ST AN2383 Application note

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