Flyback diode: it has one job, but mine sucks at it, why?

[righteous]

Gentlemen,

Merry Christmas and happy new year to everyone!

Please have a look at my simple circuit:


And here is the scope shot, as you can see, not working at all with 20V spikes building up, I would expect the diode to remove all/most spikes:

And here is a closer view:



What do I do? How do I make the diode work? The problem is the same is I use SB220 or 1N4148 or PMEG2020 (Ultra Fast 20V 2A).

 

[KlausST]

Hi,

Did you think about / calculate / measure the diode current?

Klaus

 

[betwixt]

...and please explain what the circuit is supposed to do.
It will always give strange results when you connect a small germanium diode across an inductor like that. All it can possibly do is trap back EMF but even then it may be exceeding it's ratings.

Brian.

 

[schmitt trigger]

Is it only me the one that has noticed that the output is connected to the input?

What exactly are you attempting to do?
Also as Brian has noted, utilizing a Germanium small signal diode in a switchmode supply is wrong.

 

[righteous]

Brian,

...and please explain what the circuit is supposed to do.

It is supposed to switch the coil at 820KHz and eliminate any bemf, nothing else.

It will always give strange results when you connect a small germanium diode across an inductor like that. All it can possibly do is trap back EMF but even then it may be exceeding it's ratings.

As I said, I also tried with a SB220 (40V 2A) and a PMEG2020 (Ultra Fast 20V 2A), please forget the "small germanium diode" the problem is the same regardless of diode and current rating.

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Is it only me the one that has noticed that the output is connected to the input?

What exactly are you attempting to do?

Where else could you connect the output? To the Drain of the MOSFET? Would it make any difference to the buildup of spikes?

It is an experiment, It is supposed to switch the coil at 820KHz and eliminate any bemf, nothing else.

Also as Brian has noted, utilizing a Germanium small signal diode in a switchmode supply is wrong.

As I said to Brian, I also tried with a SB220 (40V 2A) and a PMEG2020 (Ultra Fast 20V 2A) and a 1N4148, 1N4007 (both silicon), please forget the "small germanium diode" the problem is the same regardless of diode, type and current rating.

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Hi,

Did you think about / calculate / measure the diode current?

Klaus

Klaus,

Nope, it escaped my mind, but I will measure it right away and let you know.

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Hi,

Did you think about / calculate / measure the diode current?

Klaus

The current is about 1A through the diode, scope shot (AC) is across a 1 Ohm resistor after the diode, and with the PMEG2020 (Ultra Fast 20V 2A) now mounted, it should be well within operating range:

And a closer view:

 

[CataM]

I think the problem is not the diode, but the decoupling. Add a 100 uF bulk cap and 100 nF decoupling very close to the inductor and also the whole layout very tight.

 

[d123]

Hi,

I'd remove the input to output connection, there is nothing normal or useful about it - choose one path (or the other), and then check what's happening. It would seem the MOSFET is intended as a switching shunt element. I would have thought a bulk capacitor (maybe additionally preceded by a low value series resistor, i.e. a low pass filter) at the output would remove spikes, not a diode.

Switching supplies, whatever name you may want to gild them with, have "spiky" outputs.

As a 30-minute experiment aside it might be interesting to make a discrete voltage doubler (or voltage inverter by reversing the diodes) - two diodes, two capacitors and a squarewave input, then zoom in on the output signal.

 

[CataM]

I'd remove the input to output connection, there is nothing normal or useful about it

Switching supplies, whatever name you may want to gild them with, have "spiky" outputs.

This is not a switching supply. Redrawing the circuit like below, might help.

 

[righteous]

Hi,

Did you think about / calculate / measure the diode current?

Klaus

Sorry, I'm not used to my scope yet. Here are the probes turned the right way. And peak is about 3A which is acceptable since the PMEG2020 is 2A average.

 

[dick_freebird]

Inductance looks like the problem, often flyback and
boost converters need a snubber to shave off the
leading edge energy.

 

[schmitt trigger]

Thanks CataM. For drawing the circuit in a way which makes sense.

By the ultra-high frequency of the ringing, these are classic pitfalls in a switchmode supply: parasitic elements ringing together.

 

[righteous]

I think the problem is not the diode, but the decoupling. Add a 100 uF bulk cap and 100 nF decoupling very close to the inductor and also the whole layout very tight.

I'll try that in a jiffy, in the mean time, here's the layout:


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I think the problem is not the diode, but the decoupling. Add a 100 uF bulk cap and 100 nF decoupling very close to the inductor and also the whole layout very tight.

Well, that (100 uF bulk cap and 100 nF decoupling very close to the inductor) didn't do much good.


It is notable that the first 2-3 pulses are just fine, then the ringing starts indicating something is getting tired or depleted.

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Inductance looks like the problem, often flyback and
boost converters need a snubber to shave off the
leading edge energy.

Dick, by "snubber" do you mean Resistor-Capacitor-Diode (RCD) Snubber? If yes, it still has to go through a diode, so the problem would remain.


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Hi d123,

I'd remove the input to output connection, there is nothing normal or useful about it - choose one path (or the other), and then check what's happening.

Why? Would it do any good? I will stick the output up my hiney if it would help.

"normal" - it's not one of my core competencies I'm afraid, so thats out of the question, I leave that to normal people.

"useful" - I guess it depends on the eyes that see.

Switching supplies, whatever name you may want to gild them with, have "spiky" outputs.

"spiky" in comparison to what? Linear?

As a 30-minute experiment aside it might be interesting to make a discrete voltage doubler (or voltage inverter by reversing the diodes) - two diodes, two capacitors and a squarewave input, then zoom in on the output signal.

I can't really see what you say, can you please draw it?

 

[CataM]

1) What is the ringing frequency after you added the decoupling parts ?
2) Does the ring still goes to 20 V as it did before, if you scope it for a long time ?
3) Show the image with the same zooming as in post #1 for better comparison.

 

[d123]

Hi,

It helps to see the correct schematic drawing, thanks CataM. Yes, in comparison to linear. This is a close-up of simulated results of a discrete voltage inverter waveform, this one is quite neat compared to other ones I've messed around with.

 

[righteous]

1) What is the ringing frequency after you added the decoupling parts ?

About 23MHz

2) Does the ring still goes to 20 V as it did before, if you scope it for a long time ?

Yes more or less.

3) Show the image with the same zooming as in post #1 for better comparison.

Well, there is no real difference, with caps

Without caps


PS. in post #1 the diode was a 1N60, I have since then swapped it for a PMEG2020 in post #5

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Hi,

It helps to see the correct schematic drawing, thanks CataM. Yes, in comparison to linear. This is a close-up of simulated results of a discrete voltage inverter waveform, this one is quite neat compared to other ones I've messed around with.

View attachment 143738

Thank you, it's clever, I will keep that on file - but since it will be a part of a battery operated circuit, I would rather like to recover the power and not dissipate it in a resistor.

Which brings me back to the question; there is current galore (3A spikes) going through the diode, but the voltage remains on the other side, why?

 

[CataM]

Move the inductor closer to the FET. You have big leads for the inductor, which likely are the reason for the 21.4 nH stray inductance. Inductor-FET-freewheeling diode --> All close together.

 

[betwixt]

Which brings me back to the question; there is current galore (3A spikes) going through the diode, but the voltage remains on the other side, why?

I'm still puzzled by what you are trying to achieve. It seems your circuit has no output and all you are doing is pulsing current through the coil then wondering why it shows a damped oscillation. The diode is connected directly across the coil and with the exception of it's turn-on time just clamps one polarity. Also note that the absolute maximum VDS of that MOSFET is 12V which you are exceeding. The layout is probably responsible for the 23MHz ringing, mounting the MOSFET on a small PCB and running wires to it is asking for trouble.

What drives the gate? We can't see the type when it is on it's edge. 820KHz is going to need quite a lot of gate drive power.

I also find it strange that a small air cored coil like that has 3 Ohms resistance - should be much less than that and almost certainly needs more inductance to be efficient.

Brian.

 

[FvM]

Besides the dubious circuit purpose, it's quite difficult to read the experiment results. I don't see a single waveform with a clear description which voltage or current has been actually probed.

 

[dick_freebird]

Dick, by "snubber" do you mean Resistor-Capacitor-Diode (RCD) Snubber? If yes, it still has to go through a diode, so the problem would remain.
View attachment 143735

RC or RCD. "The problem" may or may not pertain to the
presence of the diode but rather its attributes (like, is
Rs anything like low enough to help?) or the attributes
of its surroundings (series inductance in leads and traces).

 

[cascoder]

Hello righteous,

The ringing is indeed caused by parasitic components associated with the physical realisation of this circuit, namely L and C that are not shown in the circuit schematic, but which exist in reality due to various factors. May I make 4 points that may help reduce the ringing effects, with reference to the sketch below (the quality of which I aplogise for, but a pencil and paper is all that I had).

1. Minimise the stray inductance contained in the loop enclosed by these three components: (a) the capacitor (b) the MOSFET (c) the diode. This is done by keeping component lead lengths short and minimizing the **area** of the circuit.

2. To reduce the parasitic L of the capacitor, connect a number of capacitors (of smaller value) in parallel rather than use one capacitor of larger value.

3. The length and the layout of the wires that connect the inductor to the circuit is not that important. The coil is already an inductor, so adding more parasitic L via longer wires is not going to contribute to the ringing. Use this fact to help acheive goals 1 and 2 above. The same can be said for the connection to the power supply, however if that voltage source is connected via significant parastic L then ensure there is plenty of C to "hold-up" the voltage of the loop mentioned in 1.

4. The connection of the gate driver to the MOSFET gate is important. In particular, the connection to the source terminal must be done as close to the component as possible to minimise including any L that carries the load current within the gate circuit. The gate drive should be low impedance, so standard circuit layout techniques should be applied such as (a) gate drive circuit to have good power suppy de-coupling capacitors, (b) small loop area from the de-coupling caps to the gate driver to the MOSFET (use twisted wires if necessary).

Even after all these actions have been applied, there still may be ringing, in which case standard snubbers (clamping type, and ramp-limiting type) may need to be applied, depending on what your final application is.

Hope this helps.

I just tried to load the image, but I am not sure it worked. If it did not, then I will try to load the image in my next post.