Inverters vs wallwarts

Pretty sure that Reynolds number cannot be defined for an inviscid liquid. But yeah, turbulent flows are difficult however you look at them.

Gosh, real numbers. Thanks.

I just got this. It's a high voltage half-bridge test board for frying inductors and film caps. People were doing another proto board so I hung this on the end as a v-score breakaway.

My point exactly.

Cheers

Phil Hobbs

One good way of frying film caps is to thermally couple them to hot chokes or power resistors.

Like a lot of other parts, they depend on thermal conduction to the PCB.

RL

Sure. But is my plate 0.2 k/w, or is it 20 k/w?

That's inelegant. I plan to pump in amps of 250 KHz triangle wave.

We will have a lot of forced air flow, at least 200 f/m acoss both sides of our plugin boards. That will reduce cap temp rise by at least a factor of 2.

Power resistor data sheets sometimes mention power vs air flow. I have seen suggestions of between 2:1 and 10:1 more power with air flow.

I'll fire up the board and pump amps into various 4.7u radial film caps, blow some air on them, and measure temps. What else is a boy to do?

I should try to evaluate how much cooling we might get through the leads. We could use big power pours on the pins to slurp out some heat. My first guess is that lead cooling will be minor.

A zero-order approximation would be to assume that the boundary layer is ~5 mm thick, and the plate behaves like still air of that thickness connected to an infinite heat sink at the outlet temperature.

You can compute the outlet temperature using the mass flow rate, heat capacity of air, inlet temperature, and power dissipation.

I'd expect things to improve faster than linearly with flow rate, because the boundary layer should thin down as well as the outlet temperature falling.

Cheers

Phil Hobbs

Film and foil caps work a lot better for that sort of use than metallized-film ones.

Cheers

Phil Hobbs

On that basis, with 200 ft/min flowing across both sides of my 4" square plate, I get just around 1 K/W. Sounds optimistic, but I'll try it.