Power MOSFET: Working & Its Applications
Table of Contents
Power MOSFETs employ similar semiconductor processing techniques to current VLSI circuits, but at significantly different levels of device voltage and current. The original FET (field-effect transistor), introduced in the 1970s, is primarily where the MOSFET gets its inspiration. Because of the BJTs’ limitations, the power MOSFET was developed in part, and it was currently the preferred device for power electronics applications. Despite the fact that any component that can switch at a minimum of 1A qualifies as a power device, it is not possible to categorize the operating limits of a power device completely.
One type of electronic device that is controlled by current supply is the bi-polar power transistor. The base drive current must be high compared to the 1/4 of the collector current in order to keep the device in an ON state. In order to achieve a quick turn-off, high reverse base drive currents are also required. A power MOSFET’s general operation is covered in this article.
What is a Power MOSFET?
Power MOSFET is one type of MOSFET that can handle large amounts of power. These MOSFETS perform significantly better than standard MOSFETs in the lower voltage range thanks to their rapid switching. It operates on the same principles as standard MOSFETs. The three most common power MOSFET operating modes are n-channel depletion mode, p-channel depletion mode, and p-channel enhancement mode. The power MOSFET frequency can reach 100 kiloHertz, which is high. Below is a picture of the power MOSFET symbol.
These three-terminal silicon devices function by applying a signal to the gate terminal, which then regulates the flow of current between the source and drain terminals. The breakdown voltage ratings range from 10 to 1000 volts and the current conduction capacities are thousands of amps. Power MOSFETs can also be found in a variety of structures, including trench-MOS (UMOS), DMOS (Double-Diffused MOS), VDMOS (Vertical Diffused MOS), etc.
The power MOSFET used in integrated circuits is a lateral device with source, drain, and gate terminals on the device’s pinnacle, where the current flowing within a lane is parallel to the outside current. The substrate of a device is used as a drain terminal by the VDMOS (Vertical Double Diffused MOSFET). Discrete MOSFETs will have a lower on-resistance in ICs than MOSFETs. Where the gap is at a premium, power MOSFETs are available in SOIC (Small Outline IC) packages. Also available are larger through-hole TO-247, TO-220, and the surface mountable D2PAK otherwise known as SMD-220.
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
These MOSFETs function similarly to regular MOSFETs in that they switch and regulate the current flowing between two terminals, such as the source and drain, by varying the voltage on the gate terminal. After applying voltage to the gate terminal, a channel can form between the source and the gate terminals, allowing current to flow. The channel will function better and the ID (drain current) will rise if the VGS voltage (gate-source) is increased. Here, the equation below will determine the main relationship between the two voltages, such as the gate and drain.
ID = K (VGS – VT)2
‘ID’ is a drain current
‘K’ is a device constant
‘VGS’ is a gate voltage
‘VT’ is a threshold voltage
Specifications & Standards
We must take into account two important specifications when choosing products based on power MOSFETs: VGS(Off), which stands for gate-source cutoff voltage, and IDSS, which stands for drain saturation current. When the voltage of the drain source equals the voltage of the gate source, which is denoted by the letters VGS, the drain saturation current, denoted by the symbol IDSS, is said to occur.
When a MOSFET’s drain current reaches its maximum level, it waits to see if the drain-source voltage rises. A depletion layer at the drain end of the gate terminals can accommodate this extra voltage. Therefore, this state is known as drain current saturation (IDSS), which refers to the highest current value. The drain current value that results in a gate-source voltage value that is close to zero is known as VGS(Off), or gate-source cutoff voltage. A variety of associations and societies test the standards that are used to manufacture power MOSFETs. JEDEC JEP 115, BS IEC 60747-8-4, and JEDEC JESD 24 are the principal examples.
Power MOSFET Testing
The following techniques can be used with a multimeter to test power MOSFETs. Let us check the presence of N- and 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
- Short the transistor’s drain & gate terminals using a screwdriver or meter probe. The device’s internal capacitance will primarily continue to discharge completely.
- Place the red color probe of the meter toward the transistor’s drain and the black probe toward the source.
- So an open circuit sign can be observed on the digital multimeter.
- The red color probe should then be removed from the drain terminal and temporarily connected to the MOSFET’s gate terminal before being placed back on the drain terminal. At this point, keep the black color probe pointed toward the source.
- 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
- You must reset the transistor by shorting the drain and gate terminals using a meter probe in order to carry out the aforementioned steps repeatedly.
Testing P-Channel MOSFETs
- For P channel MOSFET, the above five steps are the same except the meter polarities will vary.
- After that, remove the black color probe from the drain, briefly touch the transistor’s gate terminal, and then reattach it to the MOSFET’s drain while avoiding touching the RED color probe from the source.
- So this time, the multimeter will display continuity otherwise a less value on the multimeter.
- In order to draw the conclusion that MOSFETs are trouble-free and in good condition. Any other reading will indicate a defective MOSFET.
Power MOSFET Construction
Power MOSFETs are typically enhancement types. The voltage rating for an enhancement MOSFET is increased by using a drift layer. The power MOSFET has a vertically stacked structure with four layers. The main purpose of this kind of structure is to reduce the current flow region. Therefore, the on-state resistance & on-state loss will be reduced by this structure.
In the MOSFET structure, the middle layer like p-type is called as body whereas n- layer is called as the drift region. Compared to the source and drain layers, this layer is only lightly doped. The breakdown voltage for this MOSFET will be determined by this drift region. Both the first and last layers in the construction of a power MOSFET are n+ layers. The source layer is the top layer in this instance, and the drain layer is the bottom layer. The n channel MOSFET in enhancement mode makes up the n+ p n- n+ structure. However, a p-channel MOSFET’s structure includes a completely different doping shape. Because there is an oxide layer between the metal and semiconductor in this design, acting as a dielectric layer, the gate terminal is not connected directly to the p-type.
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
Below is a diagram of the power MOSFET circuit. The source and drain of this circuit serve as the main terminals. The zero voltage from the gate terminal to the source controls the direction of the current flow, which will be from the drain terminal to the source. A positive voltage is present at the drain in comparison to the source. As a result, a current of up to perhaps a few hundred volts will be blocked.
A negative charge can be induced over the silicon surface underneath the gate terminal if a positive voltage, such as 3V, is applied to the gate terminal. As a result, the P layer will induce a N layer and become a channel for charge carriers like electrons. In order to allow current to flow from the drain terminal to the source, a positive gate voltage creates a surface channel. Here, the induced channel depth will be determined by the voltage at the gate terminal, and the current flow can be calculated in this way.
Power MOSFET Characteristics
The following diagram illustrates a power MOSFET’s VI characteristics. Here, the characteristic curve between the voltage at the drain and the current at the source, denoted by VDS & Id, is drawn. The cut-off, ohmic region, and saturation are the three regions of this curve. The MOSFET will operate within the ohmic and cut off regions when switched ON/OFF appropriately when used as a switch in any application. To reduce the dissipation of power within the active state, the process can be avoided in the saturation region.
Power MOSFETs enter the cut-off region once the voltage of the gate-source is low compared to the threshold voltage. The breakdown voltage from the drain to the source must be greater than the applied voltage in order to prevent a breakdown. Avalanche breakdown will therefore happen. When the power MOSFET enters the ohmic state, power dissipation is minimal. In the saturation state, the voltage of the drain to source relationship has little effect on the drain current.
It simply depends on the voltage between the source and gate terminals. The gate terminal voltage is higher than the threshold voltage. When the voltage from the gate to the source rises, the drain current will as well.
The advantages of power MOSFET include the following.
- The next breakdown does not occur.
- Very simple to switch ON & OFF
- Very simple Gate driving circuit
- Thermal stability is good due to the positive temperature coefficient of power MOSFET
- It uses a high switching frequency to operate
- Less on-state resistance
- Small size
- Less expensive
- It is a voltage-controlled device
- Switching speed is fast
- Needs small power to hold it within the activated condition.
- For commutation, an extra circuit is not necessary
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.
- They require special care while using or else they can be damaged because of the fixed electricity.
- 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.
The applications of power MOSFET include the following.
- UPS (Uninterrupted Power Supplies)
- Relay driver
- SMPS (Switch Mode Power Supplies)
- 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?