplify Your Circuit,” AN-940, International Rectifier, http://www.eetasia.com/ARTICLES/2000MAY/2000MAY04_ICD_WLP_AN.PDF?SOURCES=DOWNLOAD.

4. Mohan, N., Robbins, W. and Undeland, T. M. “Power Electronics — Converters, Applications and Design,” John Wiley & Sons, 2nd Edition.

5. “P-Channel MOSFETs, the Best Choice for High-Side Switching,” AN804, Vishay Siliconix, March 10, 1997, http://www.datasheetcatalog.org/datasheet/vishay/70611.pdf.

6. “Linear Power Amplifier Using Complementary HEXFETs,” AN-948, International Rectifier, http://home.eunet.cz/rysanek/pdf/irf-fet-amp.pdf.