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Forum Index : Electronics : 150V 45A MPPT - roll your own

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poida

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Joined: 02/02/2017
Location: Australia
Posts: 603
Posted: 04:29am 06 Apr 2020
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ta, Mike.

I have found a nice solution for surpression of oscillation.

All tests with 5A output at 15V, 30V in.
Dark Blue is gate drive for the switch, Yellow is voltage at the

I tried a large bead on the MOSFET switch drain, nothing on diode.



no bead on the switch, nothing on the diode.



no bead on switch and a synchronous rectifier instead of diode.



no bead on switch, a small bead on cathode of diode
(as suggested by Wiseguy. Mike, please check if this is so?)
It seems to help a bit...



finally, no bead on switch, a small bead on the diode ANODE or source pin.



This last configuration is the best I have seen so far. How will it go when the current levels are 10x ? That is the question now.

I tried a synchronous rectifier with the bead on the (now) MOSFET source pin
and it looks nearly as good.



Efficiency:
no bead 92.9%
bead on anode 92.3%
sync rect plus source bead 94.3%


nickskethisniks:
the small bead is again of unknown provenience. LCR meter at 10kHz with short thick copper wire as in last photo has it at 0.67uH - 0.42uH (offset of the test leads)
5 small beads on the wire take it to 1.72uH - 0.42 offset or about 0.26uH

And with this I find it now Beer O'Clock.
Edited 2020-04-06 14:33 by poida
wronger than a phone book full of wrong phone numbers
 
renewableMark

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Joined: 09/12/2017
Location: Australia
Posts: 1465
Posted: 05:50am 06 Apr 2020
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these are the large beads
Cheers Caveman Mark
Off grid eastern Melb
 
poida

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Joined: 02/02/2017
Location: Australia
Posts: 603
Posted: 06:16am 06 Apr 2020
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  renewableMark said  these are the large beads

thanks Mark.
wronger than a phone book full of wrong phone numbers
 
Solar Mike
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Joined: 08/02/2015
Location: New Zealand
Posts: 582
Posted: 11:51am 06 Apr 2020
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Interesting displays there Poida



One has to ask, is it the drain oscillations feeding back into the gate via miller capacitance or the gate parasitic's causing oscillation and thus affecting the drain output.

You could measure that nice high freq oscillation and use an RC snubber from the formula in that PDF article to dampen it out.

Or see if the gate is causing it by increasing the gate drive resistance by several ohms, I would start at say 2r and increase to 10r in 1r steps, see what happens, if not much change place your bead on the gate drive mosfet pin; and make sure your scope probe earth lead is replaced by a short length of copper wire, its easy to get misleading measurements.

Cheers
Mike
 
poida

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Joined: 02/02/2017
Location: Australia
Posts: 603
Posted: 12:30pm 06 Apr 2020
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I am afraid I am in no way ready to perform further experiments with buck converters for the evening. I have had beer. I am enjoying Walter Lewin's 8.02 physics lectures he produced for MIT Boston 15 years ago. time for some R&R as it were. 10:30pm and  need to be up at 6:30am.

In a coupla days I will do what you want.

When you ask is it Miller capacitance driving things, this is something I have always wondered. There are strong forces at work in a MOSFET when we drive the gate from 0 to 15V in 1us. non-zero capacitance of the 3 pins... a nightmare.
And the Vds chage rate induces voltage changes in the gate. And more.

As an aside, the above post with 6 DSO captures used identical measurement instrumentation. I altered nothing, even down to avoiding moving the two unsheilded wires going to the differential probe inputs. Common mode interference, etc. is a major worry and I think if I just accept there will be some, then just make one change and compare the difference. This should give me something to think about.

The above tests use an IR12844 driver, into 4.7 Ohms.
I will change it in 5R steps I think, going 1R at a time is too slow for me.

I am well aware of scope earth lead inductance. With zero change in probe lead position, you can see profound changes in the switch/inductor node voltage in the above 6 captures. Probably I will investigate the health of the converter when built on the PCB. Then I will use good probe tech. Now I only want to get a feel for what might be good to build.
wronger than a phone book full of wrong phone numbers
 
Solar Mike
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Joined: 08/02/2015
Location: New Zealand
Posts: 582
Posted: 01:26am 07 Apr 2020
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  poida said  ... When you ask is it Miller capacitance driving things, this is something I have always wondered. There are strong forces at work in a MOSFET when we drive the gate from 0 to 15V in 1us. non-zero capacitance of the 3 pins... a nightmare.
And the Vds chage rate induces voltage changes in the gate. And more.


Good one Poida, beer is definitely required, I'm just about to bottle a 23L brew of Irish Stout.
Here is a dia. of how the drain capacitance affects the gate drive, IGBT shown, but same for Mosfet.



keep the driver 21844 output to 10mm of your mosfet if possible, this reflects a lower impedance across the gate/source and helps prevent the gate voltage rising to turn on levels during any oscillation; Prob not possible without a final pcb design.

Cheers
Mike
 
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