Second International Conference on Electrical Engineering

25-26 March 2008

University of Engineering and Technology, Lahore (Pakistan)

978-1-4244-2293-7/08/$25.00 ©2008 IEEE.

Abstract--This paper proposes a high efficiency solid state

welder used for induction welding. The cause of low efficiency

of the welder is the switching losses in the inverter section

because of high switching frequency of the power devices. The

efficiency of this part is improved by using the soft switching

technique known as zero current switching. Using this

technique the other performance parameters are also

improved like the voltage stresses on the switching devices, the

current form the source is nearly sinusoidal so the power

quality of the system improves, which in turn reduces the

electromagnetic interference. The welder is simulated with the

power IGBT modules as switching devices, and reduction of

the cost in terms of cooling is also observed because smaller

size of heat sinks are needed for small heat conduction of

switching devices.

Key Words—Induction Heating, Soft Switching.

I. INTRODUCTION

eveloping solid-state induction heating welder

involves the design of high frequency inverter. Major

challenges in this regard center on addressing issues

such as commutation, efficiency, and high current

switching techniques [1]. Previous approaches involved

designing Tube type welders based on triodes used as class

C amplifiers. Solid-state methods, though devised since

long, still wait for solution to above problems before their

wide deployment in industry. No one has yet set theoretical

bounds on inductive load on such an inverter, which would

result in commutation failure and hence resulting in

unreliable operation [2].

In the past, many approaches have been in common use

in the design of induction heating welders. A few of them

are; rectifiers with phase control-using thyristors, triode

based Tube-type welders etc. More recently, use of IGBTs

and MOSFETs has started appearing in the design of

inverters. Solid-state inverters using IGBTs have been

realized up to 150 kHz at 1.5 MW. More research is

continuing towards increasing these figures to 500 kHz and

at increased power [3]. MOSFET solutions have been

proposed but they are very costly due to large silicon area

and internal diode problem of MOSFET. Some modular

techniques using IGBTs have been practiced successfully

but they serve from the problems of switching transients

and huge losses [4]. There are plenty of heuristics in the

literature, which result in improved efficiency based on

topological changes. Combining various modules in series,

parallel and in various complex configurations like H-

bridges and then using phase shifted gating has shown to

increase the efficiency and output power of inverters

designed using modular design methods [5]. So the basic

circuit to build a high frequency induction welder is to

build a high frequency inverter.

Different welding techniques are used in the industry

based on the power requirement and on the basis of socio-

economic grounds. But induction welding is mostly used in

the industries because of large number of advantages.

Induction welding is a form of welding that uses

electromagnetic induction to heat the workpiece. The

welding apparatus contains an induction coil that is

energised with a radio-frequency electric current. This

generates a high-frequency electromagnetic field that acts

on either an electrically conductive or a ferromagnetic

workpiece. In an electrically conductive workpiece, such as

steel, the main heating effect is resistive heating, which is

due to magnetically induced currents called eddy currents.

In a ferromagnetic workpiece, such as plastic doped with

ceramic particles, the heating is caused mainly by

hysteresis as the magnetic component of the

electromagnetic field repeatedly distorts the crystalline

structure of the ferromagnetic material. In practice, most

materials undergo a combination of these two effects.

In the Induction welding technique high frequency

induction coil surrounds joint that is to be welding. When

high frequency current passes through the induction coil, it

induces eddy current at the joint leading to very localized

heating, and melting. Pressure is applied to expel the

molten metal as show in the figure1.

Mudassar Iqbal, Noor M. Sheikh,

Department of Electrical Engineering, University of Engineering & Technology, Lahore.

Performance Improvement of High Frequency

Solid State Welder Using Zero Current

Switching Technique

D

2

Fig. 1. Induction Welding

II. CIRCUITS FOR INDUCTION WELDING

Recently, solid-state welders have become more popular

due to various advantages they possess. Wide spread use of

them in various industrial processes like; Annealing,

Bonding, Hardening, and Forging etc. is becoming more

and more prevalent. A major component in the design of a

solid-state welder is inverter, which is basically used to

enhance heat losses due to induction, which are

proportional to frequency. So, the idea is to first convert

AC into DC and then reconvert it back into AC but at

higher frequency [6]. Former operation being performed by

rectifiers is already at its climax in terms of design and

efficiency but latter being performed by inverter still owes

a plenty of work for its efficiency and maturity. A major

problem that requires attention is; switching of high

currents generated during induction sparks [7], [8]. Here

also arises the problem of commutation which is basically a

short circuit produced at an inductive load. A high value of

inductance results in unreliable operation. Hence there is a

need to determine bounds on the value of an inductive load

that can be put upon such an inverter. Another area of

research is to explore relationship between inductive load

bounds and frequency i.e. whether such bounds become

tighter or the other way round and then by what rate i.e.

exponentially, logarithmically, etc. Also, the effect of

frequency on the performance still has not been explored.

Lastly, the issue of discovering optimal waveforms for high

current switching is still open to debate. Here, tradeoff

between speed and performance comes into play [9].

There are plenty of heuristics in the literature, which

result in improved efficiency based on topological changes.

Combining various modules in series, parallel and in

various complex configurations like H-bridges and then

using phase shifted gating has shown to increase the

efficiency and output power of inverters designed using

modular design methods [10]. So the basic circuit to build a

high frequency induction welder is to build a high

frequency inverter. So the basic block diagram of the

induction welder is shown in figure 2.

Fig. 2. Block Diagram of Induction Welder

A. Rectifier

All the power supplies inverter have a converter sections

that converts line frequency alternating current to direct

current. All induction heating power supplies use one of

the basic converters. The simple circuit for the rectifier

could be the single-phase rectifier that is directly connected

with the supplier’s main supply. The rectifier can be built

by using either high power diodes (uncontrolled) or

controlled rectifier using controlled switches. In this

investigation a three phase PWM rectifier is used.

B. Inverter

Different topologies can be used for this section. In the

half bridge topology inverter requires only two switching

elements for its operation. In the full bridge topology, 4

switching elements are used

For the process of induction heating two types of

inverters can be used, which current fed inverter are having

a capacitor in connection to a parallel load and voltage fed

inverter with a capacitor with series load. In this paper, for

the inverter section, voltage fed inverter has been used and

has been simulated at a frequency of 65 kHz to 70 kHz.

The switching devices used are the IGBTs and soft

switching technique has been used to improve the

efficiency of the inverter.

The major issues that are used to measure the

performance of the system are.

1. Performance

2. Reliability

3. Low cost

4. Effect on Environment (RFI, Noise)

5. Efficiency

C. Performance

The performance consists of the transient and steady

state parts. The important transient performance measures

are the transient current from the source, short circuit

current from the source, surge protection, switch stresses

(highest voltage and highest rate of rise or fall of voltage

across the switching device) and harmonic profile.

D. Reliability

The system parameters, components and design must be

selected that is system is always reliable under worst-case

condition. The system components and design must be able

to work in different environment conditions in the same

way and with the same efficiency.

E. Low Cost

Now a day an important aspect of the design is the cost

of the system. The system must be designed in a way that

the cost of the system is minimum possible that is achieved

by considering different aspects of power electronics. For

example: we can use components in the modular form that

are cheap as compared to discrete components.

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F. Effect on Environment

In power electronics because the switching of the power

devices takes place at very high frequency and because of

the presence of the inductive components, the current and

voltage waveforms are non-sinusoidal. Because of the non-

sinusoidal nature of these waveforms, high frequency

harmonics are generated. The system must be designed so

that it meets the Radio Frequency Interference Standards

(RFI standards).

G. Efficiency

Efficiency of the system can be improved by reducing

losses in the system. Particularly the power switch losses

are the major losses that reduce the efficiency of the

system. As the switching frequency increases, these losses

also increase. These losses are very important in case of

inverter section because the power devices are switched on

and off at very high frequency. So in order to improve the

efficiency of the system these losses must be minimized

[11].

The main switch losses that can occur in the switch are

1. Forward conduction losses

2. Blocking state losses

3. Switching losses

Ideal switch has zero voltage drop across it during turn-

on (Von). Although the forward current (Ion) may be large,

the losses on the switch are zero. But for real switches, e.g.

BJT, IGBT, GTO, SCR have forward conduction voltage

(on state) between 1- 3V. MOSFET has on state voltage,

which is characterized by the RDS(ON).

During turn-off, the switch blocks large voltage. Ideally

no current should flow through the switch. But for real

switch a small amount of leakage current may flow. This

creates turn-off or blocking state losses. The leakage

current during turn-off is normally very small; hence the

turn-off losses are usually neglected During turn-on and turn off, ideal switch requires zero

transition time. Voltage and current are switched

instantaneously. In real switch, due to the non-idealities of

power switches, the switching losses occur as a result of

both the voltage and current changing simultaneously

during the switching period.

The product of device voltage and current gives

instantaneous power dissipated in the device. The heat

energy that developed over the switching period is the

integration (summation) of instantaneous power over time

as shown by the shaded area under the power curve. The

average power loss is the sum of the turn-on and turn off

energies multiplied by the switching frequency. When

frequency increase, switching losses increases. This limits

the usable range of power switches unless proper heat

removal mechanism is employed.

III. REDUCING SWITCHING LOSSES & IMPROVING

EFFICIENCY

To reduce the losses high di/dt or dv/dt can be used but

this causes EMI (electromagnetic interference). The

disadvantage is extra losses in the passive components

resistance, capacitance and diode (RCD). But that increases

losses in the passive elements Figure 3 shows the

comparison of losses with and without snubber.

Fig. 3. Hard commutation and aided commutation

IV. SWITCHING

Increasing switching frequency will result in more

switching losses and electromagnetic interference (EMI).

To avoid these losses soft switching is used instead of hard

switching.In hard switching, current is flowing during the

transition and there is a voltage present. This combination

causes power to be dissipated. When power transistor is

made to “hard-switch” the unregulated input voltage the

whole raw input voltage is developed across it as it changes

state.

During the actual switching interval there is a finite

period as the transistor begins to conduct where the voltage

begins to fall at the same time as current begins to flow.

This simultaneous presence of voltage across the transistor

and current through it means that, during this period, power

is being dissipated within the device. A similar event

occurs as the transistor turns off, with the full current

flowing through it [12].

In soft switching, either the voltage (Zero Voltage

Switching ZVS) or the current (Zero Current Switching

ZCS) is zero during the transition and no power is

dissipated. Switching occurs at either the zero current or

zero voltage point in the waveform. Soft switching reduces

switching losses. This means that switching frequency can

be increased, which, in turn, allows smaller components to

be used. Lower losses also mean that the power supply runs

cooler and reliability increases. Soft Switching is the most

4

common technique, which ensure that the actual energy

being dissipated by the active device is reduced to nearly

zero.

Soft switching can be achieved by using the parasitic

output capacitance of the power transistors (typically

MOSFETs or IGBT) and the parasitic leakage inductance

of the power transformer as a resonant circuit. Using this

resonant circuit, the output inductance, the parasitic drain-

source body diodes of the MOSFETs or IGBT, and an

appropriate switching sequence allows the voltage across

each transistor to swing to zero before the device turns on

and current flows. Likewise, at turn-off, the voltage

differential across the transistor swings to zero before it is

driven to a non-conductive state. With this scheme, current

is only flowing through the transistors when they are fully

“on”, and doing useful work transferring energy to the

output of the supply.

Today the standard IGBT is designed and optimized for

hard switching and snubberless operation. But the

sensitivity against switching overvoltages caused by the

large di/dt and the not avoidable set-up stray inductance

require small protecting snubber networks, especially when

single IGBT modules instead of integrated half-bridges or

six packs are used. Resonant switching topologies make

such snubbers unnecessary. As mentioned above the

passive components of the resonant circuit are used to

define the voltage and/or current shapes during a resonant

cycle. Due to the softened slopes of the output voltage the

reliability of the whole converter can be improved because

the components are stressed less.

Figure 4 depicts typical trajectories in the safe operating

area SOA for resonant switching in comparison to hard

switching. In case of hard switching a wide area is enclosed

while in the resonant switched case the trajectories are

close to the axis. The shaded area corresponds to soft

switching while grid area corresponds to hard switching.

Fig. 4. Safe Operating Areas

V. SIMULATION MEETHOD

A. Selection of Different Devices

The main objective is to design an inverter that is based

on soft switching so that the losses are minimum and the

efficiency of the inverter is maximum. The power circuit is

a voltage fed inverter for which the following is to be

designed:

1. Proper IGBT Selection.

2. Proper Fast Recovery Diodes.

3. Snubber Circuit

4. Resonant inverter inductor and capacitor.

The IGBT must be switched at a frequency between 40

kHz to 50 kHz, so the selected device for the simulation is

SKM75GAL06. Another important parameter is the

maximum value of load current which it can bear that is

100 A for this IGBT. The rated maximum voltage stress for

this device is 600 V. So this device meets all the

requirements. Similarly the turn on and turn off time of this

device is very small. So this power device can be used at

the required switching frequency.

Similarly the internal diodes used work as the fast

recovery diodes, so external diodes are not required.

Snubber circuit is always required for aided

commutation. But as in our thesis the circuit is based on the

resonant Zero Current Switching inverter. So in this case

there is no need of using the snubber circuit. Similarly for

the resonance the proper value of inductor and capacitor is

selected to ensure Zero Current Switching (ZCS).

In this simulation, the improvement in the efficiency is

observed using the soft switching technique, which is

obtained by using resonant inverter.

B. Switching Circuit

Figure 5 shows the desired circuit for resonant full

bridge inverter that is used in the simulation and

measurement.

Fig. 5. Resonant soft-switched inverter

5

C. Simulation Results

The circuit shown in figure 5 is simulated at 50 kHz

frequency with a line voltage of 400 volts that is coming

from the rectifier. The main objective was to achieve Zero

Current Switching (ZCS) to minimize the switching losses.

The voltage and current waveforms are plotted for different

values of load current. The simulation results are shown in

the figure 6.

Fig. 6. Load current = 50 A

Fig. 7. Voltage and current in T1 Switching IGBT

In figure 7, the voltage across the IGBT has spikes. The

reason for the spikes is the internal inductance of the IGBT.

If we ignore this inductance in the model then these spikes

are not observed.

Figure 8 shows the load current and voltage and current

in the switching device. But in this case the value of the

load current is smaller that equals 30 A.

Fig. 8. Load current = 32 A

Fig. 8. Voltage and current in T1 Switching IGBT

Fig. 9. Load current = 12 A

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Fig. 10. Voltage and current in T1 Switching IGBT

Fig. 11. Current drawn from the source

In figure 11, the current from the source has smaller

value of rate of change. So that means the higher frequency

harmonics are not there. And total harmonic distortion is

smaller as compared to hard switching where the current

from the source is in the form of pulse, either Im or zero

In the figure 12, the efficiency of the hard switched and

soft-switched resonant inverter is compared. The graph

shows that the efficiency of the soft-switched inverter is

always larger for any value of load current.

Fig.12. Current Comparison of efficiency, hard

switched and soft switched welder

VI. CONCLUSION

The design and analysis of the high frequency resonant

inverter has been presented and has been simulated using

the MATLAB software. Zero Current Switching (ZCS) has

been achieved, which results in the improvement in the

efficiency. The circuit is simulated for the values of Vdc =

400 Volts, Ls =9 H and Cs =0.1 F.

For the satisfactory working of this circuit, this is

simulated for different values of load currents and for these

values of load currents the voltage and current waveforms

across the switching devices are plotted which shows the

maximum stress. For this case no extra stress (Voltage

across the switching device) are imposed on the switching

devices. At the end the comparison is made between this

soft-switched resonant inverter and hard switched inverter

and it is observed that there is significant improvement in

efficiency in the former case.

VII. REFERNCES

[1] K. H. Liu, R. Oruganti, and F. C. Lee, “Resonant

switching topologies and characteristics,” IEEE Power

Electronics Specialists Conference Record, pp. 106-

116, 1985.

[2] V.L Rundev, R.L.Cook, D.L.Loveless, “Induction Heat

Treatment”, Marcel Dekker Inc, 1997.

[3]]CA. Tubdury, “Basics of Induction Heating” Rider,

New York, 1960

[4].-S. Lai, “Practical design methodology of auxiliary

resonant snubber inverters,” in Proc. IEEE PESC’96, June

1996, pp. 432– 437.

[5] Discrete Application Power Device Division. “

Induction Heating System Topology Review”, Fairchild

Semiconductor. July, 2000.

[6] Bimal K. Bose, “Modern Power Electronics

Handbook”, Butteroworth-Heinemann Itd. 1990 2nd

Edition.

[7] V.L Rundev, R.L.Cook, D.L.Loveless, “Induction Heat

Treatment”, Marcel Dekker Inc, 1997.

[8] Zinn, “Elements of Induction Heating, Design, Control

7

and Applications”, ASM Int. 1988.

[9] F. P. Dawson, and P. Jain, “A Comparison of Load

Commutated Inverter Systems for Induction Heating and

Melting Applications,” IEEE Transactions on Power

Electronics, vol. 6, pp. 430-441.

[10]Discrete Application Power Device Division.

“Induction Heating System Topology Review”, Fairchild

Semiconductor. July, 2000.

[11] J.S. Lai, “Practical design methodology of auxiliary

resonant snubber inverters,” in Proc. IEEE PESC’96, June

1996, pp. 432– 437.

[12] Xiangning He, Kuang Sheng, Barry W. Williams,

Zhaoming Qian and Stephen J. FinneyHe, “A soft

switching inverter circuit,” IEEE TRANSACTIONS ON

INDUSTRIAL ELECTRONICS, VOL. 48, NO. 1,

FEBRUARY 2001