A

A High Voltage Power Supply Operating under a Magnetic Field
Yoshiaki Shikaze,Masatosi Imoril, Hideyuki Fuke, Hiroshi Matsumotol and Takashi Taniguchi'
Department of Physics, Faculty of Science, University of Tokyo
7-3-1Hongo, Bunkyo-ku, Tokyo 113-0033,Japan
International Center for Elementary Particle Physics, University of Tokyo
7-3-1 Hongo, Bunkyo-ku, Tokyo 113-0033,Japan
'High Energy Accelerator Research Organization (KEK)
1-1 Oho, Tsukuba-shi, Ibaraki-ken 305-0801, Japan
Abstract
The article describes a network of high voltage power
supplies where the high voltage power supply incorporates a
ceramic transformer utilizing piezoelectric effect to generate
high voltage. The ceramic transformer ,is constructed from a
ceramic bar and does not include any magnetic material. So
it is expected that the high voltage power supply can operate
under a magnetic field. The high voltage power supply was
tested under a magnetic field of 1.5 tesla. The performance of
the power supply was almost intact in the magnetic field.
The power supply includes feedback to stabilize the high
voltage output, supplying from 1500V to 2500V with a load
of more than lOMR at a efficiency higher than 50 %. From
a supply voltage of 2V,the power supply can provide about
3000V at a load of 20 MR. A supply voltage of 5V is large
enough to provide 4000V at the same load.
The high voltage power supply is equipped with an interface
chip with a network. Most functions of the high voltage power
supply are kept under the control of the chip, and are then
monitored and controlled through the network.
I. INTRODUCTION
The article describes a network of high voltage power
supplies. A number of high voltage power supplies is intended
to produce high voltage efficiently under a magnetic field. The
high-voltage power supply incorporates a ceramic transformer
instead of a conventional magnetic one. The ceramic
transformer, being constructed from a ceramic bar, utilizes the
piezoelectric effect to generate high voltage. As no magnetic
material is present, no leakage of magnetic flux occurs such
that the transformer can be operated under a magnetic field. An
inductance element is also needed to obtain efficient voltage
conversion, being implemented by an air-core coil that can be
operated under a magnetic field.
By using the ceramic transformer and the air-core coil, the
power supply can be expected to operate under a magnetic
field [2]-[4]. The high voltage power supply was tested
under a magnetic field of 1.5 tesla. The performance of the
power supply was almost intact in the magnetic field [I].
An ultrasonic motor, utilizing a piezoelectric element and
operating in the same principle as the ceramic transformer,
rotates in a magnetic field of 5 tesla. So the high voltage power
supply might work in a similar magnetic field.
11. CERAMICPIEZOELECTRICTRANSFORMER
The ceramic transformer is manufactured by NEC
Corporation [5]. The transformer is shaped symmetrically
in the lengthwise direction and operated in a third order
longitudinal vibration mode, featuring low internal resistance.
The third order vibration mode locates electrodes, which are
connected to input and output electronic terminals with wire, at
nodes of the vibration mode.
The input of the transformer is amplified in voltage at the
output. The input to output ratio in voltage is an amplitude
ratio. The amplitude ratio shows a resonance as a function of
a driving frequency. Its Q-value depends on load connected
to the transformer. The Q-value is about 2000 when no load
is connected. The Q-value is roughly a hundred when the
transformer is loaded with a few hundreds of kilo-ohm [2].
The transformer amplifies voltage by about 120times when the
load is about 200kR. Maximal output wattage is about 4W.
A. Optimal Resistance
Efficiency of the ceramic transformer is a ratio of the power
supplied to the transformer to the power dissipated at the load.
Let w be an angular velocity of the driving frequency and c d '
be an output capacitance of the transformer. Assuming that
the load of the transformer is a resistor denoted by Rload,
the efficiency of the transformer is expressed by a simple
function of q, where q = w Rload Cd?. The maximal efficiency
is achieved when q = 1, that is, the resistance is equal to the
impedance of the output capacitor at the driving frequency [3].
Let Roptimal be the optimal resistance which realizes the
maximal efficiency, then
1
Roptiinal = -.Ld Cd?
B. Energy Reservoir
When energy is stored in inductance L, which is loaded
with resistance R, the time constant is L/R. In order to maintain
the voltage across the resistance, energy is injected to the
inductance at intervals of a switching time, which is roughly
equal to the time constant. So the large resistance increases the
switching frequency. The load resistance of the high voltage
power supply tends to be large. In the sense, the inductance is
not a good energy reservoir for the high voltage power supply.
When energy is stored in capacitance C, the time constant
0-7803-6503-8/01/$10.002001 IEEE
9-254

Figure 1: Schematic circuit of the high voltage supply,where an error amplifier is implementedby INA122 and OPA234and a voltage controlled
oscillator by pPD5555 and a flip-flop.
is RC. Then the switching time required to maintain the
voltage becomes long as the increase of the resistance. So the
capacitance is a good energy reservoir for the high voltage
power supply. The piezoelectric ceramic transformer stores
energy as mechanical vibration. Energy dissipation at the
load decays the vibration. The transformer is similar to
the capacitance in that the time constant of the decay is
proportional to the load resistance.
111. HIGHVOLTAGEPOWERSUPPLY
The high voltage supply provides a high voltage ranging
from 1500 V to 2500 V against a load of more than 10 M Q
where the load is a breeder of the photomultiplier.From a view
point of power, the ceramic transformer delivers power large
enough to drive the load of 20 MR up to 4000V. So it will be
easy to see that simply changing the ratio of the divider resistors
suffices to produce the output high voltage ranging from 3000V
to 4000V.
A. Circuit
The high voltage supply is composed of a reference voltage,
an error amplifier, voltage controlled oscillator (VCO), a
driver circuit, the ceramic transformer, a Cockcroft-Walton
(CW) circuit, and divider resistors, which is shown in Fig. 1 .
The output high voltage of the power supply is stabilized by
feedback. The output high voltage is divided by the divider
resistors and fed back to the error amplifier to be compared
with the reference voltage. The voltage difference between the
input of the error amplifier is amplified and fed to the VCO.
The VCO produces a carrier wave which carries the driving
frequency for the ceramic transformer. The carrier wave is
frequency-modulated by the output of the error amplifier. The
carrier wave is applied to the ceramic transformer through the
driver circuit.
The driver circuit generates a sinusoidal carrier wave so
that input capacitance of the ceramic transformer could be
efficiently driven. The sinusoidal carrier wave applied to the
ceramic transformer is amplified in voltage. The amplified
voltage induced between the output terminals of the ceramic
transformer is then further amplified in voltage and rectified
by the CW circuit. Ideally the CW circuit multiplies the input
voltage by six times at the output, by which discrepancy
of resistance between the optimal resistance and the load
resistance is reduced. Sothe CW circuit works as an impedance
converter between the transformer and the load [3]. The
resistance of the load viewed from the input of the CW circuit
is about 200 kQ when the load is 20 MO.
I ) ErrorAmplifier
A resonance frequency of the ceramic transformer is about
120 kHz as is seen from Fig. 2. Then the driving frequency
intervenes between 120 kHz and 130 kHz. Noises on the
9-255

output high voltage include voltage ripples due to cyclic
charge/discharge of capacitors. The ripples are the sawtooth
component of the noises synchronized with the driving
frequency.
In order not to amplify the ripples at the error amplifier, a
capacitance C1 is introduced which is spanned between the
input and the output of the amplifier. C1, selected to b.7 InF,
limits the bandwidth of the amplifier between DC to a few
ten kilohertz, which is narrow enough to reduce the ripples
synchronized with the driving frequency.
A range of the driving frequency utilized for the feedback
intervenes between DC to a few kilohertz. So a second zero is
introduced by a capacitance CZ connected in parallel with 50
kR at the input of the amplifier. The second zero reduces the
phase shift in the frequency range, which can improve stability
of feedback. So far C2 is selected to be 2nF lest the second
zero should interfere with an amplitude response implemexted
without the second zero.
2 ) Eliminating Simultaneous Turn-offof MOS FETs
Delay circuits (Fig. 3) are added to the input of TPS2811
lest both of MOS FETs (2SK2796Ls) should be switched
off simultaneously. Simultaneous switch-off of MOS FETs
raises a large voltage spike at the output of the driver circuit,
consequently causing noises at the high voltage output of the
power supply. The delay circuit postpones the falling edges of
pulses, which eliminates the simultaneous switch-off.
B. Feedback
The feedback utilizes the frequency dependence of the
amplitude ratio. The amplitude ratio of the transformer
depends on the driving frequency. The range of the driving
frequency is designed to be higher than a resonance frequency
of the ceramic transformer. As shown in Fig. 2, the feedback
increases the driving frequency when the output voltage is
higher than the reference voltage at the input of the error
amplifier.
working range
on initializatione, 6 0
40
U
116 118 120 122 124
Frequency kHz
Figure 2: Resonancecurve of the ceramic transformer.
I ) Breakdown of Feedback
Providing the load of the high voltage power supply falls
within an allowable range, the driving frequency is kept to be
higher than the resonance frequency such that the feedback
is negative as designed. While the allowable range of load
is sufficient in most cases, it cannot cover, for example,
short-circuiting the output high voltage to ground. When the
load deviates beyond the allowable range, the driving frequency
may decrease below the resonance frequency; a condition that
will not provide the required negative feedback, i.e., positive
feedback locks the circuit such that it is independent of load.
2) Protection of Circuit
The breakdown of the feedback caused by an unaffordable
range of load flips the frequency of the VCO beyond the
resonance frequency. The flip of the frequency, accompanied
with the breakdown of the feedback, lowers the output high
voltage. Thus the flip of the frequency works as protection
against, for example, the short circuit of the output high
voltage.
IV. IMPROVINGEFFICIENCY
Performance bf the ceramic transformer is susceptible to
,stray capacitance, especially around the output terminal. It
is important to reduce the capacitance between the output
terminals of the transformer. The capacitance between the
output terminals is mainly due to the junction capacitance of
the diodes in the CW circuit. The diodes are therefore vital for
the efficiency of the high voltage power supply [2],[3].
The key to improve efficiency is to realize the zero-voltage
switching (ZVS) at the driver circuit. A range of the driving
frequency was studied where the ZVS was realized. The
ZVS was maintained against a wide range of the driving
frequency [l]. It is also important to match the resistance of
the load viewed from the input of the CW circuit to the optimal
resistance of the transformer. Reducing the discrepancy of
resistance contributes to improving the efficiency.
A. Diode
Efficiency of the ceramic transformer is susceptible to
stray capacitance, especially around the output terminal. The
output impedance of the transformer effects the efficiency
in that efficiency is degraded by capacitance between the
output terminal and ground. Lower equivalent resistance of
the transformer provides lower impedance at the output, which
reduces degradation due to the capacitance.
A diode in the CW circuit is important for the efficiency
because thejunction capacitance of the diode is loaded between
the output terminal of the transformer and the ground. The
diode employed in the CW circuit is ESJA98, a high voltage
diode featuring high speed switching up to 1 MHz. The diode
was developed for high voltage rectification in the deflection
system of a high definition TV. Thejunction capacitance is less
than 2 pF at 1 MHz. Characteristics of the diode have been
measured for Spice parameters which indicate that the junction
capacitance is smaller than 1 pF.
B. Zero-voltage Switching
It is indispensable to apply a sinusoidal carrier wave
to the input terminals to efficiently drive the transformer.
The driver circuit generates this wave which eliminates the
power consumed by the capacitance mainly produced by the
9-256

transformer. The driver circuit includes two identical resonant
circuits (Fig. 3), each of which consists of the inductance
implemented by an air-core coil and a MOS FET. The FETs
are simultaneously switched on and off alternately. When the
FET is off, the inductance in the resonant circuit resonates
with the capacitance; and when it is on, the inductance stores
the current. One resonant circuit makes a half sinusoidal
wave, while the other identical circuit alternately generates
the other half sinusoidal wave. Accordingly, the driver circuit
generates a quasi-sinusoidal carrier wave at the transformer
input terminals.
Inductanceimplemented
by air-corecoil
2SK2796Lx2
Ceramic
Transformer
b IT P Z 1P 1-7:Delay circuit "CO
I
v i I
1,
0
-I - Delay circuit
*
Figure 3: Delay circuits,MOS FETs,air-corecoilsand supply voltage
at the driver circuit.
The ZVS is realized if the FETs switch over when the
voltage applied across the FETs is close to 0 V. The FETs are
simultaneously driven by the driving frequency. Let a ZVS
frequency be defined by the self-running frequency at which
the inductance resonates with the capacitance autonomously,
then the ZVS is realized when the driving frequency is equal
to the ZVS frequency. The ZVS is also realized while the
driving frequency is lower than the ZVS frequency. So
efficiency is expected to be rather constant while the driving
frequency intervenes between the resonance frequency and the
ZVS frequency, where the efficiency is the ratio of the power
supplied to the driver circuit to the power dissipated at the load
resistance.
C. Dependence of EfJiciency on Inductance
The efficiency of the power supply was measured at
several values of inductance. The output high voltage and the
efficiency were plotted against the driving frequency for each
inductance. The plots show that the efficiency increases as the
frequency come close to the resonance frequency, reaching
a plateau of more than 50 %. The plateau, as is expected,
corresponds to the ,driving frequency between the resonance
and the ZVS frequencies. The plot shows furthermore that the
plateau is wider in frequency for smaller inductance and that
larger inductance reaches a higher plateau in efficiency.
The ZVS frequency is a function of the inductance in
the resonator circuit. So it is important not to select such
the inductance that locates the ZVS frequency close to the
resonance frequency. Placing the ZVS frequency at an
adequate frequency contributes to improving the efficiency of
the power supply. The inductance value could be determined
by sufficient tolerance maintaining the ZVS against a wide
range of the driving frequency.
V. PERFORMANCE
When the load is 20 MR, an equivalent resistance of the
load viewed from the input of the CW circuit is about 200 kS2.
So the amplitude ratio of the ceramic transformer is expected
to be about 120 at the resonance frequency. In practice, the
amplitude ratio was found to be about 80 around the resonance
frequency [ 11.
A. Dependencies on Supply Voltage
The efficiency of the power supply was measured at several
supply voltages. The efficiency was plotted against the driving
frequency for each supply voltage. The plot shows that the
efficiency does not depend on the supply voltage and that the
efficiency mainly depends on the driving frequency.
Similarly, the output high voltage was measured at several
supply voltages. The output high voltage was plotted against
the driving frequency for each supply voltage. The plot shows
that the amplitude ratio at a specified driving frequency is rather
independent of the supply voltage.
The amplitude of the sinusoidal carrier wave applied to the
ceramic transformer is about three times the supply voltage.
The output voltage of the transformer is furthermore multiplied
by about six times in the CW circuit. When the supply voltage
is 2 V, the output high voltage reaches 2800 V at a load of 20
MO. Then the amplitude ratio of the transformer is estimated at
about eighty around the resonance.
I ) Plot ofE@ciency versus Output High Voltage
The efficiency is plotted against the output high voltage
for each supply voltage in Fig. 4. It can be seen that the
correspondence between the efficiency and the output high
voltage is identical in shape among the supply voltages in the
sense that widening the correspondence at the 2 V supply
voltage by two times in a lengthwise direction produces the
correspondence at the 4 V supply voltage. The shape of the
correspondence is due to the dependence of the efficiency
on the driving frequency. ,The efficiency dependence on the
driving frequency is defined by the inductance at the driver
circuit.
Fig. 4 shows that the efficiency of producing the output high
voltage depends on the supply voltage. The supply voltage
should be selected according to the range of the output high
voltage. Assuming that the load of the high voltage power
supply is 20 MR, a 2V supply voltage is efficient for the output
high voltage ranging from l00OV to 2000V. Similarly a 3V
supply voltage for 1500V to 2500V. A 5 V supply voltage is
good for 3000 V to 4000 V at wide range of the load.
9-257

C. Current Monitor
When the output high voltage is assigned, the frequency,
at which the transformer is driven, depends on the supply
voltage. Then when the supply voltage is known beforehand,
it is possible to estimate the output current from the driving
frequency. The VCO outputs the driving frequency on a square
wave. So a simple logic circuit enables the chip to count
pulses. The driving frequency, obtained by counting the pulses
in a fixed time interval, suffices to calculate the output current
togethdr with the output high voltage and the supply voltage.
,___I_-__ __ - .. -- - - -
>r
.-6 0 9 0 2 0 0 V 3 0 0 V 4 0 0 V
0 s2 25V 3 25V * 4 50V
5 0 8 ~2 SOV 3 50V 50OV
.-w-
r 2 7 5 v 3 7 5 v
0 7
0 6
* . I '
VII. ACKNOWLEDGMENTS
0 3
0 2 " A
0 1
a . *
We are grateful to NEC Corp. for providing us with the
. " ceramic transformers, without which it would have been
? * impossible to develop the presented device, and to NEC
'0- 500 1000 1500 2000 2500 3000 employees Messrs. Toshiyuki Zaitsu, Yasuhiro Sasaki,
and Atsusi Ochi for their supports. We thank Prof. Akira
. .
output HV (V)
Figure 4: Plot of efficiency against output high voltage for supply
voltages
VI. NETWORK
The high voltage power supply includes a Neuron chip', a
programming device processing a variety of input and output
capabilities. The chip can also communicate. with other Neuron
chips over a twisted-pair cable , which allows establishing a
network consisting of a number of power supplies that each
incorporate the chip. The functions of the power supply under
the control of the chip are managed through the network. Then
the high voltage power supplies are monitored and controlled
through the network.
A. Output High Voltage
The reference voltage is generated by a digital-to-analog
converter which is kept under the control of the chip. Then
the reference voltage and therefore the output high voltage of
the high voltage power supply will be controlled through the
network.
B. Recovery From Feedback Breakdown
A VCO voltage, which is the output of the error amplifier,
is supplied to the VCO and controls the driving frequency.
The flip of the frequency is caused by the deviation of the
VCO voltage from its normal range. The deviation is detected
by voltage comparators, which interrupts the Neuron chip.
Being awakened, the chip reports the frequency flip through
the network. Under the control of the network, the chip enters
into recovery from the frequency flip. The recovery begins
with resetting the reference voltage. When the reference
voltage is reset, the driving frequency is initialized. After the
initialization of the driving frequency, the chip increases the
reference voltage to a prescribed value and restores the output
high voltage, which completes the recovery
'Neuron is a registered trademarkof EchelonCorporation.
. _
Yamamoto for providing facilities for tests in a magnetic
field. We also thank members of the BESS collaboration for
encouraging us to employ the ceramic transformer, as well as
all personnel supporting BESS.
VIII. REFERENCES
[l] Y. Shikaze, M. Imori, H. Fuke, H. Matsumoto and T.
Taniguch, Performance of a High Voltage Power Supply
Incorporating a Ceramic Transformer, Proceedings of
the 6th Workshop on Electronics for LHC Experiments,
September 2000, Krakov, Poland
[2] M. Imori, T. Taniguchi and H. Matsumoto, Performance
of a Photomultiplier High Voltage Power Supply
Incorporating a Piezoelectric Ceramic Transformer,
accepted for publication in IEEE Transactions on Nuclear
Science, Proceedings of Nuclear Science Symposium,
November 1998, Toronto, Canada.
[3] M. Imori, T. Taniguchi and H. Matsumoto, a
Photomultiplier High Voltage Power Supply Incorporating
a Ceramic Transformer Driven by Frequency Modulation,
IEEE Transactions on Nuclear Science 45(1998)pp.-78 1.
[4] M. Imori, T. Taniguchi and H. Matsumoto, a
Photomultiplier High Voltage Power Supply Incorporating
a Piezoelectric Ceramic Transformer, IEEE Transactions
on Nuclear Science 43(1996) 1427-1431.
[5] S. Kawasima, 0. Ohnishi, H. Halcamata et. al.,
Third Order Longitudinal Mode Piezoelectric Ceramic
Transformer and Its Application to High-Voltage Power
Inverter, IEEE Ultrasonic Sympo., Nov., 1994, Cannes,
France. pp.525-530.
[6] 0. Onishi, Y. Sasaki, T. Zaitsu, et. al., Piezoelectric
Ceramic Transformer for Power Supply Operating in
Thickness Extensional Vibration Mode, IEICE Trans.
Fundamentals. Vol. E77-A, No. 12 December 1994. pp.
2098-2105.
[7] T. Zaitsu, T. Inoue, 0. Onishi and A. Iwatani, 2 M
Hz Power Converter with Piezoelectric Transformer,
INTELEC'92 Proc., pp.430-437, Oct. 1992.
9-258

A

More Related Content

What's hot

Viewers also liked

Similar to A

A