In power electronic systems such as switching power supplies, motor drives, and power conversion, the selection of MOSFET (Metal Oxide Semiconductor Field Effect Transistor) directly affects the efficiency, thermal management, and reliability of the circuit. Incorrect selection may lead to overheating, low efficiency, and even system failures. Shysemi from the perspective of practical engineering, analyzes several key principles for MOS tube selection, helping you optimize the design and enhance product performance.
1.Voltage Stress
In the application of power supply circuits, the selection of the source-drain voltage VDS is usually the first consideration. The basic principle here is that the maximum peak voltage between the source and drain in the actual working environment of the MOSFET should not exceed 90% of the nominal source-drain breakdown voltage specified in the device's specifications. That is:
VDS_peak ≤ 90% * V(BR)DSS
Note:
Generally, V(BR)DSS has a positive temperature coefficient. Therefore, the V(BR)DSS value at the lowest operating temperature of the equipment should be taken as the reference.
2. consider the selection of the drain current
The basic principle is that the maximum cycle drain current in the actual working environment of the MOSFET should not exceed 90% of the nominal maximum drain-source current specified in the specification; the peak value of the drain pulse current should not exceed 90% of the nominal peak value of the drain pulse current specified in the specification, that is:
ID_max ≤ 90% * ID
ID_pulse ≤ 90% * IDP
Note:
Generally, ID_max and ID_pulse have negative temperature coefficients, so the values of ID_max and ID_pulse at the maximum junction temperature of the device should be taken as references. The selection of this parameter for the device is highly uncertain - mainly due to the mutual constraints and influences of the working environment, heat dissipation technology, other parameters of the device (such as on-resistance, thermal resistance, etc.). The final determination basis is the junction temperature (i.e., the "dissipated power constraint" as stated in the sixth item below). Based on experience, in practical applications, the ID specified in the specification sheet will be several times larger than the actual maximum operating current, because of the limitations and constraints of dissipated power and temperature rise. During the initial calculation period, this parameter must also be continuously adjusted according to the dissipated power constraint in the sixth item. It is recommended that the initial selection be around 3 to 5 times ID = (3 to 5) * ID_max.
3.Drive Requirements
The drive requirements of MOSFEF are determined by the total charging quantity (Qg) parameter of the gate. Under the condition of meeting other parameter requirements, choose the smaller Qg to facilitate the design of the drive circuit. The drive voltage should be selected to be as small as possible while ensuring a distance from the maximum gate-source voltage (VGSS) (generally using the recommended value in the device specification sheet).
4.Loss and Heat Dissipation
A small Ron value is conducive to reducing the loss during conduction, and a small Rth value can reduce the temperature difference (under the same dissipated power condition), which is beneficial for heat dissipation.
Initial Calculation of Loss Power
The detailed calculation formula for MOSFET losses should be determined based on the specific circuit and operating conditions. For example, in the application of synchronous rectification, the losses during the forward conduction of the internal diode and the reverse recovery losses when the diode turns off also need to be considered.
Switching losses are a key factor affecting the efficiency of the power supply, and they can be mainly divided into two categories:
Turn-on Loss
When the power transistor switches from the off state to the on state, there is a time difference between the voltage drop and the current rise, and the energy loss occurs in the overlapping area of the current and voltage waveforms.
Turn-off Loss
During the process of the power transistor switching from on to off, the energy loss is caused by the current trailing effect and the in-phase difference between the voltage and current rise.
Comparison of Hard Switching vs Soft Switching Technologies
- Hard Switching: In the traditional working mode, the switching transistor bears the complete on-state/off-state losses, including the additional losses caused by the charging and discharging of parasitic capacitance.
- Soft-Switching: Through resonant technology, zero-voltage switching (ZVS) or zero-current switching (ZCS) can be achieved, which can reduce more than 80% of the switching losses and significantly improve the efficiency of the power supply.
5.Dissipation Power Constraint
The steady-state power dissipation of the device, PD,max, should be based on the maximum operating junction temperature limit of the device. If the operating environment temperature of the device can be known in advance, the maximum dissipation power can be estimated as follows:
PD,max ≤ ( Tj,max - Tamb ) / Rθj-a
Where Rθj-a represents the total thermal resistance from the device junction to its operating environment, including Rθjuntion-case, Rθcase-sink, Rθsink-ambiance, etc. If there are insulating materials in between, their thermal resistances must also be taken into account.
6.Industry application scenarios
Switching power supplies, with their high power density and low heat generation characteristics, are widely used in: PC power supplies, industrial control systems, new energy, inverters, LED drivers, etc. They are the best solution for replacing linear power supplies.