Relation between RC time constant of psu cap and slew rate - diyAudio
 Relation between RC time constant of psu cap and slew rate
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 10th February 2013, 05:39 AM #1 diyAudio Member   Join Date: Aug 2012 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.
 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
 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...
 10th February 2013, 11:34 AM #4 diyAudio Member     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
 10th February 2013, 11:45 AM #5 diyAudio Member     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. __________________ Results aside, the ability to have complete faith in another human being is one of the finest qualities a person can possess - Haruki Murakami
 10th February 2013, 11:57 AM #6 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.
 10th February 2013, 01:45 PM #7 diyAudio Member     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
 10th February 2013, 02:56 PM #8 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.
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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.

 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?

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