1 Introduction
Generally speaking, the resonant circuit of a resonant converter consists of a capacitor, an
inductor, and resistance. Two types of resonant converters are generally used: a series
resonant circuit and a parallel resonant circuit.
The Induction Cooking evaluation board has been provided with a series resonant circuit.
The component L is the cooking element itself.
AN2384
Applica t ion note
How to size the resonant tank
of the Induction Cooking Evaluation Board
September 2006 Rev 1 1/10
www.st.com
Contents
How to size the resonant tank of the Induction Cooking Evaluation Board
Contents
1 Introduction . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 1
2 The resonant converter . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 5
3 The half bridge driver configuration . . . . . . . . . . . . . . . . . . . . . . . . . . . . 7
4 References and related materials . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 8
5 Revision history . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 9
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How to size the resonant tank of the Induction Cooking Evaluation Board
List of figures
List of figures
Figure 1. The series resonant circuit . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 5
Figure 2. The resonant curve. . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 6
Figure 3. The power stage. . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 7
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List of tabl es
How to size the resonant tank of the Induction Cooking Evaluation Board
List of tables
Table 1. Document revision history . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 9
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How to size the resonant tank of the Induction Cooking Evaluation Board The resonant converter
2 The resonant converter
Figure 1. The series resonant circuit
Formula 1
Formula 2
Formula 3
i
v
ZR2ωL
LC
XL = jωL = j2πfL [Ω]
1
X
-------- -
==
c
jωC
=
-------------- -
j2πfC
⎛⎞
+
⎝⎠
1
1
-------–
ωC
R
AI12618
Ω
2
Ω
At the resonance frequency, the inductive reactance XL of Formula 1 and the capacitive
reactance X
when the switching frequency becomes identical to the resonance frequency. In the case of
the Induction Cooking evaluation board, the switching frequency is set by MCU, and the
signal sent directly to the IGBT driver.
The resonance frequency can be calculated as shown in the following formula:
of Formula 2 become the same. The current in the circuit reaches its peak
C
Formula 4
2πfL
There are two areas around the resonant frequency: the area where the switching frequency
is lower than the resonant frequency and the area where the switching frequency is higher
than the resonant frequency. In an Induction Cooking application, the system works always
on the right side of the curve, therefore in the inductive area (Figure 2).
The value of the capacitive reactance becomes smaller according to Formula 2. In th is
situation, a higher switching frequency is followed by an i ncrease in impedance (Formula 3),
causing the output energy to be lower (as shown in Figure 2).
1
------------ -= bf0=
2πfC
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1
------------------ -
2π LC
Hz[]⇒
The resonant converter How to size the resonant tank of the Induction Cooking Evaluation Board
Figure 2. The resonant curve
Current
Capacitive Inductive
fo
Frequency
AI12620
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How to size the resonant tank of the Induction Cooking Evaluation Board The half bridge driver configuration
3 The half bridge driver configuration
Figure 3. The power stage
+325V
L2
80µH
TR1
1.5KE
R19
470K
1W
C10
3µF
400V
FST4
SCREW
C11
680nF
800V
C12
680nF
800V
1
C14
33nF
800V
C13
33nF
800V
FST5
SCREW
STGY40NC60VD
STGY40NC60VD
Note: D17 and D18 are not installed.
C16, R26 must be as close as possible to the driver.
The DC link voltage is applied directly to the IGBTs. 220 VAC coming from MAINS are
rectified by a diode bridge (not present in the Figure 3) and le v e led by capacitor C10.
Note that this capacitor is too small in capacity to level the voltage properly. In an y case, t he
Induction Cooking board does not require a bigger capacitor to get the voltage more level. In
fact, the primary pur pose of the system is to generate heat. Moreover, the rugged shape of
the DC link voltage improves the system power factor , which is around 0,98. Capacitor C10
is used as a filter, preventing the high frequency current from flowing toward and entering
the input circuitry.
1
Q1
R20
10K
Q2
R23
10K
T2
TDK_CT034
1:200
4 1
32
R22
11R
R21 47R
C15
C36
1µF
47n
R25
D9
11R
STTH102
R24
47R
D8
ST
TH102
R18
100R
8
7
6
5
V
BOOT
HVG
V
OUT
LVG
D17
STTH102
V
DT/SD
GND
U2
L6384
CC
C16
2n2
+15V
+
C17
10µF
35V
R27
1K
C37
47nF
AI12619
PWM0
R28
4K7
D18
STTH102
1
IN
2
3
4
R26
220K
The DC link voltage is applied to the load through the IGBT half bridge at high switching
frequency. The high frequency harmonics contained in the signal are eliminated by the
resonant tank. The circuitry creates a magnetic field around the resonant inductor, aff ecting
the load (pot on the plate).
Capacitors C13 and C14 are placed in parallel to the IGBTs. They act as lossless turn off
snubbers keeping the switching losses to a minimum.
The resonant capacitor has been divided in two identical capacitors, C11 and C12 (C11 =
C12). In this way, the amount of current flowing through each capacitor is reduced by half,
while the voltage across the capacitor remains the same.
A current transformer T2 has been placed in series with the cooking element (shown in
Figure 3 as FST4 and FST5). This does not affect the resonant tank, and provides the MCU
with the feedback information needed to control the system.
The gate pulses necessary for the IGBTs are sent and controlled by the half bridge driver
U2. The MCU provides the driver with the proper PWM signal, with 50% constant dutycycle. The operating frequency is in a range between 19 and 60 kHz.
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References and related materials How to size the resonant tank of the Induction Cooking Evaluation Board
4 References and related materials
For further information strictly related to the basic functionality of each integrated circuit,
please refer to the following documentations:
1. ST7LITE datasheet
2. L6384 datasheet
3. STGY40NC60VD datasheet.
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How to size the resonant tank of the Induction Cooking Evaluation Board Revision histo ry
5 Revision history
Table 1. Document revision history
Date Revision Changes
05-Sep-2006 1 Initial release.
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How to size the resonant tan k of the In duction Coo kin g Evaluation B oard
y
y
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