High Voltage Forum

General electronics => Electronic Circuits => Topic started by: Anders Mikkelsen on May 02, 2021, 07:45:41 PM

Title: Testing and teardown of a commercial induction heater
Post by: Anders Mikkelsen on May 02, 2021, 07:45:41 PM
I recently found a CEIA Power Cube 45/900 for 200 dollars on eBay. This was just the power unit, but I already had a suitable workhead from a previous eBay purchase, and I thought it could be an interesting unit for a teardown. These heaters are interesting in that they are made to work at a pretty high frequency (750 - 1150 kHz) with low-inductance work-coils (in the 100 nH range). Their main application seems to be fine brazing and soldering of connectors, spectacle frames and other miniature parts.

The power unit is specified to deliver up to 45 kVAR and 3.6 kW running from single phase mains. The VARs are provided by parallel capacitor in the workhead, and the supply only provides the real power, up to 144 V and 25 A in this case.

The unit is pretty compact and dense, with a single pair of buttons to control the heating power and a two-digit seven segment display to show status codes and operating power. The front panel design is a bit unusual in that it contains the operating manual in English and French - convenient if not very aesthetically pleasing.

I cut the workhead cable at some point in the past to figure out how it was constructed. Four cord heavy litz wrapped in what felt like nomex paper, plus a shielded four-core signal cable, all within an outer shielding. So splicing it back together was not fun, but I did a half-hearted job using fibreglass sleeving, heatshrink and all too much kapton tape. Not very elegant, but good enough for testing.

The next issue to tackle was the work-coil, as the one that came with the head was an impractically long hairpin coil that was kinked and bent. I figured a two-turn 30 mm coil would resonate with the operating range of the unit, based on the measured 300 nF workhead capacitance. I wound the coil from 1/8 refrigeration tubing and silver-soldered it in place. Sweeping the head with an impedance analyzer showed parallel resonance at just over 800 kHz, a bit on the low side but within the range of the unit so I left it at that.

I wired up a control switch to the external control input and plugged it in. The display showed error A3 "Absence of water cooling". Turning up the water flow, the unit made a loud beep and the display showed "H1/99", ready to operate. Running the heater, it quickly became obvious that this thing is no joke. It reduces nuts and bolts to molten metal in seconds. Even solid M12 threaded stainless rod is easily molten in less than half a minute.

It looks like this one is worth keeping, so I bought a new mil-spec connector (MS3106A18-19P-RES) for the workhead cable and terminated it properly. The original cable was 3 meters long, so this leaves me with a complete workhead with a 1.5 meter cable, plus a spare 1.5 meter cable with a connector on it. The unit supports two heads so this is a good excuse for me to build a second workhead.

This concludes the testing, in the next episode we'll open it up and try to figure out how it works.
Title: Re: Testing and teardown of a commercial induction heater
Post by: davekni on May 03, 2021, 04:18:29 AM
Impressive what can be done with high frequency and the resulting ability to concentrate power into a small object!  Sure makes the little DIY induction heaters look wimpy.
Title: Re: Testing and teardown of a commercial induction heater
Post by: Mads Barnkob on May 03, 2021, 08:14:37 AM
Great job Anders. Getting such a powerful unit running again for such few money is a score!

You had me intrigued at "teardown", do you have pictures from underneath the lid? I would like to see the power electronics :)
Title: Re: Testing and teardown of a commercial induction heater
Post by: Anders Mikkelsen on May 03, 2021, 09:38:30 PM
Thanks! The combination of low tank impedance, high operating frequency and moderately high reactive power, gives it pretty decent power density.

I realized I forgot to add an image of the back of the unit, so I'll start with that to help give an idea how everything fits together.

From the left, we have standard mil-spec connectors for the two workheads, cooling water in, cooling loops for the workheads and cooling water out. A digital input is switched to ground to enable the heating for each head (only one can be used at a time), and a 0 - 10 V analog input can be used for power control. The unit also has an RS-232 port for monitoring and basic control.

Let's have a look inside. From the top, there's nothing really surprising. A two-stage EMI filter, with one stage enclosed in a magnetic shielding can, a potted toroidal transformer for control power, and the DC bus filtering caps on the main power PCB are also visible. The edge of the control board can be seen along the top of the image.

Looking from the side, the unit is built around a solid plate of aluminium on which everything is mounted. This plate also serves to transfer heat from the electronics to the water cooling block on which the transistors are mounted. Nice and clean design, but it does explain why this unit feels so heavy for its size.

The lower board contains the main power electronics section. There's a rectifier, some electrolytic DC bus caps a completely separate sub-board for the MOSFET full-bridge. The bridge has body diode isolation Schottky diodes and fast antiparallel-diodes. This makes sense, as it's easy to exceed the body diode recovery dI/dt rating on MOSFETs if the bridge current phase angle ever goes lagging, which it is bound to do under some operating conditions. Some modern devices are more robust on this front, but this is still a major concern in MOSFET-based resonant converters.

It's starting to look like this is an LCLR heater. Given the lack of a matching transformer in the workhead, it was either this or a current-fed topology. Measurements show that the DC bus capacitors are directly connected to the bridge, so LCLR it is. I made a quick sketch of the power circuitry, including the critical values.

Interestingly enough, it looks like they made their own DC blocking capacitor. It's a taped up stack of copper foils and mica, the yellow unit on the center-right of the bottom view. It's probably the biggest surface-mount capacitor I've seen, and it measures 50 nF.

The output transformer and matching inductor are both planar, made on what looks like a standard PCB process. The matching inductor is 3.2 uH and the transformer seems to be around 4:3, with a 90 uH primary magnetizing inductance. The matching inductor is placed before the transformer, so the output transformer has to carry the reactive power developed by the matching inductor. They are both bolted to the aluminium cooling plate using some fancy cast aluminium parts, pretty neat.

The final part of the power board is the output filtering and switching, hidden inside a shielding can. Some X- and Y-capacitors, a common mode choke made with a PCB used as a planar busbar, a relay for selecting which output to activate, and the output connectors.

Lastly, we have the front panel board, which is more-or-less what one would expect. An obscure but unremarkable 32-bit microcontroller, a CPLD, and a MOSFET full bridge to drive the gate drive transformers. The main microcontroller had the markings burned off by laser, but the part number was still readable. I'll honor their spirit by not mentioning the part number, but I don't think it would have made a difference even if it wasn't readable.

In the next post we'll fire it up and do some measurements.
Title: Re: Testing and teardown of a commercial induction heater
Post by: T3sl4co1l on May 05, 2021, 04:51:47 PM
Nice. I've been thinking about making a planar transformer for something like this.  Just that, without a product in mind or anything, it's very, very back burner... ::)

SimplePortal 2.3.6 © 2008-2014, SimplePortal