inductor sizing for DC stepdown switcher

Discussion in 'Electronic Design' started by Ken Muldrew, Jun 23, 2005.

1. Ken MuldrewGuest

I want to charge a bank of four 12V, 100Ah lead-acid batteries using a
56V, 5A transformer (so about 78V DC). My plan is to reproduce the
charging stages provided by the UC3906 (but scaled up for 48V) by
using PWM to control the voltage seen by the batteries. The logic is
easy enough to implement in a microcontroller but I'm stuck trying to
figure out what size to make the inductor (basically, the PWM controls
a FET that turns the supply on and off with an inductor in series and
a diode between the FET and the inductor allowing conduction from
ground). Art of Electronics (2nd ed.) gives a single example but I'm
afraid it's been a long while since I've done anything with inductors.
Can anyone give me any hints?

Ken Muldrew

(remove all letters after y in the alphabet)

2. John PopelishGuest

It is all based on the definition of inductance:
V=L*(di/dt) or the applied voltage equals the inductance in henries
times the rate of change of current in amperes per second.

If you have chosen your cycle time, then you have to decide what
current ripple you are satisfied with during the cycle. You already
know the voltages involved.

Take a look at this basic switching regulator tutorial:
http://www.national.com/appinfo/power/files/f5.pdf

3. JoergGuest

Hello Ken,

To add to John's explanation: You also need to figure out what the
highest expected peak current in the inductor will be and size it
accordingly. Else you'd be treated to an impressive pyrotechnic scenario.

In a prototype you can watch the inductor current on a scope. If you see
a marked rise in slope steepness (like a shark fin) near the current
peaks that would indicate that the inductor's core might be too close to
saturation.

Often you only see typical saturation data published such as "inductance
drops to 75% at x amps". Then you have to contact the mfg to find out
what the worst case would be.

Regards, Joerg

4. martin griffithGuest

On Thu, 23 Jun 2005 15:47:05 GMT, in sci.electronics.design
try this penguin site, It's a calculator page