Switcher Inductor Ripple Current

Pull it out of your ass?

Seriously, there's a lot of different factors that go into choosing the number. The optimum is broad, and what's best and most economical for you may not be so for the next guy. So you're (a) free to choose from a broad range of options, and (b) under different constraints from everyone else anyway.

I'd try a number of different ratios and see what impact it has on my circuit cost and estimated reliability. Then I'd choose one, serene in the knowledge that no matter what, I've screwed up somewhere else in the circuit anyway, so it's not like I have to worry about destroying perfection with my inductor choice.

I've got 4 150$ lamps and no money to replace them. I'd like two remaining when I'm done. One in the laser and one spare. That is the problem and why I want to get it right. They show up on ebay every once in a while for 50$, but I'd need to order 10 pcs or so to have a run done, so it does NOT PAY to blow lamps.

Everybody else, "pulled it out of their ass" with no justification.

So far it looks like 180 millihenry for one fet string, or 360 for two fet srings in parallel.

There is enough other fun for things to go wrong, the lamps need a cap charged to 1000V, limited by 40 ohms, to aid in starting and a 27 Kv ignite pulse in series with the lamp.

Ripple current isn't a constant.

It's explained in most controller datasheets how you select an acceptable ripple and the impact the ripple has on the converter. Transient response,cost ,size,component stresses etc.

See here near the bottom of the page.

and here TPS40200 VMC controller.Pg 21 (tps40200.pdf, 1143 KB)

If it's still not clear check out more controller data sheets.

Otherwise known as trial and error.

It took several iterations on a .3 inch toroid before we found that 3 was bad, and 4 was good, and 5 was bad (turns), and we had to step through a few different types of core media as well.

Litz wire made it operate better still, and it was only at 57kHz.

Inductor ripple current will reflect in the output ripple, as current or voltage dependant on the type of regulator and the total filtering method.

It will also reflect in inductor flux swing and AC flux induced core losses contributing to temperature rise in this component.

Different ball-park figures reflect the different core materials and application voltages and operating frequencies that the figures are originally applied to.

Flux swing is a product of applied voltage and operating frequency.

deltaB = V . t / ( N . Ae )

delta B is peak to peak flux swing, in Teslas V = applied voltage, in volts. t = period of applied voltage in one switching cycle, in seconds. N = number of physical wire turns supporting the applied voltage. Ae = core cross-sectional area of the core structure, in meters^2.

The peak flux swing vs loss per volume, over a potential range of operating frequencies, is one common chart of material properties provided by core and core material sources.

Core losses

- reduce as turns increase

- reduce with volt-seconds-per-turn

- generally increase with frequency for most materials.

RL

Ah, now we get some decent math!

THANKS GUYS!!