Calculate Conduction Power Loss

[jaus tail]

Homework Statement

Homework Equations

power loss = I²R
power loss = V²/R

The Attempt at a Solution

voltage across diode is 0.7V, current is 100A. resistance = 0.01 ohm.
Power loss is I²R - 100 * 100 * 0.01
= 100 W
Power loss across IGBT = V²/R. V across IGBT = V across Diode = 0.7.
R for IGBT = 0.02 ohm
So loss is 0.7 * 0.7 / 0.02 = 24.5W
Total loss is 100 + 24.5 is 124.5 W

But they've done something like:
No current through IGBT
Conduction loss = V_tI_av + I_rms²R_on
= 0.7 * 100 + 100² * 0.01 = 170W.

Why did they do V * T + I²R? Shouldnt they do either one? and I'm not sure which one.

 

[gneill]

Look at the VI curve for the diode. As the current increases the potential drop across the diode junction will increase beyond 0.7 V. So the effective resistance of the diode rises and the power loss increases, too.

See if you can't write an expression for the junction potential drop versus current.

 

[jaus tail]

Oh yeah... (v₂-v₁)/(i₁-i₁) = .01

So (v2 - 0.7 / (100 - 0) = .01
This gives V2 as 1.7 V.
So power loss is V * I = 1.7 * 100 is 170 W.
Well we get same answer. I guess the way the book has gone about solution is confusing.

 

[gneill]

Good.

They chose to treat the diode as two components in series. The first being the fixed potential drop 0.7V and the second a resistance. They summed the power associated with each.