The (high-cap.) unregulated PSU for chipamps - Page 66

Rainwulf · 6th March 2009, 11:58 AM

There is a lot of design notes that specify just caps across diodes. In fact i used to do the same thing myself, until i did some research and found out that just caps by themselves across the diodes actually add to the diode capacitance, which makes perfect sense.

Two caps in parallel add together. The resistor is added to actually dampen the oscillations.

As for the cap across the secondary, read the following.
http://en.wikipedia.org/wiki/LC_circuit

By putting a cap across an inductor, you are creating a perfect LC tank circuit, being fed by noise from the mains, and from noisy rectifier diodes. This oscillation is a lot higher then the diodes can themselves rectify, because they are too slow, and get passed right along the DC chain direct to the amp module.

As for regulated power supplies, i cant help you there. I would personally think if the power supply can supply the max current needed by the amp, it should be fine. It may have something to do with the reaction time of the regulator module. Maybe, when the amp requests a large current surge, the regulator module takes too long to respond.

I actually got the information about tank circuits of a link posted in these forums about DC power supply design. Cant find it now can i... damn.

sasmit · 6th March 2009, 12:01 PM

exactly what I thought...but for some reason this datasheet does a lot of things contrary to common knowledge...or may be I misinterpreted some info...

sasmit · 6th March 2009, 12:03 PM

Quote:

Originally posted by sasmit
...but for some reason this datasheet does a lot of things contrary to common knowledge...or may be I misinterpreted some info...

A L & C in paralell form an oscillator is common knowledge for anyone who has read physics at the school level...but surely guys at national know that much..

Rainwulf · 6th March 2009, 12:13 PM

Perhaps they thought a low value cap across the secondary will limit noise coming through the transformer.

Its kind of strange. Its a much better idea to limit the noise as it comes IN to the transformer, because the transformer then limits noise ( due to its inductance and design at 50/60hz) and because any noise will also get transformed down just as the actual voltage is.

Try it with, and without and see what happens. I might even do it myself and use my scope to check out the noise/oscillation if any at the bridge.

My personal philosophy is to reduce noise at its source. So that means a RFI filter at the mains input, and snubbers across the actual diodes.

The big huge core of a toroid will do a lot to reduce noise.

audio1st · 6th March 2009, 03:08 PM

Hello Rainwulf,

Could you clear a point, is it a 100r in series with a 100nF or a 10r to 20r in series with 100nF as the Diode snubber?

Thanx..

AndrewT · 6th March 2009, 03:30 PM

Hi,
there can be a lot of variation in the value of both components in the snubber. These are very dependant on the parameters of the transformer.
Accurate measuring of the transformer and modeling of the damping effect of the snubber or measuring the reaction of the completed PSU to fast changing loads are required to find suitable component values for the snubber.

Rainwulf · 6th March 2009, 10:12 PM

Yea, you need a good capture scope to work out if your snubber is doing the perfect job.

But for a good roundabout figure, 100ohm (100r) in series with 100nf across the diode.

AndriyOL · 6th March 2009, 11:21 PM

My supply voltage at peak is approximately 29-30 VDC (peak load).
Do you know the 100r in snubber will increase or reduce high frequency pole against 2.7r?
Thx.

Rainwulf · 7th March 2009, 11:22 AM

The resistor in the snubber isnt part of an RC time circuit. Its only role is to dampen any high frequency oscillations that the capacitor lets through.
If the resistor is too low, there wont be enough dampening, and if its too high, there will be none. There will be a sweet spot that only a digital capture scope can let you find, but a good value after research on the internet is the 100r 100nf value that goes with standard silicon recitifier diodes.

ionomolo · 7th March 2009, 01:52 PM

Quote:

Originally posted by Rainwulf
Perhaps they thought a low value cap across the secondary will limit noise coming through the transformer.

Its kind of strange. Its a much better idea to limit the noise as it comes IN to the transformer, because the transformer then limits noise ( due to its inductance and design at 50/60hz) and because any noise will also get transformed down just as the actual voltage is.

Try it with, and without and see what happens. I might even do it myself and use my scope to check out the noise/oscillation if any at the bridge.

My personal philosophy is to reduce noise at its source. So that means a RFI filter at the mains input, and snubbers across the actual diodes.

The big huge core of a toroid will do a lot to reduce noise.

No, toroids let all the mains noise thru. That's why i use a separate laminated transformer for the small-signal stages.

6th March 2009, 03:30 PM	#656
AndrewT diyAudio Member Join Date: Jul 2004 Location: Scottish Borders	Hi, there can be a lot of variation in the value of both components in the snubber. These are very dependant on the parameters of the transformer. Accurate measuring of the transformer and modeling of the damping effect of the snubber or measuring the reaction of the completed PSU to fast changing loads are required to find suitable component values for the snubber. __________________ regards Andrew T.

Thread Tools	Search this Thread
Show Printable Version Email this Page	Search this Thread: Advanced Search

New To Site?	Need Help?
Register to Participate Search Privacy Statement Contact Us	Frequently Asked Questions Did you forget your password? Mark Forums Read


	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

6th March 2009, 11:58 AM	#651
Rainwulf diyAudio Member Join Date: Feb 2009	There is a lot of design notes that specify just caps across diodes. In fact i used to do the same thing myself, until i did some research and found out that just caps by themselves across the diodes actually add to the diode capacitance, which makes perfect sense. Two caps in parallel add together. The resistor is added to actually dampen the oscillations. As for the cap across the secondary, read the following. http://en.wikipedia.org/wiki/LC_circuit By putting a cap across an inductor, you are creating a perfect LC tank circuit, being fed by noise from the mains, and from noisy rectifier diodes. This oscillation is a lot higher then the diodes can themselves rectify, because they are too slow, and get passed right along the DC chain direct to the amp module. As for regulated power supplies, i cant help you there. I would personally think if the power supply can supply the max current needed by the amp, it should be fine. It may have something to do with the reaction time of the regulator module. Maybe, when the amp requests a large current surge, the regulator module takes too long to respond. I actually got the information about tank circuits of a link posted in these forums about DC power supply design. Cant find it now can i... damn.

6th March 2009, 12:01 PM	#652
sasmit diyAudio Member Join Date: May 2006	exactly what I thought...but for some reason this datasheet does a lot of things contrary to common knowledge...or may be I misinterpreted some info...

6th March 2009, 12:13 PM	#654
Rainwulf diyAudio Member Join Date: Feb 2009	Perhaps they thought a low value cap across the secondary will limit noise coming through the transformer. Its kind of strange. Its a much better idea to limit the noise as it comes IN to the transformer, because the transformer then limits noise ( due to its inductance and design at 50/60hz) and because any noise will also get transformed down just as the actual voltage is. Try it with, and without and see what happens. I might even do it myself and use my scope to check out the noise/oscillation if any at the bridge. My personal philosophy is to reduce noise at its source. So that means a RFI filter at the mains input, and snubbers across the actual diodes. The big huge core of a toroid will do a lot to reduce noise.

6th March 2009, 03:08 PM	#655
audio1st diyAudio Member Join Date: May 2005 Location: Foxton, CAMBS..	Hello Rainwulf, Could you clear a point, is it a 100r in series with a 100nF or a 10r to 20r in series with 100nF as the Diode snubber? Thanx..

6th March 2009, 10:12 PM	#657
Rainwulf diyAudio Member Join Date: Feb 2009	Yea, you need a good capture scope to work out if your snubber is doing the perfect job. But for a good roundabout figure, 100ohm (100r) in series with 100nf across the diode.

6th March 2009, 11:21 PM	#658
AndriyOL diyAudio Member Join Date: Jul 2008 Location: Lviv	My supply voltage at peak is approximately 29-30 VDC (peak load). Do you know the 100r in snubber will increase or reduce high frequency pole against 2.7r? Thx.

7th March 2009, 11:22 AM	#659
Rainwulf diyAudio Member Join Date: Feb 2009	The resistor in the snubber isnt part of an RC time circuit. Its only role is to dampen any high frequency oscillations that the capacitor lets through. If the resistor is too low, there wont be enough dampening, and if its too high, there will be none. There will be a sweet spot that only a digital capture scope can let you find, but a good value after research on the internet is the 100r 100nf value that goes with standard silicon recitifier diodes.

Currently Active Users Viewing This Thread: 1 (0 members and 1 guests)