MICRO-ENERGY PULSE POWER SUPPLY WITH
NANOSECOND PULSE WIDTH FOR EDM
Fang Ji
1
, Yong-bin Zhang
2
, Guang-min Liu
3
and Jian-guo He
3
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
10
-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
0
W
is related to
instantaneous discharging voltage
)(tu
,
instantaneous discharging current
)(ti
and pulse
width
T . Their relation may be written as
=
T
dttituW
0
0
)()(
. 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
RC
W
stored in the
capacitor may be described as the relation between
the capacitance
C
of capacitor, the capacitance
'C
of circuit and the discharging voltage
U
:
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
Copyright
c
SciTePress
2
)'(
2
1
UCCW
RC
+=
. 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
'C
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
R
Discharging
clearance
1
2
Q1
E
C
Q1
Discharging
clearance
1
2
E
R
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
st
and 2
nd
waveform.
The 3
rd
waveform is the best one. Hereby, the 3
rd
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
4
Overshot
Undershot
(b)
(a)
3
2
1
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
=
T
dttituW
0
0
)()(
and reaches to 10
-7
joule.
(c)
(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
10
-7
joule and the least pulse width is less than 60
nanoseconds.
REFERENCES
Ao Ming-wu, Zhang Yong, Li Zhi-yong, 2003, Research
on a micro-energy pulse power source used by micro-
EDM, Aviation Precision Manufacturing Technology.
Chi Guan-xin, Di Shi-chun, 2004, Study on key
technology for intricate micro parts on MWEDM,
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,
Foundational research on pulse generator technology
in micro-EDM, Electromachining & Mould.
Cao Feng-guo s, 2005, Electro discharge machining
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
209