Low฀Side฀Switching Small฀NPN฀Switch

Figure฀3-14฀depicts฀a฀simple฀low฀side฀switch.฀When฀RB0฀is฀high,฀

Q1฀is฀forward฀biased฀into฀conduction฀and฀current฀flows฀through฀the฀

load.฀Let’s฀work฀through฀a฀few฀design฀concerns฀with฀this฀simple฀

circuit.฀We฀will฀assume฀the฀load฀is฀a฀100฀ohm฀resistor฀and฀V฀is฀12฀

volts.฀Hence,฀the฀current฀being฀switched฀is฀120฀mA.฀We’ll฀treat฀the฀

current฀through฀this฀circuit฀as฀a฀constant฀and฀ignore฀the฀voltage฀drop฀

across฀the฀switching฀device฀to฀simplify฀our฀calculations.฀Since฀our฀

switch฀circuits฀will฀operate฀with฀a฀voltage฀drop฀of฀well฀under฀0.5V,฀

our฀simplifications฀will฀not฀introduce฀appreciable฀error.฀

Voltage฀rating—When฀RB0฀is฀low,฀Q1฀appears฀as฀an฀open฀cir-cuit฀and฀thus฀has฀the฀full฀load฀supply฀voltage฀V฀across฀it.฀In฀a฀

transistor฀data฀sheet,฀the฀maximum฀voltage฀that฀may฀safely฀be฀applied฀in฀this฀mode฀is฀V_CEO฀or฀maximum฀

collector฀to฀emitter฀voltage,฀base฀open.฀Our฀particular฀device,฀a฀2N4401฀is฀rated฀for฀40฀V฀V_CEO.฀It฀should฀

be฀safe฀to฀use฀it฀up฀to฀about฀25฀volts,฀applying฀a฀reasonable฀safety฀margin฀to฀the฀rated฀value.฀Our฀12฀V฀

switching฀example฀will฀be฀well฀within฀Q1’s฀ratings.

Leakage฀current—When฀the฀2N4401฀is฀cut฀off—that฀is,฀the฀base฀voltage฀is฀less฀than฀about฀0.4฀V,฀some฀leak-age฀current,฀I_CEX,฀will฀still฀flow฀through฀the฀device’s฀collector.฀I_CEX฀is฀rated฀not฀to฀exceed฀0.1฀µA฀in฀the฀

2N4401,฀a฀negligible฀value฀in฀the฀context฀of฀our฀circuits.

Saturation฀voltage—When฀the฀base฀drive฀is฀sufficient฀to฀saturate฀a฀bipolar฀transistor,฀the฀voltage฀drop฀be-tween฀the฀collector฀and฀emitter฀is฀approximately฀a฀constant,฀referred฀to฀in฀data฀sheets฀as฀V_CE(SAT),฀0.4฀V฀

for฀a฀2N4401฀at฀current฀levels฀near฀100฀mA.฀

Collector฀current฀and฀device฀power฀dissipation—The฀2N4401฀has฀a฀maximum฀continuous฀collector฀current฀

rating฀of฀600฀mA,฀and฀a฀maximum฀power฀dissipation฀rating฀of฀625฀mW฀at฀room฀temperature฀(25°C).฀The฀

saturated฀collector฀voltage,฀V_CE(SAT)฀is฀400฀mV฀at฀150฀mA฀and฀750฀mV฀at฀500฀mA.฀The฀thermal฀resistance฀

junction฀to฀ambient฀R_θJA฀is฀200°C/watt฀and฀the฀maximum฀junction฀operating฀temperature฀is฀+150°C.

We’ll฀assume฀adequate฀base฀drive฀to฀saturate฀Q1,฀hence฀we฀expect฀the฀collector฀voltage฀at฀120฀mA฀to฀be฀400฀

mV฀or฀less.฀We’ll฀also฀assume฀Q1฀is฀to฀be฀on฀continuously—continuously฀in฀this฀context฀means฀long฀enough฀

for฀thermal฀equilibrium฀to฀be฀reached,฀a฀matter฀of฀a฀few฀seconds฀for฀a฀2N4401฀size฀device.฀Hence:฀

฀ The฀device฀dissipation฀will฀be฀120฀mA฀×฀400฀mV,฀or฀48฀mW.฀

The฀junction฀temperature฀rise฀over฀ambient฀will฀thus฀be฀200°C/watt฀×฀0.048฀watts,฀or฀9.6°C.฀As-suming฀the฀ambient฀air฀temperature฀is฀120°F฀(49°C),฀the฀maximum฀junction฀temperature฀will฀thus฀

be฀48°฀+฀9.6°฀=฀57.6°C.฀

To฀determine฀case฀temperature,฀we฀use฀the฀thermal฀resistance฀junction฀to฀case฀R_θJC฀specification,฀83.3°C/

watt.฀The฀case฀will฀thus฀be฀at฀83.3°C/watt฀×฀0.048฀watts,฀or฀4°C฀above฀ambient฀temperature.฀Our฀design฀thus฀

is฀well฀within฀the฀safe฀operating฀parameters฀of฀the฀2N4401.

If฀the฀2N4401฀is฀being฀cycled฀off฀and฀on฀at฀a฀rapid฀rate,฀the฀duty฀cycle฀will฀enter฀into฀certain฀of฀these฀ratings.฀

For฀example,฀suppose฀the฀2N4401฀is฀driving฀a฀multiplexed฀LED฀display,฀on฀for฀2฀ms฀and฀off฀for฀8฀ms,฀for฀a฀

duty฀cycle฀of฀0.20.฀The฀average฀power฀dissipation฀of฀Q1฀will฀thus฀be฀20%฀of฀the฀peak฀power฀and฀the฀permis-sible฀peak฀power฀dissipation฀limit฀may฀be฀as฀much฀as฀five฀times฀the฀continuous฀value.฀Of฀course,฀for฀this฀

averaging฀effect฀to฀work,฀the฀on฀time฀must฀be฀short฀compared฀with฀the฀time฀it฀takes฀for฀the฀device฀to฀reach฀

thermal฀equilibrium.฀

Figure฀ 3-14:฀ 2N4401฀ NPN฀ low฀ side฀

switch.

Base฀current฀drive—As฀a฀rough฀approximation,฀we฀may฀regard฀Q1฀as฀a฀current฀operated฀switch—that฀is,฀for฀

every฀milliampere฀of฀current฀we฀wish฀to฀be฀sunk฀by฀Q1’s฀collector,฀we฀must฀inject฀into฀the฀base฀a฀certain฀

current฀level.฀(This฀is฀a฀highly฀simplified฀approximation฀of฀semiconductor฀operation,฀but฀adequate฀for฀

our฀purpose.)฀The฀ratio฀of฀collector฀current฀to฀base฀current฀is฀known฀as฀h_FE฀or฀“DC฀current฀gain.”฀The฀

DC฀current฀gain฀varies฀from฀device฀type฀to฀device฀type,฀is฀not฀well฀controlled฀from฀example฀to฀example฀

of฀the฀same฀device฀type฀and,฀finally,฀varies฀with฀current฀even฀for฀a฀particular฀transistor.฀2N4401฀devices,฀

for฀example,฀have฀an฀h_FE฀that฀varies฀from฀20฀to฀300,฀depending฀on฀the฀collector฀current.

If฀we฀are฀not฀concerned฀with฀switching฀time,฀or฀power฀minimization,฀the฀simplest฀design฀approach฀is฀to฀as-sume฀the฀worst฀case฀h_FE฀and฀design฀accordingly.฀To฀sink฀120฀mA,฀for฀example,฀since฀the฀minimum฀specified฀

h_FE฀at฀100฀mA฀is฀100,฀the฀target฀base฀current฀should฀be฀1.2฀mA.฀However,฀we฀note฀that฀at฀both฀500฀mA฀and฀

10฀mA฀collector฀currents,฀the฀minimum฀h_FE฀drops฀to฀40.฀Hence,฀as฀a฀matter฀of฀perhaps฀excessive฀caution,฀and฀

to฀ensure฀Q1฀is฀driven฀well฀into฀saturation,฀we฀will฀design฀for฀h_FE฀of฀40,฀representing฀3฀mA฀base฀current.

The฀base฀to฀emitter฀junction฀voltage,฀V_BE,฀for฀a฀2N4401฀is฀specified฀at฀750฀mV฀for฀a฀base฀current฀of฀15฀mA฀and฀

collector฀current฀of฀150฀mA,฀so฀we฀will฀use฀this฀value฀in฀calculating฀the฀base฀resistor,฀R2.฀(Since฀the฀base฀to฀

emitter฀junction฀is฀modeled฀as฀a฀forward฀biased฀silicon฀diode,฀700฀mV฀is฀a฀commonly฀used฀rough฀estimate฀for฀

the฀base฀to฀emitter฀voltage฀over฀a฀wide฀range฀of฀base฀currents฀for฀all฀silicon฀bipolar฀junction฀transistors.)฀R2’s฀

value฀(neglecting฀the฀PIC’s฀approximately฀85฀ohm฀series฀resistance฀when฀sourcing฀current)฀is฀thus:

฀ ฀ ^{R V V}

Switching฀Speed—We’ve฀alluded฀to฀Q1’s฀switching฀speed฀

concerns฀several฀times฀in฀our฀design.฀If฀we฀are฀switching฀

an฀LED,฀or฀relay฀or฀stepper฀motor,฀these฀problems฀are฀

unlikely฀to฀concern฀us.฀However,฀there฀are฀times฀where฀it฀

is฀critical฀to฀switch฀a฀load฀as฀fast฀as฀possible.฀Figure฀3-15฀

shows฀what฀happens฀when฀very฀short฀switching฀intervals฀

are฀used฀in฀the฀circuit฀of฀Figure฀3-14.฀RB0฀emits฀a฀fast฀rise฀

and฀fall฀200฀ns฀wide฀pulse฀and฀Q1฀turns฀on฀with฀less฀than฀

20฀ns฀delay.฀However,฀when฀RB0฀goes฀low,฀Q1฀exhibits฀

nearly฀500฀ns฀turn฀off฀delay.฀The฀turn-off฀delay฀results฀

from฀the฀“stored฀charge”฀effect,฀where฀excess฀minority฀

carrier฀charge฀is฀stored฀in฀the฀base฀region฀of฀the฀transistor฀

junction฀structure฀and฀must฀be฀removed฀before฀the฀transis-tor฀turns฀off฀and฀collector฀current฀ceases.฀We฀assume฀that฀

anyone฀desirous฀of฀switching฀speeds฀in฀the฀sub-microsec-ond฀range฀knows฀about฀stored฀charges฀and฀the฀mitigating฀techniques฀to฀deal฀with฀the฀problem.฀

One฀final฀point฀should฀be฀noted฀with฀the฀bipolar฀transistor฀design฀of฀Figure฀3-14—the฀voltage฀V฀being฀

switched฀is฀immaterial.฀Of฀course,฀Q1฀must฀be฀rated฀to฀withstand฀the฀voltage,฀but฀with฀a฀suitable฀transis-tor,฀the฀circuit฀of฀Figure฀3-14฀could฀switch฀500฀volts฀as฀easily฀as฀it฀switches฀5฀volts.฀The฀current฀required฀to฀

saturate฀or฀cut฀off฀Q1฀is฀not฀affected฀by฀the฀voltage฀it฀switches.

Small฀N-Channel฀MOSFET฀Switch

At฀the฀risk฀of฀considerable฀oversimplification,฀Q1฀in฀Figure฀3-14฀may฀be฀thought฀of฀as฀a฀current฀controlled฀

switch;฀current฀injected฀into฀the฀base฀causes฀the฀collector฀to฀be฀pulled฀close฀to฀ground฀potential.฀There฀is฀

a฀similar฀voltage฀controlled฀switch,฀the฀MOSFET,฀whereby฀voltage฀applied฀to฀the฀device’s฀gate฀causes฀the฀

drain฀voltage฀to฀be฀pulled฀close฀to฀the฀source,฀or฀ground฀potential฀in฀a฀low฀side฀switch.฀

Figure฀3-15:฀2N4401฀switching฀time฀Ibase฀=฀6฀

mA,฀Ic฀=฀40฀mA;฀Ch1:฀PIC฀pin฀to฀base฀drive;฀Ch2:฀

2N4401฀collector.

Figure฀3-16:฀2N7000฀NPN฀Low฀Side฀Switch.

Figure฀3-17:฀2N7000฀predicted฀on-resistance฀

variation฀with฀gate฀voltage฀and฀drain฀current.

Figure฀ 3-18:฀ 2N7000฀ driven฀ by฀ PIC฀ turn-on/

turn-off฀speed฀Ch1:฀PIC฀output;฀Ch2:฀2N7000฀

drain.

Figure฀3-16฀is฀the฀MOSFET฀counterpart฀of฀Figure฀3-14.฀A฀2N7000฀MOSFET฀com-pares฀favorably฀with฀the฀2N4401฀in฀the฀maximum฀permissible฀voltage,฀with฀a฀60฀V฀

rating.฀However,฀the฀2N7000฀is฀rated฀at฀200฀mA฀maximum฀continuous฀current฀and฀500฀

mA฀maximum฀pulsed฀current฀with฀a฀total฀device฀maximum฀dissipation฀of฀400฀mW.฀

Let’s฀look฀at฀the฀areas฀of฀difference฀between฀the฀MOSFET฀and฀

NPN฀bipolar฀transistor.

When฀saturated,฀a฀MOSFET฀acts฀like฀a฀low฀value฀resistor฀

between฀the฀drain฀and฀source,฀referred฀to฀R_DS(ON) ,฀with฀the฀cor-responding฀voltage฀between฀the฀drain฀and฀source฀determined฀

by฀the฀product฀of฀the฀drain฀current฀I_D฀and฀R_DS(ON)฀.฀Recall฀that฀

in฀the฀2N4401,฀the฀corresponding฀voltage฀V_CE(SAT) ฀is฀approxi-mately฀a฀constant฀value฀over฀a฀wide฀current฀range.

The฀relationship฀between฀R_DS(ON) ฀and฀the฀gate฀voltage฀is,฀as฀il-lustrated฀in฀Figure฀3-17,฀complex.฀The฀point฀to฀be฀taken฀away฀

from฀Figure฀3-17฀is฀that฀since฀we฀can฀drive฀Q1’s฀gate฀only฀to฀

+5฀volts฀with฀a฀high฀on฀an฀output฀pin,฀we฀exit฀the฀saturation฀

region฀with฀only฀modest฀drain฀current.฀

Let’s฀run฀through฀the฀same฀120฀mA฀sink฀design฀we฀did฀for฀the฀

2N4401.฀We’ve฀already฀determined฀that฀the฀2N7000฀meets฀

our฀open฀circuit฀voltage฀requirements฀and฀that฀120฀mA฀is฀

less฀than฀the฀maximum฀permissible฀continuous฀drain฀current.฀

With฀a฀gate฀drive฀of฀+5V฀and฀120฀mA฀drain฀current,฀Figure฀

3-17฀shows฀RDS(ON)฀will฀be฀about฀3.2฀ohms.฀Since฀VDS฀is฀the฀

IR฀drop฀across฀RDS(ON),฀we฀may฀calculate฀it฀as฀0.120฀A฀×฀3.2฀

ohms,฀or฀0.38฀volts,฀very฀similar฀to฀our฀2N4401฀NPN฀bipolar฀

transistor฀design.฀

The฀power฀dissipated฀in฀Q1฀equals฀I_D฀×฀V_DS,฀or฀0.38฀volts฀×฀

0.120฀mA฀or฀46฀mW,฀almost฀identical฀in฀value฀with฀the฀48฀

mW฀we฀found฀for฀the฀2N4401฀bipolar฀switch฀and฀well฀within฀

the฀device฀ratings.฀The฀2N7000’s฀thermal฀resistance฀junction฀

to฀ambient฀R_θJA฀is฀312.5°C/watt฀and฀the฀maximum฀junction฀

operating฀temperature฀is฀+150°C.฀The฀temperature฀rise฀at฀

the฀case฀will฀thus฀be฀312.5°C/watt฀×฀0.046฀watt,฀or฀14.4°C.฀

Assuming฀the฀ambient฀air฀temperature฀is฀120^ºF฀(49°C),฀the฀

maximum฀junction฀temperature฀will฀thus฀be฀48°C฀+฀14.4°C฀=฀

62.4°C,฀all฀quite฀acceptable฀values.฀

If฀we฀were฀to฀repeat฀this฀series฀of฀calculations฀for,฀say฀a฀

400฀mA฀load,฀we฀will฀find฀R_DS(ON)฀is฀3.6฀ohms,฀V_DS฀is฀1.44฀

V฀and฀Q1’s฀dissipation฀is฀576฀mW,฀well฀over฀the฀maximum฀

permissible฀value฀for฀a฀2N7000.฀The฀problem฀is฀that฀5฀V฀is฀

inadequate฀gate฀voltage฀to฀fully฀turn฀the฀MOSFET฀at฀400฀mA.

When฀looking฀at฀nanosecond฀switching฀with฀a฀2N4401,฀we฀

found฀significant฀turn-off฀problems฀due฀to฀stored฀charge.฀As฀Figure฀3-18฀shows,฀both฀the฀turn฀on฀and฀turn฀

off฀times฀for฀a฀2N7000฀are฀quite฀respectable.฀But,฀a฀close฀examination฀of฀the฀leading฀edge฀of฀the฀PIC฀output฀

foreshadows฀the฀main฀difficulty฀of฀driving฀MOSFETs,฀gate฀charge.฀The฀small฀plateau฀or฀kink฀in฀the฀rise฀time฀

of฀the฀PIC฀output฀reflects฀the฀fact฀that฀the฀gate฀of฀a฀MOSFET฀behaves฀like฀a฀nonlinear฀capacitive฀load฀to฀its฀

driving฀circuitry.฀Accordingly,฀the฀switching฀time฀performance฀is฀defined฀by฀the฀ability฀of฀the฀driving฀PIC฀

to฀change฀the฀gate฀voltage.฀The฀2N7000’s฀gate,฀assuming฀a฀5฀V฀gate฀drive฀and฀120฀mA฀load,฀looks฀approxi-mately฀like฀a฀200฀pF฀capacitor.฀We฀will฀examine฀MOSFET฀gate฀driving฀problems฀when฀we฀look฀at฀a฀higher฀

power฀device,฀the฀IRF510.฀For฀now,฀we฀simply฀note฀that฀a฀PIC฀is฀capable฀of฀directly฀driving฀a฀2N7000฀to฀

quite฀respectable฀switching฀speeds.

High฀Power฀Bipolar฀Low฀Side฀Switching

Both฀the฀2N4401฀and฀2N7000฀are฀low฀power฀devices,฀good฀for฀con-tinuous฀currents฀of฀a฀few฀tenths฀of฀an฀ampere฀at฀most.฀Suppose฀we฀

wish฀to฀switch฀an฀ampere฀or฀two฀with฀a฀bipolar฀transistor.฀

In฀most฀instances,฀we฀will฀not฀be฀able฀to฀build฀a฀high฀power฀switch฀

by฀simply฀replacing฀the฀2N4401฀with฀a฀high฀power฀device,฀as฀the฀25฀

mA฀or฀so฀maximum฀current฀output฀of฀a฀high฀PIC฀pin฀isn’t฀enough฀

base฀drive฀to฀reliably฀saturate฀a฀simple฀bipolar฀power฀transistor.฀

Consider฀Figure฀3-19.฀The฀TIP31’s฀maximum฀current฀rating฀is฀3.0฀

A,฀but฀at฀this฀level฀the฀minimum฀guaranteed฀h_FE฀is฀only฀10,฀so฀300฀

mA฀base฀drive฀would฀be฀required฀for฀saturation.฀Even฀at฀1.5฀amperes฀

collector฀current฀the฀PIC฀is฀short฀of฀achieving฀full฀satura-tion฀with฀the฀maximum฀possible฀PIC฀output฀current฀as฀its฀

base฀drive฀(This,฀of฀course,฀would฀require฀h_FE฀to฀be฀at฀least฀

60.).฀Figure฀3-20฀shows฀the฀V_CE฀is฀920฀mV,฀instead฀of฀the฀

expected฀300฀mV฀shown฀in฀the฀TIP31’s฀data฀sheet฀for฀1.5A฀

collector฀current.

A฀Darlington฀transistor฀solves฀our฀base฀drive฀problem.฀As฀

reflected฀in฀Figure฀3-21,฀a฀Darlington฀transistor฀uses฀one฀

transistor฀as฀a฀emitter฀follower฀current฀booster฀to฀drive฀the฀

base฀of฀the฀second฀transistor.฀The฀Darlington฀configura-tion฀may฀use฀two฀separate฀devices฀or,฀as฀in฀the฀case฀of฀the฀

TIP120,฀the฀driver฀transistor฀may฀be฀integrated฀onto฀the฀

same฀die฀as฀the฀power฀transistor.฀The฀composite฀h_FE฀of฀the฀

Darlington฀pair฀is฀approximately฀the฀product฀of฀the฀h_FE฀ of฀the฀two฀transistors.฀Since฀the฀driver฀transistor฀doesn’t฀

handle฀high฀currents,฀its฀h_FE฀may฀be฀very฀large,฀thus฀ensuring฀a฀

composite฀h_FE฀in฀the฀thousands.฀For฀a฀collector฀current฀of฀1A,฀the฀

TIP120’s฀h_FE฀is฀approximately฀3300,฀permitting฀collector฀saturation฀

with฀a฀base฀current฀of฀as฀little฀as฀300฀µA.฀

So฀far,฀so฀good.฀The฀TIP120฀is฀rated฀at฀60฀V฀V_CE,฀5A฀maximum฀

collector฀current฀I_C฀and฀65฀watts฀dissipation,฀with฀an฀appropriate฀

heat฀sink.฀The฀price฀to฀be฀paid฀for฀the฀extra฀h_FE,฀though,฀is฀found฀

in฀the฀saturation฀voltage,฀V_CE(SAT).฀For฀an฀I_C฀of฀3A,฀V_CE(SAT)฀is฀2.0฀V฀

while฀at฀5A,฀V_CE(SAT)฀is฀4.0฀V.฀A฀quick฀glance฀in฀Figure฀3-21฀reveals฀

why฀V_CE(SAT)฀is฀so฀poor.฀The฀driver฀transistor฀relies฀upon฀V_CE(SAT)฀as฀

its฀source฀of฀current฀to฀supply฀base฀drive฀to฀the฀output฀transistor.฀

Figure฀3-19:฀High฀current฀switching฀with฀

TIP31.

Figure฀ 3-20:฀ TIP31฀ at฀ 1.5A฀ IC฀ Ch1:฀ Base฀ Ch2:฀

collector.

Figure฀ 3-21:฀ Switching฀ with฀ TIP120฀

Darlington.

Recall฀that฀the฀base฀of฀the฀output฀transistor’s฀base฀must฀be฀

at฀about฀0.7V฀above฀its฀emitter฀before฀base฀current฀flows.฀

And,฀the฀driver฀transistor฀itself฀introduces฀another฀0.4V฀or฀

so฀minimum฀voltage฀drop฀even฀if฀it฀is฀saturated.฀Hence,฀if฀

V_CE(SAT)฀drops฀below฀1.1V,฀the฀driver฀transistor฀no฀longer฀

can฀supply฀base฀current฀to฀the฀output฀stage.฀

Another฀area฀we฀expect฀to฀see฀a฀performance฀problem฀with฀

the฀Darlington฀is฀turn฀off฀time฀as฀power฀transistors฀are฀

slower฀than฀the฀small฀switching฀devices฀we฀have฀looked฀at฀

so฀far.฀Figure฀3-22฀confirms฀our฀pessimism,฀as฀the฀TIP120’s฀

turn฀off฀time฀is฀nearly฀two฀microseconds.฀However,฀we฀

should฀remember฀that฀for฀many฀applications,฀2฀µs฀turn฀off฀

time฀is฀more฀than฀adequate.฀

Calculating฀the฀base฀series฀resistor฀Rbase,฀and฀the฀other฀

parameters฀follow฀the฀approach฀used฀for฀the฀2N4401฀and฀won’t฀be฀repeated.฀However,฀attention฀must฀be฀

paid฀to฀proper฀heat฀sink฀selection฀for฀power฀devices.฀Although฀the฀TIP120฀is฀rated฀at฀65฀watts฀dissipation,฀

that฀value฀presumes฀an฀adequate฀heat฀sink.฀Absent฀a฀heatsink,฀its฀rating฀is฀only฀2฀watts.฀Operating฀without฀a฀

heatsink,฀in฀continuous฀operation,฀a฀TIP120฀should฀not฀sink฀more฀than฀1A฀or฀so—and฀even฀that฀is฀pushing฀it.

High฀Power฀MOSFET฀Low฀Side฀Switching IRF510฀Switch

Just฀as฀there฀are฀high฀power฀relatives฀of฀the฀2N4401,฀the฀2N7000฀has฀many฀big฀brothers฀as฀well.฀We’ll฀

quickly฀examine฀one฀of฀the฀original฀power฀MOSFETs฀but฀then฀turn฀our฀attention฀to฀a฀new฀power฀MOSFET฀

device฀that฀solves฀many฀of฀the฀shortfalls฀of฀earlier฀devices.

The฀IRF510฀is฀one฀of฀the฀earliest฀inexpensive฀power฀MOSFETs฀and฀is฀still฀in฀production.฀It’s฀rated฀at฀100V฀

V_DS,฀maximum฀I_D฀of฀5.6A฀and฀a฀power฀dissipation฀of฀43฀watts,฀assuming฀proper฀heat฀sinking.฀It฀is฀supplied฀

in฀a฀TO220฀package.฀R_DS(ON)฀ is฀under฀1฀ohm฀at฀room฀temperature.฀(Modern฀devices฀are฀in฀the฀tens฀of฀mil-liohm฀range,฀but฀we’ll฀stick฀with฀the฀IRF510฀to฀illustrate฀the฀point฀on฀gate฀voltage฀concerns.)

When฀we฀substitute฀the฀IRF510฀for฀the฀2N7000,฀as฀shown฀in฀Figure฀3-23฀again฀we฀are฀limited฀to฀5฀V฀gate฀

drive.฀If฀we฀consult฀the฀R_DS(ON)฀versus฀gate฀voltage฀and฀current฀plot฀of฀Figure฀3-24,฀we฀see฀that฀for฀5฀V฀V_GS฀and฀

Figure฀3-22:฀TIP120฀turn฀off฀time฀Ch1:฀PIC฀output;฀

Ch2:฀TIP120฀collector.

Figure฀3-23:฀Low฀side฀switching฀with฀IRF510. Figure฀ 3-24:฀ IRF510฀ predicted฀ on-resistance฀

variation฀with฀gate฀voltage฀and฀drain฀current.

1A฀I_D,฀we฀expect฀R_DS(ON)฀to฀be฀about฀0.5฀ohm.฀But,฀suppose฀we฀wish฀to฀sink฀3.0A฀of฀current฀representing,฀say฀a฀

12฀volt฀supply฀and฀a฀4฀ohm฀load,฀well฀within฀the฀IRF510’s฀ratings.฀As฀Figure฀3-24฀shows,฀the฀minimum฀V_GS฀to฀

obtain฀saturation฀at฀3A฀exceeds฀5฀V.฀If฀all฀we฀have฀to฀drive฀

the฀gate฀is฀the฀+5฀V฀high฀from฀our฀PIC,฀R_DS(ON)฀will฀be฀many฀

ohms฀indicating฀the฀IRF510฀is฀out฀of฀saturation฀and฀operat-ing฀in฀the฀linear฀region.฀In฀fact,฀under฀these฀conditions฀with฀

V_GS฀=฀5฀V฀we฀will฀find฀V_DS฀approximately฀6.2฀V฀and฀I_D฀1.4A.฀

The฀IRF510฀will฀dissipate฀8.6฀watts฀and฀its฀R_DS฀is฀4.4฀ohms.฀

If฀we฀expected฀the฀IRF510฀to฀act฀as฀a฀saturated฀switch,฀with฀

an฀R_DS(ON)฀of฀a฀few฀tenths฀of฀an฀ohm,฀we฀will฀be฀in฀for฀a฀

surprise.฀And,฀if฀our฀design฀didn’t฀use฀a฀heatsink฀because฀it฀

wasn’t฀necessary฀with฀a฀low฀R_DS(ON),฀we’ll฀have฀an฀even฀greater฀

surprise฀as฀the฀IRF510฀destroys฀itself฀from฀overheating.

Finally,฀rounding฀out฀the฀difficulties฀in฀driving฀an฀IRF510฀

directly฀from฀a฀PIC,฀we฀see฀the฀expected฀turn฀on฀problem.฀

Figure฀3-25฀shows฀nearly฀750฀µs฀turn฀on฀delay.฀

IPS021฀Switch

Many฀suppliers฀have฀addressed฀the฀shortcomings฀seen฀when฀we฀examined฀the฀IRF510.฀We’ll฀look฀at฀one฀

particular฀device,฀International฀Rectifier’s฀IPS021฀intelligent฀power฀MOSFET฀switch฀which฀is฀rated฀at฀50฀V฀

V_DS฀and฀has฀several฀interesting฀features:

•฀ Logic฀level฀input฀with฀built-in฀voltage฀multiplier฀and฀level฀

converter฀to฀ensure฀saturation฀of฀the฀switching฀MOSFET;

•฀ RDS(ON)฀0.125฀ohms฀or฀less฀at฀room฀temperature฀and฀5฀V฀

input;

•฀ Over-temperature฀protected฀with฀automatic฀shutdown;

•฀ Over-current฀protected,฀at฀5.5฀A฀nominal;

•฀ Built-in฀snubbing฀protection฀for฀inductive฀loads.

The฀IPS021฀is฀supplied฀in฀a฀TO-220฀package฀and฀has฀the฀same฀

pin฀connection฀as฀the฀IRF510.฀In฀almost฀all฀cases฀it฀can฀be฀

directly฀substituted฀for฀an฀IRF510฀with฀little฀or฀no฀change฀in฀

circuit฀design฀or฀physical฀layout.฀As฀shown฀in฀Figure฀3-26,฀

is฀connected฀in฀the฀same฀fashion.฀International฀Rectifier฀

recommends฀a฀series฀resistor฀of฀500฀ohms฀to฀5K฀ohms฀

between฀the฀driving฀pin฀and฀the฀input฀pin฀of฀the฀IPS021,฀

although฀it฀may฀not฀be฀necessary฀in฀all฀cases.

What’s฀not฀to฀like฀about฀the฀IPS021?฀The฀main฀issue฀is฀

switching฀speed,฀with฀turn-on฀and฀turn-off฀speeds฀in฀the฀

several฀microsecond฀range.฀For฀many฀purposes,฀where฀a฀

few฀microseconds฀of฀turn-on฀or฀turn-off฀time฀is฀immaterial,฀

the฀IPS021฀is฀a฀good฀choice.฀Figure฀3-27฀shows฀excellent฀

performance฀in฀switching฀1.5A฀in฀the฀circuit฀of฀Figure฀3-26.฀

The฀series฀resistance฀of฀240฀milliohms฀computed฀from฀

Figure฀3-27฀includes฀wiring฀and฀plugboard฀components฀as฀

well฀as฀the฀IPS021’s฀R_DS(ON).

Figure฀3-25:฀PIC฀Output;฀Ch2:฀IRF510฀Drain.

Figure฀3-26:฀Low฀side฀switching฀with฀IPS021฀

intelligent฀power฀switch.

Figure฀ 3-27:฀ IPS021฀ Switching฀ 1.5A฀ Ch1:฀ PIC฀

Output;฀Ch2:฀IPS021฀Drain.

High฀Side฀Switching Small฀PNP฀Switch

Figure฀3-28฀depicts฀a฀simple฀high฀side฀switch.฀When฀RB0฀is฀

low,฀Q1฀is฀forward฀biased฀into฀conduction฀and฀current฀flows฀

through฀the฀load.฀Let’s฀work฀through฀a฀few฀design฀concerns฀

with฀this฀simple฀circuit.฀We฀will฀assume฀the฀load฀is฀a฀47฀ohm฀

resistor฀and฀V฀is฀2฀volts.฀Hence,฀the฀current฀being฀switched฀is฀

approximately฀45฀mA.฀

We฀calculate฀the฀base฀resistor฀and฀power฀dissipation฀exactly฀as฀

for฀a฀low฀side฀switch,฀but฀we฀must฀be฀careful฀to฀select฀the฀correct฀

voltage฀sources.฀The฀2N4403฀h_FE฀at฀–2V฀V_CE฀and฀–150฀mA฀I_C฀is฀

specified฀at฀a฀minimum฀of฀100.฀We฀will฀wish฀the฀bias฀current฀to฀

be฀at฀least฀450฀µA.฀(In฀a฀PNP฀transistor,฀the฀collector฀is฀nega-tive฀with฀respect฀to฀the฀emitter;฀hence฀the฀sign฀of฀the฀voltages฀

and฀currents฀are฀reversed฀from฀the฀NPN฀case.)฀When฀

conducting,฀the฀base฀bias฀source฀(RB0)฀is฀at฀0฀volts฀and฀the฀

emitter฀is฀at฀V฀volts฀(2.0฀V฀in฀our฀design฀example)฀positive.฀

Hence฀in฀our฀example฀the฀voltage฀to฀drive฀the฀base฀current฀is฀

2.0฀V.฀We฀will฀use฀the฀standard฀700฀mV฀assumption฀for฀Q1’s฀

base-emitter฀junction฀voltage฀drop฀and฀we’ll฀ignore฀RB0’s฀

base-emitter฀junction฀voltage฀drop฀and฀we’ll฀ignore฀RB0’s฀

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