2.5 kW MMA welding machine

AN3200
Application note
2.5 kW MMA welding machine
Introduction
In this application note a power AC-DC converter, suitable to be used as the current supply
for the MMA welding process, is described.
The MMA welding method is the most common method used for welding iron or steel.
Usually this method, also the most simple, is dedicated to small home repairs but also to
light professional use.
In recent years, the availability of low cost and reliable solutions, based on power
electronics, has moved the approach to this application from a standard magnetic solution,
based on a low frequency iron transformer, to a high frequency switching solution. This
gives the possibility of having compact and light solutions at a relatively low price. The
electronic control used on modern welding machines allows a simple and safe use, so even
an unskilled person can use it with good results in terms of weld quality.
September 2010
Doc ID 17388 Rev 1
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Contents
AN3200
Contents
1
MMA welding process . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 4
2
MMA welding power supply . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 6
3
State of the art forward converter . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 8
4
Current control implementation . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 9
5
Welding machine hardware . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 11
5.1
Power board description . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 11
5.1.1
5.2
6
2/19
Power board schematic description . . . . . . . . . . . . . . . . . . . . . . . . . . . . 12
Control board schematic description . . . . . . . . . . . . . . . . . . . . . . . . . . . . 15
Revision history . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 18
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List of figures
List of figures
Figure 1.
Figure 2.
Figure 3.
Figure 4.
Figure 5.
Figure 6.
Figure 7.
Figure 8.
Figure 9.
MMA welding process. . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 4
Output characteristic of an MMA welding power supply . . . . . . . . . . . . . . . . . . . . . . . . . . . . 5
Schematic diagram of a double switch asymmetric forward converter . . . . . . . . . . . . . . . . . 6
Some significant acquired waveforms . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 7
Block diagram of current control circuit. . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 9
MMA welding machine demonstration board STEVAL-ISW001v1 (side view) . . . . . . . . . . 11
MMA welding machine demonstration board STEVAL-ISW001v1 (top view). . . . . . . . . . . 12
Power board schematic . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 14
Control board schematic . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 15
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MMA welding process
1
AN3200
MMA welding process
In the following paragraph a general description of the Manual Metal Arc (MMA) welding
process is explained.
The process is very simple. A plasma arc, able to generate a very high temperature in a
restricted area, is used to fuse iron or metal.
The plasma arc is created by a high current at low voltage. Two wires, one fixed to the iron or
metal pieces to be welded by a metallic clamp, and one with a special tool on its end called
an electrode holder, and able to fix a metallic rod, are used to be connected to the power
supply’s negative and positive outputs, respectively.
A shielded electrode is used at the same time to conduct the current, necessary to sustain
the plasma arc and to introduce additional welding material into the welding area.
The electrode is shielded by a particular material that assures, when melted, the production
of an inert gas environment to protect the weld from an immediate oxidation process due to
high temperatures.
In Figure 1 the MMA process outline is shown.
In order to switch on a high temperature low voltage plasma arc, necessary to weld, a
current source able to provide a high current level at low voltage is required.
Starting from the AC net (185 Vac to 265 Vac) the system must perform a conversion from
AC to DC and from high voltage to low voltage. Proportionally, by fixing a maximum power
from the main, the output supplied current is gained. Another important function that the
welder power supply must assure is the galvanic insulation between the main net and the
output ground. In fact, during the welding process the negative output is directly connected
to the iron piece being welded, therefore, for safety reasons, insulation is needed.
Figure 1.
4/19
MMA welding process
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AN3200
MMA welding process
In Figure 2 a classic current vs. voltage characteristic of an MMA power supply is depicted.
Figure 2.
Output characteristic of an MMA welding power supply
The output characteristic of a suitable MMA power supply shows that a high DC open load
voltage is present on the welding torch in open circuit condition.
As soon as the current drained by the welding pool increases, the voltage decreases.
Around the welding operating point the characteristic suggests a current limited generator
behavior.
The plasma arc, and so the welding process, starts when contact is made with the metal
pieces, connected to ground with the electrode. In this condition a short-circuit is present at
the output of the power supply. This condition has to be managed by limiting the SC
(shortcut) current in order to protect the hardware against this transient.
As soon as the plasma starts, the electrode is no longer in contact with the metal, a voltage
at the output of the power supply (across the plasma) appears and the welding process
works by melting the iron.
In this condition, the current is still limited by the power supply at the level set by the
operator according to the type and thickness of the metal piece.
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MMA welding power supply
2
AN3200
MMA welding power supply
The following are some electrical specifications of an MMA welding power supply, for a small
machine usually used in home appliances or light professional use:
●
Vin (AC): 185 Vac to 265 Vac
●
Vout_peak: 80 V
●
Iout_max: 135 A
●
Fswitching: 70 kHz
●
Efficiency: > 75 %
The topology chosen to develop this kind of power supply is a double switch asymmetrical
forward converter.
This topology is suitable for high power delivered to the secondary side of a ferrite high
frequency transformer and at the same time can be considered simple, cheap, and robust.
Figure 3 shows a schematic diagram of a double switch asymmetrical forward converter.
Figure 3.
Schematic diagram of a double switch asymmetric forward converter
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The circuit consists of two power switches used to energize the primary side of a ferrite
transformer and two freewheeling diodes needed to discharge the magnetization inductance
of the primary side winding of the transformer. At the secondary side, two power diodes
perform the rectification. The output inductor gives a continuous-mode operation to the
current.
The two power switches are switched on simultaneously allowing a current path from the
high voltage DC bus. The two power switch gates are driven by a square wave with variable
duty cycle. The variation in duty cycle allows the possibility to modulate the voltage on the
output of the power supply and, according to this, the welding current. It is important that the
duty cycle does not surpass 50 %, in order to assure a full demagnetization of the ferrite
transformer during TOFF period.
In Figure 4 some significant waveforms are depicted.
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AN3200
MMA welding power supply
Figure 4.
Some significant acquired waveforms
The green trace Ch4 represents the current on the power switches that during the TON
switching period is the same on the primary side of the transformer. The purple trace
represents the voltage across the primary side of the transformer. It may be noted that
during TON the voltage is positive and equal to the DC bus voltage (around 300 V). At the
end of the TON period the switches are switched off and the magnetization current of the
transformer starts to circulate through the two freewheeling diodes (see the schematic in
Figure 8).
The freewheeling diode action causes the inversion of the voltage across the primary side
off the transformer, thanks to this the magnetization inductance can be discharged. At the
end of this period, and before a new “on” period, the voltage across the transformer remains
zero. For this reason, on double switch forward topology, the hard switching stress on the
power devices is only the OFF transition, as during the ON transition the switches start to
commutate from an intrinsic soft switching zero current condition.
The yellow trace is the gate drive signal, and above, the output current is represented with a
blue trace.
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State of the art forward converter
3
AN3200
State of the art forward converter
A forward converter is an insulated DC-DC converter that transfers the energy from the
primary side of a transformer to the secondary side during the TON period of a power switch
operating on the primary side.
Assuming the transformer and the power switch are ideal, during the time period TON, the
diode D3 is conducting, as D4 is interdicted, (refer to Figure 3).
The voltage across the output inductor is assumed positive; the current flowing on the
inductor increases linearly according to the following equations:
Equation 1
vL =
N2
VIN − VOUT
N1
0 < t < ton
Equation 2
v L = −VOUT
ton < t < T S
Equation 3
ton
∫
ton
v L dt = -L
0
∫
0
dil
dt
During the TOFF time period the voltage across the output inductor is negative and equal to
the output voltage. Diode D4 conducts and D3 is interdicted.
The current flowing on the inductor decreases linearly.
Assuming the voltage on the inductor is equal to zero, if integrated in the whole switching
period, and assuming the duty-cycle is equal to:
Equation 4
δ=
ton
ton
=
ton + toff Ts
The relationship between the input and output voltage can be expressed as:
Equation 5
V0 N2
=
δ
Vd N1
At this point it is simple to show that only during TON the current measured on the primary
side of the transformer is directly dependent on the current on the output, so it is practically
impossible to have precise control of the output current, knowing only the input current on
the primary side of the transformer.
Those relationships are helpful to understand the control circuit implemented on this power
supply.
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4
Current control implementation
Current control implementation
In Figure 5 the block diagram of the current control circuit is shown. The suggested control
method has been patented by STMIcroelectronics.
Figure 5.
Block diagram of current control circuit
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Reading only the input primary side current on the ferrite transformer is not sufficient to have
a precise current control on the welding pool. In fact, the output voltage of the converter,
equal to the welding voltage, is not fixed and can change during the process due to a variety
of factors. The consequence is a power control on the primary side but not a current control
on the output.
The patented idea is to reconstruct the waveform and value of the output current only by
reading the input current on the primary side of the transformer and the output voltage
reflected on the output inductor during the TOFF time period of the switching cycle.
The voltage reflected across the output inductor, during TOFF, is the output voltage of the
converter and so the voltage across the plasma welding arc during the welding process.
It is understood from the equation of the forward converter that the current on the output
inductor during TOFF and TON can be expressed as follows:
Equation 6
Il (t ) =
1
L
Ts
∫ v dt
l
ton
Equation 7
v l = − v out
Equation 8
1
Il (t ) =
L
ton
∫ v dt
l
0
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Current control implementation
AN3200
Equation 9
v l = v in
N2
− v out
N1
Where Ts-TON=TOFF.
The result of these integrals is a function which describes the slope of the output current
during TON or TOFF, in particular, precious information comes from the reflect output voltage
on the inductor that allows the slew rate of the current during TOFF, otherwise unpredictable,
to be known.
In fact the output voltage isn't controlled and depends on the welding arc, as already
explained above.
Remember that during TOFF the diode D1 (Figure 5) is interdicted and there isn't any
connection between input and output.
This means that if the voltage across the output inductor could be read, by integrating it, the
slope of the output current during TOFF would be known.
To solve the integral equation it is still necessary to calculate the value of the current, and
integration constant, to define the border condition.
This information comes from the input current transformer that measures the peak of the
input current representative of output current.
Equation 10
Iout _ peak =
N1
N2
Iin _ peak
Therefore, the key of this patented system, is to use the information from the current
transformer applied on the primary side, to obtain information about the current peak on the
inductor; and some signals turn on the output inductor to obtain information about the
current slope, that, once the value of L is fixed, depends only on the voltage across the
plasma arc.
The two pieces of information are combined together in order to have a waveform that
reconstructs the output current (see Figure 5).
T2 on the primary side is the current transformer which senses the value of the input
current. This information is applied at input to a peak detector circuit to freeze the peak
value of the current. The voltage sensed by some turns applied on the output inductor, once
integrated, give information about the output current slope.
Combining these two elements, at the output of the sum node 3 you have a scaled
reconstruction of the output current (scaled due to the sensing and post processing block
constant). This signal can be used in input to a classic PI regulator to control, and maintain
as fixed as possible, the output current against the load and output voltage variation.
10/19
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5
Welding machine hardware
Welding machine hardware
Here below in figure the welding machine. This board is one of the STMicroelectronics
demonstration board available on stock with the code STEVAL-ISW001v1.
Figure 6.
MMA welding machine demonstration board STEVAL-ISW001v1 (side
view)
The welding machine demonstration board is made up of a power board, including all the
power parts and onboard power supply necessary to supply all the circuits of the system,
and a control board installed on the power board through a 26-pin connector.
The control board can therefore be considered a “sister board” and can be replaced using
other control methods or devices, or moved to a control based on a microcontroller,
maintaining the power part unchanged.
5.1
Power board description
In Figure 6, on the left-hand side, the input mains section can be found, including the input
EMI filter. The white screw connector is the input for the Vac mains; the green screw
connector is used to connect an external fuse holder, to be installed on the rear or front
panel of a suitable case that contains the hardware. From left to right, the two big heatsinks
seen hold the 4 IGBT power switches. The IGBTs are paralleled, two for the high side
section of the converter and two for the low side.
In the middle of the power board a small control board is installed through a double in-line
connector, as previously mentioned.
On the right-hand side the output power diodes are installed.
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Welding machine hardware
Figure 7.
AN3200
MMA welding machine demonstration board STEVAL-ISW001v1 (top
view)
In Figure 7 the insulated ferrite transformer can be seen in the centre of the board. On the
left-hand side the output coil with the signal auxiliary winding is present, necessary for the
sensing of the reflected voltage.
Two copper bars with bolts are used to connect the external cable with the ground clamp
and the electrode holder.
5.1.1
Power board schematic description
In Figure 8, the schematic of the power board for the welding machine is depicted.
Starting from the AC mains input, you find an EMI common mode choke and the NTC
necessary to limit the inrush current due to the electrolytic capacitor present after the input
diodes bridge. The auxiliary power supply is based on the Viper16 and provides, using a
flyback transformer, two insulated voltages. One of the two low voltages is common ground
connected to power and control ground. The insulated +15 V is used to supply the high side
part of the L6386E gate driver. In this case this solution was preferred to the boot strap
solution because of current availability reasons.
The L6386E gate driver is also used to implement a hardware peak current protection
thanks to the internal comparator present inside this gate driver.
In order to increase the current capability of the gate driver, two small integrated push-pull
transistors are used (STS01DTP06), one for the high side and one for the low side power
switch couple.
In series with the high frequency ferrite transformer a current transformer is inserted. The
voltage across R44 is sent through a resistive partition to the Cin input of the L6386E gate
driver. As soon as the voltage on this pin is equal to the fixed internal threshold of 500 mV,
the gate driver shuts down the two gate outputs. In this way, a valid and fast hardware
current protection is implemented to protect the power part. With the net used, the current
protection is set to about 32 Apk.
The output rectification stage is made up of the power diodes D29, D30 and D31.
12/19
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Welding machine hardware
On the output inductor, a secondary winding is present in order to read the reflected welding
voltage (voltage across the plasma arc) as previously explained.
The optocoupler U7, senses the output shortcut condition. This information is sent to the
control logic to change the current reference and to start the shortcut condition delay. The
trimmer R60 allows the voltage threshold to be set between the correct voltage on the
plasma and recognition of the shortcut condition.
R60 should be trimmed according to some parameters such as, for example, the length of
the output cable.
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Figure 9.
5.2
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Welding machine hardware
Control board schematic description
Control board schematic
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Welding machine hardware
AN3200
In Figure 9 the schematic diagram of the control board is shown.
The control of the MMA power supply is based on some analog blocks.
Three basic blocks can be identified in the schematic of the welding machine control board.
●
Analog amplification and computation block based on the TSM102
●
PWM generator based on the SG3525
●
Welding current dynamic reference based on discrete components
The analog amplification and computation block is based on the TSM102. This chip has two
operational amplifiers, two voltage comparators and a voltage reference inside. The
operational amplifiers are used to implement an analog current control composed by a PID
and an integrator, (see Figure 5).
One of the two amplifiers is used to implement the error amplifier for the current control. A
voltage representative of the output welding current is subtracted to a voltage level
representative of the wanted target current level. The error signal is amplified and
represents the PWM level to be set as the control variable to the power converter.
This is a standard PID control.
As already explained in Section 4, the measurement of the output current is obtained
through an indirect estimation. The value of the peak current on the primary side of the
ferrite transformer, measured by current transformer T2, is sent in input to an analog adder
realized by the op amp inside the TSM102 connected to pins 5, 6, and 7, non inverting input,
inverting input, and output, respectively. The same op amp is used as an integrator to
calculate the current output ripple value. This final signal, representative of the output
welding current, is present on C7. The second op amp, connected to pins 12, 11, and 10 of
the TSM102, implement the error amplifier for the final current regulation block.
The output error signal is finally sent to the PWM generator SG3525. This chip provides an
output square wave signal with a duty cycle value proportional to the voltage present on pin
2. The frequency of this signal can be set by the value of C11 and R35. The signal SD
coming from the power board is pulled down by the comparator inside the L6386E driver in
case of overcurrent detection. The PNP Q2, in this case, immediately discharges C12, in
these conditions the PWM generator switches off the output. As the overcurrent condition
elapses the SD signal is released, Q2 switches off and the slow charge of C12 implements a
soft-start.
By using the voltage comparator connected to pins 1, 2, and 3 of J1 (TSM102) an
overtemperature protection is implemented. The voltage signal TS, coming from a
temperature sensor, is compared to a fixed voltage obtained by the resistive partitions R10
and R11. As the temperature on the heatsink goes over the threshold, the signal Shut_down
is pulled down and the PWM generator is switched off.
The second comparator inside the TSM102, connected to pins 16, 15, and 14, is used to
implement an anti-stick circuit. At startup, when the electrode comes into contact with the
metal piece and the arc is not yet on, it’s possible that the electrode may stick to the metal
piece being welded. Under this condition a hard short-circuit is present at the output of the
generator. This condition, sensed by the optocoupler U7, discharges the capacitor C2
through R9. This gives a delay of about two seconds before the output of the comparator
(pin 16) goes low, setting the target current coming from the potentiometer to a low level.
This protects the generator and allows the operator to remove the electrode from the metal
piece under a low current condition (avoiding sparks).
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Welding machine hardware
The last block to describe is the welding current dynamic reference which is based on the
optocoupler and the bipolar transistor Q1 plus some passive components and signal diodes.
The welding current dynamic reference block is dedicated to giving the right target to the
current control, according to the MMA process status.
During the welding process some fused metal drops can cause a momentary short-circuit of
the plasma; this causes the electrode to stick to the metal piece being welded. This forces
the operator to remove mechanically the electrode from the metal, or simply cause a
discontinuous plasma action which gives bad results in terms of weld quality. To avoid these
phenomena, an automatic and dynamic quick increase in the welding current must be
performed, as a shortcut condition is sensed by the optocoupler in order to fuse, as soon as
possible, any cause that tends to switch OFF the plasma.
During this momentary short-circuit, the voltage drops at the output cause the bipolar
phototransistor inside the optocoupler to be switched off. Q1 is immediately switched on,
clamping the welding current reference set by the trimmer R2 to its VCESAT. In this
condition the voltage reference set by the trimmer R4 is free to get into the current regulation
block. Otherwise, under normal welding conditions, the bipolar inside the optocoupler is ON
so the voltage set by R4 is low and the voltage set by R2 is free to enter the current
regulation block.
The result is the possibility to differentiate the reference for the current control in the two
different working conditions: plasma ON (voltage at the output in a valid range), short-circuit
(plasma OFF low voltage at the output).
This behavior is totally automatic and dynamic thanks to the signal generated by the
optocoupler U7.
Two small trimmers can be found on the control board; short-circuit current (R4) and
maximum duty cycle (R32).
A further trimmer is dedicated to setting the welding current during normal operation (R2).
The electrical connection of R2 is also replicated on the power board so it is possible, using
wire assembling, to mount it on the front panel of the welding machine for easy setting
during operation.
The objective of the maximum duty cycle trimmer is to set the maximum duty cycle in open
load condition. In fact, in open load condition, the current control forces the maximum
possible duty cycle (50 % for a forward converter). Acting on this trimmer, it is possible to
limit the duty cycle to a value different from the maximum allowed by the PWM generator.
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Revision history
6
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Revision history
Table 1.
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Document revision history
Date
Revision
06-Sep-2010
1
Changes
Initial release.
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