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

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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

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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

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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

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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	PLATE FEEDBACK
	5 / 15V
			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.

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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.

3.1Mains, DC link voltage and zero voltage switching

Figure 2. Mains and +325 volt DC link

FST1

NEUTRAL

ACN

+325V

1

+5V

2

C 2

2

TP14

1

4

R6

R11

C1

RV1 R1

1µF

470K 2

3

3n3Y1

C4

C5

7

3

+15V

220K

4K3

50Hz

R16

275V X2

C 3

1µF

1µF

4

6

R4

RV2

R7

R12

2K2

FST2

460V 1W

L1

275VX2 275VX2

~

220K

4K3

1

8K2

275V

LI N E 1

3n3Y1 AC L

5

+

R10

R14 R15 R17 D13

8

D1

DL1

-

R8

R13

2

TDKFT2510H

RL1

12V

~ D2 270K

4K7

4K3

4K3 4K3 4K3 1N4007

1N4007

R5

1

4

LED

VLINK

FS T3

RELAY

16A250VAC

Q3

470K

25A

R9

C6

3

EARTH

R 2

R3

BC337

1W

22nF

ACL

2 ISO2

1

8K2

TP17

4K7

10K

2

PC817

AI12612

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

AN2383	Schematic

3.2Isolated power supply

Figure 3. Isolated power supply, 5 and 15 volt

L3

T1

D10

+15V

330µH

TRONIC

STPS2H100

ACL

3

8

TP 15

D15

R3 0

+ C26

PKC-136

n.c.

33 0µF

4

5

35V

8

7

6

5

2

U6

+5V

Drain4

Drain3

Drain2

Drain1

R29

D14

+15V

+15V

~

L7805CV

4

10

BAR46

TP 16

1 VIN

VOUT 3

–

+

Source2

Source1

VDD

1

R38

R47

3

C4 1

GND

1K

24K

~

FB

2n2Y

2

D16

C22 +

C23 +

ACN

1,5A

10µF

10µF

2

1

U4

C27

C28

400V

400V

VIPer22A

3

4

R39

330nF

100 nF

1

2

1K

C24

R46

ISO1

100nF 4K7

815

+

C38

C29

+

+ C25

C40

U5

100µF

10µF

10µF

22nF

TL431I

R48

35V

16V

35V

4K7

F

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.

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Schematic	AN2383

3.3Power stage

Figure 4. L6384 IGBT driver and power stage

PLATE

FST4

FST5

T2

TDKCT034

SCREW

SCREW

1:20

0

1

1

4

1

L2

80µH

3

2

+3 25V

D17

+15V

STGY40NC60VD

R22

D8

STTH102

D18

Q1

11R

STTH102

ST TH102

C11

C13

R2147R

R27

PWM0

8

1

1K

680n

33nF

VBOOT

IN

800V

800V

R20

C36

C15

7

2

HVG

VCC

10K

47nF

1µF

3

TR1

R19

C10

STGY40NC60VD

R25

D9

6 VOUT DT/SD

15KE.

470K

3µF

5 LVG

GND

4

1W

400V

Q2

11R

STTH102

R26

C16

C17

U2

+

C37

R28

C12

680nF

L6384

220K

2n2

10µF

47nF

4K7

800V

C14

R24

35V

33nF

R23

800V

47R

10K

AI12613

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

3.4Feedbacks

Figure 5. Current peak, current phase and alarm

+15V

+15V

T2

D4

R41

C31

TP19

R37

STTH102

100K

+8

100nF

5

TDK_CT03 4

7

10K TMP2

1:200

1

R18

D6

STTH102

6

–

4

4

100R

R31

NTC2

R40

R44 U3B LM 258

D7

33R

PT1000

62K

1M

3

2

D5

STTH102

1W

Temperature control

STTH102

for plate with

+15V

PT1000 sensors

+15V

+5V

R32

TP20

TP18

C21

100nF

NTC4

R68

2K7

R35

PT1

8

R45

1K

TMP1

I-CTRL

2K2

3

1

12KALARM

47K

50K

+

R69

C44

2

–

U3A

R33

C18

R34

R36

C19

4

100K

10nF

n.c.

22nF

1K8

4K7

22nF

LM 258

C20

TH1

Alarm management

22nF 110

AI12610

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.

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Schematic	AN2383

3.5MCU pin configuration

Figure 6. MCU pin configuration

			+15V
				J8
	C3 0			FAN
		+	D3	1
	100µF
			1N4007	2
	35 V

+5V C35 220µF 16V

+

C8

100nF

TP1 I-CTRL TP2 KEYS TP3 TMP1 TP4 TMP2 TP5 VLINK

R51

1M

+5V	Q5
	STS5NF60L

				1	8
			R50	2	7
				3	6
	ExternalI nterrupt request:	TP12	10K
				4	5
	PA0 -16- ei0 - ALARM
U1			R49
	PA7 - 9 - ei1 - 50Hz
ST 7F LITE09
			100

1

16

ei0 ALARM

Fan control

VSS

PA0 (HS)/LTIC

TP6

2

15

DATA

VDD

PA1(HS)

TP7

PWM0

3

PA 2(HS)/ATPWMO

14

TP8

RESET

4

13

SCLK

SS/AIN0/PB0

PA3(HS)

TP9

/LE

5

SCK/AIN1/PB1

PA 4(HS)

12

TP10

+5V

6

MISO/AIN 2/PB 2

PA5(HS)/ICCDATA

11

+5V

7

MOSI/AIN3/PB3

PA6/MCO/ICCCLK

10

8

CLKIN/AI N4/PB4

PA7

9

R52

10K

	ei1			2	1
				4	3
				6	5
		50Hz		8	7
C9			TP13	10	9
		RELAY
10 nF	TP11				J7

Reset Pin Hints:

R51 is mandatory if residual voltage is still on Reset Pin.

R52 is not mandatory, its functionality has to be checked dur ing tests.

CON10A

I CC Programmer

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.

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