MICRO-ENERGY PULSE POWER SUPPLY WITH

NANOSECOND PULSE WIDTH FOR EDM

Fang Ji

, Yong-bin Zhang

, Guang-min Liu

and Jian-guo He

Institute of Machinery Manufacturing Technology, Mianshan Road 64#, Mianyang, Sichuan, 621900, China

Keywords: Electrical discharging machining, Micro-energy, Pulse power supply.

Abstract: Micro-energy pulse power supply is required in order to manufacture workpiece with micro-nano meter

precision in electrical discharging machining (EDM). The paper analyzes two kinds of typical pulse power

supplies and their important elements. Afterwards, another one kind of new micro-energy pulse power

supply is presented. The experiments and analysis have been done for the new power supply. Accordingly,

some important circuits have been modified, for example, the discharging circuit and driving circuit for the

metallic oxide semiconductor field effect transistor (MOSFET). The modification improves the performance

of the new pulse power supply so that the pulse width of the new pulse power supply could be less than that

of the typical pulse power supply for electrical discharging machining. The least pulse width is obtained. It

is less than 60 nanoseconds and its least energy of single pulse is less than 10-6 joule. Subsequently, the

pulse waveform is adjusted considering the impedance matching of the discharging circuit in order that the

pulse waveform has no oscillation and no overshot. The adjusted pulse waveform is good to detect

discharging status correctly and sensitively.

1 INTRODUCTION

The independent pulse power supply and RC pulse

power supply are two typical pulse power supplies

applied in electrical discharging machining. The

independent pulse power supply has the advantages:

discharging frequency may be high, pulse

parameters may be adjustable, self-adapted control

may be easy. But it still has the disadvantages:

maintaining voltage limits the energy of single pulse

to decrease, the energy of single pulse is more than

-7

joule. The disadvantages make independent

pulse power supply difficult to manufacture

workpiece with micro-nano meter precision. The

other typical pulse power supply is RC pulse power

supply. It is easy to obtain small energy of 10

-7

joule

of single pulse. But, it is difficult to adjust the pulse

parameters. It has no channel to release residual

charge between two electrodes. It is not easy to

control energy of pulse. Electrical arc discharging

happens frequently and the discharging consistency

is not good. The researches show that the machining

mass in micro-nano meter scale needs a kind of

micro-energy pulse power supply. Its pulse

parameters must be easy to control and its lowest

energy of single pulse must reach 10

-7

joule.

The independent pulse power supply is shown in

Figure.1. The energy of single pulse

is related to

instantaneous discharging voltage

)(tu

instantaneous discharging current

)(ti

and pulse

width

T . Their relation may be written as

∫

dttituW

)()(

. According to the relation, it is

obvious that there are three ways to decrease energy

of single pulse: decreasing the voltage, decreasing

the current or increasing the frequency. However,

there exists maintaining voltage which is the least

voltage to discharge between electrodes. The

discharging voltage must be larger than the

maintaining voltage. It limits the decrease of the

discharging voltage. Additionally, the increase of

frequency may be restricted by the frequency

response of the MOSFET. Therefore, it is difficult to

acquire less energy of single pulse for independent

pulse power supply and the lowest pulse energy can

only reach 10

-6

joule. The RC pulse power supply is

shown in Figure.2. The energy

stored in the

capacitor may be described as the relation between

the capacitance

of capacitor, the capacitance

of circuit and the discharging voltage

205

Ji F., Zhang Y., Guang-min L. and He J. (2009).

MICRO-ENERGY PULSE POWER SUPPLY WITH NANOSECOND PULSE WIDTH FOR EDM.

In Proceedings of the 6th International Conference on Informatics in Control, Automation and Robotics - Signal Processing, Systems Modeling and

Control, pages 205-209

DOI: 10.5220/0001966202050209

 SciTePress

)'(

UCCW

. Hereby, there are two ways to

decrease the energy of single pulse: decreasing the

capacitance and the voltage. Normally speaking, the

first way seems better. But, the capacitance

usually varies from 100pF to 10000pF in the circuit

and it is hard to reduce. Thus, decreasing

discharging voltage may be the most important way

to reduce the energy of single pulse. The new

research shows that the discharging voltage will not

be limited by maintaining voltage and may be as low

as 7 volts for RC pulse power supply. The minimum

energy of single pulse may reach 10

-7

joule.

Whereas, the present RC pulse power supply is

difficult to control and residual charge is easy to

accumulate between electrodes, which is not good

for consistency of machining. The low discharging

voltage will brings the discharging distance to be

close which is not good to remove the leftover.

At present, there are many researchers who are

developing the micro-energy pulse power supply for

EDM. The least pulse width is 90 nanoseconds

developed by Zhao, but there exists obvious

electromagnetic oscillation; The least pulse width is

80 nanoseconds developed by Han, but the width is

width of current, not width of voltage. It is well-

known that the pulse width of current is less than

that of voltage because of the discharging delay. The

paper presents one kind of micro-energy pulse

power supply which integrates the advantages of

both independent pulse power supply and RC pulse

power supply. Its least energy of single pulse can

reach 10

-7

joule. It has a special circuit to release

residual charge between electrodes.

Figure 1: Schematics of independent pulse power supply.

Figure 2: Schematics of typical RC pulse power supply.

Figure 3: Shematics of micro-energy pulse power supply

with nanosecond pulse width.

2 PRINCIPLE OF THE

MICRO-ENERGY PULSE

POWER SUPPLY

The micro-energy pulse power supply presented in

the paper is shown in Figure.3. The current will

charge capacitor C through MOSFET Q4 and

resistor R2 when the switch K1 is disconnected and

the switch K is connected. The energy in capacitor is

decided by the charging time and it can influence the

machining mass of single pulse. Then, the energy in

capacitor will transfer to the discharging clearance

between workpiece and tool electrode when Q4 is

disconnected and the Q3 is connected. Afterwards,

Q3 will be disconnected and the releasing residual

charge circuit will remove the residual energy in

order to avoid unnecessary discharging between

electrodes. At last, the releasing residual charge

circuit will be disconnected. The whole work period

of single pulse is over and the next period may

begin. A programmable logic element is applied in

the system circuit to control the MOSFETs. Thus,

some logic operations are done by hardware rapidly,

which may reduce the delay time and decrease the

pulse width. In addition, a special high-speed micro-

control unit (MCU) is configured as counter for

pulse so that fuzzy control may be done according to

the number of pulse. The main elements in the

system circuit are high-speed MOSFETs. They can

work at high frequency. They influence the

minimum energy of single pulse and the machining

efficiency of the pulse power supply. However, there

exists nonlinearity between gate voltage and source

voltage during charging and discharging because of

capacitance in MOSFET. Therefore, the internal

wastage will increase and the reliability will

decrease. This is a disadvantage. But, it can be

reduced by high-speed driving circuit for the

MOSFET. The driving circuit has instantaneous

Releasing

residual charge

iit

Discharging

clearance

Discharging

clearance

ICINCO 2009 - 6th International Conference on Informatics in Control, Automation and Robotics

206

strong current so that the MOSFET may be

connected or disconnected quickly. Thus, the pulse

width less than 60 nanoseconds may be obtained.

3 EXPERIMENTS AND

ANALYSIS

Some experiments have been done in the paper.

Then, analysis and improvements have been given

for optimized pulse power supply, for example, the

modification of driving circuit, the comparison of

MOSFETs, the impedance matching, discharging

circuit and detection of discharging status, etc.

3.1 Modification of the Driving Circuit

The system circuit adopts a driving chip with

complementary emitter follower which has strong

capability to drive gate of MOSFET. Otherwise, the

impedance of the driving circuit is low. Thus, the

charging and discharging for the gate of MOSFET

can be finished quickly. There are four waveforms in

Figure.4. They are different because their

impedances are not equal. The impedances vary

from large to small by the order 1 to 4. It is obvious

that decreasing impedance may improve the

waveform. The fourth waveform is adopted in the

paper. The overshot at rising edge and undershot at

falling edge are good for connecting and

disconnecting of MOSFET.

3.2 Comparison of the MOSFET

Comparison experiments of MOSFETs have been

done to select the best kind of MOSFET when the

parameters of the system circuit are of no difference.

Three kinds of MOSFETs are selected for further

experiments after some previous experiments. The

corresponding waveforms of different MOSFETs are

shown in Figure.5. It can be found that there still

exist some undershots for the 1

and 2

waveform.

The 3

waveform is the best one. Hereby, the 3

MOSFET is fixed for the pulse power supply.

3.3 Matching of the Impedance

There are two special results during the experiments

of pulse power supply: Firstly, the waveform of

MOSFET will be different when the voltage is

different, even though the electronical elements are

all same. It is shown in Figure.6. The overshot will

Figure 4: Different waveforms of driving circuit with

different impedances. The impedance varies from large to

small by the order 1 to 4.

Figure 5: Different waveforms of different MOSFETs.

appear at the rising edge with the increase of

voltage. The reason is that the rise of voltage results

in the rise of varying rate of current. The capacitor

will be charged more quickly, which makes overshot

at rising edge of voltage. Secondly, the waveform of

MOSFET will also be different when electronical

element is changed for different machining currents.

It is shown in Figure.7. The overshot will appear for

some elements. The reason is that the impedances

are different for different elements. The less the

impedance is, the larger the overshot will be.

Waveform 1

Waveform 2

Waveform 3

Falling edge

Waveform 1

Waveform 2

Waveform 3

Overshot

Undershot

(b)

(a)

MICRO-ENERGY PULSE POWER SUPPLY WITH NANOSECOND PULSE WIDTH FOR EDM

207

However, their falling edges are similar because of

the releasing residual charge circuit. Therefore, the

impedance must be matched for the optimized

waveform and detection accuracy of discharging

status.

Figure 6: Waveforms with different voltages and same

electronical element.

3.4 Discharging Experiment at High

Frequency

The least pulse width of the micro-energy pulse

power supply described in the paper can be less than

60 nanoseconds after some experiments and

optimization above. The discharging experiments

Figure 7: Waveforms with same voltage and different

electronical elements.

have been done subsequently. The open waveform

and discharging waveform are shown in Figure.8. It

is easy to watch the spark between the anode and the

cathode during the experiments. There exist some

discharging marks on the surface of the workpiece.

The energy of single pulse can be calculated by the

equation

∫

dttituW

)()(

and reaches to 10

-7

joule.

(c)

(b)

(a)

(b)

(a)

ICINCO 2009 - 6th International Conference on Informatics in Control, Automation and Robotics

208

Figure 8: Open waveforms and discharging waveforms of

the pulse power supply.

3.5 Detection of Discharging Status

Detection of discharging status is very important in

the pulse power supply. Its result will be fed back to

the control center. It provides the main information

used to adjust the parameters of pulse power supply

timely. The corresponding circuit must be modified

accurately. But, the signal from discharging

clearance is periodic and changed quickly, which

will bring variance, even oscillation, to the detection

circuit. The signal is shown in Figure.9. Thus, some

filters are applied in the circuit. Some noise is

limited and the signal is improved evidently. It is

shown in Figure.10.

Figure 9: Voltage of open clearance with interference.

Figure 10: Voltage of open clearance without interference.

4 CONCLUSIONS

The micro-energy pulse power supply is the key part

of EDM in micro-nano meter scale. The paper

analyzes the characters of two present typical pulse

power supplies of EDM, gives the driving circuit

consisted of low impedance element and

complementary emitter follower. The paper also

adds active releasing residual charge circuit, selects

the best MOSFET and matches the impedance for

different waveforms. At last, optimized waveform is

obtained without overshot and undershot. The

minimum energy of the pulse power supply reaches

-7

joule and the least pulse width is less than 60

nanoseconds.

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Ao Ming-wu, Zhang Yong, Li Zhi-yong, 2003, Research

on a micro-energy pulse power source used by micro-

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Chi Guan-xin, Di Shi-chun, 2004, Study on key

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Manufacturing Technology and Machine Tool.

Y.S. Wong a,., M. Rahmana, H.S. Lima, H. Hanb, N.

Ravib, 2003, Investigation of micro-EDM material

removal characteristics, Journal of Materials

Processing Technology.

He Guang-ming, Zhao Wan-sheng, 1999, Research of

Nanosecond-class pulse generator, Electromachining.

Han Fu-zhu, Chen li, Zhou Xiao-guang, 2005,

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technology, Beijing: Chemical Industry Pres.

Pei Jing-yu, Guo Chang-ning, Deng Qi-lin, 2004, Dual-

Channel MOSFET Sub- Microsecond Micro- Energy

Pulse Power Source Used in Electrical Discharge

Machining, Journal of Shanghai Jiaotong University.

(a)

Discharging

Short

Open

(b)

MICRO-ENERGY PULSE POWER SUPPLY WITH NANOSECOND PULSE WIDTH FOR EDM

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