Help needed on boost converter circuit

Hello Masa,

Many chips state a limit of 10A, for example the LM3478. But since you only have to add 7V (or slightly above 100W) worst case you could ask National whether it'll work. IIRC the main reason why they place such limits is the gate capacitance of the FET it has to drive.

Else, look at the former Unitrode UC chips. Now at TI and there are some excellent application notes, almost cookbook style.

Regards, Joerg

How long are the pulses on the output? Can you use a large capacitor to handle them so that the regulator only needs to provide the average current?

At these currents, you may be better off with a PWM chip and power MOSFET. Although it is more parts, it allows you to keep the 15A currents away from the ground of the PWM chip.

Go get a copy of switcher cad from Linears web site. It is very helpful for checking out your design before you commit it to real parts.

You need to perform some calculations:

We are boosting 11V to 18V so the inductor has 11V on it during the on time and 7V during the off time.

For the smallest inductor, you want the current in the inductor to just stop before the switch turns on again. This means that the peak current must be:

Ip = 2 * 15A * (Ton + Toff)/Toff

But:

Ton = Toff * 11V / 7V

So:

Ip = 30A * (Toff * 11/7 + Toff)/Toff

Ip = 30A * 18/7 = 77A

This is likely to be the biggest problem in the whole design. You need an inductor that is good for a fair bit more than 77A and is also good at the switching frequency. I'd look for this from

or

--
--
kensmith@rahul.net   forging knowledge

LTspice free from

suggests LT1270A

martin

ooops, misread 15 A for 5A sorry

martin

11 to 14 volts... Sounds like you're putting this thing in a car. I would suggest, instead of pulling 270 watts through a switcher, take power directly from the car battery (if that's what it is) and build a switcher with an isolated output to provide the extra 7 volts you need. Use the switcher's output in series with the battery. That way you only have to build a 105 watt switcher instead of a 270 watt switcher. If it was me I'd use the SG3525 chip. Drive a transformer with a center-tapped primary in push-pull. The data sheet for that chip kind of s***ks, though. You might find example circuits by going to and doing a search. Don't forget to ground pin 10!

Thanks for all the advice and links everyone! Kell, yes this is going into a car

- it will boost the voltage going to the coils to say 16v regulated instead of the fluctuating 12v. I'll look into the suggestions and see what I can find. thanks

By "coils", do you mean "ignition coils" or "spark coils"? If so, then it sounds like all that you're looking for is a spark booster of some kind, in which case this is almost exactly the wrong approach.

I'd seriously suggest looking up a local performance shop and ask them what they'd recommend for a hotter spark.

Good Luck! Rich

Yup.