Power loss calculation and junction temperature rise of PMOSFET in inverters

Calculating conduction loss, switching loss, and junction temperature rise of a power MOSFET in a traction inverter application involves several steps. Here's a detailed guide on how to perform these calculations:

Identify Operating Conditions:

Determine the key parameters of your traction inverter application, including input voltage, output current, switching frequency, duty cycle, and ambient temperature.

Select the MOSFET:

Choose a suitable power MOSFET for your application based on voltage and current ratings, switching speed, and thermal characteristics. Refer to the MOSFET datasheet for specific parameters.

Calculate RMS Current (I_RMS):

Determine the root-mean-square (RMS) current flowing through the MOSFET. If the current waveform is not sinusoidal, you may need to use numerical methods or simulations to calculate I_RMS.

Calculate Conduction Loss (P_conduction):

Conduction loss is the power dissipated due to the MOSFET being in its on-state. Use the formula:

P_conduction = I_RMS^2 * R_DS(on)

I_RMS: RMS current

R_DS(on): On-state resistance of the MOSFET

Calculate Switching Loss (P_switching):

Switching losses occur during the transition between the on-state and off-state. These losses depend on the switching frequency, gate drive characteristics, and MOSFET parameters. Typically, you'll need to break down the switching losses into two components:

a. Turn-On Losses (E_on):

Use the formula:

E_on = 0.5 * V_DC * I_RMS * t_on

V_DC: DC-link voltage

t_on: Turn-on time (provided in the MOSFET datasheet or can be estimated)

b. Turn-Off Losses (E_off):

Use the formula:

E_off = 0.5 * V_DC *

I_RMS * t_off

t_off: Turn-off time (provided in the MOSFET datasheet or can be estimated)

c. Total Switching Loss:

P_switching = (E_on + E_off) * switching frequency

Estimate Thermal Resistance (RθJA or RθJC):

Refer to the MOSFET datasheet for thermal resistance values (junction-to-ambient or junction-to-case).

Calculate Junction Temperature Rise (ΔTj):

Use the formula:

ΔTj = (P_conduction + P_switching) * RθJA

ΔTj: Junction temperature rise

Calculate Junction Temperature (Tj):

Tj = Tambient + ΔTj

Tambient: Ambient temperature (in Celsius)

Check for Safe Operating Conditions:

Compare the calculated junction temperature (Tj) with the maximum allowed junction temperature specified in the MOSFET datasheet. Ensure that Tj does not exceed this limit to prevent MOSFET overheating.

Optimize and Iterate:

If the calculated temperature is too close to the maximum rating, you may need to adjust the design parameters, such as heatsinking, MOSFET selection, or cooling methods, to ensure safe operation.

Please note that these calculations provide a general guideline, and real-world applications may involve additional complexities. Consult the MOSFET manufacturer's application notes and consider using simulation software for more accurate and detailed analysis, especially in high-power or high-frequency applications.