Alternately Poled Piezoelectric Transformers Using Rectangular

Ceramic Plates

Kouichi Kanayama

Faculty of Engineering, Yamagata University, Yonezawa, Japan 992

Nobuhiro Maruko

Mitsui Petrochemical Industries, Ltd., Sodegaura, Japan 299-02

SUMMARY

In order to apply a piezoelectric transformer to an

LCD backlight inverter, piezoelectric transformers are in-

vestigated [1, 2]. The load is a cold cathode fluorescent

lamp (CCFL), whose impedance is low, about 100 kW. But

the step-up ratio of the conventional Rosen-type piezoelec-

tric transformer is small and inadequate for this application.

We have developed a new piezoelectric transformer struc-

ture which has an alternately poled structure in a rectangular

single-layer piezoelectric ceramic plate. Its step-up ratio is

twice that of the Rosen-type transformer. Two types of

samples were fabricated and analyzed using equivalent

circuits, and the vibration characteristics and the electrical

characteristics were evaluated. © 1998 Scripta Technica,

81(12): 29�36, 1998

Key words: Piezoelectric transformer; resonance;

inverter; step-up ratio.

1. Introduction

The piezoelectric transformer was developed by C.

A. Rosen in 1957 [3]. It has a simple structure, and is made

from a piezoelectric ceramic plate with some surface elec-

trodes. As its step-up ratio is very high, it was investigated

for application as the high-voltage transformer in a TV set

[4]. But there were some difficulties. One is the inability to

apply the conventional drive circuits for magnetic trans-

formers, and the other is the insufficiency of output power

capacity for this application.

Presently, LCDs are used in many electronic devices

due to their low consumption. As an LCD does not radiate,

a backlight is necessary. A CCFL is used as a backlight lamp

to illuminate an LCD. The features of a CCFL are thinness

and long life. For startup, a high voltage of about 1.5 kV

must be applied to it, and about 500 V is needed to keep it

lit. One of the LCD applications is a display for a notebook

PC. This application requires thinness and small size, low

consumption, and high efficiency. Therefore, piezoelectric

transformers have been studied for this use, because they

have essentially thin shape and high efficiency. But the

step-up ratio of the conventional Rosen-type transformer is

too small to generate high voltage from the approximately

12 V of an internal battery, especially while lighting. This

is because of the heavy load, about 100 kW of a CCFL, when

it remains lit. For this reason, we proposed a new piezoelec-

tric transformer structure to realize a higher step-up ratio

and to feed energy from a low-voltage battery [5]. It has an

alternately poled structure in a rectangular single-layer

piezoelectric ceramic plate.

CCC1042-0967/98/120029-08

© 1998 Scripta Technica

Electronics and Communications in Japan, Part 3, Vol. 81, No. 12, 1998Translated from Denshi Joho Tsushin Gakkai Ronbunshi, Vol. J80-A, No. 10, October 1997, pp. 1688�1693

29

In this paper, two types of piezoelectric transformers

which were designed as alternately poled structures are

shown, and the equivalent circuit parameters are seen to be

comparable to each other. Then, the working characteristics

are discussed. Piezoelectric transformers generate heat

when they work at high power. Therefore, the upper limit

of the drive power is defined by the temperature rise. Heat

generation of piezoelectric ceramics was reported as a

function of vibration speed [6, 7]. The vibration velocity of

the fabricated samples was measured, and the load depend-

ence of the efficiency was evaluated for each sample.

Finally, it was shown that the newly developed pie-

zoelectric transformer, which we call the alternately poled

piezoelectric transformer (APT), can feed electric power at

half the input voltage of the Rosen-type transformer.

2. Alternately Poled Piezoelectric

Transformer

2.1. Structure

Figure 1 shows the structure of the fabricated piezo-

electric transformers. Panels (a), (b), and (c) show a Rosen-

type transformer with a wavelength mode, an APT with a

wavelength mode, which we call type A, and an APT with

1.5 wavelength mode, which we call type B, respectively.

The wavelength of these samples is 26.4 mm. The size of

both the Rosen-type transformer and the type A device is

26.4 ´ 7.6 ´ 1 mm, and that of the type B device is 39.6 ´

7.6 ´ 1 mm.

Type A has two pairs of input electrodes; one is half

wavelength and the other is almost quarter wavelength.

Type B has two pairs of half-wavelength input electrodes.

The poling directions are opposite each other in the input

electrode region. This poling structure is in tune with the

resonant vibration mode. By applying these structures to a

piezoelectric transformer, the capacitance of the input elec-

trodes can be increased, by 1.5 times for type A and twice

for type B compared with the Rosen-type transformer. As

a result, the input impedance decreased less than that of the

Rosen-type transformer. Of course, the same effect can be

obtained by using the same poling directions in the input

electrode region and connecting cross-opposite side elec-

trodes as to obtain the same resonant mode.

Some constants of the piezoelectric ceramics from

which the samples were fabricated are shown in Table 1.

2.2. Equivalent circuit

The equivalent circuit of a piezoelectric transformer

is shown in Fig. 2. As it works in a resonant mode, this

equivalent circuit is useful for understanding the working

state [8]. Here, m, sm, and rm indicate equivalent mass,

equivalent stiffness, and mechanical loss, respectively, and

Cd1, Cd2, A1, and A2 indicate the clamped capacitance of the

input part, that of the output part, the force factor of the

input part, and that of the output part, respectively. The

mechanical terminal is an end of the piezoelectric trans-

former. Figure 3 shows the modified equivalent circuit of

Fig. 1. Structure of the piezoelectric transformers.

Arrow is poling direction. (a) Rosen type with a

wavelength mode. (b) Type A with a wavelength mode.

(c) Type B with 1.5 wavelength mode.

Table 1. Constants of the piezoelectric ceramics

Material Pb[(Ni, Zn)1/3 Nb2/3]O3-Pb (Zr, Ti)O3-MnO2

TC 286 °C

Qm 1470

d31 �141 ´ 10�12

m/V

d33 310 ´ 10�12

m/V

Y1E

8.68 ´ 1010

N/m2

Y3E

6.30 ´ 1010

N/m2

e33T/ e0 1380

k31 0.376

k33 0.705

r 7.97 ´ 103 kg/m

3

30

the piezoelectric transformer which was used for simula-

tion. Table 2 shows the equivalent circuit parameters for

each sample.

2.3. Definitions of efficiency, vibration

velocity, and step-up ratio

The efficiency, vibration velocity, and step-up ratio

can be defined by the equivalent circuit parameters as

follows:

Efficiency

The efficiency h of a piezoelectric transformer is

defined by using the internal loss Pr and the output power

P0 to load, as follows:

By using the current Im flowing in R and the current Ioflowing in R2, the current flowing in C2 can be derived as

(Im - I0). We thus obtain

Therefore, Im is

From Eq. (3), the internal loss Pr can be obtained as follows:

The output power P0 is

From these results, the efficiency h is

Here, g2 = C2 /C, Qm = wL/R = (wCR)-1, r2n = R2 / R2n, and

R2n = (wC2)-1.

Equation (6) indicates that efficiency increases as g2

decreases or Qm increases. Table 2 shows that the relation-

ship of g2 is Rosen type < type B < type A. However, Qm

depends on both shape and fabrication. Therefore, the maxi-

mum value of the efficiency must be confirmed by meas-

urement.

Vibration velocity

The vibration velocity nm of a piezoelectric trans-

former at one end is shown as follows:

Step-up ratio

Before a CCFL is turned on, the impedance is very

high. Therefore, the output terminal condition of a piezo-

electric transformer can be considered to be open. The

step-up ratio N at a resonant frequency, which is determined

by L, C, R, and C2, is

Fig. 2. Equivalent circuit of the piezoelectric

transformer.

Fig. 3. Modified equivalent circuit of the piezoelectric

transformer.

Table 2. Equivalent circuit parameters of piezoelectric

transformers

(1)

(2)

(3)

(4)

(5)

(6)

(7)

31

The step-up ratio of the Rosen-type transformer is ex-

pressed as follows, using the parameters in Table 2:

From the fact that l / t > 1, R2n /R >> 1, and Qm > 1000, the

step-up ratio can be considered to be at least a few hundred.

Thus, the piezoelectric transformer works in the high-volt-

age generation mode.

On the other side, the impedance of a CCFL is about

100 kW during continuous lighting, and the piezoelectric

transformer works in power supply mode. In this case, the

output power depends on the load impedance, and can be

obtained by Eq. (5).

3. Working Characteristics

3.1. Measurement system

Figure 4 shows the measurement system for evalu-

ation of piezoelectric transformers. A thermotracer (NEC

San-ei TH1101) is used for measuring the surface tempera-

ture distribution of a piezoelectric transformer. The radia-

tion depends on the surface condition. Therefore, the

temperature was measured on the side surface, which has

no electrodes and is uniform. A laser vibrometer (Ono sokki

LV-1200) was used to obtain the vibration velocity at the

end of a piezoelectric transformer. Input power was meas-

ured by a digital power meter (Yokogawa 2532) in order to

obtain the effective power, including the power factor. The

output power is obtained by the calculation of I2RL. Here, I

is the load current and RL is the load resistance.

3.2. Load dependence of the efficiency

Figure 5 shows load dependence of the efficiency.

The output power was 0.5 W during measurement. The

calculation is obtained from an equivalent circuit analysis.

The matching load impedance is

The matching load impedance Zmat obtained by Eq. (10)

was about 350 kW for both the Rosen-type transformer and

type B, and about 100 kW for type A. But the adequate load

impedance to obtain maximum efficiency was about 100

kW for each. In the impedance range below the adequate

load impedance, the measured efficiency is lower than

calculated, and the difference increases as the load imped-

ance decreases. As later described, the vibration velocity

increases as the load impedance departs from the matching

load impedance. This phenomenon is considered to be

caused by the change of vibration velocity. In the imped-

ance range above the adequate load impedance, the meas-

ured efficiency too is lower than the calculated value.

Moreover, the measured values are not on a smooth line. It

is considered that this phenomenon depends on stray ca-

pacitance around the load and wiring.

As mentioned above, it was reported that the loss of

a piezoelectric resonator depends on vibration velocity.

Figure 6 shows the load dependence of the measured vibra-

tion velocity at both ends of a piezoelectric transformer and

that of the vibration velocity calculated by equivalent cir-

cuit analysis.

First, the results of calculated vibration velocity are

discussed. The calculated results in Figs. 5 and 6 indicate

(8)

(9)

Fig. 4. Measurement system.

Fig. 5. Relationship between load resistance and

efficiency. Input power is 0.5 W.

(10)

32

that the vibration velocity is minimal when the load imped-

ance is equal to the matching impedance Zmat, and then the

efficiency is maximal. Moreover, vibration velocity in-

creased as the difference between load impedance and

Zmat increased. The vibration velocity of type A is the largest

among these three samples when the load impedance is

equal to Zmat.

Second, the measured vibration velocity of a piezo-

electric transformer at both ends is discussed. As the load

impedance departed from the adequate impedance, the

vibration velocities at the ends increased nonsymmetrically.

The vibration velocity of type A was largest among these

three samples at the adequate impedance load. As for the

Rosen-type transformer, the vibration velocity of the input

part was larger than that of the output part when the load

impedance was 100 kW. On the other hand, as for both types

A and B, the vibration velocity was symmetric.

In order to obtain the relationship between vibration

velocity and loss, the temperature distribution was meas-

ured with a radiation thermometer while the piezoelectric

Fig. 6. Relationship between load resistance and vibration velocity. Input power is 0.5 W.

33

transformer was working. Figure 7 shows the results. The

temperature peaks were localized in vibration nodes. The

temperature distribution of the Rosen-type transformer was

non-symmetric, and the temperature of the driving part was

higher than that of the generating part. On the other hand,

the temperature distribution of both types A and B was

almost flat. Accordingly, it is seen that temperature distri-

bution and vibration velocity are mutually related. Tem-

perature distribution in a piezoelectric transformer causes

thermal stress, and decreases its toughness. From these

results, it is considered that both vibration velocity and

temperature distribution must be studied in order to design

a high-power piezoelectric transformer.

3.3. Input voltage dependence of output power

Figure 8 shows the relationship between input volt-

age and output power at a load of 100 kW; the lines show

the calculation results and the symbols show measured data.

In order to obtain an output power of 2.5 W, input voltages

of 63, 36, and 32 V are needed for the Rosen-type trans-

former, type A, and type B, respectively. The step-up ratio

of type B is 15.6. It is apparent that the step-up ratios of

both types A and B are higher than that of the Rosen-type

transformer, and that of type B is almost twice as great. The

measured input voltage was lower than the calculated input

voltage for the same output power. This tendency was most

pronounced for type A.

Figure 9 shows the relationship between the input

voltage and temperature rise of piezoelectric transformers.

The temperature rise was measured with a radiation ther-

mometer. The temperature rise was defined as the differ-

ence between the maximum surface temperature of

piezoelectric transformers and room temperature. From the

results in Figs. 8 and 9, it was seen that the difference

between the measured input voltage and calculated input

voltage depends on the temperature rise. Accordingly, it

was considered to be caused by the temperature dependence

of piezoelectric ceramics.

As mentioned above, the step-up ratio of an alter-

nately poled piezoelectric transformer is almost twice that

of a Rosen-type transformer, and it can be used as a step-up

transformer of a CCFL inverter which generates high volt-

age with assistance of a prestep-up inductor whose step-up

ratio is about 3. When the internal battery voltage is 12 V,

type B with a prestep-up inductor can generate an output

voltage of 561.6 V, which is sufficient to light a CCFL.

The temperature rise of type A was the greatest of

these three samples. But its size is comparable with that of

the Rosen-type transformer, in spite of its high step-up ratio.

Accordingly, type A is useful for a small-consumption

CCFL inverter. Although type B is larger than the Rosen-

type transformer, its step-up ratio is twice that of the Rosen-

type transformer, and it can be operated at the same output

Fig. 7. Temperature rise distribution of piezoelectric

transformers. The input end is x = 0.

Input power is 0.5 W.

Fig. 8. Relationship between input voltage and output

power at RL = 100 kW.

34

power as the Rosen-type transformer. Accordingly, type B

is useful for a large-consumption CCFL inverter.

4. Conclusions

A new piezoelectric transformer structure, the alter-

nately poled piezoelectric transformer, was proposed. De-

vice types A and B, whose structure was based on the newly

developed structure, and a Rosen-type transformer were

fabricated and evaluated. The relationship of the maximum

efficiency was type A < type B < Rosen type, but the figures

were over 95% for all cases and the differences were small.

The efficiency of type A decreased most rapidly as the load

impedance departed from the adequate impedance, and

both the type B and Rosen-type transformers had almost the

same load dependence of efficiency. In order to obtain the

relationship between the load dependence of efficiency and

piezoelectric vibration, the vibration velocity at the end of

a piezoelectric transformer was investigated. The vibration

velocity was reduced to a minimum at the adequate load

impedance which gives maximum efficiency. The vibration

velocity at both ends of a piezoelectric transformer was

measured at a load of 100 kW, and compared with the

temperature rise distribution. The vibration velocity at both

ends and the temperature rise distribution of types A and B

was flat and almost symmetric, but that of the Rosen type

was not symmetric. Moreover, it was observed that the

temperature rise distribution depends on the vibration ve-

locity.

The step-up ratio of both types A and B was almost

twice that of the Rosen-type transformer.

Acknowledgments. The authors thank Professor Y.

Tomikawa and Assistant Professor S. Hirose (Yamagata

University) for helpful suggestions. Also, they are grateful

to the members of the Electronics Devices Group, High

Performance Materials and Products Research Laborato-

ries, Mitsui Petrochemical Industries, Ltd. for their encour-

agement throughout this study.

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Fig. 9. Relationship between input voltage and

temperature rise at RL = 100 kW.

35

AUTHORS (from left to right)

Kouichi Kanayama (member) received his B.E. degree in precision engineering in 1978 from Osaka University. In 1988,

he joined Mitsui Petrochemical Industries, Ltd., and he has been enrolled in the doctoral course at Yamagata University since

1996. His research focuses on piezoelectric devices.

Nobuhiro Maruko (nonmember) received his B.S. and M.E. degrees in material science from Hiroshima University and

Kyushu University in 1988 and 1990, respectively. In 1990, he joined Mitsui Petrochemical Industries, Ltd. His research focuses

on piezoelectric devices.

36