Relation between RC time constant of psu cap and slew rate - diyAudio
Go Back   Home > Forums > Amplifiers > Solid State

Solid State Talk all about solid state amplification.

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 10th February 2013, 05:39 AM   #1
diyAudio Member
 
Join Date: Aug 2012
Default Relation between RC time constant of psu cap and slew rate

how fast the capacitors has to be to give higher slew rate?

Is there any connection regarding the slew rates and the psu RC time constant?

Example:
I just have a doubt that in general the amplifier just acts like modulator for the reserve power at the psu. Now I just wanted to know when an amp has slew rate of 50V/us the psu should deliver the required power that much fast and it has to come from the psu. When its coming from the psu it has to be from the capacitors. Here the discharge time from the capacitor is calculated from the RC time constant which is take an example:

if we take a decent psu cap which is 10000uf and with esr of 10 milli ohm the discharge time is 100ms which is way too slow discharge than the slew required. Is this calculation correct?
so do we need seriously fast capacitors or with capacitors having super low esr like 0.000001 to achieve the slew?

Last edited by rhythmsandy; 10th February 2013 at 05:59 AM.
  Reply With Quote
Old 10th February 2013, 10:09 AM   #2
diyAudio Member
 
Join Date: Jan 2011
Location: Fosser, Aurskog-Holand, Akershus, Norway.
Speed of the amp is related to very much else than the PSU-caps RC-time.
Take the TANDBERG TR2075, wich have a slewrate of 25V/uSecond.
Try figure out what the ESR would need to be if the caps in the PSU should have any influence to that figure.
__________________
Sooner or later you end up with TANDBERG
  Reply With Quote
Old 10th February 2013, 11:06 AM   #3
diyAudio Member
 
Join Date: Aug 2012
ohh man is that you? the legendary Tandberg?!@#!....

the esr has to be 25u ohms if its based on RC time constant...
  Reply With Quote
Old 10th February 2013, 11:34 AM   #4
diyAudio Member
 
sofaspud's Avatar
 
Join Date: Nov 2005
Location: San Antonio
I'm not sure of this thread, but shouldn't the question be, how much current can that capacitor supply in that first microsecond, and is it enough to achieve the amp slew?
__________________
It is error only, and not truth, that shrinks from enquiry. - Thomas Paine
  Reply With Quote
Old 10th February 2013, 11:45 AM   #5
diyAudio Member
 
vacuphile's Avatar
 
Join Date: Apr 2011
Location: Seaside
Plus, an amp with good PSSR can deal with sagging power lines under the influence of load, just like it can deal with other nasties that come out of the PS.
  Reply With Quote
Old 10th February 2013, 11:57 AM   #6
gootee is offline gootee  United States
diyAudio Member
 
Join Date: Nov 2006
Location: Indiana
Blog Entries: 1
The high-power output current must come from the PSU. Where else?

Unless your music only plays during the charging pulses, the current comes from the PSU caps, or from decoupling caps.

The slew rate of the CURRENT is not governed by the RC time constant. The INITIAL current amplitude seems to depend only on the ESR, plus any other series R in the circuit, and the change in the voltage across the cap. That simple impedance relationship, delta i = delta V / ESR+R is only valid for a short time. The CURRENT slew rate during that short time is probably limited mostly by the ESL plus the inductance of the conductors. The current can usually slew rather-blindingly fast. But placing decoupling caps very near the point of load is necessary, for the fastest response (and the least rail voltage change).

We are lucky that the current's slew rate is not tied to the voltage's slew rate, or vice versa, since we want the voltage to not change quickly, or by much.

Last edited by gootee; 10th February 2013 at 12:08 PM.
  Reply With Quote
Old 10th February 2013, 01:45 PM   #7
diyAudio Member
 
sofaspud's Avatar
 
Join Date: Nov 2005
Location: San Antonio
So the time constant is a voltage function. The capacitor doesn't want to give up its voltage, and will supply current to maintain it. The slewing amp will take that supplied current and convert it to the voltage it needs. Is that more or less correct, and does it help visualize the action?
__________________
It is error only, and not truth, that shrinks from enquiry. - Thomas Paine
  Reply With Quote
Old 10th February 2013, 02:56 PM   #8
DF96 is offline DF96  England
diyAudio Member
 
Join Date: May 2007
The capacitor is very happy "to give up its voltage". The only thing preventing it is issues like resistance and inductance; these come from both the circuit and the capacitor internals. Amplifier slew rates are rarely limited by supply caps, but often by amplifier loop stability measures such as dominant pole caps. In the rare case where the PSU is the limit, just raise the transformer secondary voltage a little to give more headroom or improve the wiring to the caps.
  Reply With Quote
Old 10th February 2013, 03:48 PM   #9
JMFahey is offline JMFahey  Argentina
diyAudio Member
 
JMFahey's Avatar
 
Join Date: Mar 2009
Location: Buenos Aires - Argentina
Quote:
if we take a decent psu cap which is 10000uf and with esr of 10 milli ohm the discharge time is 100ms which is way too slow discharge than the slew required. Is this calculation correct?
so do we need seriously fast capacitors or with capacitors having super low esr like 0.000001 to achieve the slew?
To see it from a different (but matching) point of view: PSU RC constants do not matter much, if any.

Why?

1) ESR is just "another" resistance in series with the load.
.001 ohm or .01 ohm, won't make a difference.

Now, 0.1 ohm will start to be noticed, not as a "Slew Rate limiter" but simply as a lossy resistor in series.

2) capacitance determines how much will voltage drop during that current pulse.

Simply stated, higher PSU capacitance will not make the amp any "slower" .

If any, it will "help" it by keeping a steadier supply voltage.
  Reply With Quote
Old 10th February 2013, 05:20 PM   #10
diyAudio Member
 
Join Date: Aug 2012
so my question is that how does caps like Siemens Sikorels have faster transients?
  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


Similar Threads
Thread Thread Starter Forum Replies Last Post
Epcos Sikorel slew rate expectations and big Metalized Film Cap bypass rhythmsandy Solid State 2 29th June 2013 09:00 AM
The high slew rate design that time forgot? ingenieus Solid State 14 15th September 2011 12:49 PM
RC time constant of bootstrap AndrewT Solid State 6 26th July 2009 11:40 AM
Tube rise time and slew rate gain wire Tubes / Valves 11 26th October 2007 02:11 AM
Slew rate value Luke123 Chip Amps 5 7th April 2005 12:30 AM


New To Site? Need Help?

All times are GMT. The time now is 11:05 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