Development of 13-V, 5000-A DC Power Supply
with High-Frequency Transformer Coupling
Applied to Electric Furnace
Toshihiko Noguchi, Senior Member, Kosuke Nishiyama

Yoshihisa Asai, and Toru Matsubara

Department of Electric, Electronics, and Information Engineering
Nagaoka University of Technology
1603-1 Kamitomioka, Nagaoka, Niigata 940-2188, Japantnoguchi@vos.nagaokaut.ac.jp

Nagaoka Headquarter and Works
MACOHO Co., Ltd.
525 Isurugi, Aza-Kanazawa, Nagaoka, Niigata 940-2032, Japan

Abstract—This paper describes a low-voltage and large-current
DC power supply with a high-frequency transformer coupling,which is applied to electric furnaces. The power supply is simply
composed of a full-bridge inverter, an amorphous-core step downtransformer and a Schottky diode rectifier. The magnetizing
frequency of the transformer is raised up to 15 kHz and the
maximum output rating of this system is 13 V and 5000 A. The
most important key feature of this system is reduction of line
inductance and leakage inductance in the whole power circuit,which exist mainly in wire harnesses, the transformer windings,
the output DC bus, etc. Even such inductance of micro-Henryorder detrimentally affects the total output power to the load
because a long overlapping period in commutation of the final
rectifying stage can be caused by the large current and suchminute inductance. The maximum total efficiency between theutility AC power source and the load is 89.9% and the maximum
total input power factor at the front end stage of the prototype is
83.5%, respectively. Although the output voltage and the currentof the system are considerably low and large, the confirmed
experimental results demonstrate rather excellent performanceand feasibility of the developed system, compared withconventional thyristor-based power supplies.

In order to overcome the problems described above, it isabsolutely necessary to raise the operation frequency of thepower converter to kHz-order. Recent high-speed powerswitching devices such as IGBTs and MOSFETs are desirable
for current ripple and acoustic noise reduction as well as
physical downsizing of the system by enlarging their operating
frequency. The most important point to develop such lowvoltage and extremely large-current power converter using
high-frequency AC coupling is reduction of the line inductanceand the leakage inductance at every part of the whole power
conversion circuit, i.e., wire harness, the transformer windings,
the output DC bus, etc. Since such inductance prevents totransfer the effective power to the load and causes a longoverlapping period in commutation of the final rectifying stage,even µH-order of the inductance should be eliminated from thepower conversion circuit. In this paper, 13-V, 5000-A DC
power supply is described and its experimental results arepresented to show advantages in operation performances overthe conventional power supply systems.

Keywords-DC power supply; low-voltage and large-current;
high-frequency transformer; Schottky diode rectifier; line
inductance; leakage inductance; overlapping period.

I.

INTRODUCTION

The authors have been investigating a low-voltage andlarge-current DC power supply system for an electric furnace,
which is used for sintering. Sintering is often performed by adirect resistive heating method and requires a low-voltage andextremely large-current power supply, e.g., nearly ten volts andseveral thousand amperes. In such applications, thyristorbased rectifiers have commonly been employed to generatesuch low voltage and large current. However, the efficiency
and the input power factor of the thyristor-based system are at
most 60% and 40%, respectively. To make matters worse,
large transformers and large smoothing reactors in the output
DC bus are required to achieve power conversion because of
low frequency of utility power source, which results in
disadvantages in physical dimensions and weight of the whole
power supply system.

II.

OUTLINE OF LOW-VOLTAGE AND LARGE-CURRENT DC
POWER SUPPLY SYSTEM FOR ELECTRIC FURNACE

Sintering is a kind of techniques that create contacts andbonds between minute particles by heating the molded powdermaterial below its melting point. This technique allows
producing ultra hard and high-melting point materials, which
are considerably difficult to process, and is remarkably usefulto create functional composites, functionally graded materials,
etc. The powder materials, which are often boron, tungsten
carbide, alumina, etc., are filled in a mold case with applying acertain amount of pressure and draw thousand amperes of DC
current for heating up. Since the mold case filled with thepowder materials has as low resistance as several mΩ, only
tens-volt output of the DC power supply is sufficient for the
thousand amperes of current draw. However, enlarging themagnetizing frequency of the step down transformer, which isinserted in the power conversion process, detrimentally affects
the power transfer to the load mold because voltage drop of theline inductance and the leakage inductance caused by the highfrequency and the extremely large current is never negligible.

Unit 1
IGBTs 600 V-200 A

0.5 mH
AC 3φ
50 Hz-200 V
11600
μF

+

#1 IOUTTo DC exitation
surpression circuit#1 iINV
17:1

InterphasereactorD1#1 vINV

Gate 1Gate 3

IOUT

VOUT

13 V
5000 A

D2

Schottky diodes
40 V-360 A#1 Gate2#1 Gate4
Same circuit as shown above

Unit 2Gate 1Gate 3#2 Gate2#2 Gate4#2 iINV#2 IOUT

Fig. 2. Schematic diagram of 13-V, 5000-A DC power supply.

Fig. 1. Overview of sintering system.

Fig. 3. Photograph of current balancer at output DC bus.

To make matters worse, the leakage inductance of thetransformer windings and the output DC bus causes anoverlapping phenomenon in commutation of the final
rectifying stage, where all the rectifying Schottky diodes turnon at the same time. This overlapping phenomenon generates
zero-voltage across the output DC bus and makes the output
voltage limitation applied to the load rather low. Therefore, it
is indispensable to reduce the total line inductance and theleakage inductance that exist in everywhere of the power
conversion circuit, e.g., the wire harness, the transformer
windings, the output DC bus, etc.

This sandwich structure allows effective reduction of the line
inductance of the output DC bus.

Fig. 1 shows an overview of the whole sintering system.
There are a vacuum chamber on the left hand side, a hostcontroller in the middle of the photograph, and the 13-V, 5000A DC power supply behind the host controller. The developed
DC power supply occupies less than quarter space of thethyristor-based system. The mold case in the vacuum chamberis connected to the DC power supply through a sandwich DC
bus of 3-m long, 200-mm wide and 10-mm thick copper bars.

III.

CIRCUIT CONFIGURATION AND CONTROL METHOD

A. Power Conversion Circuit Configuration
Fig. 2 shows a schematic diagram of the power circuit ofthe developed 13-V, 5000-A DC power supply for the sinteringsystem. This DC power supply consists of two identical units
connected in parallel with each other that has a 13-V, 2500-A
rating per unit. Output terminals from the both units areconnected at the final output DC bus through a current balancer(an inter-phase reactor) shown in Fig. 3. The current balancer
is composed of a single rectangular-shaped iron-core, where an
output line from each unit just passes through one time.
Principal electrical specifications of the whole DC power
supply system are listed in TABLE I. The front end of the DC
power supply is composed of an ordinary three-phase diodefull-bridge and a LC filter. On the other hand, a single-phaseTABLE I SPECIFICATIONS OF 13-V, 5000-A DC POWER SUPPLY SYSTEM.
AC 3φ, 50 Hz, 200 V

Inverter frequency

15 kHzOutput voltage

0-13 V+*

Output current

0-5000 A

I OUT

Load

1-2 mΩI OUT

TABLE II SPECIFICATIONS OF HIGH-FREQUENCY STEP DOWN TRANSFORMER.

Gate1

15 kHz#1 PhaseshifterPI-#1 iINV#1 IOUT

∫

+

-

+

#2 Phaseshifter

Dead time and isorated gate drives

Power source

Gate3#1 Gate4#1 Gate2#2 Gate4#2 Gate2

Capacity

30 kVA

Core size

100 mm×155 mm×85 mm#2 IOUT

Primary windings

Thickness 0.2 mm, Width 22 mm,
8 ParallelFig. 4. Block diagram of 13V-5000A DC power supply controller.

Secondary windings

Thickness 0.2 mm, Width 22 mm,
35 pieces laminated, 8 Parallel

Turn ratio

N1 : N2 = 17 : 1

inverter consists of a 600-V, 200-A IGBT full bridge
generating a 15-kHz rectangular voltage and is connected tothe step down high-frequency transformer. The average output
voltage of the inverter can be adjusted by controlling its
voltage pulse width. The transformer has 30-kVA capacity, of
which turn ratio is 17:1. The secondary circuit of thetransformer has a center tap structure and is composed of 8parallel one-turn windings. 40-V, 360-A Schottky diodemodules are employed in the final rectifying stage to reduce theconduction loss caused by their forward voltage drop. A pairof the diode modules is connected to each secondary winding,so the total number of the diode modules is 16. Each pair has asnubber circuit to absorb surge voltages during its switching
moment.
B. Control Method
Fig. 4 illustrates a simplified block diagram of thecontroller. Basically, this controller regulates the output DC
bus current to the load with a current feedback loop byadjusting the average voltage across the primary windings ofthe two transformers. The controller provides the same gate
signals to both of the 13-V, 2500-A units. The only differenceis that the gate signals to Unit 2 are modified by the output
current error between the two units. A 50-% duty, 15-kHz
reference rectangular pulse is generated to operate the inverters
and to magnetize the transformers. The gate signals “Gate 1”
and “Gate 3” are commonly provided from the reference
rectangular pulse generator to the both units. On the other
hand, the gate signals “Gate 2” and “Gate 4” are created by thephase shifter, of which inputs are the original reference
rectangular pulse and the current regulator output. Both of“Gate 2” and “Gate 4” signals also have 50-% duty but haverelative phase shift with respect to the reference rectangularpulse. According to the relative phase shift amount governedby the current regulator output, the conduction duration, i.e.,the voltage pulse width or the average output voltage, of each
inverter is determined to balance the output current from thetwo units. In addition, each 13-V, 2500-A unit has an anti-DC-#2 iINV∫

Secondary Amorphous Primaryiron core windings
windings

Center tap

Fig. 5. Cross section diagram of high-frequency step down transformer.

excitation function for the transformer. This function is
individually achieved by slightly changing the duty of “Gate 2”
and “Gate 4” signals around 50%, detecting the DC componentof each inverter output current.
C. High-Frequency Transformer and Final Rectifying Stage
The specifications of the high-frequency transformer are
listed in TABLE II. As can be seen in the table, power densityof the prototype is much higher than that of conventionalcommon transformers, owing to its enlarged magnetizing
frequency of 15 kHz. The transformer has an amorphous coreof which maximum flux density is 1.5 T and the average lengthand the cross sectional area of its magnetic path is 359 mm and2450 mm2, respectively. The primary winding consists of 8parallel thin copper sheets of which maximum current density
is designed to be 4.3 A/mm2 and the number of turn is 17. The
secondary winding has a laminated structure using 35 sheets ofthe same copper plates. The number of turn of these secondary
windings is only one and its maximum current density is 4.1
A/mm2 at 2500-A output per unit. Fig. 5 illustrates a crosssection of the transformer. The secondary windings are woundaround the amorphous core, closely overlapping the primarywindings around the core to avoid increase of the leakage
inductance as well as to improve the magnetic couplingPrimary
windings

Amorphousiron core

Secondarywindings#1 vinv
200 V/div0

t 20 µs/div
Time
Center tap#1 iinv
150 A/div0
Schottky
Diodest 20 µs/div
020

40 Time 60

80

100

Vout 20 V/div

Fig. 6. Photograph of transformer and final rectifying stage.

0
t 20 µs/div
Time

D1

#1 Iout
900 A/div
0
D2

t 20 µs/div
Time
Iout
2000 A/div

Fig. 7. Photograph of final rectifying stage and secondary copper plate.

coefficient. The secondary windings are closely placed overthe primary windings to achieve the same goal, i.e., deductionof the leakage inductance. Every junction between the centertap and the secondary windings is welded to reduce the contact
resistance. A parameter measurement test result indicates only
3.9- µH leakage inductance in the prototype transformer.
Fig. 6 shows a photograph of the transformer and the final
rectifying stage with a heat sink which is cooled down by someelectric fans. As shown in this figure, the final rectifying stage
is securely mounted on the heat sink, inserting an insulation
sheet with high thermal conductivity. Electrical insulation
among the windings is sustained by polyimide tapings, which
is much superior to any other materials in high thermal
resistance and high insulation resistance. The final rectifying
stage is composed of the Schottky diodes and a copper plate of
the output DC bus. The Schottky diodes are directly mounted
on the copper plate for effective heat conduction from thediodes because even slight forward drop (typically 0.45 V inthis system) of the Schottky diodes causes a large conduction
loss due to thousand amperes of current. Resistors used in thesnubber circuits are also implemented on the heat sink for
effective cooling. Fig. 7 is a photograph of the copper plate
and the mounted Schottky diodes without the transformer. As

0t 20 µs/div
Time
Fig. 8. Waveforms of 15-kHz inverter and output DC bus.

shown here, 16 diodes of 40-V and 360-A rating are placedside by side on the 10-mm thick copper plate. D1 or D2 in Fig.
2 corresponds to a set of the 8 parallel-connected Schottky
diodes shown in this figure.
It is possible to reduce physical dimensions of thetransformer core because of the high-frequency magnetizationof 15 kHz. However, the overlapping period in commutating
the final rectifying stage occupies more in a magnetization
cycle, which detrimentally affects the output voltage and theoutput power from the DC bus. Since the overlapping
phenomenon is caused mainly by the leakage inductance of thetransformer, the magnetic coupling between the primary andthe secondary windings must be enhanced by implementing thetwo windings around the core as closely as possible.
IV.

EXPERIMENTAL RESULTS OF PROTOTYPE SYSTEM

A. Operating Waveforms at Rated Output
Fig. 8 shows operating waveforms of the prototype systemat the full load condition. From the upper to the bottom, thefigure depicts an inverter output voltage of Unit 1, an inverter

Total efficiency (%)

100
80
60
40
5000-A Power Supply
Thyristor Power Supply

20
0

0

1000

2000

3000

4000

Output current (A)Fig. 9. Total efficiency.

Input power factor (%)

100
80
60
40
5000-A Power Supply
Thyristor Power Supply

20
0

0

1000

2000

3000

4000

Output curret (A)Fig. 10. Total input power factor on utility power source side.

Others
4.38%
Schottky Diode
44.9%

Front-End Diode Rectifier Bridge
18.3%

Transformer
5.38%
INV Swching
10.8%

INV Forward Drop
16.3%

Fig. 11. Loss analysis result at 80-% (4000-A) load.

output current of Unit 1, a total output DC bus voltage, an
output current of Unit 1 DC bus, and a total output DC bus
current. As can be seen in these waveforms, the developed
prototype achieves 5000-A DC current output. The voltage
and the current waveforms of the inverter demonstrate 15-kHz
operation and properly pulse-width-controlled inverter output.
In addition, since the inverter current does not contain DC
component, it can be confirmed that the DC magnetization of

the transformer is successfully compensated by the controller.
The output current waveform of Unit 1 DC bus isapproximately 2500 A, which implies that both output currentsof the two units are appropriately balanced by the electronic
controller and the current balancing reactor. As described in
the previous section, the load is connected to the DC power
supply through the 3-m long sandwich DC bus. Although the
sandwich structure is inherently effective to reduce the line
inductance, 8.3-µH inductance is still measured; hence thisinductance smoothes the output DC bus current as shown in
Fig. 8. On the other hand, short pulse voltages are observed
before every leading edge of the inverter output voltage
waveform as indicated in Fig. 8. These pulse voltages are
generated when the conduction path of the inverter changes
due to a lock-out period to prevent a short circuit between thehigh-side and the low-side IGBTs.
B. Total Efficiency and Total Input Power Factor
Fig. 9 and Fig. 10 show total efficiency between the threephase utility AC power source and the load, and total input
power factor on the AC power source terminals, respectively.
These figures also indicate characteristics of the conventional
thyristor-based DC power supply for performance comparison.
As can be seen in the operation characteristics, the developed
prototype achieves the maximum efficiency of 89.9% and the
maximum power factor of 83.5%, which perfectly surpasses
those of the thyristor based system.
Fig. 11 represents a loss analysis result at 80-% (4000-A)
load condition. Input and output power of the front-end fullbridge rectifier, the single-phase inverter, the step-down highfrequency transformer and the Schottky diode rectifier aremeasured with some digital power meters. The power losses of
the transformer can be separated into a copper loss and a ironloss as known well. The copper loss was calculated, using themeasured winding resistances and the measured current, while
the iron loss was estimated on the basis of the no-load test
result. Furthermore, the power loss of the Schottky dioderectifier is obtained from difference between the load power
and the power dissipation of the transformer.
As illustrated in Fig. 11, the power loss of the Schottky
diode rectifier occupies most part of the total losses. Although
the forward voltage drop of the Schottky diodes is only 0.45 V,
total conduction loss of the diodes reaches 1.3 kW.
Synchronous rectifying using MOSFET instead of the Schottky
diodes might be effective to reduce the conduction loss, but it
is necessary to implement so many devices in parallel and gate
drive circuits; hence this solution is impractical for low-voltage
and large-current power supplies. As for losses of the inverter,
the switching loss occupies approximately 11% of the total
power dissipation due to 15-kHz operating frequency, while
the conduction loss does near 16%. The saturation voltage
between an emitter and a corrector of the IGBTs is typically
1.8 V, which increases the conduction loss of the inverter
circuit. The power loss of the front-end rectifier is not
negligible because it exceeds 18% of the total loss. However,
the three-phase full bridge structure is very attractive tothe final rectifying stage enables the developed system todeliver 13-V and 5000-A output.
V.
vinv
100 V/div0

Vout
10 V/div

0

overlapping period

2 µs/div

Time
Fig. 12. Waveforms of overlapping period in commutation.

simplify the circuit configuration, so it is worthy to introduce
this structure, sacrificing the conduction loss. To contrast withthe power losses described above, it can be found that thetransformer loss is suppressed down to approximately 5%.
This fact proves that highly efficient power transfer is achieved
through the prototype transformer and employing an
amorphous core effectively reduces the iron loss in spite of 15kHz magnetizing frequency.
C. Overlapping Phenomenon
Fig. 12 shows an overlapping period of the developed
system at 80-% (4000-A) load condition. As can be observedin this figure, the output voltage is clamped at zero and its
pulse width is shortened even though the inverter output
volatage is applied to the transformer, which is the fatal
drawback of the overlapping phenomenon. However, theoverlapping period of the developed system is as short as 1.2µs; thus it occupies only 1.8% of one inverter output cycle.
This result proves feasibility of the total system configuration
to reduce the line inductance as well as the leakage inductance,
which greatly contributes availability of larger current andhigher output power regardless of the low output voltage. The
unique mechanical and electrical design of the transfomer and

CONCLUSION

In this paper, a prototype of the low-voltage and largecurrent DC power supply applied to an electric furnace wasdeveloped and its operation characteristics were experimentally
examined. The unique configuration of the step down highfrequency transformer and the final rectifying stage enabled the
DC power supply to reduce the line inductance and the leakage
inductance, which was indispensable to shorten the overlapping
period in commutation of the final rectifying stage. This
technique made it possible to raise the operating frequency of
the inverter and the magnetizing frequency of the transformer
up to 15 kHz. In the experimental tests, 13-V, 5000-A DC
current output was achieved and the 89.9-% maximum
efficiency and the 83.5-% maximum total input power factor
were confirmed. Every experimental result demonstrated
higher superiority of the developed system to the conventional
thyristor-based DC power supply.
REFERENCES
[1]

[2]

[3]

[4]

[5]

[6]

Ryota Nakanishi, Toshihiko Noguchi, Isao Takahashi, and Minoru
Tanaka, “Development of Parallel Operation of Low-Voltage LargeCurrent DC Power Supply,” Proceedings of IEE-Japan National
Convention, vol. 4, p. 1439, 2000.
Ryota Nakanishi, Toshihiko Noguchi, Isao Takahashi, and Minoru
Tanaka, “Development of Low-Voltage and High-Current DC Power
Supply Featured Small-Size and High-efficiency,” Proceedings of
Semiconductor Power Conversion Technical Meeting of IEE-Japan,
SPC-00-61, p. 37, 2000.
Keiichi Ishida, and Toshihiko Noguchi, “Development of Low-Voltage
and Large-Current DC Power Supply with High-Frequency Transformer
Coupling,” Proceedings of IEE-Japan Industry Applications Society
Annual Conference, vol. 1, p. 493, 2003.
Keiichi Ishida, and Toshihiko Noguchi, “Operation Performances of 13V, 1250-A DC Power Supply,” Proceedings of IEE-Japan Hokuriku
Branch Annual Conference, p. 35, 2003.
Keiichi Ishida, and Toshihiko Noguchi, “Loss Analysis of 13-V, 1250-A
DC Power Supply with 15-kHz Transformer Coupling,” Proceedings of
IEE-Japan Niigata Branch Annual Conference, p. 69, 2003.
Keiichi Ishida, Toshihiko Noguchi, Yoshihisa Asai, and Atsushi Iobe,“Development of 13-V, 1250-A DC Power Supply with High-Frequency
Transformer Coupling,” Proceedings of Semiconductor Power
Conversion Technical Meeting of IEE-Japan, SPC-04-45, p. 57, 2004.