13.56 MHz RF source for Metcal iron - Page 1
Forum section: electronics / projects

author: ifonlyeverything
author_rank: Regular Contributor
timestamp: September 16, 2025, 06:35:23 pm
content: I'm interested in DIYing a Metcal-style soldering iron power station. The requirements seem to be a sinusoidal output 13.56 MHz, 80-100W peak into a ~50 Ohm load. The designs I've studied, which use silicon FETs, seem to follow 1) class E pre-amp into a class C or 2) just class C.In consideration of a simpler and more efficient/robust design, has anyone tried "cheap" GAN FETs (nexperia GAN190-650 or similar) in a transformer-coupled class D amplifier? Any tips (gate driver suggestions) or pitfalls that I might be overlooking?

author: mtwieg
author_rank: Frequent Contributor
timestamp: September 16, 2025, 07:30:45 pm
content: This thread might be of interest to you:https://www.eevblog.com/forum/projects/diy-metcal-13-56-mhz-rf-supply/Sure, GaN FETs would probably be great for a class D/E amplifier. One thing to be careful of is that the load impedance will change drastically for different operating conditions. You could design a circuit with fantastic efficiency at full load (i.e. a cold tip), but promptly self destructs once the tip reaches the curie temperature, or when the tip is removed while the amplifier is on.

author: ifonlyeverything
author_rank: Regular Contributor
timestamp: September 16, 2025, 07:44:59 pm
content: Quote from: mtwieg on September 16, 2025, 07:30:45 pmYes, I've studied that thread in detail.I did read that class E might not be such a good idea due to the load having a variable impedance, as you pointed out. The DIY designs seem to have an SMPS that outputs between 10-20V DC into the RF amplifier stage (with feedback from RF stage back to SMPS). My thought was something similar with a class D amplifier maybe with a coupling transformer?

author: mtwieg
author_rank: Frequent Contributor
timestamp: September 16, 2025, 10:32:02 pm
content: By class D you are referring to the type that uses a 50% duty cycle switching and bandpass filter to produce RF at the output, or the type which uses PWM and a lowpass filter to produce low frequency at the output?The original idea behind the class E amplifier is that it should be more robust to variations in the load and operating frequency, due to having zero slope Vds at turn-on.... but I would suspect that if the variation is very large (i.e. a total open-circuit load) then class D would fare much better.

author: thm_w
author_rank: Super Contributor
timestamp: September 17, 2025, 12:08:15 am
content: If you look at hakkos FX100 implementation, to the eye it looks far simpler than what metcal does. That might give a few ideas. A 100W+ GaN version of that would be amazing.These were the components I saw:IRFI510G – 100V 4.5A N-CHIRFB4020 – 200V 18A N-CHLM2576 ADJ – 3A buckHJ4052 – high speed analog mux74HCU04A – hex inverter (I think this was the main 13MHz clock gen, tweaked with a bunch of pots)They also have some sort of granular control over the output, where it "boosts" on startup:

author: ifonlyeverything
author_rank: Regular Contributor
timestamp: September 17, 2025, 12:54:39 am
content: Quote from: mtwieg on September 16, 2025, 10:32:02 pmI meant the former, 50% duty cycle with a filter to create a sinusoidal output.Can class E work in this situation? Granted I'm learning ALL of this as I go, but I thought class E was "highly tuned" for a specific load impedance, and the changing load impedance (cold/warm tip not at its Curie point vs. hot tip at its Curie point) therefore disrupts the tuning and precludes the FETs from switching at the ideal 0V across drain and source?

author: coppercone2
author_rank: Super Contributor
timestamp: September 17, 2025, 01:02:18 am
content: In regards to hakko vs metcal circuit, I can't help but think of all the broke metcal stations we seem to see on the forum.Could this be one of those places where the simplicity is deceptive and actually better engineered (reliability wise)?Soldering irons really strike me as a place where they over-did the features by alot. There might be slight benefits over very large samples, but it really feels like a roided out VCR clock type featureset.... the biggest benefit is to noobs at soldering (without instructors)Though, maybe they both break alot, but I saw much less posts about hakko RF irons then metcal RF irons.

author: mtwieg
author_rank: Frequent Contributor
timestamp: September 17, 2025, 11:52:39 am
content: Quote from: ifonlyeverything on September 17, 2025, 12:54:39 amOkay just checking. Not many engineers are familiar with the "original" class D amplifiers.You also have the choice between voltage mode class D (zero-current switching) and current mode class D (zero voltage switching).QuoteHard to say without describing the specific "situation".For example, if load resistance increases a lot, class D won't feel much stress, and should roughly maintain its ZCS/ZVS operation. Efficiency will probably suck, but dissipated power should be less than fully loaded condition. Class E, on the other hand, will totally lose ZVS and dissipation in the FET will increase drastically.For low load resistance, voltage mode class D should maintain ZCS while current mode class D will start to lose ZVS. As you get close to short circuit condition, both will supply unlimited current/power to the load until something breaks. For class E, load power will decrease, and dissipated power will increase somewhat. Peak drain voltage will increase somewhat as well.

author: ifonlyeverything
author_rank: Regular Contributor
timestamp: September 23, 2025, 01:55:54 am
content: After playing around with some class e calculators, I drew this in LTspice. No idea how practical that current transformer is but for now I'll just roll with it.Cold tip (50 ohm load) with 35V VDD, observed ~880mV RMS at CT+:Code:Power in: AVG(-v(vdd)*i(v1))=85.4175Power out: AVG(v(filt_out)*i(rload))=82.8289Efficiency: pout/pin=0.969695Losses: (1-efficiency)*pout=2.51012Hot tip (12 ohms) regulated to 880mV RMS at CT+, yielding 11.4V VDD:Code:Power in: AVG(-v(vdd)*i(v1))=25.2004Power out: AVG(v(filt_out)*i(rload))=19.9243Efficiency: pout/pin=0.790635Losses: (1-efficiency)*pout=4.17145So I think this looks okay in simulation land? The next question is can this GaN FET handle dissipating 4W under worst case ideal conditions? I kind of haphazardly chose GAN190-650. The data sheet is here.https://www.digikey.com/en/products/detail/nexperia-usa-inc/GAN190-650EBEZ/19109876I'm having trouble understanding the thermal section of the data sheet.I see Rth(j-a) is given at 35.9 K/W and there is a derating curve as a function of mounting base temperature e.g., 40% of 125W rated power with 100C base temperature. But what is this in reference to, like how many square inches of copper?

author: thm_w
author_rank: Super Contributor
timestamp: September 23, 2025, 02:08:48 am
content: Quote from: ifonlyeverything on September 23, 2025, 01:55:54 amNormally they'd give you some hint as to the board. I would guess its some jedec standard 4 layer board with 2 solid plane layers. So to achieve 4W you might want an additional heatsink, active cooling, an aluminum core PCB, etc.https://ww1.microchip.com/downloads/en/Market_Communication/05_35man.pdfhttps://www.allaboutcircuits.com/technical-articles/junction-to-ambient-thermal-resistance-ic-package-thermal-performance/https://www.onsemi.com/pub/Collateral/AND9596-D.PDFQuote

author: ifonlyeverything
author_rank: Regular Contributor
timestamp: September 23, 2025, 02:34:12 am
content: Thanks for the quick reply. Maybe I'll look into using a different package or using enough in parallel to have reasonable thermals. I thought about a heatsink but trying to desolder a blown 8x8mm package glued to a heatsink sounds miserable.

author: Zoli
author_rank: Frequent Contributor
timestamp: September 23, 2025, 06:15:12 am
content: Quote from: ifonlyeverything on September 23, 2025, 01:55:54 amChange "K1 L3 L4 1" to "K1 L3 L4 0.9"; is much closer to the homemade transformers.

author: mtwieg
author_rank: Frequent Contributor
timestamp: September 23, 2025, 12:43:41 pm
content: Quote from: ifonlyeverything on September 23, 2025, 01:55:54 amAre those losses all in the FET? If so that sounds very high (for the hot tip). You could attach your simulation files (.asc plus any models/symbol files you're using) and I can take a look.QuoteThe derating curve in Fig 1 applies to Ptot, which is 125W at Tmb=25C. This is not a very useful metric, even with the derating curve (how would you predict Tmb?).The 35.9 K/W Rth(j-a) value is more practical, though they don't describe the board design the number is based on. Hard to say if it's overly conservative or not.

author: holko
author_rank: Contributor
timestamp: September 23, 2025, 03:29:16 pm
content: I did reverse engineer a broken MX5000 station (repo says 5200 but it is wrong) and made a few impedance measurements of various tips while heating it with some old 3D-printer parts.This information is definitely incomplete but still might prove useful.https://github.com/holkono/mx5200-and-tips

author: mtwieg
author_rank: Frequent Contributor
timestamp: September 23, 2025, 03:45:00 pm
content: Quote from: holko on September 23, 2025, 03:29:16 pmInteresting, but it looks like VSWR is actually lower when hot, opposite of how it should be? For example, for STTC-145 at 30C VSWR=1.75, but at 360C VSWR=1.13. Or maybe the nominal impedance of the system is not 50 ohms?

author: ifonlyeverything
author_rank: Regular Contributor
timestamp: September 23, 2025, 04:47:49 pm
content: Quote from: mtwieg on September 23, 2025, 12:43:41 pmOn Zoli's suggestion I changed the coupling constants of both transformers to 0.9 and it brought up the "hot tip" efficiency drastically.Step 1 is a hot tip and Step 2 is a cold tip.Code:Measurement: pinstepAVG(-v(vdd)*i(v1))FROMTO121.632502e-006284.007502e-006Measurement: poutstepAVG(v(output)*i(rload))FROMTO119.995602e-006281.00702e-006Measurement: overallefficiencysteppout/pin10.92433420.964283Measurement: overalllossesstep(1-overallefficiency)*pout11.51322.89336Measurement: fetgatelossesstepAVG(v(gate)*ix(u1:gate))FROMTO10.046982902e-00620.034957502e-006Measurement: fetdrainlossesstepAVG(v(sw)*ix(u1:drain))FROMTO11.6067502e-00622.9327102e-006Measurement: fettotallossesstep(fetgatelosses + fetdrainlosses )11.6537422.96767

author: holko
author_rank: Contributor
timestamp: September 23, 2025, 05:10:13 pm
content: There might be several errors in my measurement due to stuff like thermal coupling, bad calibration, poor usage of averaging.But due to impedance transformation I believe the voltage in the feedback-detector is actually higher when the VSWR is lower. And that voltage is again injected into to the feedback of the regulator to lower the output voltage to the amplifier.When I compare this schematic with the MX500 schematic found online and several of the Metcal patents it looks like the new designers didn't quite understand what they where doing and therefor just copied the old design and slapped on "best practices" everywhere they could. But again, who knows..

author: ifonlyeverything
author_rank: Regular Contributor
timestamp: September 23, 2025, 05:44:50 pm
content: Looks like I forgot to attach the LTspice file. Here it is, if anyone wants to play around.

author: thm_w
author_rank: Super Contributor
timestamp: September 23, 2025, 09:26:01 pm
content: Quote from: ifonlyeverything on September 23, 2025, 02:34:12 amDoesn't have to be glued on, a heatsink can be screwed orclippedonto the top or bottom of the board. Or bolted to an enclosure. Thermal pad instead of paste.Once the heatsink is removed, desoldering a DFN8 with hot air is not difficult. If you were to use an aluminum core PCB then its a bit harder and would want a preheater.Quote from: mtwieg on September 23, 2025, 12:43:41 pmYou can find some sample board images here:https://www.ablic.com/en/semicon/package/archive/dfn-82030-pp008-a/

author: mtwieg
author_rank: Frequent Contributor
timestamp: September 24, 2025, 12:53:45 pm
content: Quote from: ifonlyeverything on September 23, 2025, 05:44:50 pmHard to give feedback on the class E waveforms because you're not really following the "canonical" class E circuit, where the series LC is followed by a real-valued load impedance (at the operating frequency, anyways). Therefore the common parameters for class E design (Q, etc) aren't well-defined.

author: richard.cs
author_rank: Super Contributor
timestamp: September 24, 2025, 03:06:16 pm
content: Quote from: mtwieg on September 23, 2025, 03:45:00 pmI made some measurements and posted them in the other thread (wow, 12 years ago):https://www.eevblog.com/forum/projects/diy-metcal-13-56-mhz-rf-supply/msg246402/?topicseen#msg246402

author: ifonlyeverything
author_rank: Regular Contributor
timestamp: September 24, 2025, 03:55:03 pm
content: Quote from: mtwieg on September 24, 2025, 12:53:45 pmI used an online calculator as a starting point and just tweaked stuff until efficiency came up. I'm not sure how useful this is, anyways, considering the real life circuit will undoubtedly behave in an entire different manner.

author: mtwieg
author_rank: Frequent Contributor
timestamp: September 24, 2025, 03:59:13 pm
content: Quote from: richard.cs on September 24, 2025, 03:06:16 pmThat looks much more in line with what I'd expect.But a class E amplifier does not have a 50 ohm output impedance, therefore SWR/S11 measurements referenced to 50 ohms aren't meaningful by themselves.I haven't gone over that thread in detail, but was looking at the oldMX500P schematic, wondering about how it senses/regulates power. I see the peak AC voltage from the amplifier is sensed with diodes D8 and D9, and that's used to automatically adjust the supply voltage to the power amp. So that would make the RF act as a regulated voltage source (low impedance). If that's true, then this means one would expect a lower load impedance to absorb more power from the source, even if its SWR is high. Maybe the pi filter formed by C32, C41, and L7 form a quarter wave impedance transformer, changing that low impedance into a high impedance? But the component values don't seem right for that....

author: ifonlyeverything
author_rank: Regular Contributor
timestamp: September 24, 2025, 04:07:44 pm
content: Quote from: mtwieg on September 24, 2025, 03:59:13 pmI read somewhere that the 13.56 MHz, 80W models use a current transformer. Regulating for constant voltage inside the filter only halves the power which would mean 40W into an already hot tip. The MX-500 is rated at 40W peak output so halving that is apparently tolerable without risking a burned up tip.

author: mtwieg
author_rank: Frequent Contributor
timestamp: September 24, 2025, 08:21:44 pm
content: Quote from: ifonlyeverything on September 24, 2025, 04:07:44 pmRegulating the RF voltage will result in absorbed powerincreasingas the tip heats up and resistance drops.

author: thm_w
author_rank: Super Contributor
timestamp: September 24, 2025, 09:01:57 pm
content: Quote from: ifonlyeverything on September 24, 2025, 04:07:44 pmYou can see in the Hakko photo above, T2 is a current transformer which feeds into L5 then into the output.

author: SnowFox
author_rank: Newbie
timestamp: October 06, 2025, 02:48:34 am
content: I made a power supply for Metcal using a GAN transistor (IGT40R070D1). Of course, the GAN transistor still generates heat, but it's not very hot. Thanks to the low gate capacitance, I can eliminate the preamplifier and drive the gate at 13.56 MHz using a fast driver (2EDN7424). But the design is not yet complete. I want to make it run from a USB PD power supply or power bank, and have the power supply negotiate the permissible power with the power source. Thanks to its excellent heat transfer and efficiency, the Metcal soldering iron can easily solder even with a power limit of 15W. And if you use the full power of USB PD (65/120W), there are no limits at all.

author: thm_w
author_rank: Super Contributor
timestamp: October 06, 2025, 10:03:55 pm
content: Quote from: SnowFox on October 06, 2025, 02:48:34 amNice work, negotiation can be done on the simple end with something likeIP2721 or CH224.Is the design "safe" in that if you disconnect the tip nothing will burn up? I see it has voltage and current measurement.

author: SnowFox
author_rank: Newbie
timestamp: October 06, 2025, 11:49:33 pm
content: I want to use the FUSB302 for USB PD, and I also wanted to support power supplies with QC.The design is safe. The transistor is rated for 400V, and I installed a snubber that activates when the voltage on the transistor exceeds 200V. I also made a simple circuit to detect when the soldering iron tip is disconnected, which stops the generator.Video:https://streamable.com/d5owb3

author: ifonlyeverything
author_rank: Regular Contributor
timestamp: October 07, 2025, 06:23:01 pm
content: How is your output filter calculated? It looks like it's pulling double duty: acting as a low pass filter and an impedance matching network from the amplifier output to the hand piece?

author: SnowFox
author_rank: Newbie
timestamp: October 07, 2025, 08:40:14 pm
content: Initially, the calculations were as shown in the figure.This may be incorrect, as I previously read Metcal's patents on Google Patents, and there was a reference to a patent I couldn't find, specifically about this circuit, where it was called something like a "high-frequency voltage-to-current converter."But later, I experimented, selecting filter frequencies slightly above and below 13.56 MHz, based on the voltage waveform at the GAN transistor's drain, the transistor's heating, and the current consumption (the DC/DC converter must be able to supply this).The impedance of my soldering iron (at the cable connector) with STTC-026 varies as follows:Cold (+21°C) 34+28jHot (+300°C) 50+20jHot on magnetic stand (+300°C) 16+35jCold on magnetic stand (+21°C) 18+35jSo I tried matching it to a 50 ohm load.

author: ifonlyeverything
author_rank: Regular Contributor
timestamp: October 08, 2025, 01:59:16 am
content: What sort of waveforms are you targeting at the drain? This looks kind of like a class E amp, so is this ZVS?Is reactance = 4*RL some sort of a rule of thumb? I'm not familiar with this.

author: SnowFox
author_rank: Newbie
timestamp: October 08, 2025, 03:46:47 am
content: Yes, 4*RL is a rule of thumb taken from a Class C amplifier design manual. It works.https://www.rcarc.org/presentations/AB2EW%20Class%20C%20club.pdfPage 14I'm not particularly knowledgeable about RF circuit design, but if I understand correctly, even with full power delivered to the soldering iron, it operates in Class E mode. Although it's still a Class C amplifier, the ZVS voltage waveform is only when the soldering iron is delivering power to heat the circuit. When idle or on the stand, the waveform is worse, and the transistor losses are higher.Waveform:blue - GAN drainyellow - GAN gate (I have an oscilloscope with 50 MHz probes, so this is distorted)