Property of transistor

[king.oslo]

I am building a 0-15V 7A power supply with current limiting.

The plan is to use a buck controller and some to3 transistors to take the voltage from 18V to circa 1V above the output voltage. Then use linear regulation to get rid of ripple.

My question is: to do the linear regulation, can I connect an adjustable linear voltage regulator to the base of a power resistor, or is it there a better way? Perhaps taking all the current through the regulator?

Thanks.M

 

[Grossel]

What about having a resistor (and smooting cap) BEFORE the buck stage to "kill" the output whenever the current raises above a certain level?

It sounds pretty weird to just add another linear regulator to get rid of the ripple. Wouldn't the whole point of using a buck be lost then?

 

[duffy]

My question is: to do the linear regulation, can I connect an adjustable linear voltage regulator to the base of a power resistor, or is it there a better way?

I think you mean through the base of a power transistor, don't you? The answer's yes. For instance, you can use a PNP power transistor capable of handling at least 7A and drive the base .7v higher than your intended output, but lower than your switcher's output, which is what I think you are after. With the 1v collector to emitter drop, the pass transistor will only burn off about 7w, which I think is what you are after.

Tracking and adjustment are going to be issues. You will want both the switcher and the linear to read the voltage at the tap, this will probably be going to your voltage control pot. A linear regulator like the common LM317 wants to maintain a 1.25V differential between the output and its "adj" pin, the buck regulator is going to expect something different. A ganged pot would be very helpful with that setup.

As long as you have current regulation on the switcher side, power-up to the load through the linear should be ok.

Have you considered just connecting up a big pi filter with a couple of huge caps and a big inductor to the switcher? I've seen this done many times where the filter coil is actually bigger than the coil used in the buck regulator itself. It's not perfect (I had a bear of a time with switcher noise and active filters once) but the kinds of things that pull 7A aren't usually fiddly sensitive op amp circuits.