hi this is Silicon Soup I would like to share with you the theory and the design of induction
cookers from the physics point of view, circuit and the firmware points of views. If you are an
R&D engineer, we can compare notes and see if my explanation is accurate and useful,
but even if you are not into R&D, please stay tuned as this video will explain how to operate
your home induction cooker so that the IGBT is under the least stress and therefore last longer.
Induction cookers use magnetic induction to induce current in the cookware and the induced
eddy-current working against the cookware's resistance produces the heat needed for the
cooking. This is the induction coil, the heart of the induction cooker and the coil is wounded in
the spiral shape. When an electric current passes through the coil it sets up a magnetic field.
This is the magnetic field distribution in the coil viewed from the side.
The magnetic field achieved its maximum strength in the center and gradually decreases in strength
along the radius from the center to the edge. When the current changes in strength
or direction, then the magnetic field follows and changes the strength and direction together.
For example, when the coil current flows in the counter-clockwise direction, the B field
points in the upwards direction. When the current increases, the B field increases proportionally.
From Maxwell's Faraday equation, changing magnetic field induces electric field. When the
coil current increases in the counter-clockwise direction, the changing magnetic field induces an
electric field in the clockwise direction. When the cookware is placed in this electric field, the
electric field drives the current and the current working against the resistive path produces heat.
Notice that the eddy current always flows in the direction to resist the change of the current
in the coil. The Eddy current also produces a magnetic field in the direction resisting the
change of the magnetic field produced by the coil. The resultant magnetic field would become weaker
compared to the case if the eddy current were not there, and the self-induced EMF which depends on
the total changing magnetic field would become lower too. But before this actually happens the
lower self-induced EMF invites additional source current to flow into the coil in such a rate that
the additional magnetic field it produces cancels the opposing magnetic field from
the eddy current so that the resultant magnetic field in the coil is back to its original value.
So Eddy current in the cookware increases the current in the coil simultaneously even though
when there is no electrical connection between the cookware and the coil.
An interesting side note, we know that when two currents flow in parallel to each
other but in opposite direction both current carrying wires experience a repulsive force.
In the case of induction cooker the coil current and the induced eddy current flow in opposite
direction, so can we expect a force to push the cookware away from the coil? In this video, the
aluminum foil got pushed away from the induction coil therefore verifying that there was indeed an
induced eddy current in the foil flowing in the opposite direction of the coil current.
The induction coil and the cookware work like a loosely coupled transformer with the cookware
acting like a secondary coil short circuited. When the cookware is removed and therefore cut
off the secondary current, the primary current in the coil reduces accordingly, and from the
drop in the current we can determine that the cookware has been removed during the cooking.
The same mechanism also happens in the conventional transformer resulted in the primary
and secondary currents related by the turn ratio. Therefore we can use the transformer equivalent
circuit to simulate the induction circuit. Another point to note is that the current in the induction
coil is switched on and off by pwm and the duty cycle controls the power delivered for the
cooking. The advantage of pwm is that the IGBT is either fully turned on or fully turned off,
and won't get heated up, at least not in the ideal case. This is because when the collector current
is maximum , the collector-to-emitter voltage is almost zero, and when the collector-to-emitter
voltage is maximum, the collector current is zero, and in both cases consuming almost zero power.
This is the circuit of a commercial induction cooker. The AC power supply is rectified to become
a DC voltage source first, before supplying to the induction coil. The induction coil is connected to
the terminals out 1 and out 2. The coil is energized by the switching of an IGBT with
the pulse-width modulated with some suitable duty cycle. The pwm signal is generated from
the microcontroller which monitors the LC voltage and determines the appropriate timing to trigger
the IGBT. At the same time the microcontroller monitors the line voltage, coil current, IGBT
and coil temperatures, surge current and back EMF to give immediate protection whenever any anomaly
occurs. The circuit behavior of the induction coil interacting with the cookware can be simulated by
software such as TINA. As mentioned, induction coil and the cookware behave like a loosely
coupled transformer, and therefore we can borrow the transformer equivalent circuit with a load
resistance referred to the primary side. Based on actual measurement the coil inductance is about 95
mH, and the copper resistance is about 50 mohms respectively. The cookware resistance is very
small, but after impedance transformation to the primary side it is increased by a factor of the
turn ratio squared, and it becomes about 120 ohms. When the IGBT is triggered by a single pulse,
the LC circuit oscillates. When the cookware is there to absorb the energy from the inductor and
the capacitor, the oscillation attenuates rather quickly. However, if the cookware is absent, the
oscillation will sustain for many more cycles, as seen in the simulation. This is the screenshot
captured by the scope when the cookware is absent, and this is when the cookware is present.
Because of this distinctive difference, we can actually count the number of oscillations
within a fixed period of time and determine whether the cookware is present or absent.
We can also see that if the pulse width is wider, there is more energy stored in the coil, and when
the coil releases the energy, the amplitude of the oscillation is also higher. The voltage touches
the ground as it swings downwards. Conversely, if the coil is energized by a narrower pulse,
the amplitude of swings is smaller, and sometimes can't even touch the ground. To do continuous
cooking, the IGBT must continuously re-energize the induction coil every cycle of the oscillation, and
the best timing to re-trigger the IGBT is when the collector voltage touches the ground. Therefore
a hardware comparator, sometimes built-in the microcontroller, is used to monitor the voltage
swing and trigger the IGBT synchronously. The real-time power consumption is calculated on
the fly so that the microcontroller can adjust the pulse-width to get the desired power output.
This is the actual collector voltage when the induction cooker is set to the maximum output
2000 Watts. At the maximum power, the inductor is energized longer and when the IGBT is cut
off the inductor produces a flyback EMF, and CX is there to catch the current and absorb the voltage.
The voltage swing is so high that it could damage the IGBT and therefore the MCU monitors the
voltage and give protection if necessary. If the induction cooker power is set at 1400 watts, the
flyback EMF is reduced and the IGBT is relatively safe. When the power is further reduced to 1000
Watts, about 50 percent of the maximum power, the pulse-width is so narrow that the swing amplitude
is too low and the collector voltage cannot touch the ground. At even the best timing the IGBT will
have to short some significant collector voltage to the ground, which is equivalent
to shorting a charged capacitor to the ground, and the instantaneous current is huge, so huge that it can
sometimes kill the IGBT. Therefore the MCU must monitor the IGBT temperature and give the IGBT a
break when the temperature is too high, otherwise, the IGBT will break. If you have watched until
this point, congratulations to you because now you know to avoid running your induction
cooker as some 50 point of the full power, and 75 percent is the most comfortable for the IGBT.
Sometimes, using the wrong cookware could also damp the oscillation too much and kill
the IGBT. So it is better to use the cookware that comes together with the induction cooker.
The induction cooker comprises a main board and a user interface board. At the system level, the
user interface board is the master getting input from the user, sending instruction to the main
board, and receiving information from the main board for display. The main board is the slave
receiving instruction to perform the induction to the power level instructed. The main tasks
in the main boards are: number one, sampling the voltages, currents, the IGBT and coil temperatures;
number two, receiving instructions from the user interface board, and perform the induction
cooking accordingly; number three, prior to the induction, checking the presence or absence of
the cookware and turn on the IGBT only if the cookware is present; number four, using closed
loop control algorithm to achieve the desired cooking power; number five, monitoring the pan
removal while cooking, and stopping the cooking immediately if the pan is removed; number six,
protecting the IGBT in the events of over voltage, under voltage, over current, surge current, over
temperature, extreme flyback EMF. Besides doing theoretical analysis of circuit and Engineering,
I do hands-on circuit design and embedded firmware coding on a daily basis. For those who think that
my position that KVL doesn't always hold is due to my lack of practical experience, then I hope that
this video convinces you that I am not an armchair theorist. I hope this video gives you some ideas
what it takes to design an induction cooker. As usual I appreciate very much if you could click
on the Like- and the Share-button to help me promote this channel. See you in the next video