DC-to-DC Design Guide

1 AN607 vishay SiliconixDocument Number: Design GuideSergeJaunay,JessBrownINTRODUCTIONM anufacturers of electronic systems that require powerconversion are faced with the need for higher-density dc-to-dcconverters that perform more efficiently, within a smallerfootprint, and at lower cost despite increasing output meet these demands, Siliconix has combined advancedTrenchFETRand PWM-optimized process technologies,along with innovative new packages, to provide:Dlowest on-resistance for minimum power dissipationDlowest gate charge for minimum switching lossesDdV/dt shoot-through immunity1 Dimproved thermal in thermal management for increasing powerdensity are being achieved with vishay Siliconix packagingtechnologies such as the PowerPAKt(Si7000 Series), thethick leadframe D2 PAK(SUM Series),and ChipFETt(Si5000 Series).

2 DThe PowerPAK SO-8 offers the steady-state thermalresistance of a DPAK in an SO-8 PowerPAK 1212-8 is approximately half the size of aTSSOP-8 while decreasing the thermal resistance by anorder of SUM Series reduces thermal resistance by 33% overstandard D2 PAK is 40% smaller than a TSOP-6 package whileoffering lower on-resistance and lower thermal should be noted that lower thermal resistance results inhigher possible maximum current and power complete array of vishay Siliconix MOSFET-packagedproducts ranges from the D2 PAK (SUM or SUB series), DPAK(SUD Series), and PowerPAK (Si7000 Series) types ofpackages to the LITTLE FOOTR packages. These smalloutline devices range from the SO-8 down to the tiniestMOSFET available - the LITTLE FOOT MATERIALS witching CharacteristicsThe basic characteristics of a MOSFET are key tounderstanding how these devices work in switchmode reality the freewheel diode will have some form of reverserecovery effect ( a and b, Figure 2), and as a result, thecurrent through the drain source of the MOSFET (Q1, Figure1) will increase.

3 To accommodate the extra drain-sourcecurrent, VGSmust increase above the value necessary is carrying the combined load and recovery current(period ). Therefore, the recovery current of the freewheeldiodeaddstotheloadcurrentseenby thecontrollingMOSFET(Q1). At the end of period a, the reverse recovery currentfalls, along with the gate-source voltage. This is because thediode has recovered. The recovery current in turn will decay tozero, resultingin thegate voltagereducing tothe originalvaluerequired to support the load current (period b). During thisperiod, the freewheel diode starts to support voltage, and theVDSvoltage falls, and the Miller Plateaubegins. As with theideal-recovery diode explanation, this continues until thevoltage falls to its on-state value (end of ) and thegate-source voltage is unclamped and continues to theapplied gate-voltage is effectively the reverse of turn-on, apart from thatthere is no limitation by the freewheeling diode (in thisparticular circuit).

4 For turn-off the Miller Plateau indicates thestart of the rise of the drain-source voltage, and the voltage ofthe Miller Plateau willrepresent therequired VGSto sustaintheload current. The turn-off delay is the period from when thegate voltage falls from its on-state value to when it reaches theMiller Plateau value ( load-current value).A simple buck converter, shown in Figure 1, shows thebehavior of the MOSFET during turn-on and turn-off whenswitching an inductive load. During these periods, a positivestepinputis appliedto turnthe deviceon, anda steptransition,from positive to zero, is applied to turn the MOSFET a positive step-input voltage on the gate, the voltageacross the gate-source of the MOSFET (VGS) ramps upaccording to the time constant formed by the gate resistance(Rg) and input capacitance (Ciss), as shown in Figure 2a(period ).

5 Once VGSreaches the threshold voltage (Vth), thechannel is turned on, and the current through the device startsto ramp up (period ). At the end of period , there are twopossible switching transients that VGScould follow. In the firstcase, the freewheel diode (D1, Figure 1) is assumed to havean ideal reverse recovery, represented bythe solidwaveformsin Figure 2. Once the channel is supporting the full-loadcurrent, the voltage across the device can begin to decay (theend of point ) because the diode is now able to supportvoltage. As the drain-source voltage falls, the gate-sourcevoltage stays approximately constant. This phenomenon iscalled the Miller Plateau, and it continues until the voltagea)Specifically designed to prevent spurious turn-on during high rates of dV/dtb)SUM is an improved D2 PAK package, with lower rDS(on)and thermalresistanceAN607 vishay Number: 7191710-Oct-02falls to its on-state value.

6 At the end of period (Figure 2), thegate-source voltage is unclamped and continues to theapplied gate-voltage value. This additional gate voltage fullyenhances the MOSFET channel and reduces the rDS(on).In reality the freewheel diode will have some form of reverserecovery effect ( a and b, Figure 2), and as a result, thecurrent through the drain source of the MOSFET (Q1, Figure1) will increase. To accommodate the extra drain-sourcecurrent, VGSmust increase above the value necessary is carrying the combined load and recovery current(period ). Therefore, the recovery current of the freewheeldiodeaddstotheloadcurrentseenby thecontrollingMOSFET(Q1). At the end of period a, the reverse recovery currentfalls, along with the gate-source voltage.

7 This is because thediode has recovered. The recovery current in turn will decay tozero, resultingin thegate voltagereducing tothe originalvaluerequired to support the load current (period b). During thisperiod, the freewheel diode starts to support voltage, and theVDSvoltage falls, and the Miller Plateaubegins. As with theideal-recovery diode explanation, this continues until thevoltage falls to its on-state value (end of ) and thegate-source voltage is unclamped and continues to theapplied gate-voltage is effectively the reverse of turn-on, apart from thatthere is no limitation by the freewheeling diode (in thisparticular circuit). For turn-off the Miller Plateau indicates thestart of the rise of the drain-source voltage, and the voltage ofthe Miller Plateau willrepresent the required VGSto sustaintheload current.

8 The turn-off delay is the period from when thegate voltage falls from its on-state value to when it reaches theMiller Plateau value ( load-current value).D1 VOUTCLQ1 VINFIGURE circuit of a buck converterFIGURE waveforms for a typical MOSFET in a buck converterNote: The solid line shows an idealized curve with no recoveryof the anti-parallel diode. The dotted line shows the effect ofreverse recovery of the freewheel diode on the gate waveformand the corresponding switching SiliconixDocument Number: MOSFETs: N- and are two fundamental types of MOSFETs: n-channelandp-channel. An n-channel device needs a positive gate voltagewith respect to the source voltage, whereas a p-channelMOSFET requires the gate voltageto benegative withrespectto the source.

9 Due to these criteria, each device sometimesappears to be geared for specific applications, such asp-channels for load switches and n-channels for low-sideswitches. In reality it is only the drive circuits that need to of an n-channel MOSFETDGSSVVGSFIGURE of a p-channel MOSFETFor the high-side switch portrayed in the buck converter ofFigure 1, it would be possible to use either a p- or n-channeldevice; however, the operating conditions of the buckconverter will determine which is to be , consider the high-side MOSFET as shown in Figure it is turned on, the source voltage will tend towards thedrain voltage (minus the voltage across the device Vs Vd).Therefore, if the gate drive is generated from the input voltage(Vin) as the MOSFET turns on, VGSwill reduce as the sourcepin (Vs) goes to Vin(Vd).

10 Forann-channeldevice,thismeansthatthegat evoltagemustbe higher than the drain voltage to maintain VGSabove theMiller Plateau voltage to ensure that the MOSFET stays fullyon. To achieve this there are three common strategies orcircuits:a) use an isolated supply with the 0 V referenced to thesource voltage to ensure that the applied VGSis thesame as the voltage driving the gate;b) use a charge-pump circuit that generates a voltage high-er than the dc-link voltage to drive the gate; orc) use a bootstrap circuit that again generates a voltagehigher than the dc-link voltage, but which requires aswitching circuit to charge up the bootstrap capacitorafter the top device is turned method is to use a p-channel device in situations inwhich the gate voltage does not need to be higher than thedc-link voltage.