Parameters and Selection Principles of MOSFET

Posted by Rachel Ye on September 3rd, 2019

Parameters and Selection Principle of MOSFET 

I Several common parameters of MOSFET

1. VDS, the drain-source voltage, is a limit parameter of the MOSFET that represents the maximum voltage that can be withstood between the drain and source of the MOSFET. It should be noted that this parameter is related to the junction temperature(Tj). Generally, the higher the junction temperature is, the larger the value is.

2. RDS(on), the drain-to-source on-resistance, which indicates the on-resistance between the drain and source when the MOSFET is turned on under certain conditions. This parameter is related to the junction temperature and the driving voltage VGS of the MOSFET. Within a certain range, the higher the junction temperature is, the larger the RDS is; the higher the driving voltage, the smaller the RDS is.

3. Qg, the gate charge, is the charging charge required for the gate voltage to rise from 0V to the final voltage (eg, 15V) under the action of the drive signal. the gate charge required by the drive circuit from the off state to the fully on state of the MOSFET is a main parameter for evaluating the drive ability of the drive circuit of the MOSFET.

4. Id, drain current, are usually described in several different ways. According to the form of working current, there is a continuous drain current and a pulsed drain current of a certain pulse width. This parameter is also a limit parameter of the MOSFET, but this maximum current value does not mean that the drain current can reach this value during operation. It means that when the case temperature is at a certain value, if the operating current of the MOSFET is the maximum drain current aforesaid, the junction temperature will reach ts maximum value. So this parameter is also related to the device package and ambient temperature.

5. Eoss, the output capacitance, represents the amount of energy stored in the MOSFET by the output capacitor Coss. Since the output capacitor Coss of the MOSFET has a very significant nonlinear characteristic, it varies with the Vds voltage. So this parameter is very helpful to evaluate the switching losses of the MOSFET. This parameter is not available in all MOSFET manuals. In fact, most datasheets do not provide it.

6. Body Diode di/dt, the current change rate of the diode, which reflects the characteristics of reverse recovery of the MOSFET diode. Because the diode is a bipolar device, it is affected by the charge storage. When the diode is reverse biased, the charge stored in the PN junction must be removed.

7. Vgs, the gate-source maximum driving voltage, which is also a limit parameter of the MOSFET, indicating the maximum driving voltage that the MOSFET can withstand. Once the driving voltage exceeds this limit value, it will cause permanent damage to the gate oxide layer even in a very short time. In general, as long as the drive voltage does not exceed the limit value, there will be no problem. However, in some special occasions, the presence of parasitic parameters can have unpredictable effects on the Vgs voltage, which requires special attention.

8. SOA, safety operation area. Each kind of MOSFET will give its safety operation area. Different bipolar transistors, power MOSFETs will not exhibit secondary breakdowns, so the safety operating area is only defined from the dissipation power when the junction temperature reaches the maximum allowable value.

II Principle of Selecting Power MOSFET

After understanding the meaning of the parameters of the MOSFET, how to choose the product that meets your needs according to the manual tables of manufacturers’ products ? There are mainly four steps.

1. Channel selection

The first step in choosing the right device for the design is to decide whether to use an N-channel or a P-channel MOSFET. In typical power applications, when a MOSFET is grounded and the load is connected to the rail voltage, the MOSFET forms the low-voltage-side switch. In switches at the low-voltage side, N-channel MOSFETs should be used for consideration of the voltage required to turn the device off or on. When the MOSFET is connected to the bus and the load is grounded, the switch at the high-voltage side is used. P-channel MOSFETs are often used in this topology, which is also out of consideration of voltage drive.

2. Voltage and Current Selection

The higher the rated voltage is, the higher the cost of the devices will be. According to practical experience, the rated voltage should be greater than the rail voltage or bus voltage, which will provide sufficient protection so that the MOSFET will not malfunction. In selecting a MOSFET, the maximum voltage that can be withstood between the drain and the source must be determined, which is also called the maximum VDS. Another safety factor that design engineers need to consider is voltage transients induced by switching electronics such as motors or transformers. The rated voltage of different applications is also different. Usually, the rated voltage of the portable device is 20V, the FPGA power supply is 20~30V, and the 85~220VAC application is 450~600V. In the continuous conduction mode, the MOSFET is in a steady state, and the current continuously passes through the device. A pulse spike indicates a large amount of surge (or peak current) flows through the device. Once the maximum currents under these conditions are determined, all you need is to simply select the device that can withstand this maximum current.

3. Calculate Conduction Loss

The power loss of a MOSFET device can be calculated with Iload2×RDS(ON). Since the on-resistance varies with temperature, the power consumption will also proportionally change. For portable designs, it is easier (and more common) to use lower voltages. While for industrial designs, higher voltages can be used. It should be noted that the RDS(ON) resistor will rise slightly with current. Various electrical parameter changes for RDS(ON) resistors can be found in the manufacturers' technical data sheets.

4. Cooling Requirements for Calculation System

Designers must consider two different situations, the worst case and the real situation. It is recommended to use the calculation result in the worst case, which provides a greater safety margin and ensures that the system does not malfunction. There are also some measured data on the MOSFET's data sheet that should be noted, such as the thermal resistance between the semiconductor junction of the packaged device and the environment, and the maximum junction temperature.

Switching loss is also an important indicator. The product of voltage and current when the switch is turned on is quite large, which determines the switching performance of the device to some extent. However, if the system requires a higher switching performance, a power MOSFET with a smaller gate charge QG can be selected.