Electronic – Is this the correct way to calculate IGBT power loss


I am having trouble obtaining the power dissipation of IGBTs. It does not have a buit in diode and it isn't driving an inductive load. So I read in an application note that IGBT power losses are actually the sum of 2 kinds of power losses – 1. Static power loss 2. Switching loss


I figured this is the proper way to obtain overall power losses:

For my particular IGBT

  • Desired Switch Frequency = 2Hz
  • Duty Cycle = 2%
  • Vce(sat) = 2.10
  • Ic =130A
  • Eon = 2.25mJ
  • Eoff = 0.95mJ
  • Ets = 3.20mJ

Static power loss = Vce(sat) * Ic * duty cycle

2.1 * 130 * 0.02 = 5.46w

Switching loss = Ets * Switching Freq

3.2e-3 * 2 = 0.0064w

lastly overall power dissipation is = Static power loss + Switching Loss

5.46 + 0.0064 = 5.4664w

please let me know if this is correcct or please show the correct derivation.

As noted by a member the correct values must be referenced in the Eon/off curves.

The Vce(sat) is also probably varied a bit but insubstantially so it will be kept identical to the previous derivation.

The correct values for Id=130a are:
Eoff = 3mJ
Eon = 7mJ
Ets= 10mJ

Static Power Loss = 2.1 * 130 * 0.02 = 5.46w

Switching Loss = 10e-3 * 2 = 0.02w

The proper overall power dissipation turns out to be 5.46 + 0.02 = 5.48

the difference is ~2mW and it seems to make a bigger difference for higher frequency switching operation.

Best Answer

Fundamentally yes but not quite

You have captured the basic concept

\$ P_{total} = P_{cond} + P_{sw} + P_{leak} \approx P_{cond} + P_{sw} \$

I have added in leakage losses for completeness but this only becomes of significance one you get to series stacked devices.

Conduction loss
This is the subtle difference. Conceptually it is \$ P_{cond} = V_{ce} \cdot I_c\$ however, \$V_{ce}\$ might not be good enough. This needs to be the saturation voltage at your operating point. Now you may have read the datasheet and chosen \$V_{ce} = 2.1V\$ when \$I_c\$ equals 130A but there is no named part to cross-reference.

Assuming you have done this correctly then yes \$ P_{cond} = V_{ce} \cdot I_c\$ scaled by duty.

However, there is a more flexible method and that is approximating the IGBT operating point as a series connected DC source \$V_{ce0}\$ and a collector-emitter resistance \$r_c\$

enter image description here

In this example I an interested in an operating point around 20A.

1V = 18.942mm
zero @ 17.071mm
therefore Vce0 = 0.901V
dI = 20A
dv = 12.527mm = 0.661V
therefore rc = 0.00331R

The instantanious power eqates to

\$P_{cont}(t) = V_{ce0}(t)\cdot I_c(t) + r_c\cdot i_c^2(t)\$

The average loss is therefore

\$P_{cont}(t) = \frac{1}{T_{sw}}\int_{0}^{T_{sw}} V_{ce0}(t)\cdot I_c(t) + r_c\cdot i_c^2(t) dt = V_{ce0}\cdot I_{c,avg} + r_c\cdot I_{c,rms}^2\$

You can then derive the average and rms currents based upon your waveform.

A similar process can be done with the diode IF appropriate. NOTE: thermally the co-packaged diode typically becomes the limiting factor in a half-bridge

Switching loss
AS you have captured... Switching loss is the accumulation of switching energy with respect to the switching frequency

\$P_{sw} = (E_{on} + E_{off})\cdot f_{sw} \$

However... the Eon and Eoff are typically stated at a given operating point

enter image description here

This means you need to scale the switching energy with respect to your operating point AND the test point. a linear rescaling is usually good enough BUT please check the Eon,off curves to see if a linear interpolation is good enough

\$P_{sw} = (E_{on,t} + E_{off,t})\cdot \frac{V_{DC}}{V_{DC,test}}\cdot \frac{I_{peak}}{I_{I,test}}\cdot f_{sw} \$

Likewise there is is something comparable with the diode as you accumulate the reverse recovery charge.

The real fun is deriving the average and rms current waveforms and there are given equations for the profile all related to modulation depth