what is Rds?? - diyAudio
Go Back   Home > Forums > Amplifiers > Pass Labs

Pass Labs This forum is dedicated to Pass Labs discussion.

Please consider donating to help us continue to serve you.

Ads on/off / Custom Title / More PMs / More album space / Advanced printing & mass image saving
Reply
 
Thread Tools Search this Thread
Old 15th November 2003, 04:31 PM   #1
diyAudio Member
 
Join Date: Oct 2003
Location: Christchurch
Question what is Rds??

What does the Rds figure mean in? (Please dont just say on resistance) Why is better Rds lower?
  Reply With Quote
Old 15th November 2003, 04:39 PM   #2
diyAudio Member
 
li_gangyi's Avatar
 
Join Date: Jun 2003
Location: Woodlands Circle
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...
__________________
Kids in the back seat cause accidents...Accidents in the back seat cause kids...
  Reply With Quote
Old 15th November 2003, 04:44 PM   #3
Electrons are yellow and more is better!
diyAudio Member
 
peranders's Avatar
 
Join Date: Apr 2002
Location: Göteborg, Sweden
Blog Entries: 4
Default 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.
__________________
/Per-Anders (my first name) or P-A as my friends call me
Super Regulator SSR03 Group buy
  Reply With Quote
Old 15th November 2003, 05:11 PM   #4
SY is offline SY  United States
diyAudio Moderator
 
SY's Avatar
 
Join Date: Oct 2002
Location: Chicagoland
Blog Entries: 1
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.
__________________
You might be screaming "No, no, no" and all they hear is "Who wants cake?" Let me tell you something: They all do. They all want cake.- Wilford Brimley
  Reply With Quote
Old 15th November 2003, 07:57 PM   #5
diyAudio Member
 
Join Date: Jan 2001
Location: Scandinavia
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
  Reply With Quote
Old 15th November 2003, 11:16 PM   #6
diyAudio Member
 
li_gangyi's Avatar
 
Join Date: Jun 2003
Location: Woodlands Circle
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...
__________________
Kids in the back seat cause accidents...Accidents in the back seat cause kids...
  Reply With Quote
Old 16th November 2003, 12:33 PM   #7
diyAudio Member
 
Join Date: Jan 2001
Location: Scandinavia
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
  Reply With Quote
Old 16th November 2003, 01:15 PM   #8
diyAudio Member
 
li_gangyi's Avatar
 
Join Date: Jun 2003
Location: Woodlands Circle
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
__________________
Kids in the back seat cause accidents...Accidents in the back seat cause kids...
  Reply With Quote

Reply


Hide this!Advertise here!
Thread Tools Search this Thread
Search this Thread:

Advanced Search

Posting Rules
You may not post new threads
You may not post replies
You may not post attachments
You may not edit your posts

BB code is On
Smilies are On
[IMG] code is On
HTML code is Off
Trackbacks are Off
Pingbacks are Off
Refbacks are Off



New To Site? Need Help?

All times are GMT. The time now is 06:52 AM.


vBulletin Optimisation provided by vB Optimise (Pro) - vBulletin Mods & Addons Copyright © 2014 DragonByte Technologies Ltd.
Copyright ©1999-2014 diyAudio

Content Relevant URLs by vBSEO 3.3.2