P-Channel MOSFETs, the Best Choice for High-Side …

1 AN804 vishay SiliconixDocument Number: FaxBack 408-970-56001P- channel mosfets , the best Choice for High-Side SwitchingHistorically, P-Channel FETs were not considered as useful astheir n- channel counterparts. The higher resistivity of p-typesilicon, resulting from its lower carrier mobility, put it at adisadvantage compared to n-type n-type performance out of p-type FETs has meantlarger area geometries with correspondingly higherinter-electrode capacitances. Consequently, a trulycomplementary pair a P-Channel and an n- channel devicethat match in all parameters is , despite its shortcomings, the P-Channel MOSFET performs a vital High-Side switch task that the n-channelsimply cannot as a High-Side switch, a P-Channel MOSFET in atotem-pole arrangement with an n- channel MOSFET willsimulate a high -current, high -power CMOS (complementaryMOS) arrangement.

2 Although the P-Channel MOSFET cannotcomplement the n- channel in both on-resistance andcapacitance simultaneously, such combinations as thelow-threshold P-Channel TP0610 and the n- channel 2N7000together offer outstanding performance as a complementarypair. R SSwitching Ground-Return LoadsThe principal application of the P-Channel , enhancement -mode MOSPOWER FET is in switching power (or voltage) togrounded (ground return) drive the FET properly, the gate voltage must be referencedto its source. For enhancement -mode mosfets , this gatepotential is of the same polarity as the MOSFET s drainvoltage. To turn on, the n- channel MOSFET requires a positivegate-source voltage, whereas the P-Channel MOSFET requires a negative gate-source switching, a MOSFET s source voltage must remainfixed, as any variation will modulate the gate and thusadversely affect performance.

3 Figure 1 shows thisdegradation by comparing n- channel and P-Channel MOSFET High-Side VthVDD ILrDSVDDFIGURE the Performance Between N- channel and P-Channel Grounded-Load Switching(1a)(1b)AN804 vishay FaxBack 408-970-56002 Document Number: 7061110-Mar-97If an n- channel , enhancement - mode mosfet were switchinga positive-polarity voltage to a grounded load, the output wouldbe limited to VGG equations describing performance of the n-channelground-switching MOSFET with a ground-reference gate driveare based on the relationship between VDD and VGG: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 directrelation to VDD would require an isolated gate drive referencedto the source (Figure 4). The dc bias option rectifies the pulseof ac from the transformer and stores a + voltage on thegate-to-source capacitance of the MOSFET.

4 The RCdetermines the turn off the n- channel MOSFET (Figure 2) issatisfactory for short turn-on times of a few milliseconds. In thisarrangement, both mosfets must have breakdown voltagespecifications that match or exceed the supply a P-Channel MOSFET may place some severerestraints on the gate drive, since the gate must be close toVDD (Figure 1b). To return gate control to a more acceptablelogic format, add an n- channel MOSFET (Figure 3).Using an n- channel MOSFET in this way simplifies the gatedrive for a high -voltage, High-Side , P-Channel a Zener diode between the gate and supply ensuresthat V(BR)GSS will not be exceeded. Again, both MOSFET smust withstand the full rail for N-ChannelGrounded-Load SwitchingN-ChannelTTLP-ChannelLoad15 VVDD ILrDSFIGURE An N- channel Level-ShifterSimplifies Driving From LogicVDDLoadABFIGURE Gate Drivedc BiasOptionsRCAN804 vishay SiliconixDocument Number: FaxBack 408-970-56003 Half-Bridge (Totem Pole)A High-Side P-Channel MOSFET and a low- side n-channelMOSFET tied with common drains (Figure 5) make a superbhigh-current CMOS equivalent switch.

5 One fault common tosuch circuits has been the excessive crossover current duringswitching that may occur if the gate drive allows bothMOSFETs to be on 15 V+15 V 15 V+15 VVOUT+VDD VDDIDDFIGURE ComplementaryMOSPOWER ArrayProperly driving the MOSFET gates can minimize unwantedcrossover current at high supply voltages (both +VDD and VDD)(Figure 6).A resistively-coupled lower-power complementary pairoffers extremely low crossover current when the output stageuses high -power mosfets . The Zener, Z1, and resistors, R1and R3, act as a level shifter, properly driving the low-powerMOSFETs. The Zener may be selected according to theequationVZENER = 2 VDD Vth where +VDD = VDDW hatever crossover current that might occur in the low-powerdrivers is dramatically reduced by the series resistor, , driving the high -power complementary pair usingthis resistor divider scheme all but eliminates crossovercurrent in this critical output driver.

6 This increases both thedriver s efficiency and its 1. N AND P channel DUALMOSFETS IN SO 8 PACKAGEFOR OUTPUT STAGEPartNumberV(BR)DSS (V)rDS Max ( )ID (A)Si9939DY30/ +VDD VDDFIGURE Complementary Pair Driven by Logic-Compatible MOSFETAN804 vishay FaxBack 408-970-56004 Document Number: 7061110-Mar-97 TABLE 2. POPULAR DISCRETE N AND P channel mosfets FOR INPUT DRIVER STAGESAND OUTPUTSP- channel DevicesN- channel DevicesPart NumberV(BR)DSS Min (V)rDS Max ( )PackagePart NumberV(BR)DSS Min (V)rDS Max ( )PackageVP0300L 6010TO-226AA2N7000605TO-226 AAVP2020L 20020TO-226 AAVN2010L20010TO-226 AATP0101T* * * 6010TO-2362N7002* *Surface Mount Packag