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Buck converter question

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gxti:
I'm currently working on a simple but beefy DC-DC converter, that takes 12VDC and outputs 0-12VDC. It's intended to be part of a modular bench supply, so the minimum load current is unknown, but it's specified for up to 10A out yet should be able to sustain 10mA loads without too much ripple.

Here's my specifications (they are flexible):
12VDC in, 0-12VDC out
Output load 0-10A
Switching freq 100kHz
Current ripple: 20%
Voltage ripple: 1%
Synchronous rectifier topology, so forward losses are around 0.1V

So here's the question: If I design for 10A load, my calculator tells me to use a 15uH inductor. But if I lower the current, that value increases proportionally. I can find 10A, 15uH inductors, but not 10A, 1500uH inductors. What do I choose, and what kind of ill effects can I expect if I use the lower-value inductor with lower load currents? Does the larger output capacitor compensate well enough, at least when operating in constant-voltage mode?

Psi:
As i understand it, you should spec the inductor for the max output.

Here's a good app note about choosing an inductor if you really want to get into the detail

http://www.national.com/an/AN/AN-1197.pdf

amspire:
The 15uH should work fine - the switcher will just stop switching if is has to when the current is low or zero.

You just have to look at the voltage rise on the output capacitors resulting from a minimum width switching pulse.

Say your minimum ON time was 1 uSec with no load. The current into the 15uH inductor would rise from 0 to 12/15e-6 * .000001 secs = .8 A

This is an energy of 1/2 * L * I^2  = 4.8 uJ

Now this is the minimum amount of energy that capacitors have to be able to absorb without an ugly amount of ripple.

So say you had 10,000uF of capacitance with 12 V across it.  The energy already in the capacitor is already 1/2*C *V^2 = .72 J

So in comparison, the 4.8 uJ is nothing - it would equate to a voltage increase of under 3 mV.

The moral of this is that the only ripple you really have to worry about is at maximum current. You definitely don't want to use a 1500uH inductor if you want that 10A output.

Richard

Zero999:
A buck regulator is unsuitable for this application because the output voltage will be equal to the input voltage when it's set to 12V so the oscillator will have to stop and turn the transistor on continuously. There will be no regulation and some voltage loss in the transistor and inductor so the output voltage will be slightly under 12V.

gxti:

--- Quote from: Psi on August 15, 2011, 04:51:19 am ---Here's a good app note about choosing an inductor if you really want to get into the detail

--- End quote ---
This will keep me busy for a while, thanks!


--- Quote from: amspire on August 15, 2011, 04:58:42 am ---The 15uH should work fine - the switcher will just stop switching if is has to when the current is low or zero.

--- End quote ---
This is what my intuition told me, but my electronics intuition isn't as good as my programming intuition so I had to ask.


--- Quote from: Hero999 on August 15, 2011, 04:42:48 pm ---A buck regulator is unsuitable for this application because the output voltage will be equal to the input voltage when it's set to 12V so the oscillator will have to stop and turn the transistor on continuously.

--- End quote ---
This makes sense, I will accept a lower output voltage. This is the first component of a modular bench power supply, the mains part comes later so I'm planning to run this component off an ATX power supply until it's done. Once I get started on the mains part (getting ahead of myself here) I'll design for a higher intermediate voltage so that the second stage (the subject of this thread) can keep regulation at 12 volts.

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