AN2383
Application note
A single plate induction cooker
with the ST7FLITE09Y0
Introduction
This application note describes an induction cooking design which can be used to evaluate
ST components or to get started quickly with your own induction cooking development
project.
Induction cooking is not a radically new invention; it has been widely used all around the
world. With recent improvements in technology and the consequent reduction of component
costs, Induction cooking equipment is now more affordable than ever.
The design provides an opportunity to understand how an induction cooker works and to
make an in-depth examination of the various blocks and parts of this type of cooking
application such as the driving topology, how the resonant tank works, how the pot gets hot
and how to remove it safely from the cooking element.
The design is entirely controlled by a simple ST7FLITE09Y0 8-bit microcontroller, which
provides the PWM driving signals, communicates information to the user interface, and
drives the fan and relay control to the plate feedback.
September 2009 Doc ID 12433 Rev 3 1/39
www.st.com
Contents AN2383
Contents
1 Induction heating basics . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 6
2 Block diagram . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 7
3 Schematic . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 8
3.1 Mains, DC link voltage and zero voltage switching . . . . . . . . . . . . . . . . . . . 8
3.2 Isolated power supply . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 9
3.3 Power stage . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 10
3.4 Feedbacks . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 11
3.5 MCU pin configuration . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 12
4 How the system works . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 14
4.1 Standby (system off) . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 14
4.2 System on . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 14
4.3 Safety relay and fan management . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 14
5 Measurements at 50 Hz . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 15
5.1 Standby (system off) . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 15
5.2 Powering the plate (without pot) . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 16
5.3 Powering the plate (with pot) . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 17
5.4 Working level 1 . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 18
5.5 Working level 9 . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 19
5.6 Real-time current adjustment . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 19
5.7 Removing the pot . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 20
6 Measurements at PWM frequency . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 21
6.1 Powering the plate (with pot) . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 21
6.2 Working level 1 . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 22
6.3 Working level 9 . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 23
6.4 Current waveform at 50 Hz . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 24
7 Alarm management . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 25
2/39 Doc ID 12433 Rev 3
AN2383 Contents
8 User interface . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 26
8.1 Keyboard schematic . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 26
8.2 Display schematic . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 27
9 Software management . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 28
10 Thermal conditions . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 30
11 Bill of material . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 31
12 Demonstration board . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 34
13 References and related materials . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 37
14 Revision history . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 38
Doc ID 12433 Rev 3 3/39
List of tables AN2383
List of tables
Table 1. Bill of material (part 1 of 3) . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 31
Table 2. Bill of material (part 2 of 3) . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 32
Table 3. Bill of material (part 3 of 3) . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 33
Table 4. Document revision history . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 38
4/39 Doc ID 12433 Rev 3
AN2383 List of figures
List of figures
Figure 1. Induction cooking design block diagram. . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 7
Figure 2. Mains and +325 volt DC link . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 8
Figure 3. Isolated power supply, 5 and 15 volt . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 9
Figure 4. L6384 IGBT driver and power stage. . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 10
Figure 5. Current peak, current phase and alarm . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 11
Figure 6. MCU pin configuration . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 12
Figure 7. System in standby mode . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 15
Figure 8. Plate power-on (without pot on the plate) . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 16
Figure 9. Plate power-on (with pot on the plate) . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 17
Figure 10. System working at level 1 . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 18
Figure 11. System working at level 9 . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 19
Figure 12. Plate power-on (with pot on the plate) . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 21
Figure 13. System working at level 1 . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 22
Figure 14. System working at level 9 . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 23
Figure 15. Current waveform at 50 Hz . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 24
Figure 16. The analog keyboard . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 26
Figure 17. Display circuitry. . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 27
Figure 18. The six most important software events . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 28
Figure 19. Demonstration board photo (no cooking plate connected) . . . . . . . . . . . . . . . . . . . . . . . . . 34
Figure 20. User interface . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 35
Figure 21. Resonant capacitors (in blue) . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 35
Figure 22. Reverse angle. . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 36
Doc ID 12433 Rev 3 5/39
Induction heating basics AN2383
1 Induction heating basics
Put simply, an induction cooking element (what on a gas stove would be called a "burner") is
a special kind of transformer. When a good-sized piece of magnetically conducting material
such as, for example, a cast-iron frying pan, is placed in the magnetic field created by the
cooking element, the field transfers ("induces") energy into the metal. That transferred
energy causes the metal - the cooking vessel - to become hot.
By controlling the intensity of the magnetic field, we can control the amount of heat being
generated in the cooking vessel and we can change that amount instantaneously.
Induction cooking has several advantages over traditional methods of cooking:
● Speed: conductive heat transfer to the food is very direct because the cookware is
heated uniformly and from within. Induction cooking is even faster than gas cooking
● Safety: there are no open flames. This reduces the chances of fire and the cold stove
top is also more child safe
● Efficiency: around 90%. Heat is generated directly in the pot, while for electric and gas
the efficiency is around 65% and 55% respectively due to heat transfer loss.
Induction cooking functions based on the principle of the series L-C resonant circuit, where
the inductance L is the cooking element itself.
By changing the switching frequency of the high voltage half-bridge driver, the alternating
current flowing through the cooking element changes its value. The intensity of the magnetic
field and therefore the heating energy can be controlled this way.
6/39 Doc ID 12433 Rev 3
AN2383 Block diagram
2 Block diagram
Figure 1. Induction cooking design block diagram
USER INTERFACE FAN
PLATE
DRIVER 2x IGBTs
MCU
ST7LITE
RELAY
POWER
SUPPLY
5 / 15V
PLATE FEEDBACK
AI12605
The induction cooking design consists of a small number of simple blocks.
The isolated power supply is obtained directly from the mains, 220 V AC 50 Hz. 15 volts are
used to supply the IGBT driver, fan, relay and feedback circuitry, while 5 volts are needed to
supply the rest of the ICs, including the MCU.
The ST7FLITE09Y0 microcontroller controls the whole process and communicates with the
user interface (buttons and display), drives the fan and the relay, receives feedback from the
cooking element (referred to in this document as “plate” for simplicity) and generates the
PWM signal to drive the IGBTs.
Doc ID 12433 Rev 3 7/39
Schematic AN2383
3 Schematic
Although the schematic is not very complex, this section presents the different parts as
separate topics:
● Mains, DC link and zero voltage switching
● Isolated power supply
● Power stage
● Feedbacks
● MCU pin configuration
The user interface schematic is not presented in this section. It is discussed and analyzed in
Section 8.
3.1 Mains, DC link voltage and zero voltage switching
Figure 2. Mains and +325 volt DC link
FST1
FST2
LI N E
NEUTRAL
1
2
C1
1µF
275V X2
1
2
FS T3
EAR TH
1
2
RV1
460V
124
R1
470K
1W
L1
TDK_TF 2510H
AC N
C2
C4
3n3 Y1
3
C3
3n3 Y1
RELAY
1µF
275V X2
AC L
R2
4K7
C5
1µF
275V X2
RL1 12V
16A 250VAC
R3
10K
2
7
+15V
3
6
1
8
1N4 007
Q3
BC 337
D1
R4
8K2
4
5
DL1
LED
470K
RV2
275V
R5
1W
~
+
270K
D2
~
25A
R8
R9
8K2
R6
220K
R7
220K
+325V
R10
4K7
C6
22nF
TP1 4
VLINK
R11
4K3
R12
4K3
R13
4K3
R14
4K3
ACL
R15
4K3
50Hz
R17
4K3
D13
1N4007
2
PC817
R16
2K2
1
ISO2
4
+5V
3
TP17
The mains is filtered and is not applied directly to the power diode bridge: for safety reasons,
it goes through a relay. This means that the DC link voltage is not applied to the IGBT while
the system is off.
The 14 V DC relay is driven by the MCU through a classic NPN transistor. An LED is also
present.
When the system is on - and the AC line is applied to the power diode bridge - the IGBTs
are supplied with +325 V. The resistive divider sends an image of the DC link voltage to the
MCU (label VLINK). The last part of the schematic is an isolated zero voltage switching
(ZVS): a square waveform at 50 Hz synchronized with the mains (label 50 Hz).
8/39 Doc ID 12433 Rev 3
AI12612
AN2383 Schematic
3.2 Isolated power supply
Figure 3. Isolated power supply, 5 and 15 volt
AC L
AC N
L3
330µH
D15
PKC-13 6
8
~
2
V
DD
FB
Source11Source2
U4
VIPer2 2A
F
~
D16
1,5A
+
+C23
+C22
10µF
10µF
400V
400V
–
R29
10
4
Drain15Drain26Drain37Drain4
3
+C25
10µF
35V
D14
BAR4 6
C40
22nF
T1
TRONIC
348
2
1
C4 1
2n2 Y
1 2
ISO1
5
815
U5
TL431I
43
D10
STPS2H100
R38
1K
R39
1K
C24
100nF
R47
24K
R46
4K7
R48
4K7
R30
n.c.
+15V
+
TP 15
C26
330µF
35V
+15V+15V
+
U6
L7805CV
VIN1
C27
330nF
C38
100µF
35V
GND
2
V
OUT
C28
100 nF
C29
10µF
16V
AI12611
An isolated power supply is connected immediately after the mains filtering, without passing
through the safety relay. A VIPer22A and a simple voltage regulator provide 15 and 5 volts
respectively. The power supply ground is isolated from the system ground.
+5V
TP 16
3
+
Doc ID 12433 Rev 3 9/39
Schematic AN2383
3.3 Power stage
Figure 4. L6384 IGBT driver and power stage
PLATE
Q1
Q2
T2
TDKCT034
1:20
41
32
R22
11R
R2147R
C36
47nF
R25
11R
R24
47R
0
D8
STTH102
C15
1µF
D9
STTH102
8
7
6
5
V
BOOT
HVG
V
OUT
LVG
D17
STTH102
V
CC
DT/SD
GND
U2
L6384
+15V
D18
ST TH102
1
IN
2
3
4
R26
220K
C16
2n2
R27
PWM0
1K
C17
+
10µF
35V
C37
47nF
R28
4K7
AI12613
+325V
L2
80µH
TR1
1.5KE
R19
470K
1W
C10
3µF
400V
FST4
SCREW
C11
680n
800V
C12
680nF
800V
1
C13
33nF
800V
C14
33nF
800V
FST5
SCREW
40NC60V D
STGY
40NC60VD
STGY
1
R20
10K
R23
10K
The +325 V DC link voltage is applied through a filter to the upper-side IGBT only when the
safety relay is closed and the system is on. Components inside the dotted rectangle are the
core part of the power stage: the L-C resonant tank is obtained by the plate (represented in
the schematic by a spiral) and the capacitors on the left side. The resonant capacitor has
been divided in two identical capacitors, so that the amount of current flowing through each
capacitor is reduced by half, while the voltage to the capacitors remains the same.
A current transformer has been placed in series with the plate in order to provide plate
feedback information to the MCU.
The IGBTs are driven by high frequency complementary square waves with 50% duty cycle.
The PWM0 signal applied to the driver input pin is generated directly by the MCU. The
frequency varies in a range between 19 kHz and 60 kHz.
For more information regarding the dead time, charging pump capacitor and driving
topology, please refer to the L6384 datasheet.
10/39 Doc ID 12433 Rev 3
AN2383 Schematic
3.4 Feedbacks
Figure 5. Current peak, current phase and alarm
T2
03
K_
TD CT 4
1:200
41
32
TP19
TP18
C20
22nF
R44
1M
TH1
110
+15V
8
5
+
7
6
–
4
LM
U3B
Temperature control
for plate
PT1000 sensors
+15V
C21
100nF
8
3
1
+
2
–
U3A
4
LM 258
Alarm management
258
R37
10K
R45
12K
TMP2
with
ALARM
AI12610
R18
100R
D4
STTH102
D6
D7
STTH102
D5
STTH102
NTC4
47K
R69
100K
STTH102
+5V
R68
1K
C44
10nF
R31
33R
1W
R32
2K7
TMP1
R33
n.c.
C18
22nF
I-CTRL
R34
1K8
R35
2K2
R36
4K7
R41
100K
NTC2
PT1000
TP20
+15V
C19
22nF
C31
100nF
PT1
50K
R40
62K
+15V
Feedback signals are output by the current transformer placed in series with the plate, and
temperature sensors.
The most important feedback is the current signal (label I-CTRL), which sends the MCU an
image of the current flowing through the plate. This signal is used to monitor the current and
set it in accordance with the selected working level.
In addition, the signal coming from the current transformer is sent to an operational
amplifier. If for any reason the current increases too much, exceeding the alarm threshold
set by the potentiometer, the MCU immediately takes action to prevent damage to the power
stage.
A NTC has been glued to the heatsink between the IGBTs. The signal is sent to the MCU to
monitor the heatsink temperature and drive the fan accordingly. In the same way, a PT1000
is placed in the middle of the plate to monitor the plate temperature. The signal is amplified
and sent to the MCU for processing.
Waveforms and a description of how these signals interact with the MCU are given in
Section 5: Measurements at 50 Hz.
Doc ID 12433 Rev 3 11/39
Schematic AN2383
3.5 MCU pin configuration
Figure 6. MCU pin configuration
+15V
J8
C30
100µF
35V
+
D3
1N4007
1
2
FAN
+5V
C35
220µF
16V
+
C8
100nF
I-CTRL
TP1
KEYS
TP2
TMP1
TP3
TMP2
TP4
VLINK
TP5
R51
1M
Reset Pin Hints:
R51 is mandatory if re sidual voltage is still on Reset Pin.
R52 is not mandatory, its functionality has to be checked dur ing tests .
U1
ST 7F LITE 09
1
V
S
S
2
V
DD
3
RESET
4
SS/ AIN0/PB0
5
SCK/AIN1/PB1
6
MISO/A IN 2/PB 2
7
MOSI/A IN3/ PB3
8
CLKI N/AI N4 /PB4
C9
10 nF
ExternalI nterrupt request:
PA0 - 16 - ei0 - AL ARM
PA7 - 9 - ei1 - 50Hz
PA7
TP11
16
15
14
13
12
11
10
9
ei1
PA0 (HS)/LTIC
PA1 (HS)
PA 2 (HS)/ATPWMO
PA 3 (HS)
PA 4 (HS)
PA 5 (HS)/ICCDATA
PA6/MCO/ICCCLK
ei0
50Hz
RELAY
ALARM
DATA
PWM0
SCLK
/LE
+5V
R52
10K
TP12
TP13
TP6
TP7
TP8
TP9
TP10
+5V
R50
10K
R49
100
J7
CON10A
I CC Programmer
Q5
STS5NF60L
1
2 7
3
4
Fan control
+5V
12
34
56
78
910
8
6
5
AI12608
The ST7FLITE09Y0 microcontroller controls the whole induction cooking system. It can be
in-circuit programmed (ICP) via a standard 10-pin connector.
Starting from the left, going clockwise, the first input is the VLINK. It comes from the power
diode bridge and is an image of the DC link voltage applied to the upper side IGBT. Read as
analog input, this signal is used by the MCU to detect when a pot is placed on the plate or
when it has been removed.
TMP1 and TMP2 provide the MCU with the temperature information coming from the
heatsink and plate, respectively.
KEYS is an analog input read by the internal A/D converter of the MCU, and is connected to
the keyboard in the user interface. The keyboard features 3 buttons. In order to save MCU
pins, a smart schematic has been adopted, so that just one input pin is needed to read all
the keys.
The I-CTRL feedback is processed as analog input. It is an image of the current flowing
through the plate.
12/39 Doc ID 12433 Rev 3
AN2383 Schematic
ALARM has to be sent to the MCU as fast as possible, therefore this input has been
configured as an external interrupt. As soon as an alarm occurs, the MCU immediately
starts the alarm management routine so it can rapidly take the necessary actions.
DATA, SCLK and /LE are used to drive the 8-bit constant current LED sink driver present in
the user interface board. In this way, the MCU can address a 7-segment display using only 3
pins.
PWM0 generates a PWM signal with a 50% duty cycle. It is sent directly to the IGBT driver.
Depending on the working level (and therefore on the power required), the frequency of the
square waveform varies in a range between 19 and 60 kHz.
RELAY and FAN drive the safety relay in the mains circuitry and the fan, respectively. The
fan is used to cool the heatsink next to the IGBTs and the power diode bridge.
The last pin, 50 Hz, is configured as an external interrupt. It is synchronized with the voltage
mains and, every 10 ms, it captures the moment when the AC voltage crosses zero.
Doc ID 12433 Rev 3 13/39
How the system works AN2383
4 How the system works
4.1 Standby (system off)
As soon as the induction cooking system is plugged into the mains, the system is running
and the MCU goes into standby mode, or put simply, “system off”.
No controls or actions are taken, only the keyboard is scanned to capture a “button pressed”
event. The display shows "-".
In this status, putting or removing a pot from the plate has no impact on the system
functionality. The safety relay contacts are open, so no DC link voltage is applied to the
resonant tank.
4.2 System on
The system is turned on by pressing the on-off button (the first on the left in the user
interface).
Each time it is switched on the induction cooking system performs a sequence: safety relay
first, then plate power-on. The safety relay contacts close, which applies the DC link voltage
to the resonant tank.
At this point, the system temporarily powers-on the plate: a 60 kHz PWM signal is applied to
the half-bridge driver for half a second. During this time, if a pot is placed on the plate, or it is
there already, the system moves to the lowest operating power level, shown as "1" in the
user interface display. If however, no pot is detected on the plate, the system stops the PWM
signal. Another power-on sequence is performed after 10 seconds. After 5 unsuccessful
power-on sequences, the system goes back to standby mode.
When the PWM signal is applied to the half-bridge driver, the decimal point in the user
interface display turns on.
Once a pot is detected, the user can move through 9 working power levels by pressing the
buttons on the user interface. 1 is the lowest level, and 9 is the maximum.
4.3 Safety relay and fan management
The safety relay prevents the DC link voltage from being applied to the resonant tank when
the system is off. The relay contacts are connected in series with the plate, and they close
when the system is turned on. To prevent oscillation or undesired relay commutations, an
anti-bounce software routine is implemented. The relay turns off when the system turns off.
The fan helps the heatsink dissipate the heat while the system is working. It is turned on as
soon as the heatsink temperature reaches 55 °C. The fan stays on for at least one minute,
whether the system is on or in standby mode.
14/39 Doc ID 12433 Rev 3
AN2383 Measurements at 50 Hz
5 Measurements at 50 Hz
The following oscilloscope waveform readings have been taken during the different
operating phases. These signals are synchronized with the voltage mains, therefore running
at 50 Hz frequency.
5.1 Standby (system off)
Figure 7. System in standby mode
MAINS
ZVS
VLINK
I - CTRL
In standby mode, the zero voltage crossing signal is the only active one. The square wave is
sent to the MCU and used to synchronize all the events. VLINK, an image of the DC link
voltage (not yet applied to the plate), is constant. No current flows through the plate.
Doc ID 12433 Rev 3 15/39
Measurements at 50 Hz AN2383
5.2 Powering the plate (without pot)
Figure 8. Plate power-on (without pot on the plate)
MAINS
ZVS
VLINK
PDT
I - CTRL
The DC link voltage is applied to the plate and the PWM signal is applied to the half-bridge
driver for half a second. Due to the resonant tank consumption, a voltage drop appears on
the VLINK signal. The voltage drop is read by the MCU to detect if a pot is present on the
plate.
No pot is on the plate, so the voltage drop is not big enough to exceed the pot detection
threshold (PDT), set by software at 500 mV. The PWM signal is stopped, and the powering
sequence is repeated after a break of 10 seconds.
Powering the plate continuously in order to detect a pot would result in an increase in power
consumption. However, no parts would burn or be damaged. The break of 10 seconds
between one powering sequence and another reduces power consumption while keeping
full functionality and pot control.
16/39 Doc ID 12433 Rev 3
AN2383 Measurements at 50 Hz
5.3 Powering the plate (with pot)
Figure 9. Plate power-on (with pot on the plate)
MAINS
ZVS
VLINK
PDT
I - CTRL
The DC link voltage is applied to the plate and the PWM signal is applied to the half-bridge
driver for half a second. Due to the resonant tank consumption, a voltage drop appears on
the VLINK signal. The voltage drop is read by the MCU to detect if a pot is present on the
plate.
In this case, the pot is on the plate and the voltage drop is high enough to exceed the pot
detection threshold (PDT), set by software at 500 mV. A certain current is now flowing
through the plate.
The pot is detected, so the system can move to the first working level: level 1.
The waveform shown in Figure 9 was taken while a 22 cm-diameter iron pot filled with water
was placed on the plate.
Doc ID 12433 Rev 3 17/39
Measurements at 50 Hz AN2383
5.4 Working level 1
Figure 10. System working at level 1
MAINS
ZVS
VLINK
I - CTRL
As soon as the pot is detected, the system moves to level 1, the lowest power working level.
The PWM signal applied changes accordingly. The lower the working level, the higher the
PWM frequency applied to the half-bridge driver, and vice-versa.
I-CTRL signal is an image of the current flowing through the plate and is sent to the MCU by
the current transformer placed in series with the plate.
With the system working properly, there must be a certain current flowing through the plate,
as the I-CTRL waveform shows in Figure 10. Even if we are talking about current, the
waveform unit is expressed in volts and processed by the MCU as a voltage level.
The waveform shown in Figure 10 was taken while a 22 cm-diameter iron pot filled with
water was placed on the plate.
18/39 Doc ID 12433 Rev 3
AN2383 Measurements at 50 Hz
5.5 Working level 9
Figure 11. System working at level 9
MAINS
ZVS
VLINK
At level 9, the system delivers the maximum output power. I-CTRL rises accordingly.
The waveform shown in Figure 11 was taken while a 22 cm-diameter iron pot filled with
water was placed on the plate.
5.6 Real-time current adjustment
As seen, the induction cooking system works on the principle of a series L-C resonant
circuit. When the size of L and C are set, the resonant frequency is set as well.
Unfortunately, this value does not depend only on the resonant tank. In fact, the size and
material of the pot affect the resonant frequency too. This causes the system to have an
oscillating resonant frequency strongly dependent on the type of pot placed on the plate at
different times.
Therefore the 9 working levels cannot be based on constant frequency levels. The PWM
frequency must be adjusted to the selected level in order to work with the pot placed on the
plate at that moment.
So each working level does not work on a constant PWM frequency, but a constant current.
By reading the I-CTRL feedback signal, the MCU smoothly adjusts the PWM frequency in
order to keep the current constant for the selected working level. Each level has a
corresponding constant value of current.
I - CTRL
Doc ID 12433 Rev 3 19/39
Measurements at 50 Hz AN2383
5.7 Removing the pot
A pot placed on the plate may be removed at any time, including when the system is
working.
As seen before, the voltage drop present in the VLINK signal determines whether a pot is
placed on the plate or not. The VLINK signal is captured continuously while system is
working.
Lifting the pot up from the plate causes the voltage drop in the VLINK signal to decrease. As
soon as the voltage drop rises over the pot detection threshold (PDT), the MCU recognizes
that the pot has been removed.
The PWM signal is not stopped at once, but smoothly increased until the 50 kHz frequency
is reached, and then stopped. This procedure avoids current spikes on the resonant tank
line and prevents the power stage burning or being damaged.
At this point, the system is still on, without a pot on the plate. The MCU powers on the plate
5 times with a break of 10 seconds between one powering sequence and the other. If no pot
is placed back on the plate during this time, the system returns to standby mode.
This feature is very useful in cases where the user removes the pot and forgets to turn off
the induction cooking system.
20/39 Doc ID 12433 Rev 3
AN2383 Measurements at PWM frequency
6 Measurements at PWM frequency
The following scope waveforms were taken during the different working phases. These
signals are synchronized with the PWM signal, therefore running at PWM signal frequency.
The waveforms shown in Figure 12 and Figure 13 were taken while a 22 cm-diameter iron
pot filled with water was placed on the plate.
6.1 Powering the plate (with pot)
Figure 12. Plate power-on (with pot on the plate)
PVM
UP-G
LW- G
I - PLATE
The DC link voltage is applied to the plate and the 60 kHz PWM signal with 50% duty cycle
is applied to the half-bridge driver for half a second. UP-G and LW-G are the upper side gate
signal and the lower side gate signal, respectively. Of course they must be complementary
and there must be a certain dead time between the upper gate pulse and the lower one. The
dead time is set by hardware (L6384, resistor on pin 3).
Since the pot is on the plate, a certain current starts to flow through the plate (I-PLATE). The
unit in Figure 12 is expressed in volts, but the current probe connected to the scope is set at
20 amperes per division. This means that at plate power-on the system is already delivering
a 20 ampere peak-to-peak current.
Doc ID 12433 Rev 3 21/39
Measurements at PWM frequency AN2383
6.2 Working level 1
Figure 13. System working at level 1
PVM
UP-G
LW-G
I - PLATE
Level 1 is the first and the lowest power working level. The PWM signal frequency,
previously set to 60 kHz, smoothly decreases until the working current for level 1 is reached.
As seen before, the PWM frequency is not constant and is adjusted in real-time to keep the
current level constant. Natural changes such as the iron dilatation or the water warming up,
slightly modify the resonant frequency and therefore the current delivered.
For the 22 cm-diameter iron pot used for the test, level 1 means a PWM frequency of around
48.5 kHz, but variations of several kilohertz are possible and necessary in order to keep the
current level constant.
In this test, level 1 features a 40 ampere peak-to-peak current (I-PLATE, 20 ampere per
division).
22/39 Doc ID 12433 Rev 3
AN2383 Measurements at PWM frequency
6.3 Working level 9
Figure 14. System working at level 9
PVM
UP-G
LW-G
I - PLATE
Level 9 is the highest working level, with the system delivering maximum output power. To
increase output power, the PWM frequency must be decreased. Moving the working levels
up or down corresponds to a smooth increase or decrease of the PWM frequency, until the
current level for the selected working level is reached.
For the 22 cm-diameter iron pot used for the tests, level 9 means a PWM frequency of
around 25.0 kHz and a corresponding 100 ampere peak-to-peak current (I-PLATE, 50
ampere per division).
Doc ID 12433 Rev 3 23/39
Measurements at PWM frequency AN2383
6.4 Current waveform at 50 Hz
Figure 15. Current waveform at 50 Hz
In the waveforms in Section 6.1 through Section 6.3, the signals were shown at PWM
frequency.
Now, if we keep the system working and increase the scope time scale to observe a 50 Hz
frequency, the shape of the current delivered to the plate is different. As shown in Figure 15,
in phase with the mains, the current peak changes following the 50 Hz frequency, while the
current switching frequency runs at the PWM frequency.
The result is a double wave with a 100 ampere peak-to-peak current (20 ampere per
division).
With a 22 cm-diameter iron pot on the plate, the system delivers about 2500 watts.
24/39 Doc ID 12433 Rev 3
AN2383 Alarm management
7 Alarm management
The alarm circuitry is necessary for monitoring any possible malfunctions, and to prevent the
IGBTs, the driver, or any other circuitry from burning or being damaged.
The application described here features 4 different alarms: overtemperature on the heatsink
(H), overtemperature on the plate (t), overcurrent (C) and wrong pot on the plate (P).
An alarm is generated when the heatsink temperature exceeds 115 °C, or when the plate
reaches a temperature of 200 °C.
If for any reason the current flowing through the plate goes over the limit, an alarm occurs.
Similarly, while in the power-on sequence, if a non-magnetically conducting material is
placed on the plate, an alarm occurs. In alarm condition, the PWM frequency is immediately
set to 30 kHz, and then smoothly increased to 50 kHz. The system is put in standby mode
and the display shows which alarm occurred (refer to the letters in brackets).
Doc ID 12433 Rev 3 25/39
User interface AN2383
8 User interface
The user interface is implemented on a second PCB vertically soldered on the front side of
the induction cooking application. It features a 3-button keyboard and a 7-segment display.
8.1 Keyboard schematic
Figure 16. The analog keyboard
P1
TACT-2
1 4
2 3
R54
10 K
+5.V
R57
1K
P3
TA CT-2
1 4
2 3
R56
10K
C34
10 nF
R58
1K
KEYS
P2
TA CT-2
R55
10K
14
23
2V62 WHEN P1 PRESSED
5V00 WHEN P2 PRESSED
0V24 WHEN P3 PRESSED
1V78 WHEN NO KEY PRESSED
AI12607
The keyboard is designed with the primary intention of saving MCU pins. The KEYS pin is
directly connected to the analog input pin of the ST7FLITE09Y0.
The keys are placed in parallel with resistors, which means that every time a key is pressed,
it short-circuits its own resistor (P2 two resistors). This causes the voltage on the KEYS pin
to change as shown in Figure 16. Every individual keyboard status has its own related
voltage level.
The analog-to-digital converter of the MCU reads the status of the pin every 20 milliseconds.
Software sets a key-window for each key in the range of 0.5 V. For example, P2 is pressed if
the voltage applied to the KEYS pin is higher than 4.5 V.
If two or even all the keys are pressed together, there is an automatic priority selection.
For example, P2 has the highest priority. This is because this key, when pressed, connects
the KEYS pin directly to +5 V without passing through any resistor.
In addition, if the voltage applied to the KEYS pin does not fit any key-window, the voltage is
ignored and no action is taken.
26/39 Doc ID 12433 Rev 3
AN2383 User interface
8.2 Display schematic
Figure 17. Display circuitry
J12
CON6
J13
CON6
J14
CON12
For mechanical robustness only.
+5.V
6
5
4
3
2
1
1
2
3
4
5
6
12
11
10
9
8
7
6
5
4
3
2
1
DATA
SCLK
/LE
KEYS
J9
CON6
J10
CON6
J11
CON12
System Board Connectors
+5.V
6
5
4
3
2
1
1
2
3
4
5
6
12
11
10
9
8
7
6
5
4
3
2
1
DATA
SCLK
/LE
KEYS
+5.V
C32
100nF
U7
STP08CP05
1
GND
2
SDI
3
CLK
4
LE
5
OUT0
6
OUT1
7
OUT2
8
OUT3
DY1
Common Anode Display
9
d
7
c
5
b
4
a
3
A1
R-EXT
SDO
OUT7
OUT6
OUT5
OUT4
+5.V
16
V
DD
OE
dp
A2
R53
10K
15
14
13
12
11
10
9
10
e
2
f
1
g
6
8
+C33
100µF
16V
DISPLAY
AI12606
Although the initial design approach was to implement a user interface with a couple of
classic LEDs, the introduction of an 8-bit constant current LED sink driver in the display
circuitry improved the user interface, while still keeping the number of MCU pins used
relatively low.
The STP08CP05 needs just 3 pins to drive the display properly: DATA, SCLK, and /LE.
The display refreshing frequency is set at 50 Hz. Since the driver keeps the output signals
latched until the next refresh is performed, a lower frequency would not cause any flickering.
The display luminosity is set by an external resistor.
Connectors are duplicated solely for mechanical robustness, left side connectors are
parallel to the right ones.
Doc ID 12433 Rev 3 27/39
Software management AN2383
9 Software management
The MCU has to process six types of events: pot-on-plate detection, temperature, keyboard
scan, display refresh and current control. These events are processed every 20
milliseconds; in fact, they are synchronized with the zero voltage switching signal.
The ZVS circuitry generates a square wave with a frequency of 50 Hz. The signal is sent to
the ei1 MCU pin, which is configured as an external interrupt input for both rising and falling
edges. Therefore, an MCU interrupt is generated every time a falling edge or a rising edge
occurs on the pin.
Figure 18. The six most important software events
1
3456
2
The first event, shown in Figure 18 as number 1, takes place as soon as the interrupt
triggered by a rising edge on the ZVS signal occurs. Pot-on-plate detection is performed by
sampling the voltage drop on the VLINK signal during this time.
Before another interrupt occurs, there is still a lot of time to handle three other events.
Events 2 and 3 monitor the temperature of the heatsink and the plate respectively. During
event number 4, the keyboard is scanned to check if a key has been pressed (or released).
A software anti-bounce has been implemented to avoid undesired conditions.
Event number 5 takes place immediately after the interrupt generated by a falling edge of
the ZVS signal occurs. The display refresh routine is performed.
During the last event, number 6, the I-CTRL signal is scanned and compared to the look-up
table that the software refers to for each working level. If any discrepancy appears between
28/39 Doc ID 12433 Rev 3
AN2383 Software management
the sampled value and the table, the MCU adjusts the PWM frequency. The adjustment
process is performed step-by-step each period, resulting in a smooth current change.
The dotted lines in Figure 18 indicate the sequence in which the routines are performed, but
not the precise timing.
Events 2 and 3 together last less than 1 ms, and a similar time is needed for event 5.
Doc ID 12433 Rev 3 29/39
Thermal conditions AN2383
10 Thermal conditions
The induction cooking system described in this document can deliver up to 2500 W at its
maximum working level.
The IGBTs need to be mounted on a heatsink, as does the power diode bridge.
Tests performed in laboratory conditions demonstrate that even when delivering the
maximum power for a long duration, the temperature of the components does not exceed
the safe working area. The board was placed in an open space without an enclosure.
In a real application, the board would be placed inside a box. And, to save space, the plate
is usually placed over the circuitry. Therefore, the heat dissipated by the heatsink has no
easy way out, and the cooking process worsens the thermal conditions. The heatsink is no
longer sufficient to dissipate the heat. For this reason, a fan is implemented in the system
which is driven directly by the MCU.
Tests have demonstrated that while delivering the maximum power, the temperature
reaches a stable value below 90 °C, which can still be considered safe.
The fan starts as soon as the temperature on the heatsink reaches 55 °C. The fan stays on
for at least one minute, whether the system is on or in standby mode.
Fan management can be modified by software. An NTC mounted directly on the heatsink,
between the IGBTs, improves control efficiency. Working as a thermostat controlled by an
MCU, the sensor turns the fan on or off when necessary.
Any increase in the induction cooking system performance (for example, if higher output
power is required), would result mainly in adapting the cooling system, resizing the heatsink,
using a more powerful fan, or all three.
The system itself is capable of handling up to 3000 watts of power.
30/39 Doc ID 12433 Rev 3
AN2383 Bill of material
11 Bill of material
Table 1. Bill of material (part 1 of 3)
Item Quantity Reference Part Supplier
1 2 C30, C38 100 µF 35 V
2 1 C33 100 µF 16 V
3 3 C1, C4, C5 1 µF 275 VAC X2
4 1 C15 1 µF ceramic
5 2 C2, C3, C41 3,3 nF 250 VAC X1 Y1
6 4 C6, C18, C19, C20 22 nF 50 V ceramic
7 6 C8, C21, C24, C28, C31, C32 100 nF 50 V ceramic
8 3 C9, C34, C39 10nF 50 V ceramic
9 1 C10 3 µF 400 V
10 2 C11, C12 680 nF 1000 V
11 2 C13, C14 33nF 1000V
12 1 C16 2,2 nF 50 V ceramic
13 3 C17, C25, C29 10 µF 35 V
14 2 C22, C23 10 µF 400 V
15 1 C26 330 µF 35 V
16 1 C27, C40 330 nF 50 V
17 1 C35 220 µF 16 V
18 2 C36, C37 47 nF 50 V ceramic
19 1 DL1 LED red d. 3
20 1 DY1 Com. anode display
21 2 D1, D3, D13 1N4007
22 1 D2 Diode bridge 25 A
23 8 D4, D5, D6, D7, D8, D9, D17, D18 STTH102 ST
24 1 D10 STPS2H100 ST
25 1 D14 BAT46 ST
26 1 D15 PKC-136 ST
27 1 D16 Diode bridge 1.5 A
28 3 FST1, FST2, FST3 Faston vertical 6.3 mm
29 2 FST4, FST5 Screw
30 8 J1, J2, J4, J5, J9, J10, J12, J13 CON6
31 4 J3, J6, J11, J14 CON12
32 1 J7 CON10A
Doc ID 12433 Rev 3 31/39
Bill of material AN2383
Table 2. Bill of material (part 2 of 3)
Item Quantity Reference Part Supplier
33 1 J8 Fan 12 V 1,9 W
34 1 L1 TDK_TF2510H customized TDK
35 1 L2
36 1 L3 330 µH
37 1 NTC1 NTC 47 k
38 1 NTC2 PT1000
39 1 PT1 50 k vertical
40 3 P1, P2, P3 TACT-2 normally open
41 2 Q1, Q2 STGY40NC60VD ST
42 1 Q3 BC337
43 1 Q5 STS5NF60L ST
44 5 R3, R52, R53, R37, R50 10K 5% ¼ W
45 5 R20, R23, R54, R55, R56 10K 1% ½ W metal oxide
80 µH SF1-800Y10A-01PF
TDK
46 1 RL1 12 V (16 A 250 VAC)
47 1 RV1 460 V
48 1 RV2 275 V
49 3 R1, R5, R19 470 k 2 W
50 6 R2, R10, R28, R46, R48 4K 17 5% ¼ W
51 1 R36 4K7 1% ½ W metal oxide
52 1 R4 8K2 5% ¼ W
53 1 R9 8K2 1% ½ W metal oxide
54 3 R11, R12, R13 4K3 1% ½ W metal oxide
55 3 R6, R7, R26 220K 1% ½ W metal oxide
56 1 R8 270K 1% ½ W metal oxide
57 3 R14, R15, R17 4K3 1% ½ W metal oxide
58 1 R16 2K2 5% ¼ W
59 1 R35 2K2 1% ½ W metal oxide
60 1 R18 100R 2W
61 2 R21, R24 47R 1% ½ W metal oxide
62 2 R22, R25, R29 11R 1% ½ W metal oxide
63 5 R27, R38, R39, R43, R58 1K 5% ¼ W
64 1 R57 1K 1% ½ W metal oxide
65 1 R30 n. c.
32/39 Doc ID 12433 Rev 3
AN2383 Bill of material
Table 3. Bill of material (part 3 of 3)
Item Quantity Reference Part Supplier
66 1 R31 33R 2W
67 1 R32 2K7 1% ½ W metal oxide
68 1 R33 n. c.
69 1 R34 1K8 1% ½ W metal oxide
70 1 R40 62K 1% ½ W metal oxide
71 2 R41, R42 100K 1% ½ W metal oxide
72 2 R44, R51 1M 1% ½ W metal oxide
73 1 R45 12K 5% ¼ W
74 1 R47 24K 1% ½ W metal oxide
75 1 R49 100 5% ¼ W
76 1 TH1 Thermostat connection
77 19
78 2 ISO1, ISO2 PC817 optocoupler DIP4
79 1 TR1 1.5KE ST
80 1 T1 Customized trafo Tronic (CZ)
81 1 T2 TDK_CT034 TDK
82 1 U1 ST7FLITE09Y0 ST
83 1 U2 L6384 ST
84 1 U3 LM258 ST
85 1 U4 VIPer22A ST
86 1 U5 TL431I ST
87 1 U6 L7805CV ST
88 1 U7 STP08CP05 ST
TP1, TP2, TP3, TP4, TP5, TP6,
TP7, TP8, TP9, TP10, TP11,
TP12, TP13, TP14, TP15, TP16,
TP17, TP18, TP19
Test point
Doc ID 12433 Rev 3 33/39
Demonstration board AN2383
12 Demonstration board
Figure 19. Demonstration board photo (no cooking plate connected)
34/39 Doc ID 12433 Rev 3
AN2383 Demonstration board
Figure 20. User interface
Figure 21. Resonant capacitors (in blue)
Doc ID 12433 Rev 3 35/39
Demonstration board AN2383
Figure 22. Reverse angle
36/39 Doc ID 12433 Rev 3
AN2383 References and related materials
13 References and related materials
For further information related to the basic functionality of each integrated circuit, please
refer to the following documents, which are available at www.st.com:
1. ST7FLITE09Y0 datasheet
2. L6384 datasheet
3. VIPer22A datasheet
4. L7805CV datasheet
5. STGY40NC60VD datasheet
6. LM258 datasheet
7. STS5NF60L datasheet
Doc ID 12433 Rev 3 37/39
Revision history AN2383
14 Revision history
Table 4. Document revision history
Date Revision Changes
05-Sep-2006 1 Initial release.
22-Feb-2007 2 Introduction, Section on page 1, updated
Removed demonstration board ordering information.
23-Sep-2009 3
All references to part number STP08C596 have been replaced with
STP08CP05.
Minor text changes.
38/39 Doc ID 12433 Rev 3
AN2383
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Doc ID 12433 Rev 3 39/39
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