Re: gEDA-user: Alarm clocks and switching regulators

[John Doty <jpd@xxxxxxxxxxxxx>]

On Sep 24, 2007, at 9:47 AM, Randall Nortman wrote:

> On Mon, Sep 24, 2007 at 11:25:01AM -0400, DJ Delorie wrote:
>>
>>> DJ -- I notice from looking at your alarm clock boards that you seem
>>> to have tried something like this.  Would you care to share your
>>> approach and results?  I'd greatly appreciate it.  Looks like you
>>> have a diode in there too -- for preventing resonance?
>>
>> Yes, the diode is for limiting resonance, and also (in my thinking at
>> least) to prevent the other regulators and circuits from leaching off
>> the switcher's input filter.
>
> I wonder what would really happen if that diode weren't there?

Assuming you're talking about D401, it wouldn't make much difference.

>   This
> is where I start bumping into the limits of my analog design
> knowledge.  With your design, when the diode is reverse biased and
> shuts off, there doesn't seem to be any conduction path for the
> inductor -- wouldn't this cause the voltage across the inductor to
> spike and eventually hit the breakdown voltage of the diode, so that
> it conducts in reverse?

Nope. If the diode could cut off while the inductor is still  
conducting, the direction of current flow is such that the inductor  
will pull the cathode of the diode *down*, turning the diode back on.  
And because there's a diode there, you can't get current started in  
the other direction, so no spike is possible.

But the truth is, D401 isn't very useful where it is. L400 is going  
to keep it in conduction, so it just acts like a resistor of value ~ 
(kT)/(qI), or ~25 milliohms at 1 amp. That's smaller than the series  
resistances of L400 or C402, so those are where the damping's going  
to be. And it's pretty good: the characteristic impedance of the  
filter is sqrt(L/C), or about 250 milliohms. That's about the same as  
the sum of the series resistances of L400 and C402, so the Q will be  
about 1, not a serious resonance problem.

John Doty              Noqsi Aerospace, Ltd.
http://www.noqsi.com/
jpd@xxxxxxxxx




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