MOSFET Gate SourceExpectedreading

Meter–

(black lead)

+ (red lead)

Opencircuit

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52 Elektor Electronics 5/2003

Testing bipolar transistors, whether low orhigh power, is all plain sailing if you have justan ordinary ohmmeter available and know (1)the connections on the device and (2) the sixsteps involved. Such a simple test shouldspot 8 out of 10 defective bipolar transistors,although your editor had to admit defeat withsome high-power UHF RF transistors he triedto examine the other day. For example, anMRF646 50-watts ‘beast’ checked out justfine electrically but its in-circuit power gainwas nothing to write home about! Returningto the subject of our article, MOSFET testingis a different (but not necessarily nastysmelling) kettle of fish.

Whither charge and voltage?It can be argued that MOSFETs are chargecontrolled devices, because the gate, whichis the control electrode, represents a virtu-ally ideal capacitor exhibiting an extremelylow leakage current. However, the samecharge produces a voltage, which in turndetermines the degree of conduction sothese wonderful devices may equally becalled ‘voltage-controlled’.

No matter if voltage or charge-controlled,when a MOSFET is out of its circuit, anycharge stored in it will stay there, keeping thedevice on if it is positive, or keeping it off if itis negative (we are talking of most commonN-channel devices, for P-channel MOSFETs

you just have to reverse any polarityinvolved). For an N-channel device,‘negative’ also means below thethreshold necessary for switchingthe MOSFET on.

In fact when you handle a MOS-FET for testing — pulling it from thecircuit or from its protective packag-ing — your fingers, the soldering ironetc., will typically cause a randomcharge to be stored in the gate-source equivalent capacitor.

The first thing to be done is givethis charge a known value becauseonly then does it become possible tocheck the drain-source path (junc-tion) for correct on/off operation.Let’s see how this is done in practice.

Preparing for testingFirst of all you have to switch yourdigital multimeter to the DiodeCheck range. This way your multi-meter will supply the junction undertest with a voltage that’s usually ofthe order of a couple of volts (open-circuit) and a current limited to afew milliamps. This is just what weneed — don’t try to use an ohmrange since the voltage supplied isthen much lower (approx. 0.2 volts)and certainly not enough to switch

MOSFETs on and off.Now its time to lay your MOSFET

on the table surface. No matter ifthe surface of the table is conduc-tive or not, the most important pointto observe is that the MOSFETsleads do not touch anything. Alsobe sure not to touch the leads orprobe tips with your fingers so asnot to lose any stored charge. In thecase of power MOSFETs, the Draintab can be freely touched and laidon the table surface, but a safermethod is to pick up and hold thepower MOSFET by the tab, thentouch the table surface with yourother hand, and only then lay theMOSFET on the table.

Let’s test againAt his point you are ready for theactual testing, which involves anumber of steps described below.

1. In the first test we switch theMOSFET off and test its gate-source(GS) junction.

MOSFET Testingusing a MultimeterYes it can be done

By Carlo Cianferotti carloc@infol.it

This short article will deal with testing power MOSFETs using an ordinarydigital multimeter. Anybody who have ever tried this must have marvelledat strange readings and short circuits found even on surely workingdevices. Of course, there is no magic involved!

500 mV for the forward diode drop.

3. Now it’s time to switch our MOSFET on.

This way you double check that the gate isnot short-circuited. A wrong reading at thispoint is very rare, but nonetheless you shoulddiscard your device if you don’t see an opencircuit.

4. Now that we have the ‘on’ charge in thegate, all we have to do is check the drain-source junction for proper conduction. Thisshould be done using both polarities becausewhen a MOSFETs is on, it acts as a smallvalue resistor, irrespective of the direction ofthe current flow.

If you don’t get these results, the MOSFEThas a permanently open-circuit Drain-Sourcejunction and should be discarded.

If the suspect MOSFET has passed steps 1through 4 successfully it can be relied uponto work properly. As a matter of fact, voltagesand currents delivered by multimeters areusually much lower than those required foreffective testing of power MOSFETs (IRF,BUZ, etc.) but nonetheless this simple testprocedure has given very good results overyears of field tests.

If you look at the ID = f(VGS) graph in Fig-ure 1, you’ll notice that conduction starts ata gate-source voltage of 3.5 to 4 volts, whileat 5 volts (i.e., TTL High level) some 15 A isallowed to flow through the drain-sourcejunction. The graph is for a Philips BUK466-200A which can be describes as a ‘typical’example of a power MOSFET.

Any reading other than ‘open circuit’means that gate is short-circuited tothe source and that the MOSFETmay be discarded without the needfor any further testing.

2. Now that we have the ‘off’ chargein the gate we can test if the drain-source (DS) junction is opened.Most MOSFETs have an integralreverse protection diode across thedrain and source and this may betested simply by reversing the test

voltage polarity.

Any reading other than open-circuitmeans that MOSFET has a short cir-cuit and should be discarded.Meters usually give readings in mVin this range, so you can expectsomething between 250 mV and

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535/2003 Elektor Electronics

40

=

30

20

10

00 2 4 6

150 25

8 10

030031- 11

BUK 466-200A

ID[A]

VGS[V]

Tj[°C]

MOSFET Gate SourceExpectedreading

Meter+

(red lead)

–(black lead)

Opencircuit

MOSFET Drain SourceExpectedreading

Meter–

(black lead)

+(red lead)

Open circuit /Forward

diodedrop

MOSFET Drain SourceExpectedreading

Meter+

(red lead)

–(black lead)

Open circuit

Figure 1. Typical transfer characteristics of an ordinary power MOSFET, in this case,a BUK446-200 from Philips Semiconductors. Graph shows ID = f(VGS) at VDS =25 V, with two values of Tj as parameters.

28

20

24

12

16

8

4

00 2 4 6 8

030031- 12

BUK 542-60A/B

ID[A]

VGS[V]

=150

25Tj[°C]

Figure 2. As Figure 1, but for a Logic FET, in this case the BUK542-60A/B.

MOSFET Drain SourceExpectedreading

Meter+

(red lead)

–(black lead)

Short circuit

MOSFET Drain SourceExpectedreading

Meter–

(black lead)

+(red lead)

Short circuit

Other devices called ‘logic FETs’ start to con-duct at slightly lower levels of VGS, for exam-ple, at about 2 V already in the case of theBUK542-60 (see Figure 2). This makes thesedevices the perfect choice for direct insertionbetween a logic output such as a microcon-troller port line set up as an output and a (veryheavy) load like a power relay or a motor.Again, this is an example only and the graphshould not be taken to apply to ‘any old logicFET’ — see the notes below on finding exactdatasheets of the device(s) you’re working on.

P-channel devices and the ohmmeterIf you wish to test a P-channel device youshould obviously swap the polarity of the testprobes in the above tables.

Using an analogue moving coil multimeter

is also possible but these instru-ments usually do not sport a DiodeCheck range. This requires someinvestigation about the open circuitvoltage and short circuit current sup-plied by the meter. You should aim atone ohm range to get 2-3 V and 5-20mA respectively which is ideal forsafe testing. You can find out by con-necting a second multimeter to theone used for testing MOSFETs. Firstselect a voltage range and the checkopen circuit voltage on the probes,then switch to current range andnote the short circuit current. Usuallythe OHM×1 or OHM×10 range will dothe job. Finally, as we are sure youwill note at some point whenattempting to use an old fashionedohmmeter, the – (black) lead is usu-

ally positive (+) and vice versa!

Whence the pinout?At the risk of stating the obvious, youshould always know exactly wherethe gate, source and drain pins areon the device you wish to test usingthe method described in this article.Educated guesses, ‘a friend told me’ and ‘seem toremember’ are worthless in thisrespect and may lead to costly mis-takes and hours of fruitless efforts atrepairing equipment. The informationyou want should be obtained frommanufacturers’ data books or fromoriginal datasheets downloaded fromthe manufacturers’ website.

(030031-1)

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