Author Topic: DIY Metcal 13.56 MHz RF Supply  (Read 351573 times)

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Offline mamalalaTopic starter

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Re: DIY Metcal 13.56 MHz RF Supply
« Reply #125 on: February 14, 2013, 12:46:55 pm »
As for the FET ... It switches the center tap of the RF transformer to GND, while one end of that transformer is connected to the supply voltage. From a DC perspective that is like a short circuit there.

If the FET can not completely switch off in the required time, the FET will switch between "half on" and "full on", allowing a lot of DC current to flow through that half of the RF transformer. This _could_ result in too much current draw, so that the buck regulator tries to limit that, since it has builtin current limiting.

Remove the FET and check that you still get a high enough output voltage at L2, around 22 Volts. Apply a load there to check that the buck regulator is still OK, it should stay stable even whenn you draw 2-3 amperes.

Double check that there are no shorts or similar in the output path, especially across the capacitors in the filter section. If that is OK, place a IRF510 as the output FET in there and try again.

Greetings,

Chris
 

Offline mamalalaTopic starter

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Re: DIY Metcal 13.56 MHz RF Supply
« Reply #126 on: February 14, 2013, 01:10:09 pm »
Just checked the datasheet for the 30N06L that you put into the circuit. It is completely unusable!

At 13.56 MHz you have a period time of about 73 nS. However, the switching times of that FET are far slower: 60nS turn-off delay plus 110nS turn-off fall time, 15nS turn-on time plus 210nS turn-on rise time. The IRF 510 has 15nS turn-off delay, 12nS fall time, 8nS turn-on delay and 25nS rise time. Next is the gate charge, 15nC for the 30N06L vs. 5nC for the IRF510. The Ciss/oss/rss are way higher as well: 800/270/50pF for the 30N06L vs. 135/80/20pF for the IRF510.

Last but not least, the voltage rating is too low as well. It is spec'd for 60 volts only, the IRF is for 100 volts. There can be voltages in excess of 60 volts appearing at the FET, depending on load match/mismatch and supply voltage. So in the worst case it could just blow.

Really, you need to change the FET to the specified IRF510. The one you selected is completely unusable for that application.

Greetings,

Chris
 

Offline mamalalaTopic starter

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Re: DIY Metcal 13.56 MHz RF Supply
« Reply #127 on: February 14, 2013, 02:25:01 pm »
Attached are some scope images of how the waveforms should look like in this circuit. For all images the supply voltage into the buck regulator is 30 Volts. It draws 3 Amperes. A 50 Ohms dummy load is connected to the RF output during the first 7 scope images. In the last image an actual tip was connected, a STTC-147, and the image was taken after the tip heated up completely.

- Output_L2 shows the ripple on the output of L2. It is about 1 Volt peak-peak, and you can clearly see the RF ripple waveform.

- XFMR_Center shows the waveform at the center tap of the RF transformer. Since that is connected to the FET's drain pin, this is also what the FET "sees". At a 50 Ohms load it is already 40 volts peak-peak.

- XFMR_Output is the signal that appears at the output of the RF transformer, before it goes into the filter section. This is about 90 Volts peak-peak.

- L3_Input is at the input side of L3, after the output capacitors of the RF transformer. A bit over 90 Volts peak-peak.

- L4_Input is at the input side of L4 (output side of L3). My scope only goes up to 160 Volts, and this signal is in excess of that.

- L5_Input is the signal at the input side of L5 (output of L4). Roundabout 80 Volts peak-peak.

- L5_Output is the output side of L5 (which goes through the SWR bridge/coupler to the actual RF output). About 100 Volts peak-peak.

Now, all these waveforms are with a 50 Ohms dummy load, which is probably not a perfect match for this circuit (i.e. it differs from the load that a real tip cartridge presents). Using a real tip, the output voltage can get higher than that!

To show why a 60 Volts rated FET is not suitable for the output stage, look at XFMR_Center_with_Tip. This is the signal that appears at the center-tap of the RF transformer (drain of the FET) when using a STTC-147 tip cartridge, after it has completely heated up. As you can see it already reaches about 60 volts peak-peak. This rise of 20 volts compared to the dummy load is due to the reflected RF energy. It also varies depending on the actual tip used, as well as what stage of the heatup process it is in.

Greetings,

Chris

Edit: With the dummy load, the output voltage at L2 is about  23.3 Volts. I used the ground connection/mountin hole in the lower-right (when looking at the layout file), next to C47, as ground reference for all the measurements.

Edit 2: Take the readings with a huge grain of salt! My scope is half broken, i only have a shitty probe, and it's not properly compensated as well. So it's quite possible that the readings are too low or high.

Just checked another probe (again a cheap one) and the output of L5 now shows 120 Volts peak-peak instead of 100 Volts into a 50 Ohms dummy load. So, the readings in the scope images are just very rough "somewhere around that" values.
« Last Edit: February 14, 2013, 03:10:24 pm by mamalala »
 

Offline Paulinho19

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Re: DIY Metcal 13.56 MHz RF Supply
« Reply #128 on: February 14, 2013, 08:00:04 pm »
Ok, here are the results from today.

First I removed the FET and testes the L2 Output. First with no Load and then with the 50ohm dummy load connected there, everything seems fine.
 

Offline Paulinho19

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Re: DIY Metcal 13.56 MHz RF Supply
« Reply #129 on: February 14, 2013, 08:03:08 pm »
Next I soldered a proper IRF510 and put the dummy load on the RF Output.

Turned R19 all clockwise and powered the unit.
 

Offline Paulinho19

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Re: DIY Metcal 13.56 MHz RF Supply
« Reply #130 on: February 14, 2013, 08:04:45 pm »
Everything looked good to me.
 

Offline Paulinho19

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Re: DIY Metcal 13.56 MHz RF Supply
« Reply #131 on: February 14, 2013, 08:07:26 pm »
Next I started to turn R19 counter-clockwise and the voltage on L2 started to rise.
 

Offline Paulinho19

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Re: DIY Metcal 13.56 MHz RF Supply
« Reply #132 on: February 14, 2013, 08:08:42 pm »
Everything still looked good.
 

Offline Paulinho19

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Re: DIY Metcal 13.56 MHz RF Supply
« Reply #133 on: February 14, 2013, 08:10:42 pm »
As I got over 16V on L2 the FET died.  :'(
 

Offline Paulinho19

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Re: DIY Metcal 13.56 MHz RF Supply
« Reply #134 on: February 14, 2013, 08:14:52 pm »
So I put in a new IRF510 and exactly the same happened. I reach 100V p-p with around 15V on L2 and when I increase the voltage there with R19 the FET dies. :(
 

Offline Paulinho19

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Re: DIY Metcal 13.56 MHz RF Supply
« Reply #135 on: February 14, 2013, 08:16:28 pm »
Here are some better pics from my board.
 

Offline Paulinho19

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Re: DIY Metcal 13.56 MHz RF Supply
« Reply #136 on: February 14, 2013, 08:17:10 pm »
Any sugestions?

The last pic is my Dummy Load.
« Last Edit: February 14, 2013, 08:20:55 pm by Paulinho19 »
 

Offline mamalalaTopic starter

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Re: DIY Metcal 13.56 MHz RF Supply
« Reply #137 on: February 14, 2013, 10:47:33 pm »
Hi Paulo,

hmm, you are getting closer, at least ;)

OK, first i see that you have wound the coils rather loose. I have them tightly wound around the cores, as you can see in the images in the docu. This will affect the inductance to some extent, so that might be part of the reason.

Next, you have only 3 caps installed for C30/31/32. I have a third one placed on top of them, making a total of 4. I can't see it in you images, but at C20/21 only one cap has to be installed instead of two.

In your post #130, L4_Input shows 56 Volts pp, while RF_Output shows around 80 Volts pp. That can't be right. Are you sure you got those right?

In post #132, L3_Output it shows 232 Volts pp, which seems a bit high considering the output voltages. It might just be that there is too much RF reflected, which then kills the FET somehow. While i can't meassure the voltage there, a image interpolation with a lower voltage waveform tells me that i have around 200 Volts pp there in the image i posted here, resulting in about 85 Volts pp at L5 input, and about 100 Volts pp at the RF output.

Please install that extra capacitor, and make sure that there is only one installed at the input of L3 (positions C20/21), and rewind the toroids so that the windings sit really tight on the core. The XFMR looks fine already. Oh, and bend L3 away a bit from the XFMR1, in the image it literally leans onto it.

Other than that it already starts to look good. I assume you used a "trustworthy" supplier for the IRF510. I have seen cheap China FET's that didn't hold up to the specs in other projects... Keep an eye on the voltage/waveform at the drain of the IRF510, which goes into the center tap of the XFMR. It shouldnt go too much above 60 Volts pp. If it does, too much RF is reflected back somewhere.

Just to make sure, you can try to do the measurements with L6 (right at the RF Out pads) removed. There just might be something fishy going on in that section that throws of the matching. After all, that is the only part that is in direct contact with the RF output signal (besides the output filter obviously, of course).

When i was experimenting with the coils/caps in the output filter section, i blew a few FET's myself, so that filter section is a rather sensitive area.

Oh, and while it may sound stupid to ask, but i can assume you used proper capacitors in the filter section? That is, with the proper voltage rating? As you can see, rather high voltages can appear there, so the caps must have a suitable rating.

At one point i blew the cap at C13, in the supply line to the XFMR, at the moment i switched the unit off. While the circuit still worked somewhat, it also messed with the FET (and overall signal quality). That's why i now have three in that position. Lot's of current draw there. So the caps are a really important factor here.

Greetings,

Chris

Edit: Oh, and you can scale down the scope images, for example to 800x600 like i did. There isn't more information in them than that anyways. Only eats up bandwidth ;)
« Last Edit: February 14, 2013, 10:49:18 pm by mamalala »
 

Offline mamalalaTopic starter

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Re: DIY Metcal 13.56 MHz RF Supply
« Reply #138 on: February 14, 2013, 11:28:47 pm »
Just for fun, here's a small video showing the waveform at the drain of the IRF510 from power-on until the tip reaches it's temperature. It starts high, goes down and then back up again. At the same time the voltage at L2 goes from the initial 23.3 volts down to 17.6 volts. This shows that the reflected RF increases the voltage at the drain, even with a decreasing supply voltage.

So, if there is a mismatch in the RF path, and too much RF gets reflected back into the IRF, that surely will kill that poor thing.

http://www.vidup.de/v/LjF47/

Greetings,

Chris
 

Offline Paulinho19

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Re: DIY Metcal 13.56 MHz RF Supply
« Reply #139 on: February 14, 2013, 11:55:47 pm »
Hi again,

Yes I'm closer. ;)

I have 4 caps on C30/31/32, there are 2 on C30, you can't really tell by that pic, but you can see them on the 140220132894 from post #135.

I also have only 1 cap on C20/C21, you can see that on the 140220132892 also from post #135.

Regarding L4_Input from post #130, it could actually be L4_Output, dose that make more sense?

I got the IRF510 from my local electronics part dealer. They are normally "trustworthy", but I will try to get some elsewhere.

I used the following caps on the RF Output Filter:

http://pt.rs-online.com/web/p/condensadores-ceramicos-multicapa/0391038/

I will fix coils tomorrow, and if I can find some new IRF510 I will try it again. :D

Thanks for all the help,

Paulo

The problem could also be the warm weather here in Portugal :P
 

Offline mamalalaTopic starter

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Re: DIY Metcal 13.56 MHz RF Supply
« Reply #140 on: February 15, 2013, 12:04:04 am »
Alright then. If the caps are all OK, the only thing left that i can think of are the coils and the FET (assuming no funny business is going on with L6/the tip-detection circuitry).

Just to make sure i did not mess up anything with the caps in the filter section, i will check them during the weekend for the values i used. But i'm pretty sure they are correct. Of course there is still the dummy load that may be faulty, but i think that is rather unlikely. However, just to make sure you could check if it still has 50 ohms. And watch the temperature of that thing. It's rated for 15 watts average only, while this circuit can easily deliver over 35 or 40 watts into a 50 ohms load.

I'm sorry that i can't provide more accurate scope measurements... Stupid broken HP scope here...

Greetings,

Chris
 

Offline Paulinho19

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Re: DIY Metcal 13.56 MHz RF Supply
« Reply #141 on: February 15, 2013, 12:04:34 am »
 

Offline Paulinho19

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Re: DIY Metcal 13.56 MHz RF Supply
« Reply #142 on: February 15, 2013, 12:12:00 am »
The dummy load looks ok, I measured its resistance and got 49.9 Ohm. It doesn’t get very hot, the FET dies before it has time to heat up. ;)

A friend will lend me a more powerful one just in case.

Paulo
« Last Edit: February 15, 2013, 12:35:14 am by Paulinho19 »
 

Offline mamalalaTopic starter

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Re: DIY Metcal 13.56 MHz RF Supply
« Reply #143 on: February 15, 2013, 12:42:52 am »
 

Offline mamalalaTopic starter

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Re: DIY Metcal 13.56 MHz RF Supply
« Reply #144 on: February 15, 2013, 02:32:44 pm »
Just did a quick check for the cap values. As in the instruction, the C0G caps in the filter section are all 56p, the sic X7R at the XFMR output are all 47n, and for C13 i have 3 x 100n installed. The 3 caps in the peak detector (C37/38/39) are also 56p each.

The caps i got from RS are the same that you have. They actually meassure around 59pF here, but my capmeter is not that good. However, you may want to verify them as well. I already had several issues with RS, either the wrong parts in correctly labelled bags (like 4µ7 caps, when 10µ were ordered and 10µ is on the bags label), or no parts at all in sealed bags, etc. Interrestingly enough, every time that happens it is from their UK warehouse always.

So, all that's left now are the coils and the FET. And as said, temporarily remove the coil L6 from the tip detection circuitry, in case something is wrong with that section (it couls mess up the SWR if, for example, Q1 or D9 have issues, or if something is wrong with C41/R22), however, it's rather unlikely, since the 47µH inductor should block virtually all of the RF. But then, who knows...

Greetings,

Chris
 

Offline Paulinho19

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Re: DIY Metcal 13.56 MHz RF Supply
« Reply #145 on: February 15, 2013, 02:42:27 pm »
Here are my new coils, nice and tight. ;)

Paulo
 

Offline mamalalaTopic starter

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Re: DIY Metcal 13.56 MHz RF Supply
« Reply #146 on: February 15, 2013, 02:47:26 pm »
Here are my new coils, nice and tight. ;)

Paulo

Looks great! Hope you hadn't too many cramps in your fingers while winding them  ;D

Greetings,

Chris
 

Offline Paulinho19

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Re: DIY Metcal 13.56 MHz RF Supply
« Reply #147 on: February 15, 2013, 02:48:08 pm »
My caps meassure around 61 pF.

Paulo Almeida
 

Offline mamalalaTopic starter

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Re: DIY Metcal 13.56 MHz RF Supply
« Reply #148 on: February 15, 2013, 02:56:27 pm »
My caps meassure around 61 pF.

Paulo Almeida

Good. Just looking at your picture of the board above, and it seems that some solder joints at the caps are not properly flowed to the whole pads on the GND side of the caps. This could of course be misleading due to light reflections, but please double check those places. The GND pads have no thermals, and have vias nearby stitching them to the bottom GND plane. So theck away heat pretty good, which in some cases may cause faulty solder joints. Just a precaution.

Greetings,

Chris
 

Offline mamalalaTopic starter

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Re: DIY Metcal 13.56 MHz RF Supply
« Reply #149 on: February 15, 2013, 03:05:14 pm »
Oh, and to check that nothing is wrong with the dummy load, you can measure the voltages VR (pin 2 of SV1) and VF (pin 4 of SV1) when the circuit is powered on. You should, if all is fine and properly adjusted, have virtually no volrage at pin 2, and over 2 volts on pin 4 (i have 0.4mV on pin 2 and 2.4V on pin 4). But even with a much lower output power you should still have basically no voltage at pin 2 and a much higher one at pin 4. So, if any notable voltage appears at pin 2, something is wrong with either the dummy or tip detection circuitry.

Greetings,

Chris
 


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