JAERI-Conf 95-021

PA29
DCCTs for Magnet Power Supplies
Toshiyuki Ozaki
National Laboratory for High Energy Physics, KEK
1-1 Oho, Tsukuba-shi, Ibaraki-ken, 305, Japan

Abstract

low value. The operation is alike that of the SCR (siliconcontrolled

This paper describes the operation principle ofrectifier),

sincetheload

current

canbecontrolled by firing the saturable core as well as the SCR.

DCCTs which are used in the magnet power supplies of theThe· photograph in Fig.l

TRISTAN main ring.

firing angle can be controlled -by the de current of theshows the load voltage. Theprimary. The difference between the SCR and a saturable
inductor is that the SCR allows the current to pass in only

1. Introduction

one direction, whereas inductors conduct current in bothThe curr!)nt sensor used in the magnet power supply

directions. It is worth noting that the return current ismust be of high precision. In the TRISTAN main ring,nearly constant. This property depends on the B-H loop,

high-precision DCCTs comprising 2 Brentford/Hingorani

because the characteristics on the integral of the voltage(B.H.)-typeDCCTs(direct-currentcurrent

transformer)

are used.

versus the current are almost as sharply rectangular as
the B-H characteristics.

The usual textbooks explain the Kraemer type DCCT,

B.H.-type DCCTs make use of the flatness of the return

which has a well-known defect: the waveform of thecurrent. The circuit in Fig.2 can be regarded as being anoutput exhibits "notches" at the peaks of the ac voltage

equivalent ·circuit of that shown in Fig.l. The resistor (R)wave. The output therefore has a large ripple of 100 Hz.

in Fig.l is equivalent to a mixture of the resistors andAlthough capacitors can be used to smooth out this ripple

diodes in Fig.2.

so as to produce a pure de component, this sacrifices theresistors

response speed.

alternately; the rectified output across the resistor (RL) is

Dr. Hingorani and Brentford corporation in EnglandinThe bypass diodes are connected to theparallel.

Therectifiers

areswitchedshown in Fig.2. We obtain square pulses. The height isdeveloped a modified version of the Kraemer circuit inproportional to the primary current. Since the region isthe early 1960's which eliminated the current notches(refs.l-2). Although any DCCTs of this type are nowwider than 180 degrees, a pure de output can be madefrom 2 square pulses by means of a circuit functioning as

operational in power supplies, no papers explaining thean AND-gate.

principle of the B.H.-type DCCT could be found. This paper
thusgives

boththeprincipleandaprocedure

forbuilding DCCTs.

R2.

Brentford/Hingorani-type DCCT
Principle of operationThe B.H.-type DCCT comprises two saturable inductors,
fourrectifiersandthere

resistors.

Theprimaryandsecondary windings are wound on a single core (Fig.l)
The core is the key element.

A rectangular hysteresis-loop core material is employed to obtain a sharp break
between the saturated and unsaturated states.

As soon as

the flux in the core reaches the saturation region, theimpedance of the inductor changes from a high value to a-106-Fig. 1

Circuit with a saturable core

JAERI-Conf 95-021

RPSPICE analysis
In order to fully understand the operation, computer
simulations were performed. The circuit shown inRLFig.3(b) was investigated using computer code SPICE. TheJiles and Atherton hysteresis model (refs.3-4) is listed in
the library of SPICE. The model is ferrite so that thehysteresis-loop

is not rectangular.

Many computer runs were made in order to obtain asmaller ripple waveform. The resulting ripple, even in
the best case, was about 30%. This was caused by notrectangular

hysteresis.
Test ofDCCTTheFig. 2

Equivalent circuit

The DCCT used for tests has two cores with 6000 turns.
core material has a high permeability and arectangular-hy steresisloop. Theinductance was980 H,

which vias almost the upper limit of the LCR meter.
(I) The B-H loops in operation are shown in Fig.4.

In Fig.3(a), two saturable cores are combined to supply
each square pulse (shifted by 180 degrees) to a common
load resistor (RL). The core is periodically magnetized
into saturation; the role of the two cores is reversed in thenext half-cycle, providing a DCCT. Fig.3(a) is equivalent toFig.3(b).

HThe circuit arrangement shown in Fig.3(b) isfamiliar to us.

lp=10A

Ip=OA

RLV2(a )Oscilloscope presentations of the B-H loop.

Fig. 4(2) The ripple of. the output was about 8%. This unwanted
ripple component is generated by the width of thehysteresisloop.

(3) A pulsed current was measured using the DCCT and a
precision shunt. Fig.5 shows the current waveforms.
Although( b )Fig. 3

Superposition of saturable core circuits

thecarrier

frequencyof

themagnetization

source is 50 Hz, a short pulsed current can be observed,
anbecause a core in the unsaturation state serves as
ordinary- 107-transformer.

JAERI-Conf 95-021.,-- .... -- ........ ··- ........ ---------"'\:

To obtain high stability, a stabilized ac voltage source

SCRandarerequired.

Theseperformances are well documented in the literature, andLOAD

chia temperature-controlled ovenfulfill the requirements of the storage ring.

ch2chlch2~

6ms
Fig. 6Fig. 5

3.

Circuit of the high-precision DCCf

Pulse response of the DCCf

Construction of a high-presition

DCCT

The qircuit of the high-presition DCCT is shown in Fig.6.
Acknowledgment

There is one primary winding and 3 auxiliary windings. Alarge de current for making measurements flows in theprimary cable. Four windings for the carrier current areThe author would like to acknowledge Professor H.

provided in each core; 2 bias windings are provided in

Kiwakieach DCCf, which are connected in series. The output ofsuggestions.

of Kyushu

SangyoUniversityfor

his

helpfuleach DCCT is set to the proper operating level by a bias
current.

The

primarycableandcompensating winding

are wound in opposite directions to each DCCT, and areconnectedincompensatedcurrent

series.

Thebysecondarythroughtheacurrent

high-gain

topower

feedback loop. This control circuit

bemeasured

iswithamplifieramaintains a decreasein the difference between the two DCCTs. The negativefeedback

References

windingcompensating

circuit improves the linearity, dynamic range,

frequency characteristic, temperature stability, andsignal-to-noise, ratio. The burden resistor (Vrtf) converts(1) M.G.J.Fry: IEEE NS-18 (1971) 865(2)M.G.J.Fry: Proceedings of the 4th international
conference on magnet technology (I 972) 674(3) D.C.Jiles and D.L.Atherton: IEEE MAG-19 (1983) 2183
(4) J.H.B.Deane : IEEE MAG-30 (1994) 2795

the compensating current into a voltage signal (V..., ). The
voltage is proportional to the primary de current.
We constructed a high-precition DCCf.

Matched DCCfs

were chosen for a pair. Although the ripples of about 8%of the DCCTs were canceled in the input of the amplifier, aslight ripple

remained in the output voltage.

A spikeripple was caused by diodes when the current was turnedoff. Above a high amplification, an oscillation ofkHz occurred, and quickly shifted2- 3

to the voltage limit.

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