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-   -   what is Rds?? (http://www.diyaudio.com/forums/pass-labs/22770-what-rds.html)

Svetlana 15th November 2003 05:31 PM

what is Rds??
 
What does the Rds figure mean in? (Please dont just say on resistance) Why is better Rds lower?

li_gangyi 15th November 2003 05:39 PM

I'm assuming you are reffering to MOSFETs here..a lower R(ds) will mean you have a lower current drop due to On-resistance..it's just like a swicth...but with a resistor connected there..and the value of the resistor is given as the R(ds) so a smaller on will mean less energy is turned into heat..and that you can switch more effienctly due to less resistance...and the overall effieciency of the circuit will increase...hope I have answered your question...

peranders 15th November 2003 05:44 PM

Re: what is Rds??
 
Quote:

Originally posted by Svetlana
Why is better Rds lower?
It isn't... in audio. Only in the ancient 2SK135/2SJ50 Rdson was a parameter to count in. Rdson under 0.5 ohms is of no importance. It's only important in switching applications where Rdson*I*I creates losses.

SY 15th November 2003 06:11 PM

All other things being equal, lower Rds means higher input capacitance, but also higher efficiency and less heat at a given current. Like everything else, it's a trade-off, not a simple less-is-better.

Petter 15th November 2003 08:57 PM

Quote:

Originally posted by SY
All other things being equal, lower Rds means higher input capacitance, but also higher efficiency and less heat at a given current. Like everything else, it's a trade-off, not a simple less-is-better.

To be even more precise: Lower conduction losses at maximum current (current is of course between Drain and Source). In Audio, MOSFET's are typically far from fully on.

My selection criteria for audio MOSFET's start at looking for the highest Rds(on) in a given group - then I evaluate these first and select/deselect the entire group. Switching transistors and the most modern units (which are typically optimized for switching applications since this is where the money is) are not intended to be linear.

Petter

li_gangyi 16th November 2003 12:16 AM

I'm not sure about audio...but for SMPSUs...you would go for the lower R(ds) and a low gate charge...but for audio...you would typically use those audio MOSFETs right?? And even some special cases that do not need Temperature Compensation...and stuff like that...

Petter 16th November 2003 01:33 PM

Quote:

Originally posted by li_gangyi
I'm not sure about audio...but for SMPSUs...you would go for the lower R(ds) and a low gate charge...but for audio...[snip]

It is not that simple either. For normally operated non-resonant SMPS you are typically doing what you suggest above. For resonant SMPS you are taking advantage of the parasitic components to achieve ZVS (Zero Voltage Switching) or ZCS. Some manufacturers claim that predictable albeit higher Coss is useful (IRF) whereas others (Infineon) claim lower effective Coss yields better performance in resonant mode. A paper co-authored by a member of this forum can be found here


http://www.infineon.com/cgi/ecrm.dll...rent_oid=-8176

Petter

li_gangyi 16th November 2003 02:15 PM

hmmm..yeah you are right...most of the time I go along with suggested values and try to tweak it to work better...but that's only when I have nothing to do or I'm too free....hehe


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