J. Electromagnetic Analysis & Applications, 2009, 2: 108-113
doi:10.4236/jemaa.2009.12017 Published Online June 2009 (www.SciRP.org/journal/jemaa)
Copyright © 2009 SciRes JEMAA
A Novel Half-Bridge Power Supply for High Speed
Drilling Electrical Discharge Machining
He Huang, Jicheng Bai, Zesheng Lu, Yongfeng Guo
Department of Manufacturing and Automation Engineering, Harbin Institute of Technology, Harbin, Heilongjiang Province, China.
Email: huanghe@hit.edu.cn, jichengbai@hit.edu.cn, lzesn@hit.edu.cn, guoyf@hit.edu.cn
Received February 10th, 2009; revised March 12th, 2009; accepted March 20th, 2009.
High Speed Drilling Electrical Discharge Mach ining (HSDEDM) uses controlled electric sparks to erode the metal in a
work-piece. Through the years, HSDEDM process has widely been used in high speed drilling and in manufacturing
large aspect ratio holes for hard-to-machine material. The power supplies of HSDEDM providing high power applica-
tions can have different topologies. In this paper, a novel Pulsed-Width-Modulated (PWM) half-bridge HSDEDM
power supply that achieves Zero-Voltage-Switching (ZVS) for switches and Zero-Current-Switching (ZCS) for the dis-
charge gap has been developed. This power supply has excellent features that include minimal component count and
inherent protection under short circuit conditions. This topology has an energy conservation feature and removes the
need for output bulk capacitors and resistances. Energy used in the erosion process will be controlled by the switched
IGBTs in the half-bridge network and be transferred to the gap between the tool and work-piece. The relative tool wear
and machining speed of our proposed topology have been compared with that of a normal power supply with current
limiting resistances.
Keywords: High Speed Drilling Electrical Discharge Mach ining, Half-Bridge Power Supply, Zero Current Switching,
Zero Voltage Sw it c hi ng
1. Introduction
Electrical Discharge Machining (EDM), also known as
spark erosion machining, is becoming increasingly popular.
EDM sees the removal of matter from high hardness con-
ductive materials by means of a series of repeated electri-
cal discharges between electrode and work-piece, which
are separated by a discharge gap. Dielectric fluid is forced
into the discharge gap where electrical discharge erosion
occurs. When a voltage large enough is applied, the dielec-
tric fluid breaks down, the gap is ionized and electrons are
emitted from the tool (cathode). When more electrons
gather in the gap, the resistance drops, which causes elec-
tric spark to jump between the work-piece surface and the
tool. The whole sequence of operation occurs within a few
microseconds and is accompanied by a shock wave in the
dielectric. The impact of the wave on the electrode causes
high transient pressure. The current density in the dis-
charge channel is of the order 10,000 A/cm2. The tem-
perature of the central point of the channel is of the order
of tens of thousands of °C. The forces of the electric and
magnetic fields caused by the spark produce a tensile force
and tear-off particles of molten and softened metal from
this spot on the work-piece [1,2].
Within the scope of EDM, High Speed Drilling Elec-
trical Discharge Machining (HSDEDM) is an important
technology. The basic components of a HSDEDM system
are as follows and the schematic is shown in Figure 1.
Like other EDM methods, the HSDEDM process is
especially suitable for machining high strength and hard
metal materials. Generally, a rotating thin copper or brass
tube electrode is used as the drilling tool. High speed and
pressure dielectric (water) is pumped through this hollow
Figure 1. Diagram of HSDEDM process
A Novel Half-Bridge Power Supply for High Speed Drilling Electrical Discharge Machining 109
Copyright © 2009 SciRes JEMAA
electrode and injected into the discharge gap to flush out
eroded debris from working area to ensure the process
remains stable. The dielectric fluid also acts as a coolant.
Rotation of the electrode helps reduce deviation and un-
even wear, which maybe otherwise be caused by the high
pressure dielectric, on the face end of the electrode. The
servo system acts to adjust the discharge gap and feed
throughout the process. The most outstanding feature of
HSDEDM is high speed of manufacture which can be up
to 60mm/min. This is hundreds of times the speed of the
normal EDM and is exceeding th e level of the traditional
drilling. The processed apertures range from 0.3mm to
3mm and the aspect ratio can be up to 100 [3].
In HSDEDM, pulse power supply plays an important
role in providing the thermal action of the electrical dis-
charges between the electrode tool and the work-piece in
order to achieve material removal. The Metal Removal
Rate (MRR), the surface finish and Electrode Wear (EW)
mainly depend on the magnitude and duration of the
pulse discharge [4]. As the current increases, so do the
MMR and the EW, although the surf ace finish decreases.
As the discharge frequency is increased, the surface fin-
ish improves but the EW increases.
In order to improve the whole efficiency of the elec-
tronic equipment, the search for a more efficient switching
technique has been developed. For a switch such as Insu-
lated Gate Bipolar Transistor (IGBT), the zero current
switching (ZCS) technology has been frequently used,
because this technology can reduce the current rapidly
when IGBT is turned off, therefore reducing the energy
loss caused by the tail cu rrent [5,6]. It is also important, in
miniaturizing power supply, to balance switching fre-
quency and conversion efficiency. Zero Voltage Switching
(ZVS) is a powerful means for increasing the switching
frequency with small switching loss. On the other hand,
switching noise is a critical problem at high frequency
switching. ZVS is also effective in decreasing the spike
current and surge voltage when switches are turned off [6].
This research has produced a pulse power supply that
is used in HSDEDM system without an output capacitor
for charging and discharging spark energy and bulky re-
sistor for limiting current. A Pulsed Width Modulated
(PWM) half-bridge topology with ZCS and ZVS is de-
veloped in this power supply for suppressing the noise
current in dischar ge ga p a nd sur ge v olt a ge of swi t ches.
2. Developments in Power Configurations
The shapes of voltage and current pulses in the discharge
gap depend on the chosen power supply. There are three
types of power supplies that have received most interest
amongst the scientific community. They are Resistance
Capacitance (RC) power supply (A), transistor switching
circuit (B) and energy-saving power supply (C).
2.1 Resistance-Capacitance (RC) Power Supply
EDM was developed in Russia in the mid-1940-s by
Lazalenko and he used the basic RC power supply which
is still used today in many cases when a fine surface fin-
ish is required [7]. The common configuration of these
systems is shown in Figure 2.
The AC input voltage is normally fed into a variac and
isolation transformer and is rectified to produce an uncon-
trolled DC. The bulk capacitor C1 is used to filter the rec-
tified output and the output capacitor C2, which stores the
energy needed for the spark, is connected in parallel to th e
discharge gap. Every time dielectric breaks down, the
energy in C2 will be discharged and in the next cycle it
will be charged again. The current is limited by the resis-
tor, R1, which is normally a high power resistor. R1 is
where most of the losses of an EDM power supply occur.
2.2 Transistor Switching Circuit Power Supply
Figure 3 shows a different technique of implementing the
spark generator whilst utilizing the linear power supply.
The transistor is Pulse Width Modulated (PWM) to control
the total period of the charging time of the capacitor. This
circuitry provides a higher MMR than the normal rectan-
gular pulse power supply. In current research, more and
more linear converters are being used to switch the electric
energy. These improvements increase efficiency, power
density and decrease the size of the magnetic components
[1]. However this type of configuration still uses bulky
resistors. The problem of high l osses still remains.
2.3 Energy-Saving Power Supply
The power supply with the feature of energy-saving was
developed in the end-1980s. Dr. Zhao Wansheng, with
his group, developed a pulse generator for EDM with the
feature. The schematic is shown in Figure 4. They de-
signed the circuitry as a forward converter without the
output capacitor. Current limiting is achieved by sensing
the load current and feeding it back to the control circuit.
Once the current reaches a present current limit, the con-
troller shuts down the drive and starts again at next cycle.
The forward converter topology is designed for use in
low power (50W) to medium power (250W) applications
[8]. The second winding of flyback transformer, T1, is
used in the discharge circuit and acts as clamp winding.
However this high inductive reactance will induce volt-
age swing and current tail in the discharge gap.
It can be seen from the developments in Power supply,
the switch mode configuration and improvement of effi-
ciency are research focuses in EDM.
Figure 2. Traditional EDM power supply
110 A Novel Half-Bridge Power Supply for High Speed Drilling Electrical Discharge Machining
Copyright © 2009 SciRes JEMAA
3.1 Configuration
Two sets of IGBTs (TL1 to TLn and TR1 to TRn) are
used as switches and connected with each other through
the discharge side inductance L, rd, discharge wires and
discharge gap. L consists of the inevitable lead induc-
tance, self-inductance in the discharge gap and any other
stray inductances in the discharge path. The resistance of
the discharge wires and the spark gap during discharge
constitute rd. D1 and D2 are fly-wheel diodes. D3, D4,
R3, R4, C1 and C2 constitute two sets of RCD snubber
circuits which are used to protect the switches from volt-
age surge caused by the sudden turning off. Fast recovery
diodes DL1 to DLn and DR1 to DRn are anti-parallel
diodes in the switches which are also used to protect the
IGBTs. In this way, ZVS of the switch is achieved even
with high swi tching fre quency.
Figure 3. Transistorized RC power circuit
3. Half-Bridge Network Power Supply
The proposed topology is a PWM half-bridge converter,
which uses IGBTs a as switch to control the energy. The
configuration is shown in Figure 5.
3.2 ZVS of Switches
There are two sets of RCD snubber circuits parallel con-
Figure 4. Energy-saving EDM power supply developed by Harbin Institute of Tec hnology
Figure 5. Circuit diagram of the HSDEDM power generator using a half-bridge ne twork
A Novel Half-Bridge Power Supply for High Speed Drilling Electrical Discharge Machining 111
Copyright © 2009 SciRes JEMAA
nected with switches as shown in Figure 5. The snubber
circuits achieve ZVS for switches TLn and TRn. If with-
out any protection measures for switch, when switch is
turned off, the prime current ip decreases linearly, while
the collector-emitter voltage (Vce) of the switch increases
rapidly, and the surge voltage with extremely high value
will be generated. In verify experiments, the surge volt-
age usually reaches more than six times of the supply
voltage as shown in Figure 6(a). This surge voltage with
high value is harm to switches. In this proposed topology,
when switch is turned off, surge energy is stored in cap
C1 or C2 through the fast recovery diodes D3 or D4. In the
next time, when the switch is turned on, C1 or C2 dis-
charge though the resistances R3 or R4. The measured
waveforms of Vce and prime current of switch with ZVS
can be seen in Figure 6(b).
3.3 ZCS of Gap and Switching Methodology
The traditional power supplies of EDM or HSDEDM
always use bulky resistors to limit and con trol the current
in the gap. The electrical energy surges also can be con-
sumed by these resistors. For energy-saving, there are not
current limit resistors in the proposed topology. If no
action being taken to li mit the power, the current will rise
to a very high level to break the switches, especially
when short circuit occurs. When the switch is off, the
IGBTs also would be damaged by electrical surges
caused by the sudden changes in electrical flow. In this
PWM half-bridge network, when the drive IGBT is off,
the fly-wheel diodes are used to smooth out the electrical
surges caused by the sudden changes in electrical flow,
therefore the ZCS can be obtained by this topology. This
facilitates dielectric deionization quickly, therefore, short
circuit and electric arc discharging are reduced and
higher utilization of the pu lse can be gained.
For this half-bridge network, the following PWM op-
erational sequence is taken to generate the proper electri-
cal pulses for the security of the switching IGBTs. The
current in the discharge is shown in Figure 7.
The left and the right IGBTs are all switched on in
time t0. The power streams from source through the left
IGBT, discharge wir e, gap and the right IGBT to ground.
The current in the gap rises rapidly to I1 which is safe for
the switches.
The left or the right IGBT is switched off in time t1.
If the left IGBT is off, the energy stored in L streams
through the circuit which consists of D1, wire, gap and
the right IGBT to the ground. If the right IGBT is off, the
energy streams though another circuit which consists the
left IGBT, wire, gap and D2 and goes back to the source.
In that time, the current decreases to I2.
The step 1 and 2 are repeated several times. The
on-time is t2 and the off-time is still t1. The current os-
cillates between I1 and I2. While the IGBT in left or right
side is switched on and off repeatedly, the IGBT in the
other side always is turned on.
Figure 6. Waveforms of prime current in gap and Vce of s witches
Figure 7. Theoretical current waveform in the discha rge gap
The left and the right IGBTs are all switched off. The
energy stored in the wire inductance streams though
flay-wheel diodes, D1 and D2 back to source. The current
decreases rapidly to zero.
In Figure 7, a trapezoidal current waveform with
saw-tooth form on the top can be seen. Compared with
normal rectangular current waveform, the average current
peaks is (I1+I2)/2 and the width of the pulse is almost
t0+n(t1+t2)+t3, n is the repetition of switching on and off
of IGBT just in left or right side. The peak current is de-
pended on the simultaneous on-time (t0) of switches in
112 A Novel Half-Bridge Power Supply for High Speed Drilling Electrical Discharge Machining
both sides of the bridge in the beginning of the pulse. If
just one IGBT is switched on in both sides in that time then
t0 can not be set too large in order to avoid exceeding the
rated current level of the IGBT. Larger peak current can be
achieved by turning on more switches in both sides at the
beginning of the pulse to help share the current. The width
of the pulse is depended on number of the repetitions of
switching. If only one IGBT is switched repeatedly in the
middle of the pulse, the frequency is high and close to the
rated frequency of the IGBT. This will lead to the in-
creased transient thermal impedance of the IGBT, thus
more heat generated by the current will damage the switch.
In the proposed topology, several IGBTs are combined in
parallel connection with collectors and emitters on each
side. These IGBTs are switched by turns to decrease the
working frequency of every one.
Figure 8. Experimental gap voltage and current waveforms
(a) (b)
(c) (d)
(e) (f)
Figure 9. Relative tool wear and machining speed respond to 3 electrical parameters
Copyright © 2009 SciRes JEMAA
A Novel Half-Bridge Power Supply for High Speed Drilling Electrical Discharge Machining 113
Copyright © 2009 SciRes JEMAA
Figure 8 shows gap voltage and current waveforms of
normal discharges. During the on-time the output current
is around 25 A and the output maintaining voltage is
around 20 V.
It can be seen from Figure 8 that the power supply
proposed in this research has good performance in con-
trolling the energy in the discharge gap, even without
current limiting resistor. The process features of the pro-
posed power supply will be demonstrated by comparing
with the normal power supply which using bulky resistor
to limit current.
4. Experimental Results
Two types of HSDEDM power supplies were utilized to
get relative tool wear and machining speed data. The first
type is a normal power supply with current limiting re-
sistances and the second type is the half-bridge network
power supply without the resistances as proposed in this
paper. Both types of the power supplies are characterized
based on Relative Tool Wear (RTW) and EW data using
three electrical parameters: gap peak current, pulse width
and duty cycle. The comparison is shown in Figure 8. A
brass tube electrode with 1 mm in external diameter and
0.3 mm in inner diameter was used in the experiments.
The work-piece material was 40Gr steel and water was
used as dielectric. The open voltage is 80 V.
As shown in Figure 9, the proposed power supply
achieves a higher machining speed in most situations,
especially at larger electrical parameters. However, the
relative tool wear achieved by using the proposed power
supply is little larger than the normal one. In theory, the
proposed power supply characters the improvement in
electric energy efficiency, because the current-limiting
resistances are removed. Detailed analyses of tool wear,
improvement of efficiency and the methods to decrease
electrode wear are future tasks.
5. Conclusions
A power supply for HSDEDM based on a half-bridge
network without bulky current-limiting resistances in the
discharge circuit was developed resulting in a great re-
duction in weight and size. A PWM operational sequence
of switches was utilized to generate trapezoidal pulse
waveform with saw-tooth form in the discharge gap. The
electrical parameters of peak current, pulse width and
duty cycle were achieved by control the switching time of
these IGBTs in the circuit, This was also used to limit the
current in the discharge circuit to avoid the power being
too large to break the IGBTs. ZVS of switches was
achieved to decrease the surge voltage when IGBTs were
turned off. The “fly-wheel” diodes were developed to
smooth the electrical surge and the ZCS technology was
used to reduce the current rapidly when switches were
turned off, therefore reducing the energy loss caused by
the tail current. In comparative tests, it was shown that
the normal power supply with current-li miting resistances
has good performance when considering relative tool
wear and machining speed at light loading conditions.
However the machining speed of the normal one cannot
compare to the performance of half-bridge network
power supply at lager maximum poser outputs. This de-
velopment paves the way for alternative in designing
HSDEDM power supplies for high power applications.
6. Acknowledgments
This work was funded by National Science Foundation of
China with the grant number: 50875064 and supposed in
the frame of 863 research project of China with the grant
number: 2007AA04Z345 and supposed in the frame of
Heilongjiang province important science and technology
fund with grant number: GA06A501. The authors would
like to thank Y. L. Liu for help in data acquisition, G. Q.
Deng and C. J. Li for preparing the devices, and A. Hird
for invaluable advi ce c o ncer n i ng En gl i sh w r i t i ng.
[1] C. M. F. Odulio, L. G. Sison, and M. T. Escoto Jr, En-
ergy-saving flyback converter for EDM applications,
IEEE Region 10 Annual International Conference Pro-
ceedings, pp. 1-6, 2007.
[2] N. L. Kachhara and K. S. Shah, The electric discharge
machining process. Journal of the Institution of Engi-
neers (India) -Mechanical Engineering Division, No. 51,
pp. 67-73, 1971.
[3] M. R. Cao, S. C. Yang, S. Q. Yang and Y. T. Qiao, Ex-
perimental research on the process influencing machining
velocity to small holes EDM, Modem Manufacture En-
gineering (in Chinese), Vol. 4, pp. 82-83, 2005.
[4] R. Casanueva, M. Ochoa, F. J. Azcondo, and S. Bracho,
Current mode controlled LCC resonant converter for
electrical discharge machining applications, Proceedings
of the 2000 IEEE International Symposium on Industrial
Electronics, Vol. 2, pp. 505-510, 2000.
[5] G. Hua and F. C. Lee, Soft-switching techniques in
PWM converters, IEEE Transactions on Industrial Elec-
tronics, Vol. 42, pp. 595-603, 1995.
[6] K. Yoshida and T. Ninomiya, A novel current resonant
ZVS-PWM half-bridge converter, Electronics & Com-
munications in Japan, Part I: Communications, Vol. 82,
pp. 45-55, 1999.
[7] B. Sen, N. Kiyawat, P. K. Singh, S. Mitra , J. H. Ye, and P.
Purkait, Developments in electric power supply con-
figurations for electrical-discharge-machining (EDM),
The 5th International Conference on Power Electronics
and Drive Systems, Vol. 1, pp. 659-664, 2003.
[8] B. Y. Song, W. S. Zhao, and G. L. Shao, Current type of
electrical discharge machining pulse generator, China
Mechanical Engineering (in Chinese), Vol. 12, pp. 386-
390, 2001.