How to calculate power ripple, and then to add the sutable Capacitor?

[Boki]

Hi All,
My audio amplifier has a lot of noise, after I connect a VCC to
GND capacitor, the noise is gone.

But, I did a boring thing, ==> try and error to find the value.

How can I calculate the value directly?

Do I have to use scope to see the ripple first? and then calculate the
RC time?

Best regards,
Boki.

 

[Tim Shoppa]

My audio amplifier has a lot of noise, after I

connect a VCC to GND capacitor, the noise is gone.

Noise? Does this mean hum, or does it mean motorboating, or does it
mean broadband hash, or ?

But, I did a boring thing, ==> try and
error to find the value.

How can I calculate the value directly?

Do I have to use scope to see the
ripple first? and then calculate the
RC time?

Nothing wrong with using a scope, but this is pretty much chapter 1 of
any power supply design tutorial.

Tim.

 

[Jon]

Boki,

If you're using a switching supply, your best source of equations would
be the app notes for the regulator chip that you're using. For a
simple full wave rectifier-capacitor input filter type design, here is
a rule of thumb that I have found useful:
~
For ripple < 10%, the following rule applies:
.. Let R = the load resistance (Ohms)
.. Let C = the filter capacitance (Farads)
.. Let f = Frequency (Hz)
.. Pi = 3.14159......
(2PifRC) = 10 yields (roughly) 10% ripple and 10% load regulation.
This rule scales (roughly) linearly, so, for example:
.. (2PifRC) = 20 would yield roughly 5% ripple and 5% load
.. regulation
Remember this is just a rough rule of thumb, but it gets you close.
Regards,
Jon

 

[Boki]

ya, I have to review them recently.

basically, I think I can measure the ripple first, and then try to
calculate how larger RC time do I need, am I right?

but it seems that only a capacitor between VCC and GND, so what I have
to consider about R is total circuit?

Best regards,
Boki.

 

[ehsjr]

Hi All,
My audio amplifier has a lot of noise, after I connect a VCC to
GND capacitor, the noise is gone.

But, I did a boring thing, ==> try and error to find the value.

How can I calculate the value directly?

Do I have to use scope to see the ripple first? and then calculate the
RC time?

Best regards,
Boki.

This site may help:
http://hyperphysics.phy-astr.gsu.edu/hbase/electronic/rectct.html#c2

Ed

 

[Pooh Bear]

Hi All,
My audio amplifier has a lot of noise, after I connect a VCC to
GND capacitor, the noise is gone.

But, I did a boring thing, ==> try and error to find the value.

How can I calculate the value directly?

Do I have to use scope to see the ripple first? and then calculate the
RC time?

You're asking the wrong question.

Your 'noise problem' is unlikely to be related to any ripple current
requirement. The reservoir capacitor(s) in the power supply do this.

It would help if you explained what the noise sounds like though !

Graham

 

[Walter Harley]

Hi All,
My audio amplifier has a lot of noise, after I connect a VCC to
GND capacitor, the noise is gone. [...]

Given the context of prior quotes, I think you're using an LM386.

They are prone to motorboating (that is low-frequency instability) without
good supply isolation. If the noise you're hearing sounds like a pulse
train (tick-tick-tick at 5 to 50 ticks/second), that's probably what's
happening. If it sounds like hum, then it's more likely a problem of supply
filtering, that is, ripple. If it sounds like hiss, it's either noise on
the supply lines, or high-frequency instability, the latter again an
isolation problem. Ripple reduction is a matter of reading an intro text on
power supply design (it's probably covered in AoE, I don't remember
offhand). Isolation is harder to solve theoretically, because you don't
have enough information about the supply impedance or about the internals of
the LM386; trial-and-error is the best approach there.

In either case, looking at the output and at the supply line (near the chip)
with an oscilloscope can tell you a lot. Especially if the problem goes
away when you touch it with the scope probe

 

[Pooh Bear]

In either case, looking at the output and at the supply line (near the chip)
with an oscilloscope can tell you a lot. Especially if the problem goes
away when you touch it with the scope probe

Or your finger !

That kind of instability was very popular in the 70s / 80s.

If Boki used a more suitable amplifier chip the problem would likely vanish.

Graham

 

[Mac]

ya, I have to review them recently.

basically, I think I can measure the ripple first, and then try to
calculate how larger RC time do I need, am I right?

but it seems that only a capacitor between VCC and GND, so what I have
to consider about R is total circuit?

Best regards,
Boki.

It's not RC. It's I=cdV/dt. I is the max supply current, dV is the ripple
Voltage you are willing to accept, and dt is the period of the waveform on
VCC.

For example if you build a power supply from fully rectified 50 Hz, then
your waveform is 100 Hz. So you want to use 10ms as your dt. Now you can
solve for c:

c=Idt/dV

HTH

For high-current power supplies you then you have to calculate the ripple
current for the capacitor and select a part (or parts) which can handle
the ripple current.

--Mac

 

[Don Lancaster]

It's not RC. It's I=cdV/dt. I is the max supply current, dV is the ripple
Voltage you are willing to accept, and dt is the period of the waveform on
VCC.

For example if you build a power supply from fully rectified 50 Hz, then
your waveform is 100 Hz. So you want to use 10ms as your dt. Now you can
solve for c:

c=Idt/dV

HTH

For high-current power supplies you then you have to calculate the ripple
current for the capacitor and select a part (or parts) which can handle
the ripple current.

--Mac

The rule is that an 8300 microfarad capacitor has its volts of ripple
equal to its amps of current at full wave 60 Hertz.

Everything simply scales from there.



--
Many thanks,

Don Lancaster voice phone: (928)428-4073
Synergetics 3860 West First Street Box 809 Thatcher, AZ 85552
rss: http://www.tinaja.com/whtnu.xml email: [email protected]

Please visit my GURU's LAIR web site at http://www.tinaja.com

 

[John Woodgate]

I read in sci.electronics.design that Don Lancaster <[email protected]>

The rule is that an 8300 microfarad capacitor has its volts of ripple
equal to its amps of current at full wave 60 Hertz.

Everything simply scales from there.

10 000 uF for full-wave 50 Hz. Even simpler.

 

[Pooh Bear]

I read in sci.electronics.design that Don Lancaster <[email protected]>


10 000 uF for full-wave 50 Hz. Even simpler.

Actually about 8000 uF @ 50Hz and 6700uF @ 60 Hz

Don incorrectly assumed a discharge time = 1/2 cycle.

Graham

 

[John Woodgate]

I read in sci.electronics.design that Pooh Bear

Actually about 8000 uF @ 50Hz and 6700uF @ 60 Hz

Don incorrectly assumed a discharge time = 1/2 cycle.

Unfortunately for the harmonic levels on the mains, the charge time for
high-efficiency SMPS is so short now that the half-cycle assumption is
almost true.

 

[Pooh Bear]

I read in sci.electronics.design that Pooh Bear

Unfortunately for the harmonic levels on the mains, the charge time for
high-efficiency SMPS is so short now that the half-cycle assumption is
almost true.

Uh ?

Peak loading results in flat-topping which actually extends the charging time ! The
ac supply's way of 'compenating'. ;-)

Graham

 

[John Woodgate]

I read in sci.electronics.design that Pooh Bear

Peak loading results in flat-topping which actually extends the
charging time ! The ac supply's way of 'compenating'. ;-)

But I've done the measurements.

 

[Pooh Bear]

I read in sci.electronics.design that Pooh Bear


But I've done the measurements.

What were you measuring ?

Just doesn't sound right to me.

I'll take a look at an smps I'm working on right now. It certainly isn't the case
for classic line frequency transformer based psus where the charging time is as
much as 2.5ms at lower powers ( lots of DC R from the transformer ) and typically
around 2 ms at medium / high power.


Graham

 

[John Woodgate]

I read in sci.electronics.design that Pooh Bear

What were you measuring ?

Just doesn't sound right to me.

SMPS between 30 W and 75 W. Short conduction angles, like 18 degrees, or
1 ms in your terms. And it wasn't only me: all the members of the IEC
Task Force saw it done. Some, but not all, eyebrows were raised.

I'll take a look at an smps I'm working on right now. It certainly
isn't the case
for classic line frequency transformer based psus where the charging time is as
much as 2.5ms at lower powers ( lots of DC R from the transformer )

More; you can get to the magic 65 degree conduction angle (3.6 ms) that
meets the IEC/EN 61000-3-2 Class D limits. Not so easy with a toroidal
transformer, but a few ohms in series helps.

and typically
around 2 ms at medium / high power.

Transformer/rectifier supplies do have lower harmonic emissions.

 

[Pooh Bear]

I read in sci.electronics.design that Pooh Bear


SMPS between 30 W and 75 W. Short conduction angles, like 18 degrees, or
1 ms in your terms. And it wasn't only me: all the members of the IEC
Task Force saw it done. Some, but not all, eyebrows were raised.

Ok, I have a 33W smps I can compare with.

I'll bet the relatively small reservoir cap has something to do with that. I can't
recall offhand if we use 68uF or 100 uF.

Was there any R in the way at all btw ?

More; you can get to the magic 65 degree conduction angle (3.6 ms) that
meets the IEC/EN 61000-3-2 Class D limits. Not so easy with a toroidal
transformer, but a few ohms in series helps.

Transformer/rectifier supplies do have lower harmonic emissions.

I'll also take a look at an R-core based supply we have. The DC R in those is very
high indeed. The R-cores have the lowest stray flux I've ever met btw. Haven't got
my hands on an O-core yet but they should be even better.

Graham

 

[John Woodgate]

I read in sci.electronics.design that Pooh Bear

I'll bet the relatively small reservoir cap has something to do with
that. I can't recall offhand if we use 68uF or 100 uF.

Probably not enough if you have to meet the immunity requirements
against voltage dips and interruptions. People are using much the same
capacitors as for 200 W supplies. They must be, otherwise we wouldn't
get such small conduction angles.

The only impedance in series with the rectifier is to limit the inrush
current to 50 to 80 A or so, to avoid things going BANG!

 

[Pooh Bear]

I read in sci.electronics.design that Pooh Bear


Probably not enough if you have to meet the immunity requirements
against voltage dips and interruptions. People are using much the same
capacitors as for 200 W supplies. They must be, otherwise we wouldn't
get such small conduction angles.

The only impedance in series with the rectifier is to limit the inrush
current to 50 to 80 A or so, to avoid things going BANG!

I just did a quick simulation using EWB. I'm blowed if I can tweak anything much
away from a 2.5ms conduction period.

I had a small resistance in series with the supply that I varied from 0R1 to 5R.
Still didn't change much though.

Do you recall any more details ?

Graham