Introduction

IKEA had their single stand alone induction stove on sale for 40 Euro and that was cheap enough to buy it only to take it apart and possibly destroy it during experiments.

Perhabs this could be a quick and cheap jump into a 2 kW induction heater for DIY purposes.

Safety

WARNING!: Working with electricity is dangerous, all information found on my site is for educational purpose and I accept no responsibility for others actions using the information found on this site.

Read this document about safety! http://www.pupman.com/safety.htm

Considerations

Cheap and simple circuits like these, that run from 230 VAC mains often use a sketchy power supply for the logic circuit, the ground is often floating and could be several hundred Volt away from a true ground. This makes it risk filled to interface with and measure on without a differential probe.

I expect to explode some part of this induction stove, more than once.

The primary / work coil in a quasi-resonant circuit like this is live at full resonant voltage! Short-circuit of the coil or touching it while it is running can be fatal to both yourself and the circuit!

Specifications

What is in the box and a look at the electronics? (Part 1 of 5)

The box is plain, simple and white with a black stencil of the induction stove on it. The IKEA name for the product is TILLREDA and means “to prepare a meal”.

The control of the induction stove is straight forward. There is a children lock, on/off, pause and intensity buttons. The stove also has temperature protection right in the centre of the work coil.

Overall impression of the inside is that it is a very cheap and simple design with a microcontroller, logic power supply and a single switch inverter topology using a IGBT.

The brand name on boards and components are HIGHWAY, which is from the Chinese company Guangdong Highway Electronic Technology Co., Ltd.

In the following table is the manufacturers own “datasheet” for the MCU found in the IKEA induction hob. Click the model number for their own page, I also corrected the spelling errors and I think it is worth mentioning that they advertise their product specifically as Imported chip with stable performance.

A few searches quickly gave me the idea that Holtek could be the true manufacturer of the microcontroller, as these are used extensively in products from China and the above specifications also pointed me in that direction.

I have spent hours browsing through the product catalogues of Holtek Semiconductor Inc. and the closest I ever got to find a microcontroller with all the above specifications was the HT46R51A, but the pinout does not match 100%, but very close, so far my conclusion on this IC is that its a older product or simply a custom pinout IC made specifically for Highway.

The IGBT is also of HIGHWAY brand and has some of the same funny features, no datasheets or curves are needed if the transistor can take a harder beating with a hammer than Fairchild or Siemens…

It is also possible to get a 2500 Watt version at almost the same price from Alibaba: https://www.alibaba.com/product-detail/2014-2500W-small-size-mini-induction_1863654875.html

Measurements of inverter voltage, primary current and gate drive (Part 2 of 5)

Test setup is compromised of the following 3 instruments connected to a Rigol DS1054Z oscilloscope.
Inverter output voltage, measured across the work coil, is done with a 1300V Tektronix P5200 differential probe. 250V/div on the oscilloscope.
LC circuit current is measured between the work coil and resonant capacitor with a Pearson electronics model 110 current monitor. 20A/div.
Gate drive waveform is measured between Gate and Emitter with a two-channel Tektronix A6909 isolator. 10V/div.

Quasi-resonant inverter topology
The output power of the inverter can be controlled by a Pulse Frequency Modulation (PFM) with fixed off-time and variable on-time. The waveform of the resonant voltage changes whenever DC-LINK becomes LOW or there is any change in load impedance. The amplitude of DC-LINK (VDC) ranges from zero to maximum as the capacitor has a small capacity.

When observing the waveforms of the current and voltage in the resonant circuit, it can at first be very confusing as the measured amplitude seems to follow the trigger level, so it is actually easy to lock on to a stable signal, but that does not tell the whole story. With the horizontal time base at 10 us/div where the single switching can be observed, it is impossible to discover what the waveform actually looks like at 2 ms/div horizontal time base, here it can be seen that all power is drawn within the full-wave rectified mains 100 Hz envelope.

Equivalent of circuit

Single quasi-resonant cycle analysis
Mode I: t0-t1
The IGBT is turned off at t0. VCE is gradually increased by the capacitor (Cr) to become DC-LINK (VDC) at t1. Even when the IGBT is turned off at t0, the current keeps increasing to reach its peak at t1, when VCE becomes equal to VDC. At this point, the energy stored in the inductor begins to be transferred to the capacitor.

Mode II: t1-t4
As VCE gets higher than VDC after t1, the current is decreased and reaches zero at t2, while the resonant voltage reaches its maximum level. This is also the point where the transfer of the energy, stored in the inductor, to the capacitor is completed. The peak level of the resonant voltage has a direct relationship with the on-time of the IGBT (Mode IV: t5-t6). After t2, the capacitor starts discharging the energy to the inductor, which causes the resonant voltage to decrease and reach its minimum level at t3, i.e. VCE=VDC. Passing t3, the resonant current increases as VCE<VDC and the discharge is completed at t4.

Mode III: t4-t5
At t4, VCE becomes zero and the anti-parallel diode, D1, turns on naturally. Since the resonant current is flowing through D1, the voltage drop of the IGBT remains zero. Therefore, Zero Voltage Switching (ZVS) turn-on can be achieved by turning on the IGBT in this mode.

Mode IV: t5-t6
At t5, the current direction changes and flows through the inductor. Therefore the inductor starts to store the energy. At t6, the IGBT is turned off, returning to Mode I.

Pulse Frequency Modulation
Divided by the red line we have power mode 5 at the bottom and power mode 9 at the top.
It can be observed that the off-time of the purple gate drive signal is identical in both power modes, so this pulse frequency modulator operates with fixed off time.
In quasi-resonant switching, the device does not have a fixed switching frequency, which is also clear from looking at the two waveforms, due to the longer on-time at high power, the resonant frequency is lower. The microcontroller waits for one of the negative half-cycles in the collector voltage and then switches the IGBT on.

The time between IGBT turn-off and the first negative half-cycle is fixed by the resonant frequency. The time between IGBT turn-on and turn-off is set by the microcontroller.
The narrow frequency span from 22 to 25 kHz does not pose any significant problems in designing the magnetic components, but it is enough to get the resonant current to rise up the maximum power level that can be drawn from a regular mains outlet.

Hacking the control loops of the MCU (Part 3 of 5)

Check back later for updates.

Using it as a induction heater (Part 5 of 5)

Check back later for updates.

Using it as a Tesla coil (Part 5 of 5)

Check back later for updates.

Conclusion

Part 1: It is a cheap kitchen appliance and it is cheap for a reason. It is made from a absolutely minimum of parts with a custom/Chinese controller that is most likely a programmed microcontroller.

References