What is a Power MOSFET : Working & Its Applications

Power MOSFETs use the methods of semiconductor processing that are similar to present VLSI circuits except for the levels of device voltage & current are drastically different. The MOSFET mainly depends on the original FET (field-effect transistor) launched in the 70s. The power MOSFET invention was done partially through the BJTs limitations and at present, this device was the choice within the applications of power electronics. Even though it is not feasible to classify the operating limits of a power device completely because power devices can be referred to like any component that can switch as a minimum of 1A.

The bipolar power transistor is one kind of device that controls through current supply. To maintain the device in an ON state, the base drive current must be high as compared to the ¼th of the collector current. In addition, high reverse base drive currents are necessary to achieve quick turn-off. This article discusses an overview of a power MOSFET & its working.

What is a Power MOSFET?

One kind of MOSFET which handles high levels of power is known as Power MOSFET. As compared to normal MOSFETs in the less voltage range, these MOSFETS works much better by exhibiting high speed of switching. Its operating principle is the same as general MOSFETs.

The most widely used power MOSFETs are p-channel Enhancement-mode, n-channel Enhancement-mode or n-channel depletion mode & p-channel depletion mode. The power MOSFET frequency is high like up to 100 kiloHertz. The power MOSFET symbol is shown below.

These are three-terminal silicon devices that work through applying a signal toward the gate terminal so that it controls current conduction among source & drain terminals. The current conduction capacities are equal to thousands of amperes including breakdown voltage ratings from 10Volts-1000Volts.

Further, power MOSFETs are available in different structures such as VDMOS (Vertical Diffused MOS or DMOS (Double-Diffused MOS), Trench-MOS (UMOS), or VMOS, etc.

In integrated circuits, the power MOSFET used is a lateral device including source, drain & gate terminals on the pinnacle of the device where the current flowing within a lane is parallel as compared to the exterior. The VDMOS (Vertical Double diffused MOSFET) utilizes the substrate of a device like a drain terminal.

In ICs, MOSFETs will exhibit a high on-resistance than discrete MOSFETs. Power MOSFETs are obtainable in SOIC (Small Outline IC) packages which are used where the gap is at a premium. And also, larger through-hole TO-247, TO-220 & the surface mountable D2PAK otherwise SMD-220 are also accessible.

Newer packages include chip-scale devices & also the PolarPak™ & DirectFET™ packages. The fabrication process used in power MOSFET is similar to the process used in VLSI circuits but the levels of voltage, current are different

Operating Principle

Similar to normal MOSFETs, these types of MOSFETs will switch & control the flow of current in between the two terminals like source & drain through changing the voltage on the gate terminal. Once the voltage is applied to the gate terminal, then a channel can be formed in between the source & the gate terminals which allows the flow of current.

By enhancing the V_GS voltage (gate-source), the channel will become superior & the I_D (drain current) will increase. Here, the main relationship among the two voltages like gate & drain will depend on the below equation.

I_D =  K (V_GS – V_T)²

Where,

‘I_D’ is a drain current

‘K’ is a device constant

‘V_GS’ is a gate voltage

‘V_T’ is a threshold voltage

Specifications & Standards

While selecting products based on power MOSFET, we have to consider two significant specifications like and V_GS(Off) or gate-source cutoff voltage & I_DSS or drain saturation current. Drain saturation current can be defined as it is a drain current saturation measurement denoted with IDSS, which takes place once the voltage of the drain-source equals the voltage of the gate-source and is denoted with V_GS.

Once the drain current of MOSFET attains the highest value then it waits if any drain-source voltage increases; this additional voltage can be accommodated through a depletion layer situated at the gate terminals drain end. So this condition is called drain current saturation (I_DSS) like the highest value of current.

Gate-source cutoff voltage or V_GS(Off) is the gate-source voltage value that results within a drain current value that is near zero. The standards used to manufacture the power MOSFETs are tested through a various range of associations & societies. The main examples are JEDEC JEP 115, BS IEC 60747-8-4 & JEDEC JESD 24.

Power MOSFET Testing

The testing of power MOSFET can be done by using a multimeter through the following methods. Let’s test for N-channel & P-channel power MOSFETs.

Testing N-Channel MOSFET

• Fix the digital multimeter to the range of diode
• Place the power MOSFET on a wooden table
• Using a meter probe or a screwdriver, short the drain & gate terminals of the transistor. This will primarily remain the inside capacitance of the device will discharge completely.
• Place the black probe of the meter toward the source whereas the red color probe is toward drain of the transistor.
• So an open circuit sign can be observed on the digital multimeter.
• After that, keep the black color probe toward the source and remove the red color probe from drain & connect it toward the gate terminal of the MOSFET for a moment, again place it back on the drain terminal.
• So at this moment, the digital multimeter will display a short circuit.
• From the above two results, we can conclude that the MOSFET is okay.
• For proper confirmation, repeat these steps several times
• To repeat the above steps every time, you have to reset the transistor by shorting the drain & gate terminals through a meter probe.

Testing P-Channel MOSFETs

• For P channel MOSFET, the above five steps are the same except the meter polarities will vary.
• Next, without touching the RED color probe from the source, detach the black color probe from the drain & contact the gate terminal of the transistor for a moment & place it back toward the drain of the MOSFET.
• So this time, the multimeter will display continuity otherwise a less value on the multimeter.
• So that we can conclude that MOSFET is in okay condition without any troubles. Any other type of reading will specify a faulty MOSFET.

Power MOSFET Construction

Generally, the power MOSFETs are enhancement types. A drift layer is used to enhance the voltage rating for enhancement MOSFET. The structure of the power MOSFET is the vertical shape and it includes four layers. This type of structure is mainly used to decrease the region of the flow of current. So this structure will decrease the on-state resistance & on-state loss.

In the MOSFET structure, the middle layer like p-type is called as body whereas n- layer is called as the drift region. This layer is doped lightly as evaluated to the other layers like source & drain. This drift region will decide the breakdown voltage for this MOSFET. In the power MOSFET construction, both the first & last layers are n+ layers. Here the source layer is the primary layer whereas the drain layer is the last layer.

The structure of n+ p n- n+ is the n channel MOSFET in enhancement mode. But the structure of a p-channel MOSFET includes quite opposite doping shape. In this construction, the gate terminal is not connected directly to p-type as there is an oxide layer in between the metal & semiconductor which works as a dielectric layer.

It forms a metal oxide semiconductor capacitance on the MOSFET’s input which is high like above 1000 pF. The oxide layer provides excellent insulating properties by offering the silicon dioxide layer to separate the terminal from the body to the gate.

Power MOSFET Circuit

The power MOSFET circuit is shown below. In this circuit, the main terminals of this circuit are the source and drain. The flow of current direction will be from drain terminal to source and it is controlled through the zero voltage from gate terminal to the source. At the drain, a positive voltage is relative to the source. So it will consequence in a current of up to possibly a few 100 volts will be blocked.

If a positive voltage like 3V is applied to the gate terminal then a negative charge can be induced over the surface of silicon under the gate terminal. So P-layer will become an induced N layer and allows the charge carriers like electrons to flow through it. Thus, a positive gate voltage sets up a surface channel for the flow of current from the drain terminal to the source. Here, the voltage at the gate terminal will decide the induced channel depth and in this way, the flow of current can be determined.

Power MOSFET Characteristics

The VI characteristics of a power MOSFET are shown below. Here the characteristic curve is drawn between the drain to source voltage and drain current which is denoted with VDS & Id. This curve includes three regions namely cut-off, ohmic region & saturation.

When the MOSFET is used as a switch in any application then the device will work within the regions of ohmic & cut off once switched ON/OFF correspondingly. In the saturation region, the process can be avoided to decrease the dissipation of power within the active state.

Once the voltage of the gate-source is low as compared to the threshold voltage, then power MOSFET will be in the cut-off region. To keep away from a breakdown, the breakdown voltage from the drain to the source must be larger as compared to the voltage applied. So avalanche breakdown will occur.

The power MOSFET moves into the ohmic state then the power dissipation is low in this region. In the saturation state, the drain current is approximately independent of the voltage of drain to source.
It is simply dependent on the voltage of the gate to source terminals. The voltage of the gate terminal is greater as compared to the threshold voltage. The drain current will increase when the voltage from gate to source increases.

Advantages

The advantages of power MOSFET include the following.

• The next breakdown does not occur.
• Very simple Gate driving circuit
• Very simple to switch ON & OFF
• It uses a high switching frequency to operate
• Thermal stability is good due to the positive temperature coefficient of power MOSFET
• Less on-state resistance
• Less expensive
• Small size
• It is a voltage-controlled device
• Needs small power to hold it within the activated condition.
• Switching speed is fast
• For commutation, an extra circuit is not necessary

Disadvantages

The disadvantages of power MOSFET include the following.

• The on-state voltage is extremely high beyond the MOSFET. Thus, the dissipation of on-state power is high.
• The blocking capacity of this MOSFET is not symmetric so they can block high forward voltage instead of high reverse voltage. So, we need to fix a diode for guarding the MOSFET.
• They require special care while using or else they can be damaged because of the fixed electricity.

Applications

The applications of power MOSFET include the following.

• UPS (Uninterrupted Power Supplies)
• Relay driver
• SMPS (Switch Mode Power Supplies)
• Industries
• High-frequency based inverters
• Used within power amplifiers
• In motor controlling
• Display driver

Thus, this is all about an overview of power MOSFET, construction, working, characteristics & its applications. From the above information finally, we can conclude that the main difference between power mosfet and mosfet is, a MOSFET handles with less power so used for experimental purposes whereas the power MOSFET handles with huge power. So it is very dangerous to handle. These are used mostly in power electronic devices. Here is a question for you, what are the different types of MOSFETs available in the market?