Choke-Input: How do I calculate the right snubber for high-current-low-voltage - diyAudio
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Old 13th March 2004, 05:02 PM   #1
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Default Choke-Input: How do I calculate the right snubber for high-current-low-voltage

I have build a choke-input (LC)-power supply for a SS-Amp with a 40mh-choke, at 6A. The choke has a resistance of 2,2ohm. The amp is an Aleph-X, so has 6A and runs in class a, so constant current draw is guaranteed. It works very nice, but I want to add a snubber across the choke to minimize stress on the choke like vibration etc.

I read through quiet a bit of documentation like
http://www.hagtech.com/pdf/snubber.pdf

For a tube amp, I have rules of thump like 10k-resistor in series with 1uF across the choke. But there I have 10H and a feww hundres volts and only 100mA!

So how do I calculate it for low voltage with high current correctly ?

Thanks a lot
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Old 13th March 2004, 05:14 PM   #2
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The optimum snubber value is based on the recovered charge of the diode, and to a lesser extent on the circuit di/dt. The effective recovered charge in switching depends on diode type, operating temperature, forward current, and rate of di/dt in commutating the diode off. The snubber goes across the diode, to control the snappiness of the diode turn-off and damp the ringing in the circuit.

I can't really see any obvious benefit to putting some type of network in parallel with the inductor. Seems to me it would just create a parallel resonant circuit with the inductor, though with enough resistance in the RC snubber the Q would be low. The only reason I can imagine a "snubber" across an inductor would be to reduce the possiblity of voltage transients in some sensitive part of the circuit due to the voltage compliance which a pure inductor has, operating as a current source; a snubber might prevent overvoltage in some other circuit component, but it would also compromise to a degree the current regulation properties.

I've built a couple of AB style high power amps with inductor input power supply; other than the expense of getting good inductors (don't even ask what Plitron charges for 25A 15 mH inductors!), the results are very nice for a solid state amp. Only a few commercial models have done this, such as Ayre, Cello, and Music Fidelity.

One nice think about large inductors/chokes is that they are real sturdy items... I don't think you need to worry about reducing stress on the inductor. Even if you saturate the inductor with a temporary high volt seconds and go beyond the rated DC value, it will recover nicely without any lasting damage, unless it's been sitting in such a mode for so long that the heat build up is bad enough to damage the insulation. A snubber wouldn't affect this kind of overload.

My background is both SMPS design and audio, so I'm trying to look at this from both angles.

BTW, the easiest way to avoid being much concerned with diode snubbers is to use Schottky diodes, which only have a simple depletion capacitance, and no recovered charge. Then a "optimum" snubber can be calculated which doesn't depend on load conditions or operating point, just on the diode depletion capacitance.

This is why HV Silicon Carbide Schottky diodes (300V, 600V, 1200V) are becoming popular for some high density power supply applications, as they behave much closer to an ideal diode than most other available components.

Regards,

Jon
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Old 13th March 2004, 07:37 PM   #3
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Jon,

Thanks for your fast response. I have built a couple of amps, but mainly tube amps. I made there already the experience that chokes can start to buzz and that either a small cap before them to earth helps without taking away thesonic advatnages of a choke-input our a snubber can help across the choke. My chokes here in the Aleph-X are of excellent quality and there is minimum buzzing...but being a perfcetionist, I try now to get rid of that as well so that the power supply is true quiet.

My question came arose from the comments Morgan Jones had in his book "VAlve amplifiers":

"We said that the choke input power supply drew a constant current from the mains transformer, but this is not exactly true.Since the rectifier diodes require a certain voltage before they switch on, there must be a time as the input waveform crosses through zero volts, when neither diode is switched on. The current drawn from the transformer is therefore not continous, and must fall momentarily to zero. The choke will try to maintain current and by doing so will develop an e.m.f.:

E=-L*(di/dt)

A resistor/capcitor snubber network should be fitted across the choke to limit these spikes"

I must correct my statement above: He suggests a 10nF and 10K snubber across a 10H choke. HE states that high frequencis filtering will become bad as well through this snubber, so I ask myself if a small capcitor to ground is not the better soultion.

By the way: the chokes come from http://www.ae-europe.nl/smoorspoelen.htm

They get with 6A through them really hot (how hot is still OK?) and funny enough become more silent when they get so hot (after one hour).
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Old 13th March 2004, 10:06 PM   #4
mwh-eng is offline mwh-eng  United States
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"how hot is still OK?"

Depends on the core material properties versus temperature. Temperature characteristics of insulation on magnet wire. What other components could be affected by this heat. Temperature caused expansion and contraction stress.

In general, I like to keep the temperature rise to no greater than 30 degrees C above ambient. Many times this is neither practical nor necessary though, and a higher temperature rise should be considered. Obviously, reliability should be considered depending on many factors.

Measure inductor/core temperature rise and inside enclosed unit air temperature with worst case room ambient temperature, since you are a perfectionist. Not a bad idea for the non-perfectionist. Try to get the thermocouple (temp. probe) on the core with thermally conductive grease, near the center of the core or towards the center of the inductor unit. If you are building your own inductors, you can embed a thermocouple in the copper windings, also. Wait for temperature to stabilize.

Being a perfectionist can be an asset sometimes and a curse at other times, IMO.
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Old 13th March 2004, 10:10 PM   #5
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Ok, you punished me for the perfectionist stuff enough.

So, I measured 60 degree celsius at the chokes (which are of the type I posted). Good enough ?

Any thoughts on the snubber ?
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Old 13th March 2004, 10:32 PM   #6
mwh-eng is offline mwh-eng  United States
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Quote:
Originally posted by Blitz
Ok, you punished me for the perfectionist stuff enough.

So, I measured 60 degree celsius at the chokes (which are of the type I posted). Good enough ?

Any thoughts on the snubber ?

Unfortunately I'm a perfectionist, also. It did come in handy for doing worst case analysis on others' designs. Painful for them, though. Sorry, my intention was not to punish. Just joking.

If your ambient is 25 to 30 degrees C, considering the core material in the picture, that should be good enough. Is that in an enclosed unit with vented slots, etc.? Most metal laminate cores can really take the heat, in my experience.

I need more study on the snubber. I have not looked at a circuit diagram on exactly what you are doing, but I plan to after taking my dog for a hike or vice versa.
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Old 13th March 2004, 10:49 PM   #7
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To give you a better picture: I used these Diodes: http://www.schuro.de/Daten/IR/hfa25pb60.pdf

Voltage is 30 VAC at the transformer, 23VDC after the choke-input filter.
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Old 14th March 2004, 01:30 AM   #8
mwh-eng is offline mwh-eng  United States
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Seem like nice rectifiers (diodes). I'm still playing catch-up.

I'm assuming you used full wave rectification off the transformer secondary and go into the choke with a large filter cap to ground. Your choke winding resistance is 2.2 ohms and your load of 6 amps at 23VDC means a load of 3.83 ohms. I'm assuming you use a single supply voltage for your amp (Class A).

Center tapped transformer secondary and 2 diodes or no center tap and full bridge?

40mh at 100 Hz fundamental. XL = 2*pi*f*L = 25 ohms impedance approximate due to rectified AC. How much capacitance for the C part of the LC filter?

Do you see high frequency ringing anywhere? If not forget the snubber. If so, what's the frequency of this ringing?

A large choke with a lot of turns is really only good for filtering low frequencies. It will have a relatively large shunt winding capacitance that passes high frequencies like a short circuit, almost. If you have high frequency noise with these rectifiers (diodes), then you will need another method of blocking it. A small choke with a small number of turns in series with this large choke maybe. Some suggest putting small valued resistors in series with rectifiers and/or small valued capacitors in parallel. There are others on this forum with more power supply experience and more recent experience with newer rectifier technology than me. I want to learn more about reducing noise and ringing, also. Those high voltage Schottky diodes sound interesting!

I'm thinking about how much energy the capacitor in the snubber will need to absorb in order to size the capacitor. Once the capacitor value is known, calculating the resistor value is easy.
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Old 14th March 2004, 02:15 AM   #9
mwh-eng is offline mwh-eng  United States
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I realize it's bedtime in Germany.

Do you use a small value capacitor (1uf or similar) at the input of the choke? I don't yet see the need for a snubber for the choke, unless it rings at it's own low self-resonant frequency. Not seen that happen. I think rectifier ringing, if present, will be very high frequency based on lead inductance, junction capacitance, and transformer parasitics. I agree with Jon, can't see a need for a snubber for the choke, but there's a good chance you can have rectifier ringing.
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Old 14th March 2004, 08:03 AM   #10
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Quote:
Originally posted by mwh-eng
I'm assuming you used full wave rectification off the transformer secondary and go into the choke with a large filter cap to ground. Your choke winding resistance is 2.2 ohms and your load of 6 amps at 23VDC means a load of 3.83 ohms. I'm assuming you use a single supply voltage for your amp (Class A).

Center tapped transformer secondary and 2 diodes or no center tap and full bridge?

40mh at 100 Hz fundamental. XL = 2*pi*f*L = 25 ohms impedance approximate due to rectified AC. How much capacitance for the C part of the LC filter?

Do you see high frequency ringing anywhere? If not forget the snubber. If so, what's the frequency of this ringing?

Yes, full-wave with two secondaries and two bridges, balanced powersupply with +23V and -23 V, therefore two chokes and a capacitance of 100000uF behind the choke.

I have started to debug the power supply for high-frquency ringin with the osci, I first wanted to get rid of the buzzing (sound like high-frequency though) of the chokes.

A small cap (100-470uF) before the choke brings the buzzing you can hear from the chokes down from high-frequent to the more normal 100Hz

On the high-frequency filtering: As we have 6A current drawn , I would loose 3V when attaching a 0,5R into the rail...so the resistor should be smaller like 0,25R. How much additional filtering will this give compared to the DC-resistance of the choke which is already 2,2 Ohm ? And where to place it: Making it RLC, RCLC or LCRC ?

Best Regards
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