Calculate IGBT losses for a SPWM voltage source converter

[Infineon_Team]

In continuation of IGBTs: Loss calculation and junction temperature estimation for an SPWM voltage source converter, please read it before going through the content in this document.

A three-phase IGBT-based voltage source converter

A three-phase IGBT-based voltage source converter comprises three identical legs for three phases. The current and voltage waveforms in each phase are the same with a phase lag. Therefore, conduction loss and switching loss can be calculated keeping a single phase in mind, as shown in Figure 1.

Figure 1: A phase leg of three-phase voltage source converter

Total converter losses can be calculated by multiplying it by three. If i(t) > 0, T1 and D2 in Figure 1 are operational. The current waveforms for T1 and D2 can be seen in Figure 2 and Figure 3 respectively. Therefore, if i(t) > 0, calculate the power losses of T1 and D2 during the positive half period.

Figure 2: IGBT (T1) current waveform in a VSI

  Figure 3:  Diode (D2) waveform in a VSI

Because the IGBT junction temperature is almost constant during one period, it can be assumed that the junction temperature is maintained constant during the calculation.

Figure 4: SPWM scheme

Assuming the converter is working in linear modulation mode with modulation index m, frequency ω, phase lag between ac voltage and current Փ, dead time t_d as in Figure 4, the conduction time of the IGBT (∂_T) and diode (∂_D) during one switching period Ts can be calculated as follows:

Assuming the current equation as:

IGBT conduction loss for one switching period can be written as:

Equation (6) can be re-written as:

Where

And

V_CE0 and R_T can be determined from the device datasheet, as shown in Figure 1.

Similarly, diode conduction loss is calculated as:

Where

And

See the device datasheet to calculate V_d0 and R_D value.

IGBTs switching loss over one switching period is:

Assuming the junction temperature to be constant over one period of the output waveform. V_DC is also constant during the switching period, and the switching loss depends on the I_C variation. The device’s current waveform is shown in Figure 3.

Let fs be the switching frequency, then IGBT switching loss and anti-parallel diode reverse recovery loss can be written as follows:

E_On, E_Off, and E_rec can be determined from the device datasheet at the average current value calculated from Equation (8). IGBT switching loss and diode reverse recovery energy loss can be calculated as follows:                                                                                                      

Note:  See the KBA236569 for an example of IGBT loss calculation for a SPWM voltage source converter using the Infineon IGBT FF1200R12IE5P.