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Trang 1Voltage Rating Reduction of Magnet Power Supplies
Using a Magnetic Energy Recovery Switch
Takanori Isobe, Taku Takaku, Takeshi Munakata, Hiroaki Tsutsui, Shunji Tsuji-Iio, and Ryuichi Shimada
Abstract—A new concept of magnet power supplies that can
reduce voltage ratings of the power supplies is proposed Circuit
diagram and operation principles of magnetic energy recovery
switch (MERS) are described MERS consists of a capacitor and
four semi-conductor devices such as MOSFETs and IGBTs It
is connected in series to a power supply and a coil MERS is
a switch module and it has no power supply in itself Because
MERS generates a voltage required for the inductance of the coil,
the power supply only has to supply a voltage required for the
resistance of the coil Therefore, using MERS can reduce voltage
rating and capacity of the power supply Two types of power
supply using MERS and voltage rating reduction are discussed.
Comparatively small power supplies for high-repetition pulsed
magnets and alternating magnetic field coils can be designed.
Some experiments were carried out and confirmed that MERS
can reduce voltage ratings of power supplies.
Index Terms—Magnet power supply, pulsed magnet.
I INTRODUCTION
POWER supplies for magnet coils often use power
elec-tronics devices, such as MOSFETs and IGBTs In general,
a high voltage is required to control high currents at high
rep-etition rates because of an inductance Consequently the power
supply becomes comparatively large scale because the capacity
of the power supply becomes large
Therefore, some types of rapid-cycling synchrotrons are
op-erated by power supplies using LC resonance to reduce their
voltage ratings [1], [2] However, these types of power supply
can not change output current frequency In addition, aged
de-terioration of capacitor may cause some problems
The authors proposed some types of power supply using
Mag-netic Energy Recovery Switch (MERS) [3]–[5] In this paper,
power supplies for magnet coils using MERS are proposed
MERS is a switch module with a capacitor From other point
of view, it is a capacitor controlled by semi-conductor devices
The power supplies using MERS can reduce their voltage
rat-ings regardless of resonant frequencies by using forced LC
res-onance These power supplies can be applied to high frequency
coil excitations such as magnetic levitations, liner motors, and
induction heating
II MAGNETICENERGYRECOVERYSWITCH(MERS)
A Circuit Diagram
Fig 1(a) shows bi-directional type MERS Four MOSFETs
are connected in two parallel arms Each arm consists of two
Manuscript received September 20, 2005; revised November 24, 2005.
The authors are with the Research Laboratory for Nuclear Reactors, Tokyo
In-stitute of Technology, Meguro-ku, Tokyo 152-8550, Japan (e-mail: tisobe@nr.
titech.ac.jp).
Digital Object Identifier 10.1109/TASC.2006.870491
Fig 1 Magnetic Energy Recovery Switch (MERS) (a) Bi-directional type of MERS consists of four MOSFETs and a capacitor The pair of MOSFETs 1 and
2 is used to control upward current and the other pair of MOSFETs is used to control downward current (b) Mono-directional type of MERS consists of two MOSFETs and two diodes and a capacitor.
MOSFETs connected in series Four MOSFETs are connected
in reverse direction each other in both of series and parallel con-nection The middle points of series are connected to a capacitor Semi-conductor devices which can turn off current are needed for MERS Therefore, MOSFETs or IGBTs can be used because they can turn on and off in any time by gate control
In the case of upward current control in Fig 1(a), MOSFETs
1 and 2 are controlled, 3 and 4 are left turned off and used as diodes Therefore, mono-directional MERS can consist of two MOSFETs and two diodes and a capacitor as Fig 1(b) The circuit diagram of MERS is similar to full-bridge con-verter; however, there are two different points First, MERS is connected in series to circuit Since MERS is inserted between power supply and load, the MERS can control current flowing
to the load Second, the voltage of the DC capacitor of MERS is allowed to change dynamically and even becomes zero because the capacitor is not connected to DC power supply
B Operation Principle
MERS is usually used with an inductive load Fig 2 shows operation modes of MERS When a pair of MOSFETs is turned
on, current flows through in two ways as Fig 2(a) Next, when the MOSFETs are turned off, the current charges the capacitor through diodes as Fig 2(b) Current decreases gradually and
it becomes zero After this time, no current flows because of diodes as Fig 2(c) The magnetic energy stored at the inductive load is absorbed to the capacitor, and converted to electrostatic energy
Next, when the MOSFETs are turned on, the electrostatic en-ergy of the capacitor raises current as Fig 2(d) The voltage of the capacitor decreases gradually and it becomes zero After this time, the current flows through the MOSFETs and the diodes which included in MOSFETs of the other pair, so it becomes
1051-8223/$20.00 © 2006 IEEE
Trang 2(a) (b)
Fig 2 Operation modes of MERS The MERS has four modes to control
current in each direction (a) When the capacitor is not charged and a pair of
MOSFETs is turned on, current flows through two ways (b) When current
flows and the MOSFETs are turned off, the current charges the capacitor
through diodes (c) When the current becomes zero and the MOSFETs are
kept turned off, no current flows (d) When the capacitor is charged and the
MOSFETs are turned on, the capacitor discharges.
Fig 3 Circuit configuration of power supply using MERS for high-repetition
pulsed magnet A mono-directional MERS is connected in series to a DC power
source.
parallel conduction mode as Fig 2(a) The magnetic energy is
recovered from the electrostatic energy
III MAGNETPOWERSUPPLIESUSINGMERS
Power supplies for magnet coils are suitable applications for
MERS Most of the energy given to the magnet is stored on
magnetic field Therefore, in high frequency applications, most
of energy shuttles between the power source and the magnet
in each cycle By using MERS, the energy shuttles between
the MERS and the magnet, and the power source supplies only
losses at the magnet and switching devices, etc
A Power Supply for High-Repetition Pulsed Magnet
Fig 3 shows circuit configuration of the power supply using
MERS for high-repetition pulsed magnet In this circuit, a
mono-directional MERS is connected in series to a DC power
source By switching MOSFETs of the MERS, pulsed currents
are generated from the DC power source Moreover, the MERS
absorbs and supplies magnetic energy of the magnet in each
cycle
Fig 4 Schematic waveforms of generated current i and capacitor voltage v
by the circuit of Fig 3 t and t mean times to charge and discharge the capacitor respectively I and V mean peak values of i and v respectively Mode symbols are referred to Fig 2 Current waveform shown as dotted line indicates schematic current waveform when the MERS is not used.
Fig 4 shows schematic waveforms of generated current and voltage of the capacitor of the MERS In mode (b), when MOS-FETs are turned off, the current decreases and the capacitor voltage increases In mode (d), the current increases and the voltage decreases These phenomena are part of LC resonance
(1) where is the inductance of the magnet and is the capaci-tance of the MERS In general, it is much shorter than the time constant of the load
In mode (a), the current flows through the MERS and only the voltage of the power source is applied to the magnet The current raised in mode (d) is equivalent to the current shut off in mode (b) Therefore, after some cycles, becomes as
(2) where is the voltage of the power source and is the total resistance of the circuit Since the electrostatic energy stored in the capacitor when mode is (c) is equivalent to the magnetic energy stored at the magnet when mode is (a), is given by
(3)
B Power Supply for Alternating Magnetic Field Coil
Fig 5 shows the circuit configuration of a proposing power supply for alternating magnetic field coil The power supply consists of a full-bridge inverter and a bi-directional MERS con-nected in series MOSFETs , , , and are turned on and off at the same time with , , , and respectively and are always opposite condition to and and and are also always opposite condition to and The MERS absorbs and supplies magnetic energy also in this circuit Consequently the full-bridge inverter supplies only losses Fig 6 shows schematic waveforms of generated current and voltage of the capacitor of the MERS The current flowing
Trang 3Fig 5 Circuit configuration of power supply using MERS for alternating
mag-netic field coil A bi-directional MERS is connected to a full-bridge inverter.
Fig 6 Schematic waveforms of generated current i and capacitor voltage v
by the circuit of Fig 5 Gate signals to U and Y of the full-bridge inverter and
V and X of the MERS are also shown Gate signals of MERS are shifted
by t to those of the inverter Current waveform shown as dotted line indicates
schematic current waveform when the MERS is not used.
through the magnet is decreased by the MERS and it becomes
zero The magnetic energy of the current is absorbed to the
capacitor, and then used to raise the opposite direction current
The inverter supplies the maximum power when the inverter
are controlled so that forward current flows when it outputs
positive voltage and reverse current flows when it outputs
negative voltage The shifted time to realize that condition
is given by
(4) and are given by (2) and (3) respectively
C Voltage Rating Reduction of Power Supplies
Required DC voltage to raise current by a conventional
voltage source type power supply is given by
(5)
where is the inductance of the coil and is peak current and
is requested time to raise the current On the other hand, from
(2), required DC voltage with a power supply with MERS is
given by
(6)
while is the resistance of the circuit From (5) and (6), voltage reduction rate is given by
(7)
where is the time constant given by This means that the voltage rating reduction is achieved more effectively in the con-dition that the time constant is much longer than the requested time to raise the current Usually power supplies for high-repe-tition pulsed magnets and high frequency alternating magnetic field coils are under this condition
Using MERS can reduce voltage rating and capacity of power supply However, total capacity of semi-conductor devices can not be reduced since MERS also consists of semi-conductor de-vices The number of semi-conductor devices increases and total voltage ratings can not be reduced The most important point of the voltage rating reduction is that DC capacitors of the power supply can be reduced by using MERS Large DC capacitors are used in the power supply because energy storage is needed
to generate pulsed output The size of the capacitors relates to the voltage rating of the power supply Therefore, voltage rating reduction causes decreasing of the DC capacitors In general, the
DC capacitors occupy large part of the power supply in volume The DC capacitors store energy as large as several times energy
of one cycle in order to maintain the voltage within the range
of several percent On the other hand, the capacitor of MERS only stores the energy of one cycle The capacitor of MERS dis-charges all of its charge and the voltage becomes zero There-fore, the total size of capacitors is reduced by using MERS
In other words, dividing off the capacitor of MERS from the capacitors of the inverter power supply can reduce the total size
of capacitor To discuss about it, the energy supplied to the coil
is divided between the energy consumed at the coil and the en-ergy which makes a round trip between the power supply and the coil Both these parts of energy pulsate and cause voltage fluctuations of the capacitors The capacitors must maintain the voltage within a range; therefore the capacitors must store en-ergy as large as several times of both these parts of enen-ergy By using MERS, the energy which makes a round trip does not af-fect to the capacitors but relates to the capacitor of MERS The capacitors of the inverter must store energy as large as several times of only the energy consumed at the coil, and the capac-itor of MERS must store as large as the energy which makes a round trip Therefore, the total size of capacitors which relates
to stored energy is reduced
IV EXPERIMENTS
Some experiments are carried out to confirm operations of the power supply and theory about voltage rating reduction A lab-oratory pilot device for alternating magnetic field coils is made Circuit configuration is same as Fig 5 Fig 7 shows photos of experimental setup A DC power supply which can generate variable DC voltage was used as DC voltage source Two test coils which have different time constants were used for experi-ments Table I shows parameters of these coils Time constants
in the table are calculated from coil parameters and the re-sistance of MOSFETs In the experiments using the laboratory pilot device, a comparatively high frequency is used because the
Trang 4Fig 7 Photos of the experimental setup (a) Test coil is air-core coil made from
polyester enameled copper wire of 0.7 mm in diameter (b) Experimental circuit
which have an inverter and a MERS with a small capacitor.
TABLE I
E XPERIMENTAL P ARAMETERS
Fig 8 Waveforms of current and voltage applied to the coil when t is 30 s.
MERS generated about 360 V and applied it to the coil alternately The current
was controlled fast by that voltage.
time constant of the laboratory scale circuit is much shorter than
that of real applications
In the first experiment, target condition was set to 4 A with
30 rising time which is much shorter than the time constant
in the coil A By using conventional power supply, required
voltage to achieve that condition is 206.7 V from (5)
From (7), is estimated at 0.0489 and consequently the required
voltage will be reduced to about 10 V by using MERS
The capacitor of MERS is determined at 0.2 to meet that
condition from (1) In that condition, the maximum voltage of
the capacitor will be 357 V from (3)
Forward and reverse currents of 4 A were raised alternately
at 5 kHz repetition rate Experimental results confirmed that
required voltage was reduced most effectively when the gate
shift time is 30 calculated by (4) Fig 8 shows
wave-forms of current and applied voltage when is 30 It was
also confirmed that required DC voltage was reduced to 13.4
V extremely The MERS generated about 360 V and controlled
current by applying that voltage
Fig 9 Measured voltage reduction rate with varying requested rising time
T Theoretical values of as a function of T by (7) are shown as dotted lines.
In the second experiment, required voltage with varying requested rising time are measured by two coils which have different time constants Fig 9 shows voltage reduction rate from the measurement These results confirms that measured roughly agree with theoretical values given by (7)
V CONCLUSION
This paper discussed the magnet power supplies using MERS The power supplies can reduce their voltage ratings regardless of frequency by forced LC resonance MERS can not reduce total semi-conductor capacity but total amount of
DC capacitors which occupy large part of power supplies Therefore, by using MERS, comparatively small scale power supplies can be designed
Voltage rating reduction rate is described by coil parameters and target condition Large time constant and fast rising time can realize more effective voltage rating reduction This indi-cates that large scale magnets which have large time constant are suitable applications However quite large scale applications which have huge magnetic energy is not suitable because these applications do not use DC capacitors to store the energy High frequency applications are also suitable because required cur-rent rising time is fast compared with the time constant Experiments by the laboratory pilot device confirm opera-tions of this power supply and voltage rating reduction
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