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OverUnity Research > Benches > Itsu (Moderator: Itsu) > Jegs "HV Push Pull by Jeg" replication.
2024-04-18, 03:07:03
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Author Topic: Jegs "HV Push Pull by Jeg" replication.  (Read 7491 times)
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Re: Jegs "HV Push Pull by Jeg" replication. « Reply #50 on: 2023-03-27, 13:55:00 »
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Quote from: gyula on 2023-03-26, 16:47:43

very likely the use of the MOVs (or other components) to help the HV distribute more or less equally between the series connected fets

Yes indeed. I use a 10 Mohm resistance between source and drain for each of the two mosfets.
   
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Re: Jegs "HV Push Pull by Jeg" replication. « Reply #51 on: 2023-03-27, 14:16:25 »
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Hi Jeg,

I forgot you did include the 10 MOhm resistors in your circuit schema here https://www.overunityresearch.com/index.php?topic=4459.msg104395#msg104395   

Regarding the bidirectional circuit topology I had suggested to Itsu earlier,  we should not use it here.   

Gyula


Quote from: Jeg on 2023-03-27, 13:55:00
Yes indeed. I use a 10 Mohm resistance between source and drain for each of the two mosfets.
   
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Re: Jegs "HV Push Pull by Jeg" replication. « Reply #52 on: 2023-03-27, 16:48:54 »

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Jeg, did you measure the drain source voltage across your MOSFETs?

If i do that i get instable readings, depending on what MOSFET i use in what position (see the 1st screenshot here: https://www.overunityresearch.com/index.php?topic=4459.msg104456#msg104456)

I have this breadboard setup now, so i have the 10M resistors across the MOSFETs:



But the 40V across the load (bulb) is either on the top MOSFET or, if i change over the both MOSFETs, on the bottom MOSFET.
It seems there is some preference for a certain MOSFET to switch, and then it takes it all (almost like 80 / 20 % across the MOSFETs).


Itsu
« Last Edit: 2023-03-27, 21:11:08 by Itsu »
   
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Re: Jegs "HV Push Pull by Jeg" replication. « Reply #53 on: 2023-03-28, 19:51:10 »
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Hi Itsu
How do you scope your two drains? Try to connect your ground clip to the source of the lower switch and probe both drains with that same reference. The upper's drain normally would be the double of the voltage than that measured on the lower drain.

Also check your resistances if are indeed the same.
   
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Re: Jegs "HV Push Pull by Jeg" replication. « Reply #54 on: 2023-03-28, 20:47:48 »

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Jeg,

If i follow that, i have 40V on the upper MOSFET drain and 40V on the lower MOSFET drain (reference is the lower MOSFET source).

The way i measured it earlier, is scoping each MOSFET drain - source separate.

Make sure you have no ground connected to the circuit, that way you can use ONE (1) probe of your scope to scope any component, like the both MOSFET drain - source.

Itsu
   
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Re: Jegs "HV Push Pull by Jeg" replication. « Reply #55 on: 2023-03-28, 21:14:35 »

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If i make a LTspice simulation of the Jeg setup, then it works as expected (each MOSFET takes half the voltage), see the below simulation output.  (.asc file attached).

I use IRF840 MOSFETs (500V) at 300V load supply.

Itsu
   
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Re: Jegs "HV Push Pull by Jeg" replication. « Reply #56 on: 2023-03-28, 21:34:26 »

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What i did to find the problem is to swap some connections / components.

The problem moves with the MOSFET, so if the bottom MOSFET takes all the voltage, i swap it with the top MOSFET and now the top MOSFET takes all the voltage.

When swapping the gate toroid coil connections, the problem stays at the same  MOSFET.
Also swapping the 1uF cap, 1.8 Ohm and diodes between top and bottom secondary circuits keeps the problem at the same MOSFET.

So it seems to me that the problem is within the MOSFET, some parameters are different and causing this inbalans.
I tried different MOSFET pairs (IRF540, IRF840), but both type have the same problem.

Itsu
   
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Re: Jegs "HV Push Pull by Jeg" replication. « Reply #57 on: 2023-03-28, 22:03:44 »
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Hi Itsu,

Yes, there are no MOSFETs with identical parameters.  If you use say small value source resistors in series with the source pins, and the difference between them is only 0.01 Ohm, the drain-source voltages will already differ.
See the simulation attached (asc file is below it).
If you set the two resistors equal, or simply short circuit them, the drain source voltages will nicely overlay. 

The difference in the MOSFETs even within the same type or batch can cause easily what you have found in practice. 
Or If you use say IRF840LC type for the bottom fet in the simulator for instance its drain voltage will drastically get reduced to a few volts.

Gyula
   
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Re: Jegs "HV Push Pull by Jeg" replication. « Reply #58 on: 2023-03-29, 09:34:04 »

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Gyula,

thanks, i thought so, so that is why i asked Jeg to measure his drain - source voltages, because he could have the same problem.

I changed the "Ron" parameter on one IRF840 in the .Model file in the "LTspiceXVII/lib/cmp/M standard.mos" file from 850m to 500m:


.model IRF840 NMOS(Level=3 Gamma=0 Delta=0 Eta=0 Theta=0 Kappa=0.2 Vmax=0 Xj=0 Tox=100n Uo=600 Phi=.6 Rs=6.382m Kp=20.85u W=.68 L=2u Vto=3.879 Rd=.6703 Rds=2.222MEG Cbd=1.415n Pb=.8 Mj=.5 Fc=.5 Cgso=1.625n Cgdo=133.4p Rg=.6038 Is=56.03p N=1 Tt=710n mfg=International_Rectifier Vds=500 Ron=850m Qg=63n)

but that did not change the LTspice simulation outcome,  i guess that "Ron" parameter is just an indicator, not a parameter for the RDSon (Drain-source ON resistance).

Anyway, so i have to "match" 2 IRF840 MOSFETs (or any other MOSFET pair) to get it right.

I did try to use a DMM in resistance mode to measure the Ron resistance, but they both measure 0.6 Ohm which is not the specified 850mOhm RDSon.


For info: I measured the "coupling coefficient" (K) of my coils on the gate isolation toroid to be 0.99999, so very close to 1, so to have the LTspice simulation correct.


Itsu
 
   
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Re: Jegs "HV Push Pull by Jeg" replication. « Reply #59 on: 2023-03-29, 10:26:49 »
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There could be a few reasons why one of the two MOSFETs is taking more voltage than the other.

    1. Mismatched MOSFETs: The MOSFETs may not be perfectly matched in terms of their threshold voltage or on-resistance, which can cause one of the MOSFETs to conduct more than  the other.

    2. Unequal gate drive: The gate drive for one MOSFET may be stronger than the other, causing it to conduct more.

    3. Parasitic elements: Parasitic capacitance or inductance in the circuit can cause one MOSFET to turn on faster than the other, leading to an imbalance in voltage across the two MOSFETs.

Most likely is that there is a difference at ON-resistance or unequal threshold voltage.

Try replacing the MOSFET that is exhibiting the higher voltage with a new one and see if the issue is resolved. If the issue persists, then there may be an issue with the gate drive circuit or parasitic elements in the circuit that are causing the voltage imbalance.

I may have the same issue but didnt notice. It is because i use them to share the voltage of the kickback spike and so i was interesting more on what happens when switch goes OFF and not when switch goes ON as you do. I will test it though cause i am curious.
   
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Re: Jegs "HV Push Pull by Jeg" replication. « Reply #60 on: 2023-03-29, 12:24:18 »

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Thanks Jeg,

reason 2 and 3 are not in play here i think because it swapped out the gate drive parts, and it did not change the problem, also the turn on time was measured to be the same.

So indeed, probably reason 1 is the cause.

The gate voltages are above the MOSFET gate threshold level, so they should not be in the linear region, but fully on, so that leaves indeed the RDSon values.

I changed several MOSFETs, but they all show similar behavior up till now.

Itsu
   
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Re: Jegs "HV Push Pull by Jeg" replication. « Reply #61 on: 2023-03-29, 12:34:33 »

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As suggested by Gyula, here i use 2 different MOSFETs, bottom the normal IRF840, and top the IRF840LC which has some different parameters.
The effect is the same as i see on my bench circuit, 1 MOSFETs takes 80% of the voltage, the other 20% or so when measuring across each MOSFET drain - source.

Itsu
   
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Re: Jegs "HV Push Pull by Jeg" replication. « Reply #62 on: 2023-03-29, 13:38:18 »
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Hi Itsu,

Here are the MOSFET model parameters listed which LTSpice and in general Spice based simulators use:
http://www.simonbramble.co.uk/lt_spice/ltspice_lt_spice_tutorial_6.htm 

It turns out that LTSpice ignores the Ron parameter...   >:(

At the bottom of the link there is an Rdson test jig asc circuit file to play with gate bias and drain current and display ON resistance for a MOSFET model. It mentions the Kp parameter which is the transconductance (in Siemens), this shows how the drain current changes in the function of the gate-source control voltage.

But the important thing would be to match the MOSFETs in practice of course. Data sheets specify Rdson at given bias test conditions. And even so, the individual values can differ from it as we find.

You wrote: "I did try to use a DMM in resistance mode to measure the Ron resistance, but they both measure 0.6 Ohm which is not the specified 850mOhm RDSon."

 Try to bias the gate source by a PS for say 15 VDC first and then remove it. The Cgs gate source capacitance will keep this voltage for a certain time and during this short time use the Ohm meter to see the Rds resistance. (Connect the positive lead of the Ohm meter to the drain and the negative to the source.)  Sorry if you already did it so.
And then do the same with the other MOSFET(s) within the same type.   The gate-source charge will slowly discharge as the Ohmmeter imposes its own measuring current (it will be the drain-source current).

Gyula
   
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Re: Jegs "HV Push Pull by Jeg" replication. « Reply #63 on: 2023-03-29, 16:24:42 »

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Thanks Gyula,

so the "Ron" parameter is indeed ignored, just for aiding the user to compare MOSFETs.

The RDSon test jig runs OK, 700.05mOhm  for the IRF840, and 680.75mOhm for the IRF840LC (10V on gate / 4.8A  through drain), so this 20mOhm difference causes the 80/20% drain voltage difference seen.


Concerning measuring the RDSon, yes i used that method to measure the RDSon, but it seems inaccurate this way (both measure 0.6 Ohm).
I guess one has to be lucky to have 2 MOSFETs that match.

Itsu
   
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Re: Jegs "HV Push Pull by Jeg" replication. « Reply #64 on: 2023-03-29, 16:32:17 »
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Addition:

I find that the transconductance parameter Kp is the main factor in simulation. If we choose for instance MOSFET type IRF840LC for the top fet (Kp=6.626) and the IRFBC40LC for the bottom fet (Kp=4.96), these are relatively close to each other.
It is known that the use of a source resistor reduces transconductance (it gives negative feedback) and indeed adding 0.05 Ohm resistor in series with the source of the IRF840LC, we can get the desired response, see attachment. Without the resistor, the blue trace amplitude goes up to 210 V peak voltage from the desired 150 V.

Note that such "solution" is valid for a given average drain current the R3=100 Ohm load in this simulated circuit establishes from the 300 VDC supply. Changing the supply voltage and / or the load will change the common drain current and the drain source voltages will also change.

So the use of a series source resistor for the MOSFET having the higher transconductance to match it to the other MOSFET is not the best solution, may serve as a secondary solution.

Gyula
   
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Re: Jegs "HV Push Pull by Jeg" replication. « Reply #65 on: 2023-03-29, 16:42:57 »
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Well, the main problem is that RDSon does not really change linearly, it depends mainly on the actual average drain current, and this involves the actual supply voltage and load resistance.  So it is not directly the 20 mOhm difference that causes the problem.   The RDSon resistance of the two series MOSFETs make a voltage divider and these resistors change their actual value as the drain current changes by the duty cycle, by the supply voltage and by the load. But as the simulations show the transconductance is the main parameter, the IRF840 has a Kp of 11.192.

Gyula

Quote from: Itsu on 2023-03-29, 16:24:42
Thanks Gyula,

so the "Ron" parameter is indeed ignored, just for aiding the user to compare MOSFETs.

The RDSon test jig runs OK, 700.05mOhm  for the IRF840, and 680.75mOhm for the IRF840LC (10V on gate / 4.8A  through drain), so this 20mOhm difference causes the 80/20% drain voltage difference seen.


Concerning measuring the RDSon, yes i used that method to measure the RDSon, but it seems inaccurate this way (both measure 0.6 Ohm).
I guess one has to be lucky to have 2 MOSFETs that match.

Itsu
   
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Re: Jegs "HV Push Pull by Jeg" replication. « Reply #66 on: 2023-03-29, 19:36:30 »
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Itsu,

Perhaps you might want to look at techniques for balancing series connected mosfets via snubber capacitors.  See attached pdf for one example .

Pm
   
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Re: Jegs "HV Push Pull by Jeg" replication. « Reply #67 on: 2023-03-29, 20:30:52 »
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Hi Pm,

Thanks for this paper, it offers a clever idea and simple solution. 
In the hardware configuration Figure 5, diodes are shown between the fets at each source pin and the text does not mention about them. Do you know why they are used?
(I assume they block possible switching spikes.)

Gyula
   
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Re: Jegs "HV Push Pull by Jeg" replication. « Reply #68 on: 2023-03-29, 23:34:45 »
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Quote from: gyula on 2023-03-29, 20:30:52
Hi Pm,

Thanks for this paper, it offers a clever idea and simple solution. 
In the hardware configuration Figure 5, diodes are shown between the fets at each source pin and the text does not mention about them. Do you know why they are used?
(I assume they block possible switching spikes.)

Gyula

Hi Gyula,

After looking again at the paper, I'm not sure about their function!  There are a number of papers out there using the same technique and some on IEEE but my membership has lapsed.

Pm
   
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Re: Jegs "HV Push Pull by Jeg" replication. « Reply #69 on: 2023-03-30, 07:24:53 »
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Quote from: Itsu on 2023-03-29, 16:24:42

I guess one has to be lucky to have 2 MOSFETs that match.

Itsu

Hey Itsu,

I'm unsure of the power demands of your circuit but seeing that you're trying to operate a 3KV switch, you might be able to get away with a lower current FET with a higher blocking voltage. 

Here is one that might work:
IXTF1R4N450 --- MOSFET N-CH 4500V 1.4A
https://www.digikey.com/en/products/detail/ixys/IXTF1R4N450/6109391


This is the one that I personally used to make a gate isolated, HV full bridge circuit for various HV experiments:
IXTF02N450 --- MOSFET N-CH 4500V 200MA
https://www.digikey.com/en/products/detail/ixys/ixtf02n450/3737625

Take Care,

Dave
   
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Re: Jegs "HV Push Pull by Jeg" replication. « Reply #70 on: 2023-03-30, 09:21:03 »

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Quote from: partzman on 2023-03-29, 19:36:30
Itsu,

Perhaps you might want to look at techniques for balancing series connected mosfets via snubber capacitors.  See attached pdf for one example .

Pm

Partzman,

thanks for the paper, i looked at it, and could be worth to do something with those caps, but the paper is not so detailed i think.

Itsu
   
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Re: Jegs "HV Push Pull by Jeg" replication. « Reply #71 on: 2023-03-30, 09:24:46 »

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Quote from: web000x on 2023-03-30, 07:24:53
Hey Itsu,

I'm unsure of the power demands of your circuit but seeing that you're trying to operate a 3KV switch, you might be able to get away with a lower current FET with a higher blocking voltage. 

Here is one that might work:
IXTF1R4N450 --- MOSFET N-CH 4500V 1.4A
https://www.digikey.com/en/products/detail/ixys/IXTF1R4N450/6109391


This is the one that I personally used to make a gate isolated, HV full bridge circuit for various HV experiments:
IXTF02N450 --- MOSFET N-CH 4500V 200MA
https://www.digikey.com/en/products/detail/ixys/ixtf02n450/3737625

Take Care,

Dave

Hi Dave,

thanks, it looks like that would be a better solution instead of going "in series" and it was suggested before.

But i expected these HV MOSFETs to be expensive, but it turns out they are reasonably prized.

The "current" demand is not that high, so these MOSFETs you suggested could do the job i think.

Itsu 
   
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Re: Jegs "HV Push Pull by Jeg" replication. « Reply #72 on: 2023-03-30, 15:51:28 »
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Hi Pm,

Okay, thanks for the hints.  I have found some papers and among the freely available ones these two sound informative and practical:

https://ntnuopen.ntnu.no/ntnu-xmlui/bitstream/handle/11250/2472861/Peftitsis.pdf   

https://iopscience.iop.org/article/10.1088/1742-6596/2401/1/012052/pdf

Side note OF course the simplest solution would be to use a single HV switching device capable of working  directly at the needed high voltage levels.

Gyula


Quote from: partzman on 2023-03-29, 23:34:45
Hi Gyula,

After looking again at the paper, I'm not sure about their function!  There are a number of papers out there using the same technique and some on IEEE but my membership has lapsed.

Pm
   
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Re: Jegs "HV Push Pull by Jeg" replication. « Reply #73 on: 2023-03-31, 01:56:52 »
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Re: Jegs "HV Push Pull by Jeg" replication. « Reply #74 on: 2023-03-31, 09:54:27 »

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Thanks Panyuming,

a good example of a usable HV IGBT.

Itsu
   
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