The International Workshop on Mechatronics 2006
December 12-13, 2006, Supalai Resort, Saraburi, Thailand

Low-Voltage and High-Current DC Power SupplyUsing Direct Power Control
Keita Nakai, and Toshihiko Noguchi
Department of Electrical Engineering, Faculty of Engineering,
Nagaoka University of Technology, 1603-1 Kamitomioka, Nagaoka, Niigata 940-2188, Japan
Tel/Fax: +81-258-47-9510, E-mail: tnoguchi@vos.nagaokaut.ac.jp
Abstract
This paper describes a low-voltage and high-current
DC power supply with a direct-power-controlled (DPC)
AC-AC converter, which is applied to electric furnaces.
The AC-AC converter operates as positive current-source
rectifier and negative one employing the DPC switchingtable, in order to generate square wave of an output
current at the primary of a high-frequency step downtransformer. Then DC current is drawn by anotherrectifier at secondary of the transformer. According to
the simulation results, it is confirmed that the DC current
can be controlled up to 2500A, and the input power
factor is improved to more than 90% at the 15kW, 2500A
operation condition.
Keywords: DC power supply, AC-AC direct converter,
direct power control

2. Outline of Furnace
Sintering is a kind of techniques that create contacts
and bonds between minute particles by heating powdermaterial below its melting point. The powder materials
are filled in a mold case with applying a certain amountof pressure and draw thousand amperes of DC current
for heating up. The mold case filled with the powdermaterials has as low resistance as several milli-ohms.
Therefore only tens-volt output of the DC power supply
is sufficient for the thousand amperes of current draw.
3. Circuit Configuration and Principal of Operation3.1 Main circuit configuration
Figure 1 shows a schematic diagram of a main
circuit of the DC power supply for sintering. This DC
power supply consists of a utility three-phase power
supply, an LC filter, an AC-AC direct converter, a highfrequency step down transformer, and a high-current
rectifier. One of the bi-directional switches, which areneeded in the AC-AC direct converter, is composed of
two 2-in-1 IGBT modules connected antiparallel. The
frequency is converted directly from three-phase 50Hz tosingle-phase 15kHz to excite the transformer. Thetransformer has 30kVA capacity, of which winding turnratio is 17:1. A secondary circuit of the transformer has acenter tap structure and is composed of eight one-turnwindings. Schottky diode modules, which connected toeach secondary winding, are employed in the final
rectifying stage. Each diode has a snubber circuit to

1. Introduction
A conventional DC power supply for the electric
furnace has been developed and used successfully tocreate ultra hard material by means of sintering [1].
However, the basic circuit topology of the power supply
includes an AC-DC-AC conversion, which is acombination of a capacitor input type rectifier and afull-bridge inverter. Hence, the total input power factor
has limitation because of a choke coil and an electrolytic
capacitor in the DC bus. In this study, the converter has
been altered to the AC-AC direct converter. Then total
input power factor is improved.

AC200V
50Hz

vu

S1

Lf

S3

S5

S 2inv

S 4inv

ip

I out

S 6inv

Voutvv

vpvw

I out
iu iw C f

S2

S4

S6

S1inv

S3inv

17 : 1S5inv

10V max.
2500A max.

ipFigure 1. Schematic diagram of DC power supply using AC-AC converter.

S1

S3

S5ip

Positive NegativeCurrent Current

ip
S 2inv S 4inv

vp

S 6inv

ip

15kHz0V

vp(a) Primary current.

S2S4

S6
S1inv

S 3inv

S5inv

Io u tDC Current

0V(a) Positive current-source
rectifier.

(b) Positive current-source
rectifier.

(b) Secondary current after rectification.

Figure 2. Operation of AC-AC converter.

Figure 3. Generation of DC current.

absorb surge voltages at its switching moment.

3.3 Control of DPC based current-source rectifier
The DPC technique is applied to the current-source
rectifier because of its extremely high-speed response.
Figure 4 indicates a block diagram of the DPC based
control circuit. As shown in Figure 4, instantaneous
active power P and instantaneous reactive power Q are
calculated from the voltages and the currents of theutility power source as P   vα  = v
Q   βvβ  iα   ,− vα  iβ iu iv

vu vv vw

3.2 Control method based on DPC
In general, a current-source rectifier operates to keep
current constantly to the load. The AC-AC directconverter, which is illustrated in Figure 2, selects (a) for
positive current, and (b) for negative current. A primary
current ip of the transformer is a square wave of thecurrent of which frequency is 15kHz, then the rectifier at
the secondary of the transformer produces DC current Iout,
as shown in Figure 3. A switching pattern of thecurrent-source rectifier is determined so that the output
current follows to a current reference. A DPC technique
is applied to this control to select an optimum switchingstate of the converter.

I out---

3φ → 2φ

3φ → 2φvα vβiαiβPI*
VoutEq.(1)ssPQ*+ I outkdkd+++P*- +Q*+- +∆Q∆P

DC Excitation Canceller
ip ∫Oscillator

vsSQ
SPθnSwitching Table

ReferenceWaveform(CK;15kHz)

S1dpc

S 6 dpc

D-FF
-1

S1D

S6 DDivide

S1 L S6

Positive Switching
Negative Switching

S1inv L S 6inv

Figure 4. Block diagram of direct power controller.

Table 1. Optimum switching state.
(1)

where vα and vβ are two-phase voltages and iα and iβ are
two-phase currents obtained by three-phase to two-phasetransformation.
In general, it is necessary to dump the LC resonancewith a power feedback compensation, which iscomposed by differential elements skd.
An instantaneous active power command P* iscalculated by using a detected output current Iout and anoutput voltage reference Vout* which is an output of thePI controller to regulate Iout. On the other hand, theinstantaneous reactive power command Q* is provided
from outside of the controller, and is normally set at zeroto achieve a unity total power factor operation.
Control errors ∆P=P*−P and ∆Q=Q*−Q are quantized with hysteresis comparators, generating two-levelsignals Sp and Sq [2].
A phase angle of thepower-source voltage vector is also quantized to sixsectors θ n , where α-axis and β-axis denote real andimaginary parts of the vector, respectively. The quantizedSP SQ

1
1
0
0

0
1
0
1

θ1

θ2

θ3

θ4

θ5

θ6PON

OPNNPO

NOP

ONP

PNO

PNO

PON

OPN

NPO

NOP

ONP

SOO

OOS

OSO

SOO

OOSOSOONP

PNO

PON

OPN

NPONOP

Table 2. State of switching.
PNOSHigh side device (S1, S3, S5) ON (1) OFF (0) OFF (0) ON (1)Low side device (S2, S4, S6) OFF (0) ON (1) OFF (0) ON (1)phase θ n is mathematically expressed asππ(2n − 3) ≤ θ n < (2n − 1) Q n = 1, 2, L , 6 .
66(2)By using the quantized signals Sp, Sq and θ n , themost appropriate switching state of the rectifier is
uniquely determined to restrict the control errors ∆P andS1dpcS1D200Input voltage
vu (V)

CK

1000-100-200

S14. Simulation Results
Figure 6 and Figure 7 show operating waveforms ofsimulation results. In these simulations, the load was
15kW and the output current command was 2500A. As
can be seen in Figure 6, iu was controlled to be in phasewith vu, and power factor was more than 90%. Besides,
Iout of 2500A was achieved, which is well-regulated tofollow the command. Iout depends on ip, and enlarged
waveforms of ip and vp are shown in Figure 7. According
to ip, the positive current and negative current keep
constant value, which means the positive current-type
converter and negative one operate alternately to produce
the square wave of the current. Then, DC current can begenerated applying the DPC based AC-AC conversion
technique.
5. Conclusion
In this paper, a control method of operation of thelow-voltage and high-current DC power supply using theDPC based AC-AC converter was proposed. The basic
operation of the AC-AC converter is based on positive
current-source rectifier and negative one, generating thesquare wave of current ip at primary of the transformer.
Therefore, the rectified transformer output can generates
a DC current to the load. As a result of simulated
waveforms, generation of the DC current, using theAC-AC converter was confirmed. The input power factor
was improved more than 90%. In the future works, thecircuit proposed in the paper will be examined throughvarious tests.

Input current
iu (A)
Primary current
ip (A)
Output current
Iout (A)∆Q within the predetermined hysteresis bandwidth. Inorder to achieve this relay control operation, an optimum
switching state table shown in Table 1 is employed toselect a unique optimum switching state of the rectifier,according to a combination of the quantized signals Sp,
Sq and θ n [2]. Table 2 indicates relationship between theswitching states and the corresponding actual state ofswitching devices.
After Sndpc (n=0-6) passes the D-FF, SnD (n=0-6) is divided
into switching patterns for the positive current-source
rectifier (Sn (n=0-6)) and the negative one (Sninv (n=0-6)), as
shown in Figure 5. Sn and Sninv make it possible togenerate square shaped current at the primary of thetransformer.
An important point of this control is that using only
DPC switching table for the positive current-source
rectifier can control both the positive current-source
rectifier and the negative one. Sn and Sninv have the same
switching pattern, but the direction is opposite eachother.

500-50-100
200
1000-100-200
3000
2500
2000
0

Time: 20 (ms/div)
Figure 6. Waveforms of DC power supply.

Primary voltage
vp (V)

Figure 5. Switching pattern of S1 and S1inv (for n=1).

100Primary currentip (A)S1inv

4002000-200-4002001000-100-200

Time: 50 (µs/div)

Figure 7. Waveforms of vp and ip.
Acknowledgments
The authors wish to express their sincere gratitude toMACOHO Co., Ltd. for supporting development of theDC power supply.
References[1] Nishiyama，K., and Noguchi, T., 2005. Development of 13-V, 5000-A DC Power Supply with
High-Frequency Transformer Coupling Applied toElectric Furnace. PEDS, Vol. 2, pp. 1474-1479.
[2] Sato, A., and Noguchi, T., 2004. Direct Power
Control of Current-Source PWM Converter,
National Convention Record IEEJ. No.4-026.