Transcription of P-Channel MOSFETs, the Best Choice for High-Side Switching
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.
3 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. 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.
4 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).
5 The dc bias option rectifies the pulseof ac from the transformer and stores a + voltage on thegate-to-source capacitance of the MOSFET. 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).
6 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.
7 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.
8 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. This increases both thedriver s efficiency and its 1.
9 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