Journal of the Korean Physical Society, Vol. 59, No. 6, December 2011, pp. 3688∼3693

Design of High-efficiency Soft-switching Converters for High-powerMicrowave Generation

Sung-Roc Jang∗ and Suk-Ho Ahn

Department of Energy Conversion Technology, University of Science and Technology, Daejeon 305-350, Korea

Hong-Je Ryoo and Jong-Soo Kim

Electric Propulsion Research Center, Korea Electrotechnology Research Institute (KERI), Changwon 641-120, Korea

(Received 15 December 2010)

In this paper, the design of three types of power supplies for 30 kW and 60 kW industrialmagnetron drives, a high-voltage power supply, a filament power supply and a magnet power supply,are studied. The high voltage power supply was designed based on the series resonant converterdiscontinuous conduction mode (DCM) to take advantage of a clean current source that couldcover the nonlinear characteristics of a magnetron, such as a zener diode. The maximum outputspecification of a high-voltage power supply is 42.5 kW for a 30 kW industrial magnetron drive.This can also be used for a 60 kW magnetron with a parallel operation of two power supplies. Thedesign considerations and feasibility tests of a resonant converter for an industrial magnetron aregiven. Additionally, the designs of two auxiliary power supplies (a filament power supply: 15 V,120 A and a magnet power supply: 75 V, 5 A) are also provided. These were designed based onthe series resonant converter continuous conduction mode (CCM) above resonance operation forhigher efficiency. Finally, the developed power supplies were tested using both a 30 kW magnetronand a 60 kW magnetron. The results prove the design procedure and the reliability of the proposedscheme.

PACS numbers: 84.40.Fe, 84.30.JcKeywords: Industrial magnetron, High-voltage power supply, Series resonant converterDOI: 10.3938/jkps.59.3688

I. INTRODUCTION

Recently, research on the high power and high effi-ciency industrial magnetron has grown owing to its ad-vantages for various kinds of applications, such as food-stuffs, chemicals, textiles, paper, wood, materials, com-munications and the environment. In order to drive theindustrial magnetron, three kinds of power supplies arerequired, as shown in Fig. 1. The high-voltage powersupply is used to generate high-power microwaves. Thefilament power supply is used to heat the cathode forthermo electron emission. The magnet power supply isused to control the electron trajectory. These power sup-plies should be highly efficient, reliable, compact, and af-fordable from the perspective of industrial applications.Depending upon the research, studies of high-power andhigh-efficiency DC-to-DC converters are increasing [1–6]. Soft switching converter topologies were adapted forindustrial magnetron drives [7–9].

As a result of this research, this paper presents thedesign of industrial magnetron power supplies based on

∗E-mail: scion10@keri.re.kr; Fax: +82-55-280-1490

Fig. 1. Power supply connection diagram for an industrialmagnetron.

a series resonant converter. An individual output volt-age and current control method for each power supply isproposed to allow flexible driving conditions for the mag-netron; however, an interface between the three powersupply operation techniques is adapted against a faultcondition to protect the magnetron. Furthermore, a fil-ament power supply output current control algorithm,with respect to the high voltage power supply outputpower, is proposed due to heating from the reflectedwave. The relationship between the filament power sup-

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Table 1. Specifications of the developed high-voltagepower supply.

Input Voltage 3 × 380 VAC (+10/−10%)

Maximum Output Power 85 kW (42.5 kW × 2)

Maximum Output Voltage 17 kV

Maximum Output Current 5 A

Output for 30 kW Magnetron 14 kV, 3 A

Output for 60 kW Magnetron 17 kV, 5 A

Maximum Efficiency 95%

Protections O.V., O.C., O.T.

Size (Width × Depth × Height) 430 × 550 × 300 mm ( × 2)

Weight 42.5 kg

Fig. 2. (Color online) Required relationship between thehigh-voltage power supply’s output power and the filamentpower supply’s output current.

ply output current and the high-voltage power supplyoutput power is depicted in Fig. 2. The value of the realfilament current (Ifilament) can be expressed, in terms ofthe compensation gain (K), the set current (Ifilament set),and the high-voltage power supply current (Ihigh voltage)as

Ifilament = Ifilament set − K · Ihigh voltage. (1)

II. DESIGN OF POWER SUPPLIES FOR ANINDUSTRIAL MAGNETRON DRIVE

1. Design of a High-voltage Power Supply

In order to drive a 60kW industrial magnetron, an 85kW (17 kV, 5 A) power supply is required because themagnetron possesses an 80 % maximum oscillation effi-ciency. The specifications of the developed magnetronpower supply are summarized in Table 1. To generatean 85 kW output power, we proposed the parallel opera-tion of two 42.5 kW power supplies because these can beused to drive a 30 kW magnetron for single operation.The overall structure of the 42.5 kW power supply isdepicted in Fig. 3. It consists of three full-bridge series

Table 2. Summary of the design parameters for the high-voltage power supply.

Resonant Capacitor 0.56 µF

Resonant Inductor 13.5 µH

Resonant Frequency 58 kHz

Characteristic Impedance 4.9 Ω

Transformer Turns Ratio 10:100

Maximum Switching Frequency 24 kHz

Pulse Width of Switching Signal 10 µs

Fig. 3. (Color online) Overall structure of a 42.5 kW powersupply.

Fig. 4. (Color online) Picture of the designed high-voltagetransformer.

resonant converters, and each converter operates indi-vidually, except that the output part, which representsvoltage-doubled rectifier outputs, is connected in series.This modular structure allows for flexible of output volt-age, and current change, with the same power ratingsand for ease of increasing the power by using a paral-lel or a series connection of the module. Moreover, if aphase delay of the switching signal for each converter isprovided, the output voltage ripple can be decreased forthe same filter components. A three-phase switching sig-nal for the 30 kW industrial magnetron and a six-phaseswitching signal for the 60 kW industrial magnetron wereapplied. Therefore, detailed design procedure for the 85kW power supply starts from the design of the 14.2 kWseries resonant converter, which has a maximum outputvoltage and current of 5.67 kV and 3 A, respectively. Se-ries and parallel connected resonant converter operationtest, with phase shifted switching, are required.

Depending on the design specifications, the parame-ters for the 14.2 kW converter module, including its res-onant tank components and its transformer turns ratio,should be determined [9]. These results are summarizedin Table 2. As a first step in the design procedure, the

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Table 3. Specifications of the developed filament powersupply.

Input supply voltage3 × 380 VAC

(+10/−10%)

Maximum load1.8 kW

power, Pload max

Maximum operating15 VDC

voltage, Vout max

Maximum operating120 A

current, Iout max

Switching frequency at25 kHz

maximum output, fs min

Ripple of output less than 10%

voltage, Vout rip of Vout nom

Ripple of output less than 5%

current, Iout rip of Iout nom

resonant frequency should be decided, based on the def-initions of the discontinuous conduction mode operationof the series resonant converter. The value of the res-onant frequency is calculated from 2.5 times the maxi-mum allowable switching frequency of the semiconductorswitch, owing to the switching delay time and margin.In the second step, the transformer turns ratio is deter-mined from the maximum voltage of the primary side.When the transformer turns ratio is fixed, the peak valueof the resonant current at the primary side can be calcu-lated based on the required average output power. Fromthe calculated resonant current peak value and the con-verter DC input voltage, the characteristic impedance ofthe resonant tank can be determined. Finally, the valuesof the resonant capacitor and inductor were found fromthe resonant frequency and the characteristic impedance.Even though the transformer turns ratio was calculatedas 1:10, the actual turn ratio should be decided by con-sidering the saturation of the core material and the re-quired leakage inductance, which can be used for theresonant inductance. To achieve a 13.5 µH leakage in-ductance without core saturation, a 10:100 turns ratiowas determined with a Teflon bobbin to ensure insula-tion. An image of the designed high-voltage transformeris shown in Fig. 4. The pulse width of the switching sig-nal is calculated from the resonant frequency when theswitch at the diode conducting region is turned off.

2. Design of a Filament Power Supply

The overall scheme and the detailed specifications ofthe filament power supply are given in Fig. 5 and Table 3,respectively. These have a single-phase full-bridge se-ries resonant inverter structure with a transformer centertapped rectifier, which is proper for high output currentapplications owing to the reduced losses at the rectifier

Table 4. Summary of the design parameters for the fila-ment power supply.

Resonant Capacitor 0.11 µF

Resonant Inductor 660 µH

Resonant Frequency 18.3 kHz

Characteristic Impedance 76 Ω

Transformer Turns Ratio 42:2:2

Minimum Switching Frequency 25 kHz

Fig. 5. Overall scheme of the filament power supply.

Fig. 6. Overall scheme of the magnet power supply.

diodes. A detailed design of the resonant tank parame-ters and the transformer turns ratio is performed basedon the minimum component stress of the switch and theresonant tank [2,3]. The design parameters are summa-rized in Table 4. It should be noted that the value ofthe resonant inductance for above resonance zero voltageswitching type converters is relatively higher than thatof zero-current switching type discontinuous conductionmode converters. Therefore, an additional inductor isrequired, which represents lower conduction losses, dueto the higher characteristic impedance of the resonanttank.

3. Design of a Magnet Power Supply

Due to the low output power specification shown inTable 5, the half-bridge series resonant converter schemeis proposed for the magnet power supply from the single-phase AC input voltage. However, the power factor cor-rection, with the boosting circuit, is used for the DCinput voltage. Figure 6 shows the overall scheme andthe values of the designed parameters are listed Table 6.

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Table 5. Specifications of the developed magnet power supply.

Input supplying voltage 220 VAC + G (+10/−15%)

Maximum load power, Pload max 375 W

Maximum operating voltage, Vout max 75 VDC

Maximum operating current, Iout max 5 A

Switching frequency at maximum output, fs min 20 kHz

Ripple of output voltage, Vout rip less than 10% of Vout nom

Ripple of output current, Iout rip less than 5% of Iout nom

Table 6. Summary of the design parameters for the magnetpower supply.

Resonant Capacitor 0.2 uF

Resonant Inductor 400 uH

Resonant Frequency 18 Hz

Characteristic Impedance 45 Ω

Transformer Turns Ratio 28:20:20

Minimum Switching Frequency 20 kHz

Fig. 7. Experimental waveforms for 17 kW operation (reso-nant current (100 A/div), switching signal (10 V/div), outputvoltage (2 kV/div), and output current (1 A/div)).

III. EXPERIMENTAL RESULTS FOR THEDEVELOPED POWER SUPPLY

1. Experimental Results of the High VoltagePower Supply

Based on the designed parameters, a converter mod-ule for 14.2 kW series resonant converter was developedand tested with a resistor load. As mentioned earlier,one converter module has to generate the 3 A maximumcurrent for the 30 kW magnetron drive, with a 4.7 kVoutput voltage and a 5.7 kV maximum output voltage,for the 60 kW magnetron with a 2.5 A output current.Therefore, a 17 kW operation test was performed to en-sure reliable operation of the power supply for both typesof magnetrons.

Figure 7 shows the resonant current and the switching

Fig. 8. (Color online) Relationship between the outputcurrent and the switching frequency.

Fig. 9. Experimental waveforms of 13 kV operation with-out a voltage balancing capacitor (50 A/div, 5 kV/div).

signal, with output voltage (5.7 kV) and current (3 A),for a single converter module operation. It makes clearthat the switch is turned off at zero voltage and cur-rent owing to the diode conducting region and is turnedon at zero current. Furthermore, a relationship betweenthe output current and the switching frequency is ob-served. Because of the current source characteristics ofthe applied topology, the output current is almost linearin the switching frequency, independent of the outputvoltage, as shown in Fig. 8. In order to implement the42.5 kW power supply, as shown in Fig. 3, the series op-eration of three converter modules is required with three-phase switching signals. Figure 9 shows the waveformsof the resonant current for each converter with output

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Fig. 10. (Color online) Experimental waveforms of 14 kVoperation (50 A/div, 5k V/div).

Fig. 11. (Color online) Experimental waveforms of 85 kWoperation for converter 1(50 A/div, 5 kV/div).

voltage at 13 kV, 2.8 A operation. When it operates ata high switching frequency, which represents a high valueof the output current, the problem of unbalancing wasobserved for the three-phase resonant current due to theseries connection of the current source converter, eventhough each converter operated with the same switchingfrequency. This arises from the voltage difference be-tween the output voltages and results in a higher loss forone converter over the others due to the higher value ofthe RMS current. To solve this problem, we connecteda small capacitor (1 nF) to the high-voltage transformersecondary winding, which can provide the same voltageto the primary transformer, because the peak value ofthe resonant current depends on the voltage across theresonant tank and the characteristic impedance. Thewaveforms at balanced operation of the three-phase res-onant converter for 42 kW shown in Fig. 10, makes clearthat the unbalance problem in the resonant current wassolved by applying a simple method. Additionally, it wastested in maximum voltage 17 kV operation without anyunbalance problems.

The last step in developing the 60 kW industrial mag-netron power supply is the parallel operation of two con-verters, which consists of three resonant converter mod-ules. Figure 11 shows the waveforms for the 85 kW (17kV, 5 A) operation, including the three-phase resonantcurrent and the output voltage. Because of the current

Fig. 12. (Color online) Measured efficiency of the high-voltage power supply vs. output power.

Fig. 13. (Color online) Output voltage and current wave-forms of the filament power supply (5 V/div, 20 A/div.).

Fig. 14. (Color online) Output voltage and current wave-forms of the magnet power supply (20 V/div, 1 A/div).

source characteristics of the proposed topology, a par-allel operation was performed without problems. Themeasured efficiency, with respect to the output power(1 P.U. = 85 kW), is plotted in Fig. 12, and maximumefficiency of 96% was achieved.

2. Experimental Results for the Filament andMagnet Power supplies

The developed filament and magnet power supplieswere tested before combining them with the high-voltagepower supply. The maximum output voltage and currentwere measured and are depicted in Fig. 13 and Fig. 14,respectively, which show waveforms from zero to maxi-mum with a soft start. Furthermore, an interfacing testbetween the filament power supply’s output current andthe high-voltage power supply’s output power was per-formed, as shown in Fig. 15. The result is very close tothe required value given in Fig. 2.

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Table 7. Summary of the experimental results with an industrial magnetron.

30 kW Microwave 60 kW Microwave

Developed High-Voltage Power SupplyOutput Voltage: 13 kV Output Voltage: 16.5 kV

Output Current: 3 A Output Current: 4.7 A

Magnet Power SupplyOutput Voltage: 39 V Output Voltage: 53 V

Output Current: 3.6 A Output Current: 4.8 A

Filament Power SupplyOutput Voltage: 11 V Output Voltage: 10 V

Output Current: 107 A Output Current: 98 A

Total System Efficiency 74% 75%

Fig. 15. (Color online) Measured relationship between thefilament power supply’s output current and the high voltagepower supply’s output power.

3. Experimental Results with an IndustrialMagnetron

Finally, the developed 85 kW power supply was testedwith an industrial magnetron. Two kinds of test condi-tions for generating both 30 kW and 60 kW microwavesare summarized in Table 7, including the magnet and thefilament power supply output conditions and the totalsystem efficiency from input power to output microwavepower.

IV. CONCLUSION

High efficiency series resonant converter topologieswere applied to an industrial magnetron drive. A ba-sic design procedure for the series resonant converter, atboth the discontinuous conduction mode operation andthe continuous conduction mode operation, were given.Feasibility tests of the series and the parallel operationsof the converter module were done, and a solution for theunbalancing problem for each converter was provided.Finally, the developed power supply was tested with anindustrial magnetron for 30 kW and 60 kW microwavegeneration, and the maximum efficiency was measuredas 96%. These results verify that a series resonant con-verter can be effectively used as a power supply for anindustrial magnetron.

ACKNOWLEDGMENTS

This work was supported by an Energy Efficiency &Resources of the Korea Institute of Energy TechnologyEvaluation and Planning (KETEP) grant funded by theKorea government’s Ministry of Knowledge Economy(No. 2008EEL02P040000).

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