Four-Step Method for Selecting the Correct MOSFET

In electronic circuit design, the MOSFET (Metal-Oxide-Semiconductor Field-Effect Transistor), as a crucial semiconductor device, its correct selection is of vital importance for the performance and reliability of the circuit. The following will elaborate on the method of selecting the correct MOSFET through four steps.

The primary step in selecting a suitable MOSFET for a design is to determine whether to use an N-channel or a P-channel device. In typical power applications, when the MOSFET is grounded and the load is connected to the main line voltage, the MOSFET then constitutes a low-side switch. For low-side switches, the choice of an N-channel MOSFET is based on the consideration of the voltage required to turn off or on the device. The conduction principle of an N-channel MOSFET is that when the gate voltage is higher than the source voltage by a certain threshold, the device conducts, and the driving voltage required for its conduction is relatively easy to achieve. For example, in the low-side switch circuit of a common step-down DC-DC converter, an N-channel MOSFET can efficiently control the on-off of the current.

When the MOSFET is connected to the bus and the load is grounded, a high-side switch configuration is required. In this topology, a P-channel MOSFET is usually selected, also due to voltage driving factors. The conduction condition of a P-channel MOSFET is that the gate voltage is lower than the source voltage by a certain threshold, and its characteristics make it better adapt to the requirements of high-side switch applications. For instance, in the power management circuit of some battery-powered devices, a P-channel MOSFET is commonly used to realize the on-off control between the battery and the load.

The rated voltage of a MOSFET is closely related to its cost. The higher the rated voltage, the higher the cost of the device. According to practical design experience, to ensure the safe and reliable operation of the device, the rated voltage of the MOSFET should be greater than the main line voltage or bus voltage. This can provide sufficient protection for the device and prevent the MOSFET from failing due to excessive voltage. When determining the MOSFET, it is necessary to accurately identify the maximum voltage that may be applied between the drain and the source, namely the maximum VDS. In addition, design engineers also need to consider other safety factors, such as voltage transients induced by switching electronic devices like motors or transformers, which may damage the MOSFET. The requirements for the rated voltage of MOSFETs vary in different application scenarios. Generally, for portable devices, the operating voltage is relatively low, and the rated voltage of the MOSFET is approximately 20V; for FPGA power supplies, the rated voltage of the MOSFET is in the range of 20 - 30V; for 85 - 220VAC applications, the rated voltage of the MOSFET is usually 450 - 600V.

In the continuous conduction mode, the MOSFET is in a steady state, and the current continuously flows through the device; pulse spikes refer to the situation where a large amount of surge (or spike current) flows through the device. Once the maximum currents in these two situations are determined, simply select a MOSFET device that can withstand this maximum current. For example, in a circuit driving a small DC motor, it is necessary to reasonably select the current parameters of the MOSFET according to the pulse spike current at the moment of motor startup and the continuous current during normal operation to ensure the stable operation of the device under various working conditions.

The power loss of a MOSFET device can be calculated by the formula Iload²×RDS(ON). It should be noted that since the on-resistance RDS(ON) changes with temperature, the power loss will also change proportionally. In portable designs, it is more common and easier to use a lower voltage, while in industrial designs, a higher voltage is often adopted. Meanwhile, the RDS(ON) resistance also rises slightly with the change of current. The changes in the RDS(ON) resistance under various electrical parameters can be found in the technical data sheets provided by the manufacturer. For example, when designing the discharge circuit of a portable power bank, by calculating the load current and the RDS(ON) of the MOSFET, the conduction loss of the device can be accurately evaluated, thereby optimizing the circuit design, improving the power conversion efficiency, and extending the device's battery life.

When considering the heat dissipation problem of the MOSFET, designers must pay attention to two different situations, namely the worst-case scenario and the actual situation. It is recommended to use the calculation results for the worst-case scenario, as this result provides a larger safety margin and effectively ensures that the system will not fail due to poor heat dissipation. In the data sheet of the MOSFET, there are some important measurement data that need to be focused on, such as the thermal resistance between the semiconductor junction of the packaged device and the environment, as well as the maximum junction temperature. The thermal resistance reflects the difficulty of heat transfer from the semiconductor junction to the environment, and the maximum junction temperature is the highest temperature limit at which the MOSFET can operate normally. By reasonably calculating the thermal resistance and the maximum junction temperature, and combining with the heat dissipation conditions of the system, such as the size, material, and installation method of the heat sink, a circuit that meets the heat dissipation requirements can be designed to ensure that the MOSFET operates within a safe temperature range.

In addition, the switching loss is also an important indicator for measuring the performance of the MOSFET. At the moment of MOSFET conduction, the product of voltage and current is quite large, which to a certain extent determines the switching performance of the device. If the system has high requirements for switching performance, a power MOSFET with a relatively small gate charge QG can be selected. A smaller gate charge means that less energy needs to be provided by the driving circuit during the switching process, enabling a faster switching speed, reducing the switching loss, and improving the overall efficiency of the circuit.

The above has explained the MOSFET selection method in detail from four key steps. If you have any questions about a certain step or want to know specific application cases, feel free to let me know.