MOSFET Test Procedures

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pThe term MOSFET is an acronym for Metal Oxide Semiconductor Field Effect Transistor. It is a three terminal device with terminals (or leads) named Gate, Drain and Source. There are both P-channel (PMOS) and N-channel (NMOS) variants. P-channel MOSFET’s require a negative going voltage on the Gate lead to effect a change in Drain current while N-channel MOSFET’s require a positive voltage./ppAnother important consideration is whether they are enhancement or depletion mode devices. Drain current flows when a Voltage is applied to the Gate of an Enhancement mode device. This MOSFET is sometimes referred to as being a normally off type. In a digital application (with only two states) it can also be thought of as being akin to a switch closure with N/O (normally open) contacts. With a Depletion mode device, Drain current reduces as voltage is applied to the Gate. This MOSFET is sometimes referred to as being a normally on type. In a digital application (with only two states), this can be thought of as being akin to a switch closure with N/C (normally closed) contacts./ppIn Inverter and SMPS (Switch Mode Power Supply) based designs the N-Channel Enhancement mode MOSFET is by far the most prevalent. In this application, these are usually Power devices (capable of high output current). They inherently have a significant amount of capacitance (greater than 10nF) between Gate and Source terminals. Due to this, Power MOSFET’s can be considered one of the few three terminal components that can be gated utilizing a DVM (Digital Volt Meter)./ppTo perform this Functional Test (written for an N-channel enhancement mode device), the following steps need to be performed:/pp1. Ensure the MOSFET is out of circuit, then place the DVM into the Diode test position. Attach the positive lead (Red) to the Drain terminal, and the negative lead (Black) to the Source terminal. Confirm an Open circuit condition./pp2. Leaving the negative lead (Black) attached to the Source Terminal, remove the positive lead (Red) and briefly attach it to the Gate terminal. This applies a voltage to the gate of the MOSFET through the DVM’s internal battery./pp3. Repeat Step 1. This time, confirm device conduction./pp4. Attach the negative lead (Black) to the Gate Terminal, and the positive lead (Red) to the Source terminal. This turns the device off by reversing the Gating voltage (making it more negative). Repeat Step 1, and confirm an Open circuit condition./pp5. If the MOSFET has a built in Protection Diode (this is connected across the Drain and Source Terminals), attach the negative lead (Black) to the Drain terminal, and the positive lead (Red) to the Source terminal. Confirm a forward voltage drop of between 0.2 to 0.8V DC./ppThe effects of ESD (Electro-Static Discharge) should also be an important consideration during the Functional Test process. A MOSFET has an ESD damage threshold of as little as 10V DC. The component should not be touched unless suitable ESD precautions have been taken. When using a DVM to test a MOSFET, the task of providing ESD protection can be quite a feat as the DVM has no protection circuitry to help achieve this. The use of a good quality IGBT amp; MOSFET Tester is a more practical solution to the Test dilemma of confirming component Functionality while safeguarding the component from ESD events./p brbr
pDavid Willcocks is an Electronics professional, and Technical writer. For more information on Automated Test Equipment please visit a target=_new href=http://www.imagineATE.com rel=nofollow,externalhttp://www.imagineATE.com/a/p