Simple, no-math transformer snubber using Quasimodo test-jig

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To read post #1's schematic with perfect crystal clarity:
  1. View post #1
  2. Click on the fourth attached thumbnail (the V4 schematic)
  3. Click on the white "X" icon at the bottom left of the image. This is the diyAudio website's icon which means "Increase image display size to 100% of original".
You might want to print out these instructions, and keep the paper in hand while you're attempting the maneuver
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I'm grateful to member brian92fs, who pointed out this 2019 article on snubbers by Rod Elliott, on his Elliott Sound Products website. Be sure to read part 1 and also part 2 (follow the link at the bottom).

Rod starts out thinking that snubbers are stupid, and he bends over backwards never to write the word "Quasimodo" in the article, lest he point his readers towards a 20 page document full of badly misguided ideas, plus a lot of maths in the appendix.

Then as he gets rolling along, he begins to change his tone if not his tune, saying things like
This is an article where, in an attempt to prove that something was completely unnecessary, I discovered that this may not be the case.
By the end of the article, after putting up a dozen scope photos of power supplies with various kinds of snubbers, including no snubber at all, he believes that
  1. Yes transformer secondary ringing does exist
  2. Yes it is created by diode switchoff
  3. Yes different types of diodes do create different amounts of ringing
  4. Yes adding a snubber with resistive damping, does reduce ringing
  5. Yes it is possible to adjust the component values in the snubber, to make ringing disappear completely, and here's some pictures

However I am disappointed that he doesn't add up all of these thoughts, and formulate this conclusion: "Ringing certainly does make radio frequency emissions worse, nobody actually WANTS ringing, we already know how to get rid of ringing using snubbers, why don't we simply decide we will completely eliminate ringing by adding a snubber? It's cheap and it's easy: just a resistor and a couple of small capacitors. Why on earth not?
 
Rod certainly wants to emphasise that there is no audible benefit to snubbing a secondary winding for an acceptably designed and integrated power supply.

For those wanting to add a snubber circuit to a secondary winding, Rod presents an almost in-situ measurement technique. I'd suggest that those with a soundcard style signal assessment tool (eg. software with spectrum analyser, and for that matter also a scope screen) would find it easier to discern the disturbance signal, and may not even need the CR filter depending on the interface they have available (although I haven't yet done this myself).

I note that Rod's technique alters the normal operating configuration by changing the ground point location, which may have an influence on how ground path parasitic currents flow, and may not be easy for those wanting to make measurements on a built amplifier. Some oscilloscopes have a battery powered capability, and similarly for soundcard based interfaces, and so should not require an alteration to amplifier grounding if due care was taken.

Having available a means for in-situ fine-tuning a snubber seems a good outcome for those wanting to go done the snubber path.

If an amp power supply was generating a diode induced ring transient (without any secondary winding snubber), and the ringing frequency was below 90kHz (as per example 40kHz waveform shown in Fig.3 of post #1 Quasimodo jig Rev A doc) then a modern soundcard interface and spectrum analyser may well be able to discern that transient when probing the power supply and downstream.
 
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Jhofland's right. If you use a CMOS 555 chip such as the LMC-555 in the bill of materials, and if you use a low threshold MOSFET such as the ones in the BOM, then it'll work down to 3 volts or even less. You start running into Absolute Maximum Ratings when the supply exceeds 15 volts, so either (a) don't do that; or else (b) switch to the bipolar SE555 whose Absolute Maximum Rating is 18V rather than 15V. But this won't go all the way down to below 3V.
 
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Rod Elliott is pretty enthusiastic about the "HFP" (high frequency probe) technique that he developed in 2019; see the last 2 paragraphs of Part 1.

He may not know that diyAudio member peufeu published that same circuit idea, right here in this thread, five and a half years ago (Dec 2013). It's revealed in post #142, and praised by me in post #156. (link to #142). Peufeu summarizes with "it works well" and it seems to have worked well for Rod Elliott, too.
 
Now I was half asleep when I was ordering from the BOM but I could not find the gate driver and power mosfet so I ordered the below ones. Listed are the manufacturers PN's so I hope these will work.

I also ordered 6 of the boards from OSHpark company. So I could make a few of the these and learn how to do this for the snubber board to be used in my Pearl 2 build.

Thank you very much to Mark for all of his work on this, which I greatly appreciate. And thanks to all others who contributed to this.

Gate driver = MCP1407-E/P
Power MOSFET = FDP8880
 
Mark,

I did read through that first post when I was awake but when I was ordering everything unfortunately I was really no awake.

I will go back over that post you listed for me above and see if I messed up on those two parts and if so I will get the correct ones ordered.

Thank you very much for your reply and information.

James
 
Mark,

I went back and reviewed the datasheets for the original mosfet specified and the one that I purchased. The one question I have is concerning the Rds(on), but the way I am reading it is that the one I purchased meets the criteria specified in the post documented.

If you do not mind taking a quick lok I have attached a screenshot of both of the mosfet's Rds information and also the datasheet for the one I ordered.

If you could be kind enough to take a quick peek and let me know if I am good or need to go back and review this some more.

Thanks, James
 

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The FAQ directs you to post #175, which says:

... here are the required properties of the MOSFET (as discussed on p.4 and p.22 of the Quasimodo design note), which you can use to help you choose a device from your favorite distributor:

  • package: TO-220 or IPAK, pinout GDS (left to right)
  • VDSmax > 20V
  • VGSmax > 17V
  • IDSmax > 2A
  • Logic Level Gate preferred but not absolutely mandatory, see p.4
  • RDS(on) as low as possible. RDS(on) < 100 milliohms mandatory, but < 10 milliohms preferred

What does that mean?

That means you write out a little questionnaire like the one below. Then you answer each question by searching for that piece of data in the datasheet.

  1. Is it in a TO-220 package? yes or no
  2. What is the pinout, left to right?
  3. What is the maximum permitted value of VDS in volts?
  4. What is the maximum permitted value of VGS in volts?
  5. What is the maximum pernitted value of IDS in amperes?
  6. Does the first page of the datasheet mention the phrase "Logic Level Gate" ? preferred but not mandatory
  7. Is RDSon less than 0.010 ohms (< 10 milliohms is preferred)? Is RDSon less than 0.10 ohms? (< 100 milliohms is mandatory)
I will answer the first few questions to get you started. According to the datasheet attached to your post:

1. Yes, TO-220
2. Pinout is G D S
3. Maximum VDS is 30 volts
4. Maximum VGS is 20 volts

Now you answer the rest. When you've finished, if every question is answered YES, then the MOSFET is acceptable. If your MOSFET lacks a "preferred" attribute, it's acceptable too, but not insanely great spectacularly awesome. Preferred is, well, preferred. But not mandatory.

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