Re: gEDA-user: Alarm clocks and switching regulators

[John Doty <jpd@xxxxxxxxxxxxx>]

On Sep 24, 2007, at 6:00 PM, Peter Clifton wrote:

>
> On Mon, 2007-09-24 at 14:36 -0600, John Doty wrote:
>
>> The game is different with high voltage switchers using high Q
>> components like polypropylene capacitors but you don't intend to go
>> there today, I hope ;-)
>
> I'm just starting to learn this testing 1.7kV half-bridge devices  
> which
> switch unreasonably fast! The polyprop cap across the bridge seems to
> want to ring with the parasitic L going back from the test board to  
> the
> main DC link, which has its own MLC ceramics and Al Electrolytic bulk.

In these cases, it's helpful to put in components that can absorb the  
energy in unwanted modes. Nice high Q inductors and capacitors  
reflect it around the circuit, but they don't absorb it. Low Q  
inductors made with lossy ferrites are useful here. Ferrite beads are  
usually quite lossy. Or you can absorb unwanted energy with  
strategically placed resistors. A series RC "snubber" connected  
across where you want a relatively slowly varying voltage can kill  
fast ringing. A parallel LC can do the same thing for a current.

>
> I'm seeing high current edges fast enough that I'm wondering if I need
> to start thinking of my power tracks as transmission lines! They are
> certainly effective enough radiators of EMI.

There are many cases where you really should be thinking of your  
circuits as coupled multimode transmission lines. Very helpful when  
you're suffering from ringing and crosstalk.

>
> Regards,
>
> -- 
> Peter Clifton
>
> Electrical Engineering Division,
> Engineering Department,
> University of Cambridge,
> 9, JJ Thomson Avenue,
> Cambridge
> CB3 0FA
>
> Tel: +44 (0)7729 980173 - (No signal in the lab!)
>
>
>
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John Doty              Noqsi Aerospace, Ltd.
http://www.noqsi.com/
jpd@xxxxxxxxx




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