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OverUnity Research > Benches > Itsu (Moderator: Itsu) > Jegs "HV Push Pull by Jeg" replication.
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Author Topic: Jegs "HV Push Pull by Jeg" replication.  (Read 9978 times)
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Re: Jegs "HV Push Pull by Jeg" replication. « Reply #75 on: 2023-03-31, 21:13:57 »

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To be sure that the breadboard does not add some parasitic capacitance on one side of the gate / MOSFET, i put this Jeg circuit on a PCB and made it as symmetrical as possible see the picture:


I now use some HV (1700V) MOSFETs G3R450MT17D's, but there still is asymmetry in the voltages across the Drain / Sources of the both MOSFETs, see screenshot:



Itsu
   
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Re: Jegs "HV Push Pull by Jeg" replication. « Reply #76 on: 2023-03-31, 22:40:25 »
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Hi Itsu,

From Figure 11, Page 6 in this data sheet https://genesicsemi.com/sic-mosfet/G3R450MT17D/G3R450MT17D.pdf  the output capacitance COSS varies roughly between 30 pF and 120 pF when the drain source voltage, VDS changes between zero and about 70 V.  So if you feel like using a snubber capacitor across the drain source of the MOSFET which now has the higher VDS  (white wave form) it may help achieve a better balance.  For the capacitor, perhaps an air variable capacitor would be the best to use,  the 40 V should not cause any arcing between the capacitor plates. Do this only if you agree, of course.  OF course, even a fix ceramic or any good quality capacitor (of 56pF or 72pF etc) would do instead of a variable cap.

Thanks for sharing.

Gyula 
   
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Re: Jegs "HV Push Pull by Jeg" replication. « Reply #77 on: 2023-04-01, 17:44:40 »

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Gyula,

Without any snubber caps, i have 41.8Vpp across the left (highest voltage) MOSFET and 9.1Vpp across the other MOSFET (40V input).

When using a 6-100pF trimmer cap across the left (highest voltage) MOSFET Drain - Source, while scoping the voltage across its DS i did not see any noticeable difference, but when moving the probe to the other MOSFET DS, i could increase the voltage from 9.1 to 14.9V when turning the trimmer cap. to max.

I added then some ceramic caps to this trimmer cap and saw the change in voltages, see table below.

Finally i used a fixed 2nF ceramic cap across the left MOSFET instead of the trimmer cap, and its voltage dropped to 35.4V, while the voltage across the other MOSFET was increased to 34.2V.

Not what i expected as what i would expect is to have 20V across each MOSFET when balanced.

I do notice some rounded off corners on the left MOSFET with the 2nF cap, see screenshot (yellow trace, White is the original trace without any caps).

So it seems that the snubber caps are doing something, but not what we want, here is a table of the results:

Cap value |Left MOSFET|  Right MOSFET
----------|------------|---------------
   0pF      |    41.8V     |   9.1V
100pF      |    41.2V     |  14.9V
200pF      |    39.5V     |  18.0V
570pF      |    38.5V     |  25.6V
   2nF      |    35.4V     |  34.2V   with rounded off corners visible.


Itsu
   
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Re: Jegs "HV Push Pull by Jeg" replication. « Reply #78 on: 2023-04-01, 21:19:02 »
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Hi Itsu,

Thanks for the tests.

I find quasi similar behaviour in simulation.  When the voltage across the top fet's DS was 91V,  the voltage across the bottom fet's DS was 207V.  By adding a 150 pF snubber cap across the bottom fet, the voltages changed to 99V and 201V respectively. And when I used 1.3 nF snubber, both voltages changed to 150 V, the waveform nicely covered each other.  Supply voltage was 300V.

When I reduced the supply voltage to 40V for the same circxuit, the voltage across the top fet's DS was 22.5V and 17.5V across the bottom fet's DS. A 700 pF snubber cap (instead of the 1.3 nF) equlized the two voltages to 20V.  See the picture attached on the latter simulation.

Gyula
   
img  Itsu
Re: Jegs "HV Push Pull by Jeg" replication. « Reply #79 on: 2023-04-01, 21:40:31 »

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Gyula,

i do not call that a quasi similar behavior as you can balance the voltages to have half of the supply voltage over each MOSFET (20V at 40V input), but my bench does show equalization, but not to half the voltages, as i have 35V / 34V across the both MOSFETs, but with 40V input.

Itsu
   
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Re: Jegs "HV Push Pull by Jeg" replication. « Reply #80 on: 2023-04-01, 21:51:38 »
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Sorry, you are right.  More study is needed. 

Gyula
   
img  Jeg
Re: Jegs "HV Push Pull by Jeg" replication. « Reply #81 on: 2023-04-02, 16:14:11 »
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Itsu, i did some quick tests in this direction.
V=48 V dc
15Khz
Load: 42V 40W incandescent bulb.

At 50 % duty cycle the upper mosfet is at 46V, and the lower mosfet 2V!!!
As long as i decrease the duty cycle the difference starts to decrease. At 39% we have equality in voltage. As i go lower than 39 % the voltage at the lower mosfet starts to increase its voltage level at higher values than the upper. Even if i didn't try to change frequency it is obvious that the analogy will also be affected.

Thanks for pointing out this issue.

Ps. There is a technique which introduces a small delay to one of the two gate pulses and equalise voltages. It is something like a small capacitor between gate and source of one of the mosfets. Normally the one that takes the higher voltage across it.

« Last Edit: 2023-04-02, 18:53:12 by Jeg »
   
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Re: Jegs "HV Push Pull by Jeg" replication. « Reply #82 on: 2023-04-02, 19:46:55 »

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Jeg,

thanks for doing these tests, so it confirms what i see here.

But i do not have the outcome you describe, when changing the duty cycle from 50% to 10%, there is hardly any difference in the 2 voltages, in my case it stays around 41Vpp / 9Vpp with 40V input.

So any type of MOSFET could have its own behavior.

Itsu
   
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Re: Jegs "HV Push Pull by Jeg" replication. « Reply #83 on: 2023-04-02, 19:50:24 »

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I did a test using my DMM in DC voltage mode, I had the FG set to 10kHz @ 50% duty cycle, input 41.5V.

With no snubber caps installed:
Left MOSFET   15V   (Scope shows 42Vpp)
Right MOSFET  4.8V (scope shows 10.9Vpp)

With a 2nF snubber cap installed on the left MOSFET:
Left MOSFET   3.6V??!!   (Scope shows 35Vpp)
Right MOSFET  17V         (scope shows 35Vpp)


So also the DMM shows a difference in the no snubber situation, like my infrared temperature meter, Left MOSFET 22 degrees C,  right MOSFET 17 degrees C.   (17 degrees is room temp.).
Why the DMM shows 3.6V with the 2nF cap across it I don’t know, but the infrared meter still shows same temperatures across both MOSFETs 22 / 17 degrees C).

Itsu
   
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Re: Jegs "HV Push Pull by Jeg" replication. « Reply #84 on: 2023-04-02, 20:24:10 »
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Itsu normally it can also be done without capacitor but by varying the resistances between gate and source.
This will affect again the charching time of the gate input capacitor.
   
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Re: Jegs "HV Push Pull by Jeg" replication. « Reply #85 on: 2023-04-02, 21:07:26 »

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Jeg,

lets forget the solution for a moment, i made some further tests and have some questions.

When you say: "At 50 % duty cycle the upper mosfet is at 46V, and the lower mosfet 2V!!!" how did you measure that? (DMM or scope and if with the scope what was it p2p, mean or rms?)

You have a 42V 40W bulb as load, so at 48V input, what is the voltage drop across this bulb?

Thanks,  Itsu
   
img  Jeg
Re: Jegs "HV Push Pull by Jeg" replication. « Reply #86 on: 2023-04-02, 22:51:01 »
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Itsu i am talking about waveforms like yours above. 46v is the peak level. I used an oscilloscope, but by using the same reference for both of my probes. And this is the source of the lower mosfet. So when both voltages are equal then my upper probe shows exautly the double the voltage of the lower mosfet.
   
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Re: Jegs "HV Push Pull by Jeg" replication. « Reply #87 on: 2023-04-03, 21:20:21 »

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Thanks Jeg.

I see that with every change in supply voltage, frequency and duty cycle the voltages across the both MOSFETs vary, so i will be not easy to come to a stable situation across these variables.

Itsu
   
img  Jeg
Re: Jegs "HV Push Pull by Jeg" replication. « Reply #88 on: 2023-04-04, 08:53:34 »
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Quote from: Itsu on 2023-04-03, 21:20:21
Thanks Jeg.

I see that with every change in supply voltage, frequency and duty cycle the voltages across the both MOSFETs vary, so i will be not easy to come to a stable situation across these variables.

Itsu

Yes Itsu it seems that it needs to be calibrated on its final position/placement with fixed values of frequency duty and voltage. Not that bad for such a cheap mosfet switch. 
   
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Re: Jegs "HV Push Pull by Jeg" replication. « Reply #89 on: 2023-04-04, 08:59:46 »

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Jeg,

that's true, but in an experimental environment like we have where we need to vary those parameters on the fly to search for optimal results in our circuits, its not so good.

Itsu
   
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Re: Jegs "HV Push Pull by Jeg" replication. « Reply #90 on: 2023-04-08, 11:32:58 »

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Gyula pointed me to this article: http://www.power-mag.com/pdf/feature_pdf/1260811353_CT-Concept_PEE_0809.pdf in where is described some active clamping methods for stabilizing IGBT/MOSFETs.

It turns out that the gate clamping method shown in Fig. 4 also works to stabilize / balance the drain - source voltages of our series MOSFETs.

I build the below circuit, and it works fine in balancing the 40V input across the load and both MOSFETs:



It stays balanced across a wide range of frequency, duty cycle and input voltage settings.

Care should be taken to not let the zener current increase too much leading to overheating etc. and to not let the Gate - source voltage to increase above the MOSFET specifications.

The 10K resistor in series with the zener can be adjusted accordingly.

As shown in the diagram, the zener / diode combination can be changed out for a bidirectional TVS diode (Transient Voltage Suppression) like the shown 1.5KE18CA.

I am now building a HV edition of this circuit using 2 C2M0045170D MOSFETs (1700V) and some 1.5KE440CA (440V) TVS diodes to see if it can switch the 3kV from my source.

Regards Itsu
   
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Re: Jegs "HV Push Pull by Jeg" replication. « Reply #91 on: 2023-04-26, 21:53:28 »

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I build Lee his switching board as presented here:  https://www.overunityresearch.com/index.php?topic=4459.msg104430#msg104430

I compared this switching unit with another older one which i was using up till now, see picture:



But when i saw the signal when the MOSFET closes i was kind of disappointed.

here is a screenshot of a 10kHz 10% duty cycle signal taken from both circuits:



Yellow signal is Lee his circuit (using the 1700V MOSFET) and white signal the older used switching board (also a 1700V MOSFET).
Load is a 470 Ohm resistor at 41V.

I use 3.9 Ohm resistors as source / sink resistors in Lee his circuit.

I will try to use a short for the sink resistor to see if this improves the turn-off time.


Itsu
« Last Edit: 2023-04-27, 15:26:24 by Itsu »
   
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Re: Jegs "HV Push Pull by Jeg" replication. « Reply #92 on: 2023-04-27, 15:28:21 »

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Changing the sink resistor R3 for a short does not change the turn-off signal.

Itsu
   
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Re: Jegs "HV Push Pull by Jeg" replication. « Reply #93 on: 2023-04-28, 19:48:26 »

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I made some further tests, especially to test the gate driver signals, but these turn out to be OK.

So finally i changed the used 1700V MSC035SMA170B4 (TO-247-4 layout) MOSFET for a CREE 1700V C2M0045170D (TO-247-3 layout) MOSFET.

I had to modify the board / MOSFET connections due to the layout differences, but then the turn-off time was much better, see screenshot:



Yellow is the CREE drain source signal and blue the gate signal.


I cannot imagine i had a damaged MOSFET as i hardly used it, so i ask Lee if he can do the same test as i did by switching a 470 Ohm resistor load with 40 or so volt at a frequency of 10kHz and at 10% duty cycle and look at the turn-off time.
Does it have the same 1us turn-off time i had as shown in this (yellow) trace:



Thanks,   regards Itsu
   
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Re: Jegs "HV Push Pull by Jeg" replication. « Reply #94 on: 2023-04-28, 20:39:25 »
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Hi Itsu,

Sorry for the late reply to your original post about slow turn off time for the MOSFET. I only just saw it and your subsequent post. For some reason this thread wasn't highlighted as having new content on the main page.
It's certainly very peculiar that it takes so long to turn off, especially when you have shown that the Cree turns off much quicker. The MSC035SMA170B4 has a gate charge of 178nC and the C2M0045170D has a gate charge of 200nC, so if anything I'd expect the C2M0045170D to turn off slower because there is more charge to discharge.

In my experience, a MOSFET turning off slowly is due to the charge on the gate not being discharged quick enough. This would usually be caused by a high resistance between the gate and the gate driver. Reducing this resistance should improve the gate discharge time, at the expense of some ringing. Having said that, you are using 3.9ohm resistors which is what I'm also using.

What modification did you make to mount the 3 pin Cree MOSFET? Can you please post a photo?

I have some boards built with the MSC035SMA170B4, so I'll run a test tomorrow. I don't have a 470ohm load resistor but I do have a 500ohm load resistor. Is that close enough?
Best regards,
Lee
   
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Re: Jegs "HV Push Pull by Jeg" replication. « Reply #95 on: 2023-04-28, 20:42:10 »

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I used this hardly used 1700V MSC035SMA170B4 MOSFET in my old switching board and also here i have this long (1us) turn-off time:
Yellow DS signal, blue gate signal.



So either my 1700V MSC035SMA170B4 is damaged or this MOSFET is working as normal this way.

Itsu
   
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Re: Jegs "HV Push Pull by Jeg" replication. « Reply #96 on: 2023-04-28, 20:49:54 »

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Quote from: lfarrand on 2023-04-28, 20:39:25
Hi Itsu,

Sorry for the late reply to your original post about slow turn off time for the MOSFET. I only just saw it and your subsequent post. For some reason this thread wasn't highlighted as having new content on the main page.
It's certainly very peculiar that it takes so long to turn off, especially when you have shown that the Cree turns off much quicker. The MSC035SMA170B4 has a gate charge of 178nC and the C2M0045170D has a gate charge of 200nC, so if anything I'd expect the C2M0045170D to turn off slower because there is more charge to discharge.

In my experience, a MOSFET turning off slowly is due to the charge on the gate not being discharged quick enough. This would usually be caused by a high resistance between the gate and the gate driver. Reducing this resistance should improve the gate discharge time, at the expense of some ringing. Having said that, you are using 3.9ohm resistors which is what I'm also using.

What modification did you make to mount the 3 pin Cree MOSFET? Can you please post a photo?

I have some boards built with the MSC035SMA170B4, so I'll run a test tomorrow. I don't have a 470ohm load resistor but I do have a 500ohm load resistor. Is that close enough?
Best regards,
Lee

Hi lee,

the 500 Ohm will be fine.

Here a picture of my mod, it is a simple cross-over between the pins and board.

By the way, for R3 i used a 0 Ohm (short, see picture) resistor, but that did not make any difference with the earlier used 3.9Ohm resistor.

Itsu
   
img  lfarrand
Re: Jegs "HV Push Pull by Jeg" replication. « Reply #97 on: 2023-04-28, 21:32:14 »
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Well at least the MOSFET is consistent in it's inability to turn off quickly! :)

If it is behaving the same way in a completely different board with different gate driver etc. then it is either the MOSFET that is damaged in some way, or this is the inherent behaviour of this MOSFET. With that said, the spec. sheet lists a 17ns turn off time, so 1 microsecond is considerably longer.

I'll run some tests in the morning and let you know my findings.
   
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Re: Jegs "HV Push Pull by Jeg" replication. « Reply #98 on: 2023-04-29, 10:42:46 »
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Hi Itsu,

I ran the test. I placed a 500ohm load resistor across the switch and powered it with 40V from my bench power supply.

The results were..interesting! They seem to confirm your finding about slowness, although I see it as being slow to turn off rather than slow to turn on.

The pink trace is from my signal gen, and the yellow trace is a probe placed across the load resistor.
   
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Re: Jegs "HV Push Pull by Jeg" replication. « Reply #99 on: 2023-04-29, 11:57:48 »

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Hi Lee,

well, that confirms that this type of MOSFET has a slow turn-off time (as that was slow with me too all the time) only it is inverted by you because you measure across the resistor while i measured across the drain - source.

Perhaps it's only slow at this relative low drain voltage as this type of MOSFET is designed to handle HV (1700V).

I will see if i can make a measurement with those kinds of voltages.

Thanks for your test.

Regards Itsu
   
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