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Problem with Class E Modulator




 
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GW0FZY
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« on: January 24, 2018, 04:01:47 PM »

Hi All

Wondered if I can get some advice....I'm building Steve's (WA1QIX) modulator and have been testing it. Have been using it with 4.7 Ohm load which is as near as I can get to 5 and it seems OK up too about half of the 120v HV DC into it. As I increase the voltage beyond about 65-70 v on the drains of the FETs the DC voltage output as measured on the PWM output does not go higher than about 11.8v with a current draw of about 3.5A,  further increases in the HV causes the FET driver (TC4452) to blow up taking out the FETS( IRFP260's) and the optocoupler. Up until it dies it does produce a reasonable modulated voltage at the output with a 1khz audio signal.Any ideas would be appreciated!!

73
Justin GW0FZY
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vk3alk
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« Reply #1 on: January 25, 2018, 07:26:48 AM »

Hi Justin...

The output board is that Steves KIT or a homebrew board....
U501 the LM7812 pin2 is that connected to Floating Common or to PWM ground ?


Wayne
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GW0FZY
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« Reply #2 on: January 25, 2018, 07:59:53 AM »

Hi Wayne
The PWM board is homebrewed but using Steves circuit. Yes the pin 2 of that voltage regulator goes to floating common not GND. I was looking at it this morning and 1 of the 3 FETs has gone short circuit plus the opto coupler is u/s and the TC 4452 has blown itself apart. Wondering if it a problem with the fet driver as it was not mounted right next to the gates / source of the FETs
Justin
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vk3alk
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« Reply #3 on: January 25, 2018, 08:49:01 AM »

OK its a big blowup isn't it......
You might have to go back to the basics.....
It would be nice if the TC4452 was close to the FETs but shouldn't cause the problems your having...
The waveform...is that nice and square ?
I would only use one IRFP260 ( if you have any left ) ....
Apply only appox 30 volts to the drain and see if you can change the mark space ratio....it should easily go from 50:50  to 80:20 ....so the PWM output should read 15 volts down to 5 volts or so....
Check the driver voltage is 12 volts between Vcc pin and its ground pin....
Perhaps for whatever reason the gate voltage has exceeded 20 volts blowing the gate junction etc:
Noticed your still online so will post this now...

Wayne
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GW0FZY
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« Reply #4 on: January 25, 2018, 09:40:35 AM »

Thanks for those pointers Wayne. Iíll take a look at it again tonite. Solid state modulators are a bit new for me, bit of a dark art; used to working with tubes and mod trannys!!!
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vk3alk
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« Reply #5 on: January 25, 2018, 04:38:33 PM »

If the TC4452 was blown apart like that it would take a high energy source and that can only be from the FET area...
The IRFP260 were they purchased from a reliable source.....have you just bought them...maybe off ebay china stores.....
Also the insulation washes between the FET and the heatsink ... are they second hand or brand new...
You could look in your junk box and see if you have any old FETs like IRFP450/460 and use those for testing etc:
Just some thoughts and also don't connect the ground lead of your CRO probes to common ground unless the CRO is battery powered...


Wayne
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GW0FZY
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« Reply #6 on: January 26, 2018, 01:43:39 AM »

Wayne,

OK, got this going again last night.Possible cause of the problem may have been that both GND leads on the TC4452 should have been grounded (tied  to float) as I discovered after reading the data sheet!!. Anyway, as it is now I can vary the duty cycle on the PWM gen board and get from 3-15v out with a HV DC of 40v. Gate voltage (avg) is about 16v-18v, a bit worried that if I try to increase the HV DC it's going to blow again as I can't seem to get much more than 16v out from the PWM  power output despite increasing B+
BTW all the parts came from Farnell/RS  so no junk Ebay components!!!
I suppose the logical thing is to try and increase the duty cycle even more (the pot's at its max) to try and get more volts from the output...the circuit has been built exactly as per Steve's so should work as is.
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vk3alk
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« Reply #7 on: January 26, 2018, 05:32:38 AM »

Probably what I would do now is to go back to the Generator board and be confident that its working well..
I don't use Steves Generator Board anymore and cannot remember just how far you can advance the DC...
At the end of the day you should be able to achieve 50:50....there's no point going beyond that anyway as there would be no headroom for 100% positive...
I wouldn't apply any audio yet leave that to the end.....when all the rest is working ..
If the generator board is working OK advance along the circuit....
Input to the opto coupler and output and then to the drivers input/output if you get what I mean...
Finally the FETs should just follow along.....showing the same ratio....just different voltages...
A Variac controlling the voltage to the FETs is handy as you can slowly increase the voltage to them....stopping if somethings not right..
The best M/S ratio is 60:40 that is if 100 volts is on the drains of the FETs the output is 40 volts...
I will finish this post now its a bit long.......check each stage as you go along...
With all the testing of the low level stages do not apply power to the FETS....leave that to the end...

Let us know how you go....


Wayne
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vk3alk
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« Reply #8 on: January 26, 2018, 05:52:54 AM »

Just a quick one....are you using a CRO to check the waveforms ?

The voltages you state are they from the CRO...etc:....

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GW0FZY
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« Reply #9 on: January 26, 2018, 07:08:01 AM »

Thanks Wayne. Yep, thats where I was going to look at next. The voltages on the gates were measured on the scope. Anyway will let u know what happens
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vk3alk
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« Reply #10 on: January 26, 2018, 06:32:43 PM »

Just one other thing too...
For testing purposes a 4.7 Ohm Dummy Load is a little low....
Not that it matters but it would get rather hot quickly...
Mine is appox 10 Ohms....found some 100 Watt non inductive resistors on ebay and paralleled them up with fan......

Wayne
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steve_qix
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« Reply #11 on: January 28, 2018, 04:41:52 PM »

Wayne,

OK, got this going again last night.Possible cause of the problem may have been that both GND leads on the TC4452 should have been grounded (tied  to float) as I discovered after reading the data sheet!!. Anyway, as it is now I can vary the duty cycle on the PWM gen board and get from 3-15v out with a HV DC of 40v. Gate voltage (avg) is about 16v-18v, a bit worried that if I try to increase the HV DC it's going to blow again as I can't seem to get much more than 16v out from the PWM  power output despite increasing B+
BTW all the parts came from Farnell/RS  so no junk Ebay components!!!
I suppose the logical thing is to try and increase the duty cycle even more (the pot's at its max) to try and get more volts from the output...the circuit has been built exactly as per Steve's so should work as is.

There are several possibilities.  A scope picture of the PWM output signal that is being delivered to the filter (or resistor to ground if you're not using a filter yet) will be very useful.

If the PWM output does not fall to 0V when the input waveform falls to 0, then the charge pump will not function properly and you will have insufficient voltage to drive the gates of the output MOSFETs.  All sorts of nasty things will happen under those circumstances.  The DC voltage going to the charge pump input should be 18VDC under all conditions.  15V is too low and anything over about 20 or 21V is too high (the 12V regulator on the PWM output will get too warm).

The output circuit is fairly straight-forward, and there isn't a whole lot that can go wrong.

Scope pictures of waveforms will be extremely useful.    You should be able to vary the PWM duty cycle from 0% to 99% and the output voltage should follow - varying from 0V to full DC (after the filter).  The output DC must follow the duty cycle precisely.
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GW0FZY
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« Reply #12 on: January 29, 2018, 01:39:53 PM »

Hi Steve,
First let me say a big thank you for putting all your fine business info and circuit diagrams on the website. It has been very easy to follow and so far it has been fun building this rig.
I think I have more or less sorted this problem... I re-built my output board with a proper ground plane (which it didn't have before ) and mounted the optocoupler and FET driver much closer and on the same float ground plane. I had to substitute the 740l6010( which blew-up!!) for a HCPL2210 coupler as I had a load of these and they work fine (and do not need a 5v supply).So far it is behaving as expected. I'm gonna have to re-do the PWM filter as I was using a pair of T300-26 toroids which are fine at low amp levels but as the power is increased they start to allow the PWM signal to come back in.I presume that they are becoming saturated and the inductance is falling off. Luckily I've found a pair of Arnold HF300060-2's toroids on E-Bay which are the same as the CH777060's so I will re-do the filter and hopefully it all should work, fingers crossed!!I'll post some pics of the finished modulator.
73's Justin
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steve_qix
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« Reply #13 on: January 30, 2018, 12:50:22 AM »

Ok, sounds good - yeah, type 26 material is terrible for DC biased inductors such as the PWM filter.  The inductance varies tremendously with current.  Not a good core for this application.

I use 2D material quite a bit (the T300-2D core, in fact).  Works great at the kw (carrier) level very nicely.  The CH777060 is a good core for a DC (carrier) current of 8A or less.  You could stack a couple of them to handle more current.

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GW0FZY
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« Reply #14 on: January 30, 2018, 04:48:57 AM »

Steve, could you use a T300-2 rather than the double thickness 2d? I'm not looking to run anything up to a Kw carrier...
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vk3alk
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« Reply #15 on: January 30, 2018, 07:32:45 AM »

Hi Justin..

Good to see things are now all going well...
I think your power levels are similar to what ours are here in Aus...
I haven't used CH777060 cores for some time now...
I use either IE-33 ferrite cores that are used in Computer SMPS...you can buy them on ebay for US$6.00 for two sets... or RM10 cores...that are A$2.60 each from RS...
You have to gap the IE33 cores of course...I use a piece of PCB and file the copper off..
The RM cores come with gaps....
Both cores work up to appox 300Watts carrier ( with Modulation )....without any problems at all....for me anyway.......a little bit of heat glue to stop the singing thats all...
They do work and if you made another output board a little bit longer you could easily use them etc:
Can send you an email with an attachment if you like.....

Anyway maybe of interest to you....


Wayne
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GW0FZY
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« Reply #16 on: January 30, 2018, 09:28:04 AM »

Hi Wayne
Thanks for the info. My mate who works in the electronics industry gave some of these similar smps trannys to try; he reckoned they would work better than the toroids. I was nt sure so stuck with Steveís design which is proven. The RM10 cores are alot cheaper thats for sure. I doubt Iíll re-do the board again unless it blows up😱 I Ďll re-do the filter with the right toroids and hopefully should be ok


* 3A4EAF62-D7BE-48BE-8F95-578FD1E7959F.jpeg (1781.21 KB, 4032x3024 - viewed 101 times.)
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GW0FZY
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« Reply #17 on: January 30, 2018, 09:33:50 AM »

This is what the innards of the modulator looks like... yet to re-do the pwm filter


* AF358C6C-2F0A-4C61-BCE3-77DA3B4CBF2B.jpeg (2473.19 KB, 4032x3024 - viewed 127 times.)
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vk3alk
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« Reply #18 on: January 30, 2018, 07:56:22 PM »

All looks good....you should place it along side your Jet Engines as per your web site as see what generates more power  Grin

Those transformers look good...wouldn't throw them away.....
Got nice clips and bobbins as well....
There double Es so when you take them apart oneday be careful not to break them....

Have you run an audio sweep on the PWM Generator board ?
The audio filter needs to have a sharp cutoff at 5.7Khz appox........

Hope the bolt on the TC4452 is tight...on the photo looks close the the Drain Bus......but I am sure your aware of that.........

Can see your floating common on the output board too...

Wayne



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vk3alk
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« Reply #19 on: January 30, 2018, 08:11:41 PM »

Ohh I forgot to say those green PCB boards your using are very nice...
Ordered some the other day and I like them....
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steve_qix
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« Reply #20 on: January 30, 2018, 10:35:54 PM »

The Micrometals type 2 material is very stable with respect to the al value.  You definitely don't want the inductance changing with modulation !!!!

At a kW of carrier, assuming you are running 45 V at carrier, you are looking at peak currents of somewhere in the neighborhood of 70A peak.  Any filter has to be able to handle THIS value of current.

So, for the input inductor, a T300-2D works at the 1kW power level. You could stack 2 of the T300-2 cores.

For the 2nd inductor, I have stacked 2   T300-2D cores.  You can use other magnetically stable, high flux cores, but the T300-2Ds are fairly common on Ebay, and are not too expensive.

The 3rd inductor is similar to the 2nd.

Do you have the Micrometals design software?  It is free, and will show you the inductance curve over a wide current range.  The idea is to design an inductor, the inductance of which does not vary more than a few percent over the usable current range (0 to 70A).
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vk3alk
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« Reply #21 on: January 31, 2018, 01:05:34 AM »

Hi Steve

I haven't noticed any weird audio effects or aliasing at appox 300 watts...is there anything else I should look out for  Huh


Wayne
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GW0FZY
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« Reply #22 on: January 31, 2018, 03:46:06 AM »

Thanks Wayne/Steve, I had toyed with the idea of stacking 2 x T300-2ís as they are certainly alot cheaper and more abundant than T200-2dís on EBay UK.
Steve, are there any differences in using solid core wire versus multistrand wire of the same current rating for the filter? It seems that insulated solid core wire larger than about 16 swg is impossible to find over here...
BTW Wayne those Chinese pcb boards are of varying quality.. the ones I got from Aliexpress were good, however I bought some of eBay and the holes and tracks were a bit blurry.
Justin
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« Reply #23 on: January 31, 2018, 05:15:02 PM »

Hi Steve

I haven't noticed any weird audio effects or aliasing at appox 300 watts...is there anything else I should look at for  Huh


Wayne

How much current are you running, and how many poles filter are you using?  Also, what are the values of the inductors and what are you using for core material?

Regards,  STeve
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vk3alk
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« Reply #24 on: January 31, 2018, 05:29:30 PM »

I'm using IE33 Ferrite cores used in many Computer SMPS in the 350 watt - 500 watt range....
Its a 6 pole filter......the 1st inductor is 13uH .......2nd 51uH ..........3rd 37uH

The cutoff is appox 30Khz

Impedance of 6 Ohms

Voltage is around 42 volts @ 7 Amps


Wayne
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