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Old 28th March 2013, 02:37 PM   #31
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The reservoir capacitor will appear as a low impedance to any high frequency, The transformer will appear as a high impedance so most of the high frequency will appear across the transformer. What you choose to do about this depends on what you are trying to achieve. Placing a snubber across the transformer secondary will reduce the amount of high frequency that the transformer sees. Hagerman may have had a reason to reduce the high frequency the power transformer might see. Placing a snubber in parallel with the diodes will keep any high frequency local to the diodes.
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Old 31st March 2013, 10:14 AM   #32
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Some interesting findings, now that I found some time to work with PSpice.

I simulated the exact circuit that Hagerman did, using the same model he used for the 1N4007. To cut the story short, this is the circuit I used:

Hagerman method.JPG

So, firstly, I specified the diode's recovery time as 7.2us. This is what I got at the point where I put the probe:

1N4007 7.2us.PNG

After that, I left the 1N4007 PSpice model the same, except for changing the recovery time to 750ns (making it shorter). The result:

1N4007 750ns.PNG

And, up to 75ns:

1N4007 75ns.PNG

75ns is the typical value of a UF4007 diode, according to datasheets. So, this gives credit to anyone that just pays for some better diodes and does not bother with snubbers.

Anyway, these are simulation findings - what remains to be seen, is whether we can also get experimental evidence of these facts.
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Old 31st March 2013, 04:49 PM   #33
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Quote:
Originally Posted by audiostrat View Post
I simulated the exact circuit that Hagerman did
If you want to see the LC circuit's oscillatory ringing, you've got to give it a stimulus. The standard analogy is a bell: you've got to smack the bell with a mallet, to make it ring.

In this case, (L = transformer secondary leakage inductance), (C= transformer secondary capacitance + rectifier capacitance), and the stimulus is a large dI/dt. To get a large dI/dt, increase the filter capacitance to narrow the current pulse, and decrease the load resistance to make the current pulse taller. Voila, large dI/dt.

I simulated this in LTSPICE using the Vishay Semiconductor Ultrafast rectifier "UF4007" spice model, schematic below. As you can see, I was able to get oscillatory ringing. Also as you can see, Hagerman's proposed snubber resistance (470 ohms) damped the ringing perfectly.

Conclusion: Un-snubbed transformer secondaries CAN oscillate, even with UltraFast rectifiers.

.

.
Attached Images
File Type: png schem.png (8.3 KB, 383 views)
File Type: png nosnnub.png (25.1 KB, 376 views)
File Type: png yessnub.png (22.6 KB, 374 views)
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Old 31st March 2013, 05:11 PM   #34
Magz is offline Magz  United States
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Quote:
Originally Posted by transistormarkj View Post
If you want to see the LC circuit's oscillatory ringing, you've got to give it a stimulus. The standard analogy is a bell: you've got to smack the bell with a mallet, to make it ring.

In this case, (L = transformer secondary leakage inductance), (C= transformer secondary capacitance + rectifier capacitance), and the stimulus is a large dI/dt. To get a large dI/dt, increase the filter capacitance to narrow the current pulse, and decrease the load resistance to make the current pulse taller. Voila, large dI/dt.

I simulated this in LTSPICE using the Vishay Semiconductor Ultrafast rectifier "UF4007" spice model, schematic below. As you can see, I was able to get oscillatory ringing. Also as you can see, Hagerman's proposed snubber resistance (470 ohms) damped the ringing perfectly.

Conclusion: Un-snubbed transformer secondaries CAN oscillate, even with UltraFast rectifiers.

.

.
Hmmm. Sounds like another plus for choke input supplies. Much lower dI/dt, so they are less likely to induce ringing.
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Old 31st March 2013, 05:17 PM   #35
Magz is offline Magz  United States
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Hey Mark,

How does .022uF and 330ohm look in that sim? I've been using that as my default snubber and it seems to work quite well.
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Old 31st March 2013, 06:15 PM   #36
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Member I_will_run_spice_for_you might volunteer to perform that simulation, or you could run it yourself. A third option, and the one that I prefer myself, is to agree with Hagerman that this is a classical second order system with damping factor Zeta (wikipedia). Then all you need to do is calculate the Zeta of the parallel RLC circuit, whose R=330, L=1.33E-4, C=5.5E-10.

It's the 2nd boxed equation on Hagerman's page 4:
  • Rsnub = (1/(2*Zeta))*sqrt(L/C)

which yields, after rearrangement, Zeta = (1/(2*Rsnub))*sqrt(L/C) . Plugging in the numbers for this particular transformer & rectifier, we get Zeta ~ 0.73 . Would a waveform with damping factor 0.73 please you?
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File Type: pdf second_order_systems.pdf (272.6 KB, 68 views)
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Old 31st March 2013, 06:24 PM   #37
Magz is offline Magz  United States
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Quote:
Originally Posted by transistormarkj View Post
Member I_will_run_spice_for_you might volunteer to perform that simulation, or you could run it yourself. A third option, and the one that I prefer myself, is to agree with Hagerman that this is a classical second order system with damping factor Zeta (wikipedia). Then all you need to do is calculate the Zeta of the parallel RLC circuit, whose R=330, L=1.33E-4, C=5.5E-10.

It's the 2nd boxed equation on Hagerman's page 4:
  • Rsnub = (1/(2*Zeta))*sqrt(L/C)

which yields, after rearrangement, Zeta = (1/(2*Rsnub))*sqrt(L/C) . Plugging in the numbers for this particular transformer & rectifier, we get Zeta ~ 0.73 . Would a waveform with damping factor 0.73 please you?
Thanks for nothing.

I am perfectly capable of running a spice sim, but being on the road and not having my computer with me I thought you could plug the numbers into the sim you've already run.

Forget it. Happy Easter.
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Old 31st March 2013, 09:44 PM   #38
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Quote:
Originally Posted by transistormarkj View Post
If you want to see the LC circuit's oscillatory ringing, you've got to give it a stimulus. The standard analogy is a bell: you've got to smack the bell with a mallet, to make it ring.

In this case, (L = transformer secondary leakage inductance), (C= transformer secondary capacitance + rectifier capacitance), and the stimulus is a large dI/dt. To get a large dI/dt, increase the filter capacitance to narrow the current pulse, and decrease the load resistance to make the current pulse taller. Voila, large dI/dt.

I simulated this in LTSPICE using the Vishay Semiconductor Ultrafast rectifier "UF4007" spice model, schematic below. As you can see, I was able to get oscillatory ringing. Also as you can see, Hagerman's proposed snubber resistance (470 ohms) damped the ringing perfectly.

Conclusion: Un-snubbed transformer secondaries CAN oscillate, even with UltraFast rectifiers.

.

.
I could not agree more. Still, please observe my point: I said that better diodes can ameliorate the situation, not extinguish the problem completely. Sorry if I did not make that clear.

But what your simulation does show to me, is that for relatively normal load requirements, a good diode could eliminate unwanted interference. Given the numbers for leakage and parasitic capacitance are logical, of course.

For example, I am planning to build a bridge with an input cap of 4700u, giving roughly 200mA at 14-15V - a 76 ohm load. Seems like a UF4002 could be sufficient for this.

I just wanted to make this clear - to myself, too - since leakage and stray capacitance could be hard to find, given a possible lack of appropriate equipment. Or just an unnecessary pain, for a normal load condition.

I did not have time to play with values, like you did, and I was planning to do so. Thank you for this information! Maybe I will give it a try, too.

It would be interesting to test some bad but possibly real values for leakage and stray capacitance, and observe whether the design could be safe in theory for a maximum load and reservoir capacitance, when using fast diodes. Maybe I will give it some time, should I have it.
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Old 31st March 2013, 11:08 PM   #39
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Hi Magz, Happy Easter to you as well. I hope you had a chance to look at that .pdf file, especially Figure 6 on page 8. It shows damped oscillatory ringing for several different choices of the damping factor Zeta.

(Your snubber) plus (Hagerman's transformer) plus (UF4007 rectifier) gives a Zeta of 0.73 -- which, looking at this set of curves, appears to produce a very pleasant waveform indeed. Although Hagerman advocates Zeta=0.5, plenty of 2nd order systems are designed and built and put into the field with Zeta=0.717 (sqrt_0.5), and lots of other second order systems are shipped with Zeta=1.0. I myself have shipped millions of phase locked loops having 0.8 < Zeta < 1.2, and they work perfectly well out in the real world.
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File Type: png fig6.png (15.6 KB, 326 views)
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Old 31st March 2013, 11:20 PM   #40
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Be careful with simulations, I ran this as is with a 1n4007 and got a ring on diode turn off, the set the filter capacitor for a ESR of 0.1 ohms typical for a 1000uF electro and the ringing almost disappeared. Then I changed the transformer leakage inductance to 900uH which is bit low for a transformer with 0.5 Ohm effective resistance and the ringing came back bigger than ever. I have never observed ringing like this with real circuits, there are probably more loss mechanisms in the actual system than are accounted for in the simulation.
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