jtw11:
as an example, on the board are 8 transient suppressors dissipating up to ~400mW each, so we're at 3.2W.
Surely a transient suppressor should dissipate almost no power under normal conditions, and only dissipate significant power for very short periods when there is a transient?
jtw11:
Now include the 12V regulator, 5V regulator, 3.3V regulator...
If the load currents are significant, use switching regulators. What will the 12V regulator be feeding?
jtw11:
...8 MOSFETs driving not insignificant currents...
If you choose mosfets with a sufficiently low Rds(on) then you can get the power dissipation down to a low value, unless either the current is very high or you are using fast PWM.
jtw11:
...4 IGBTs that are self clamping...
They are more of a problem, if you are using them to drive inductive loads at a moderate frequency.
jtw11:
...h-bridge IC, stepper motor IC
Which ones? (not bipolar ones, I hope.)