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Anti-interference Design of Quasi-resonant Tank for MagneticInduction Heating System

Cheng-Chi Tai and Ming-Kun Cheng

Department of Electrical Engineering, National Cheng Kung University, Tainan 70101, Taiwan

Abstract— In the paper, we present the design of a quasi-resonant tank for magnetic inductionheating system, also discuss some anti-interference applications using a half-bridge inverter withoptimal construction of the filter circuits and the decoupling capacitance. The paper also presentsa method for the design of filter and the insertion of decoupling capacitor. By using this method,the problems of high-frequency voltage interference can be significantly reduced. Experimentsindicated that the magnetic induction heating system can satisfy well the heating requirement ofthe magnetic nanoparticle (MNP) with a resonant frequency of approximately 220 kHz.

1. INTRODUCTION

The electromagnetic inducing of heat has been extensively studied for the treatment of hyperther-mia, wherein case deposited magnetic particles are used to locally heat human tissues. Applicationof magnetic materials for hyperthermia of biological tissue has been known, in principle, for morethan four decades [1]. Many empirical works were done in order to confirm a therapeutic effecton several types of tumors by performing experiments with animals [2] or using cancerous cell cul-tures [3], and poor AC magnetic field parameters [3]. At least two full-sized human prototypes havebeen built by magnetic fluid hyperthermia (MFH) and will be used shortly for the first clinical trialsof hyperthermia [3]. But, these systems are too bulky and with poor heating efficiency. Therefore,we want to develop a more compact, stable and efficient heating applicator.

Electromagnetic interference (EMI) is a relevant problem in the electronic system [4–8]. In fact,devices have to meet industry standards concerned with conducted interference. To avoid disturbingthe half-bridge series-resonated (HBSR) inverter, the oscillation circuit and the digital signal forMNP heating system, which make electromagnetic compatibility (EMC) design for the MNP verychallenge. Most of the current EMI designs use experimental trial-and-error methods, which aretime consuming and difficult for system stability. In order to optimize EMC performance, the EMIcharacteristics of power lines need to be analyzed at an early design stage for a system-stable andtime-efficient design approach. Common-mode (CM) and differential-mode (DM) conducted EMInoises are related to the circuit and circuit layout, the high dv/dt and di/dt slew rates in the powerrails [9]. Therefore, for successful EMI prediction and stable system, the correctness-insertion ofthe power line filter and the decoupling capacitance circuit is necessary.

2. BASIC MAGNETIC NANOPARTICLE HEATING SYSTEM

The essential elements of the MNP heating system using power-MOSFET is depicted in Fig. 1,where the dashed-line block is the place where a proper conducted EMI filter will be inserted. TheEMI filter with the capacitance (C1 and C2) and the inductance (L3) reduces conducted EMI toinput source. The full-wave diode bridge rectifies the AC commercial power and produces a DCvoltage of HBRS power supply, which is then smoothed by the capacitance (C3 and C4) and theinductance (L2). Because of the variable switching frequency control of CD40106, IC gate driverand the power-MOSFET, the conducted EMI problem in this MNP heating system is serious, andit is a challenge to design a proper filter to reduce the conducted EMI noise to a required lowlevel. The main specifications of this MNP heating system are: input voltage 110 V, the operatingfrequency 220 kHz, and output DC voltage 154 V.

In Fig. 1, HBRS tank of power-MOSFETs (Q1, Q2) and capacitance (CS1, CS2) are connectedin a full bridge configuration and switched at 220 kHz to convert the DC to high frequency AC.The main source of the EMI noise in the MNP heating system is the inductance of the switchQ1 and Q2 (IRFP 460) interconnections shown in Fig. 1 [10]. When these power-MOS FET areoperating as a switch in the HBRS tank, the high frequency drain and source voltage swing inpower-MOS FET causes charging and discharging of the FET parasitic inductance, resulting in

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(a) (b)

Figure 1: Circuit diagram of the MNP heating system used power-MOSFET. (a) The dashed-line block isthe place the EMI filter and the wire, (b) the parasitic inductance in the real-case of HBRS tank, the wiremodel and the decoupling capacitance.

EMI noise currents flowing out through power wire and returning via the ground wire. Those partsof the HBRS tank which are mounted on its chassis via parasitic inductance and have an AC waveform flowing through them also contribute EMI noise.

In order to maintain a stable voltage supply during the fast transients of HBRS tank, the powerlines with the high frequency and the external decoupling capacitors are required. Fig. 1(b) showsa power distribution model for the power supply and the HBRS tank. The decoupling capacitors(Cd1, Cd2) are used to minimize ringing, and the decoupling capacitors are spread evenly acrossthe circuit to maintain stable power distribution.

3. NOISE REDUCTION

Because of the high-voltage HBSR inverter, most designs use high-speed power MOSFETs as theinverter switch to ensure cost effectiveness and efficient operation. However, these fast switch-ing inverters generate high voltage slew rates (dv/dt), high current peak rates (di/dt), and highcommon-mode voltage at HBSR, causing some serious EMI problems [6, 7]. The main path of theconducted EMI is the high-frequency switching noise produced by the HBSR tank feeding backthrough the power inverter circuit, and back to the AC source. One of the most popular solu-tions of the EMI filter is shown in Fig. 2(a), where L1 is the common mode choke which providesboth the common mode filtering by its leakage inductance and the differential mode filtering byits primary inductance. L1, C1, and C2 form the differential filtering network that would filter outnoise between the supply lines. The EMI filter operates according to a principle whereby induc-tance connected directly in series with the line has virtually no affect on the noise current at lowfrequencies. But at high frequencies, it demonstrates a high interruptive effect with respect to thenoise current. The two capacitors (C1, and C2) connected in parallel with the line are used as aside path to return high frequency back to the power line. The result is that normal mode noisepasses through the capacitor and is shunted back to the other line.

(a) (b)

Figure 2: (a) EMI filter and (b) smoothing filter.

In the power supply circuit, the capacitor acts as a charge storage reservoir. The smoothingfilter is performed by a large value electrolytic capacitor connected across the DC supply to act asa reservoir, and supplying current to the output when the varying DC voltage from the rectifier

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is falling. In Fig. 2(b) shows the unsmoothed varying DC and the smoothed DC. The capacitorcharged quickly approach the peak of the varying DC, and then discharged when it supplies currentto the output. The smoothing filter that consists of L2, C3−1, C3−2, C4−1, and C4−2 is to filterout noise between the supply lines. The smoothing filter operates according to a principle wherebyinductance connected directly in series with the line has virtually no affect on the noise current atlow frequencies, but at high frequencies it demonstrates a high interruptive effect with respect tothe noise current. The four capacitors (C3−1, C3−2, C4−1, and C4−2) connected in parallel with theline is used as a side path to return high-frequency components back to the power line. The biggervalue of C3−1 and C4−1 reduce the switching frequency voltage ripple of MHz range. The smallervalue of C3−2 and C4−2 reduce the switching frequency voltage ripple of MHz range. The result isthat the normal mode noise passes through the capacitor and is shunted back to the other line.

The decoupling of a high-frequency HBRS tank is paramount to power delivery and minimizationof emission. In Fig. 1(b), the decoupling capacitances amount to creating low AC impedancebetween power line and ground line. The control strategy employs a current loop of decouplingcapacitance (CC1 and CC2) with a switching noise of HBRS tank.

In the power distribution model shown in Fig. 1(b), CC1 and CC2 represents decoupling capac-itors for the power supply. The decoupling capacitors supply current to the HBRS tank duringsudden excessive current demands that cannot be supplied by the smoothing filter. The requiredCC1 or CC2 can be calculated by the following equation:

C ≥ ∆I

∆V∆t (1)

where ∆I is the maximum processor current transient, ∆V is the tolerance times the nominalprocessor voltage, and ∆t is the voltage regulator response time.

The decoupling capacitors (CC1 and CC2) should be located as close to the resonant tank powerand ground pins as possible, which to minimize resistance and inductance in the lead length. Whenpossible, use wires to connect capacitors directly to the resonant tanks power and ground pins.In most cases, the decoupling capacitors can be placed in the MOSFET on the same side of theresonant tank (top side) or the opposite side (bottom side).

4. RESULTS AND DISCUSSION

The hard-switching HBSR tank is designed for MNP heating system applications. The circuitparameters for both HBSR tanks are: input AC voltage VAC = 110 V, output DC voltage ofbridge rectifier VDC = 154 V, duty cycle = 50%, switching frequency = 220 kHz, inductance ofapplicator 56.2µH. Fig. 3 shows the diagram of the implemented MNP heating system, whichshows a suggested component placement for the decoupling capacitors. The wires connected tothe capacitor should be short and wide. The measured results for the driver circuit and applicatorwaveform are presented in Fig. 4. It is shown that magnetic induction heating system can satisfywell with the heating requirement of the MNP with a resonant frequency of approximately 220 kHz.

Figure 3: The diagram of the implemented prototype.

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VGS1

VGS2

VGS1

VGS2

VGS1

VGS2

µ

Vol

tage

Vol

tage

Time ( S)Times ( )-10

-5

0

5

10

15

20

-1.5

-1

-0.5

0

0.5

1

1.5

(a) (b)

Figure 4: Measured results of the driver circuit and the applicator waveform.

5. CONCLUSIONS

The conducted EMI filter and decoupling capacitance can be integrated into a MNP heating system.The aim of this research is to implement the MNP heating system and improve its conductedemission. The power line filter and decoupling capacitance are used to reduce the conductedemission of MNP heating system.

ACKNOWLEDGMENT

This research was supported by the grant from National Science Council, Taiwan (NSC 95-2221-E-006-016). Also, this work made use of Shared Facilities supported by the Program of Top 100Universities Advancement, Ministry of Education, Taiwan.

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