AN804 Vishay Siliconix P-Channel MOSFETs, the Best Choice for High-Side Switching Historically, p-channel FETs were not considered as useful as their n-channel counterparts. The higher resistivity of p-type silicon, resulting from its lower carrier mobility, put it at a disadvantage compared to n-type silicon. Getting n-type performance out of p-type FETs has meant larger area geometries with correspondingly higher inter-electrode capacitances. Consequently, a truly complementary pair—a p-channel and an n-channel device that match in all parameters—is impossible. The principal application of the p-channel, enhancementmode MOSPOWER FET is in switching power (or voltage) to grounded (ground return) loads. Switching Ground-Return Loads To drive the FET properly, the gate voltage must be referenced to its source. For enhancement-mode MOSFETs, this gate potential is of the same polarity as the MOSFET’s drain voltage. To turn on, the n-channel MOSFET requires a positive gate-source voltage, whereas the p-channel MOSFET requires a negative gate-source potential. Yet, despite its shortcomings, the p-channel MOSFET performs a vital “high-side” switch task that the n-channel simply cannot equal. During switching, a MOSFET’s source voltage must remain fixed, as any variation will modulate the gate and thus adversely affect performance. Figure 1 shows this degradation by comparing n-channel and p-channel MOSFET high-side switching. Used as a high-side switch, a p-channel MOSFET in a totem-pole arrangement with an n-channel MOSFET will simulate a high-current, high-power CMOS (complementary MOS) arrangement. Although the p-channel MOSFET cannot complement the n-channel in both on-resistance and capacitance simultaneously, such combinations as the low-threshold p-channel TP0610 and the n-channel 2N7000 together offer outstanding performance as a complementary pair. N-Channel P-Channel VDD VDD VGG VDD – ILrDS VGG – Vth 0 0 Load (1a) 0 Load (1b) FIGURE 1. Comparing the Performance Between N-Channel and P-Channel Grounded-Load Switching Document Number: 70611 10-Mar-97 www.vishay.com FaxBack 408-970-5600 1 AN804 Vishay Siliconix If an n-channel, enhancement-mode MOSFET were switching a positive-polarity voltage to a grounded load, the output would be limited to VGG – Vth. The equations describing performance of the n-channel ground-switching MOSFET with a ground-reference gate drive are based on the relationship between VDD and VGG: gate-to-source capacitance of the MOSFET. The RC determines the turn off time. Bootstrapping the n-channel MOSFET (Figure 2) is satisfactory for short turn-on times of a few milliseconds. In this arrangement, both MOSFETs must have breakdown voltage specifications that match or exceed the supply voltage. Using a p-channel MOSFET may place some severe restraints on the gate drive, since the gate must be close to VDD (Figure 1b). To return gate control to a more acceptable logic format, add an n-channel MOSFET (Figure 3). If VDD ≥ VGG, then VO = VGG – Vth ; If VDD < VGG – Vth . then VO = VDD – ILrDS(on). Sustaining a more acceptable gain with an output in direct relation to VDD would require an isolated gate drive referenced to the source (Figure 4). The dc bias option rectifies the pulse of ac from the transformer and stores a “+” voltage on the Using an n-channel MOSFET in this way simplifies the gate drive for a high-voltage, high-side, p-channel MOSFET. Placing a Zener diode between the gate and supply ensures that V(BR)GSS will not be exceeded. Again, both MOSFETs must withstand the full rail voltage. A R C Load B dc Bias Options FIGURE 4. Floating Gate Drive VDD VDD 15 V P-Channel VDD – ILrDS N-Channel N-Channel TTL N-Channel Load TTL FIGURE 2. Bootstrapping for N-Channel Grounded-Load Switching www.vishay.com FaxBack 408-970-5600 2 FIGURE 3. Using An N-Channel Level-Shifter Simplifies Driving From Logic Document Number: 70611 10-Mar-97 AN804 Vishay Siliconix Half-Bridge (Totem Pole) A high-side p-channel MOSFET and a low-side n-channel MOSFET tied with common drains (Figure 5) make a superb high-current “CMOS equivalent” switch. One fault common to such circuits has been the excessive crossover current during switching that may occur if the gate drive allows both MOSFETs to be on simultaneously. A resistively-coupled lower-power complementary pair offers extremely low crossover current when the output stage uses high-power MOSFETs. The Zener, Z1, and resistors, R1 and R3, act as a level shifter, properly driving the low-power MOSFETs. The Zener may be selected according to the equation VZENER = 2 VDD – Vth +VDD +15 V where +VDD = –VDD Whatever crossover current that might occur in the low-power drivers is dramatically reduced by the series resistor, R4. Additionally, driving the high-power complementary pair using this resistor divider scheme all but eliminates crossover current in this critical output driver. This increases both the driver’s efficiency and its reliability. P-Channel +15 V VOUT –15 V N-Channel TABLE 1. NĆ AND PĆCHANNEL DUAL MOSFETS IN SOĆ8 PACKAGE FOR OUTPUT STAGE IDD –15 V –VDD FIGURE 5. Low-Voltage Complementary MOSPOWER Array Properly driving the MOSFET gates can minimize unwanted crossover current at high supply voltages (both +VDD and –VDD) (Figure 6). Part Number V(BR)DSS (V) Si9939DY Si9942DY Si9928DY rDS Max () ID (A) 30/–30 0.05/0.1 3.5/–3.5 20/–20 0.125/0.2 3/–2.5 20/–20 0.05/0.11 5/–3.4 +VDD R1 P-Channel R2 Z1 R4 N-Channel TTL N-Channel R3 –VDD FIGURE 6. High-Voltage Complementary Pair Driven by Logic-Compatible MOSFET Document Number: 70611 10-Mar-97 www.vishay.com FaxBack 408-970-5600 3 AN804 Vishay Siliconix TABLE 2. POPULAR DISCRETE NĆ AND PĆCHANNEL MOSFETS FOR INPUT DRIVER STAGES AND OUTPUTS P-Channel Devices Part Number V(BR)DSS Min (V) N-Channel Devices rDS Max () Package Part Number V(BR)DSS Min (V) rDS Max () Package VP0300L –30 2.5 TO-226AA VN0300L 30 1.2 TO-226AA TP0610L –60 10 TO-226AA 2N7000 60 5 TO-226AA VP2020L –200 20 TO-226AA VN2010L 200 10 TO-226AA TP0101T* –12 0.65 TO-236 TN0200T* 20 0.4 TO-236 TP0610T* –60 10 TO-236 2N7002* 60 7.5 TO-236 *Surface Mount Package www.vishay.com FaxBack 408-970-5600 4 Document Number: 70611 10-Mar-97