MOSFETs
are a type of transistor whose primary function is to act as a control switch
or an amplification device. MOSFETs can operate in megahertz,
which makes them suitable for a wide range of applications such as motor
controllers, high-frequency inverters, operational amplifiers, and power
electronic converters. However, like any electronic device, MOSFETs
can also fail, and it is important to test them before plugging them into the
circuit or even when they are in the circuit to ensure they work perfectly. The
failure of MOSFETs can be due to many reasons, such as improper handling,
overheating, and excessive current.
This
article will provide a step-by-step guide for testing MOSFETs using a multimeter
and their in-circuit and out-of-circuit
testing procedure. This blog will also cover potential causes of MOSFET failure
along with necessary equipment required for testing and troubleshooting.
What is MOSFET?
MOSFET
stands for ‘Metal Oxide Semiconductor Field Effect Transistor’. As the
name suggests, MOSFET is made up of three layers: metal, oxide, and
semiconductor. MOSFET has three terminals: Gate, Source, and Drain. The voltage
applied on the Gate terminal of MOSFET determines the state of the transistor.
This characteristic is used to operate MOSFET as a Switching device or an
amplification device. The MOSFET can either be an N-channel or a P-channel. To
use MOSFET for your specific application, it is important to test the MOSFET
correctly to avoid any failure and potential issues.
Equipment Needs for Testing the MOSFET
MOSFET testing requires
some basic electronic equipment such as a multimeter, oscilloscope, and power
supply. Each of these equipment serves a critical role in MOSFET testing and troubleshooting.
The MOSFET datasheet is also essential for identifying pin configuration and
understanding the electrical characteristics of the device.
·
Multimeter: A multimeter, as the
name suggests, is a multi-purpose electronic measurement instrument that is
used to measure voltage, resistance, and current. A multimeter is also used for
connectivity tests in electronic circuits. A multimeter is a key instrument in
the testing and troubleshooting of MOSFET.
Oscilloscope: Oscilloscope is another
useful device used in MOSFET testing and troubleshooting. It helps us to
inspect the behavior of MOSFET by studying the high switching signals.
·
Power Supply: Power Supply is an essential
device required to test MOSFET in different operating conditions. The power supply
provides the testing circuit with the necessary voltage and current that
determine the MOSFET region of operation i.e. switch or amplifier. It is
preferred to use an adjustable power supply to meet the specifications of MOSFET
under test.
MOSFET PIN Identification
Before
start testing MOSFET, it is important to first understand and identify MOSFET
PINS. There are three Pins (terminals) of a MOSFET i.e. Gate (G), Source (S),
and Drain (D). The Gate pin controls the current flow between the Drain and
Source by applying voltage, the Drain is the output pin, and the source pin is
used as a common ground. Before
starting the testing procedure, identify these pins for making the correct
testing circuit.
How To Test a MOSFET using a Multimeter
Using
a Multimeter to test a MOSFET is an easy and effective method. Use a datasheet
to identify the MOSFET pin configuration and follow the step-by-step guide to test
a MOSFET using a multimeter.
Testing a
MOSFET using the Short-Circuit
Method:
Step 1: Set
your multimeter knob to Diode testing or beep testing mode.
Step 2: Connect the multimeter positive terminal with MOSFETs Source PIN and connect the
multimeter negative terminal with MOSFETs Drain PIN.
Step 3: Observe
the multimeter reading.
Ø If
it indicates a low resistance (a few
ohms) along with a beep sound. It shows that the Drain and Source terminals are
shorted and MOSFET is faulty.
Ø If
it indicates a high resistance along (Mega ohms) with no beep sound. It shows
that the MOSFET is working properly.
Testing MOSFET using Gate Activation Method:
Step 1: Set
your multimeter knob to measure Resistance.
Step 2: First place the positive
lead of the multimeter on the Gate PIN and the negative lead on the Source
terminal.
Step 3: Now, move the positive lead of the multimeter to the Drain PIN of MOSFET and
observe the multimeter resistance reading. Keep the negative terminal on Source
PIN.
Ø If
the resistance decreases, it indicates that MOSFET is switching properly.
Ø If
the resistance does not decrease and shows a high resistance. It indicates that
the MOSFET may be corrupted.
Testing MOSFET using Gate Threshold Voltage Method:
Step 1: Set up a MOSFET circuit in the breadboard and note its threshold
voltage (Vth).
Step 2: Connect the positive lead of the multimeter with the Gate PIN and
the negative terminal with the Source PIN.
Step 3: Apply a small voltage between the source and gate terminals.
Slowly increase the voltage. Observe the multimeter reading.
Ø If
the MOSFET starts conducting upon reaching Vth, it indicates MOSFET is working
properly.
Ø If
it does not conduct and shows, it indicates that MOSFET is faulty.
MOSFET In-Circuit VS Out Circuit Testing
In-circuit
testing of MOSFET means testing a MOSFET when it is connected to a circuit.
Out-of-circuit
means testing a MOSFET when it is not connected in-circuit or removed from the
PCB board. Both MOSFET testing method has its advantages and disadvantages
depending on your application and requirements.
In-circuit MOSFET Testing
In-circuit
tests of MOSFET are often performed to get rid of the desoldering process that
may damage the MOSFET and when quick testing and results are required. So, in
this method MOSFET tests are performed in actual working conditions to check
and acquire accurate results. This method may not give accurate results as
other circuit components affect the results of MOSFET. In-circuit testing of
MOSFET is performed with a multimeter, oscilloscope, and MOSFET tester.
In-circuit MOSFET Testing Procedure:
Step 1: Turn ‘OFF’ the circuit to prevent component damage and wrong
readings. Connect the circuit to the Power Supply.
Step 2: Pick a multimeter and set its knob in DC voltage testing mode.
Step 3: Switch ON the power supply and measure the voltage across the gate and source terminals
of MOSFET.
Ø If the voltage drops below the
threshold (refer to the datasheet for specific MOSFET’s threshold), it indicates that MOSFET is faulty.
Ø If the voltage drops above the
threshold, it indicates the MOSFET is working properly.
Out-of-circuit
MOSFET Testing
Out-of-circuit is often performed
when the detailed testing of MOSFET is required. This method is used to get
accurate results by applying detailed MOSFET analysis. Out-of-circuit MOSFET
testing is free from external component interference. Out-of-circuit MOSFET
test can also be performed with the help of Transistor testing device.
Out-of-circuit MOSFET Testing Procedure:
Step 1: First remove the MOSFET from the circuit board.
Step 2: Use a datasheet to identify the terminals of MOSFET.
Step 3: Set your multimeter knob to measure resistance. Measure the resistance across
the drain and source PINS of MOSFET.
Ø If multimeter readings indicate a
low resistance, it shows the pins are short and MOSFET is faulty.
Ø If the multimeter readings
indicate a high resistance, it shows the MOSFET is working properly.
Out-of-circuit testing of MOSFET
is a preferred choice when accuracy and precision are required. On the other
hand, if convenience and time savings are required in-circuit MOSFET testing is
the preferred choice.
Conclusion
In
conclusion, MOSFET testing is an essential tool for electrical engineers to
identify and resolve faults in electronic circuits. MOSFET testing requires
some essential equipment like a multimeter, oscilloscope, and power supply. One
can easily diagnose whether the MOSFET is working or not by following the
simple steps discussed in this article. Understanding the basic MOSFET testing
methods is
essential for engineers and technicians to ensure the smooth operation of their
circuit boards.
