The device seems to be a rebranded generic induction cooktop from China the Better China Corporation, to be exact.www.better-china.com Model BT-180K. Their resonant circuit is awesome, so two thumbs up on power management. Two thumbs down on everything else, tho sorry guys!7 screws on the back separates cooktop side from interesting side. Remove a ribbon cable connecting the two halves and youre home free. Our pic of the two open halves is blurry so its not going up sorry. The next pic is looking down into the bottom of the device to the power side. This big coil is mounted above the PCB and fan all in the base of the unit.

Fig 4 Inside the Base. Are belong to us

The work coil is 20 turns of stranded (Litz) wire. 10 gauge-ish? Who knows, a lotta strands tho. It sits on a plastic platform with a couple of embedded ferrite rectangles in the bottom. This probably keeps the inductance reasonable when no pot is applied so the resonant frequency doesnt go to 10MHz or anything. The coil attaches to the driver with a couple of lug screw connections marked IN and OUT.Interestingly, the INside of the coil goes to IN and the OUTside turn goes to OUT. Unlikely that it matters, but its the little things that entertain us.The coil measures about 55uH with no metal nearby and about 25uH with a big steel cooking pot sitting on it. The white pedestal in the center seems to be a thermistor spring-pressed against the cooking glass for overtemp protection. And a lot of thermal grease.Remove 3 bigger screws to free the coil platform from the base of the unit, and unplug the thermistor cable to remove the coil entirely. Heres the back of the coil when its set over on the cooktop side. Note the radiating ferrite bars and the thermistor connection.

Fig 5 Back of the Work Coil

With the work coil removed, we can see the guts of the power electronics. Now heres where we start salivating. Mmmm giant iron powder toroids (2 x 300uH standing up and 1 x 400uH laying flat), mystery heatsinked power stage and some hefty caps. Everything else is fluff and housekeeping.

Fig 6 Power PCB After Work Coil Removal

Lets take a quick peek at these hardcore capacitors. They are by far the nicest component in the entire device, and probably accounted for 30% of the cost. Guess they are truly necessary. Well be looking at the upper left hand side of the PCB above the heat sink.

Fig 7 Cap Glamour Shot. Note the Lugs for the Coil Between Caps.

The 8uF is the input cap located right after the bridge rectifier. They run this thing almost like a PFC so he eats a LOT of ripple at 60Hz. The 2 x 0.33uF are the resonant tank caps, connected in parallel and running at 20kHz. youd like to see another angle? Sure thing!

Fig 8 A View Past One Resonant Cap, Down the PCB

The next step on our tour is the controller board. The controller board is mounted in the top of the device, and connected to the power stage board by the ribbon cable we unplugged a little while ago. By removing a couple short screws, we can release the controller board from its flex-button prison and take a look.Umm, buttons, 7-segments, discrete logic not too exciting. That 20-DIP with the sticker on top is a micro that runs the show. Its a Samsung S3F9454BZZ and from what we can tell, its notoriously hard to hack. So no firmware mods this time folks, this is strictly a strip show. Micro takes timing data (we speculate) from the current transformer and delivers pulses to drive the gates of the IGBTs. Get your timing right and you drive the tank into resonance with tasty results. Get your timing wrong and you try to switch 500V with 20A running in your IGBT and the 10kW transition loss blows your switch with very un-tasty results. So the controller is important, just not that interesting from a hacker sense.

Fig 9 The Controller Board

Back to the mainboard. Back to the back of the mainboard, to be precise. In order to take a peek at those power components, were going to have to get the heatsink off. We can guess at their function, and we can see the IBGT markings straight away, but to be sure well have to get the heatsink off. The only problem is: one of the components was screwed into the heatsink and THEN the components were soldered in! Now the screw is trapped!

Fig 10 Back of the Power Stage PCB

If you search long enough you may find the missing hole that allows you to unscrew the heatsink without desoldering. In our board we just had to drill down into the PCB a bit and presto! The missing hole appeared.

Fig 11 The Missing Hole

The following pic shows the board with the heatsink removed. The components that are heatsinked are a GBJ3510 35A/1kV bridge rectifier (wider rectangle, all black) and two parallel FGA25N120 25A/1200V IGBTs. The circuit analysis is as follows:Starting from the upper right, you can see the AC input screw lugs, one right above the vertically-mounted xfrmr and one near the hole in the PCB a little ways in. That inboard lug passes through the black shrink-wrapped horizontal fuse and then both AC lines pass into the toroidal chokes at the upper right. One choke per line, of course 300uH each.After the chokes (which prevent switching noise from passing back into your home wiring), the AC lines are filtered by the big yellow X-cap at the top of the PCB. The Neutral line passes next into the 3000:1 current sense xfrmr (horizontal xfrmr just below the Xcap) and then both lines go to the heatsinked bridge rectifier to be converted from 60Hz AC to full-wave rectified 120Hz lumps.The full-wave rectified current is filtered by the 8uF cap and the negative side connects to the IGBT emitters as well as acting as the GND rail for the entire power stage. The positive side of the rectified current passes through the horizontally mounted toroidal choke (400uH) and then to the top lug of the work coil connection and one side of the two 0.33uF resonant caps. The other work coil screw lug is connected to the low side of the two resonant caps as well as the two IGBT collectors. The work coil, of course, attaches to the two screw lugs and is connected in parallel to the 2x 0.33uF caps. So one side of the LC tank is held high, and the other side is pulled down by the two parallel IGBTs.Check it out folks GROUND referenced resonant switching!! We were really, 100%, expecting a half bridge when we saw those two IGBTs and it was very exciting to see a nice, 1.8kW commercial IH using such a simple and hobby-friendly power stage. Next, we used a 3-turn external coil on a 50mOhm resistor to pick up the current waveforms and read them on a scope lets take a look.

Fig 12 Clear View of the Power Stage PCB

Heres the 50,000 foot view. The current rises and falls in sync with the rectified AC line, meaning that when the AC input voltage is high the current is also high. This is really great because the AC power is used for 100% of its cycle and the input load of the device just looks like a resistor its called goodpower factorand it gives two advantages.1. You can get more power from a single plug. Like we said, youre actually using all the power delivered by the AC line. This is different than when a bridge rectifier is filtered by a huge cap to try to make DC. That circuit only takes power from the AC line at its peak, and it gets no useful power for most of the AC wave so the total power available is much less.2. The power company likes it better. Only taking current at the peak draws a big surge and uglys-up the power companys nice waveform. It also means that they need to use really big wires to provide the peak surge current but the rest of the cycle is basically dead, making those wires a big waste. For a big power user like a factory, youll pay huge penalties if you do something dumb like this so good power factor is a must.For little household users like us, its mostly good manners to strive for good power factor. Our surge is a drop in the bucket, but when added up to all the homes in the neighborhood its gotta count! The more important reason is that for a fixed 20A circuit, a good power factor allows us to get more power out.The interesting thing is that this topology seems to (if implemented correctly) give PFC for free!Heres a scope shot showing the envelope of the current drive as you can see, the amplitude is directly proportional to the rectified AC line. These scope shots were obtained by winding up a cliplead into a 4-turn coil and clipping across a 50mOhm resistor. So the cliplead acts as a tiny transformer winding to pick up the electromagnetic field from the work coil. The cliplead coil measured 850nH, for those that are interested.

Fig 13 Long Timescale View of the Work Coils FieldIn this next shot, we can see a closer view of the field signal from the work coil. Now since the sense resistor was 0.050 Ohm (1%), this measurement of 500mV of signal means that our little 850nH sense coil is circulating 10A! Imagine how much current is circulating in the pan, which should only measure a few nH at best massive!

Fig 14 Closer view of the Work Coils Field

And the final scope shot shows a close-up view of the work coils field. Its not exactly sinusoidal, although its pretty close. What we are picking up here is a replica of the current through the work coil. The little glitch at the bottom right of each cycle is probably due to the diode turnoff/switch turnon, which makes sense when you consider that the current begins to rise right after the glitch the switch must be on!

Fig 15 Zoomed in View of the Work Coils Field.Stay tuned for more analysis of the circuit, and some theories on how to build a controller for this type of device. This article is already WAY too long to even begin to delve into the circuit diagram. But trust that we will be back very soon with our ratty hand-drawn schematics converted into pretty PDFs for your viewing. Until then