Current control for SMPS Arc Welding Machine

[tAhm1D]

Hi all,

I am working on making a switched mode arc welding machine with maximum 150A current and 30-40V. I have done most of the circuitry but the problem I face is the output current control. I do not want to use the bulky old linear type inductor, so what can be the solution?

I have seen in some SMPS welding machine (in schematics), current is controlled with a small ferrite core inductor. I tried to find out the calculation and procedure, but could not find out from any book or internet the required procedure for controlling output current through ferrite core inductor.

I require to control the output current to control the inrush current and also to use different type of welding rod with controlled current from 75A to 150A.

In some discussion on some forums, I found the idea of controlling output current through hall effect sensor. But nowhere it is discussed in detail. So, no concrete idea could have been formulated.

Help in this regard will be appreciated.

[Jehan]

Linear Current control in SMPS is difficult, you can use a hall effect sensor from LEM. in lower current application upto 50 Amp i had used a Current sense resistor and an OP amp OP37 as the amplifier stage , with a low offset op amp the shunt dissipation can be kept low with the sense voltage in the range of 10 millivolt, this will be a lower cost approach than a Hall based design.

[sohel]

u can read this topics for ur requerment

http://www.sonsivri.com/forum/index.php?topic=13375.0

[tAhm1D]

Linear Current control in SMPS is difficult, you can use a hall effect sensor from LEM. in lower current application upto 50 Amp i had used a Current sense resistor and an OP amp OP37 as the amplifier stage , with a low offset op amp the shunt dissipation can be kept low with the sense voltage in the range of 10 millivolt, this will be a lower cost approach than a Hall based design.

Hi,
This is current sensing, but I require to control the current flow with my requirement to use different sizes of welding rod and current control should be from 50A-150A. I think with your proposed system sensing can be done but current control can not be done. However I appreciate your reply and please let me know if any other ideas are there for switching type control. I tried with Pic Micro controller and Triac by Line Phase Angle control but in that case Voltage also reduces, which is not acceptable as Voltage should be constant for good Arc Welding.

For Sohel,

I appreciate your reply but the Link you have provided  will not serve my purpose and I have consulted that earlier.

[bbarney]

You can check Allegro MicroSystems, Inc. for hall effect products and apps
http://www.allegromicro.com/en/Products/Design/Tech_Pubs.asp#B
http://www.allegromicro.com/en/Products/Categories/Sensors/index.asp

[tAhm1D]

Hi,
Yes, I am working regarding hall effect products and think the links will be useful.

Does anybody having any idea regarding saturable reactor/core current control system?  I am working on it also and find it interesting but for designing an workable prototype requires more information/knowledge. Concrete ideas will be appreciated.

[DTiziano]

Hi

From your state " .. have done most of the circuitry" and previous post, I suppose you design the SMPS.
I saw some IC (linear technology) that use a double feedback for regulation, one from voltage or-ed with one from current. Why not use a similar approach ?
Use two signals and or then to the feedback. The voltage one will limit the maximum output the current will work during welding. More or less like a power supply when limits the output current.
The setting/adjust may be made on the reference or better on the feedback signal.

P.S.
You did not specify if your design output is AC, DC or both.

[tAhm1D]

Hi DTziano,
I appreciate your Idea. But it is not fully clear to me. If you elaborate in detail, it will be useful for me to grasp.
My design output is DC and it is SMPS type.

[beque]

Look at these patents...

http://rapidshare.com/files/180410915/Patents_-_Arc_Welding.rar

Size:10Mb

[tAhm1D]

Hi beque,

I have downloaded it and found it very resourceful. I am carefully studying it and hope will get something appropriate to fulfill my requirement. I appreciate your cooperation. If you have any other resources or knowledge regarding smps based ARC/Butt/SEAM Welding Machines or Inverter, please don't hesitate to inform. I will be highly obliged.

[okk]

Here are a few more welding power source patents (there may be the odd duplicate).

I have found them very useful over the years.

Download Link: http://rapidshare.com/files/180500549/Welding_Patents.rar

[tAhm1D]

Hi okk,
Appreciatable effort on your part to help me. This is the beauty of Sonsivri. I will study and confident that something useful will be there. If you have anything regarding saturable reactor current control system, please inform/upload.

[DTiziano]

Hi DTziano,
I appreciate your Idea. But it is not fully clear to me. If you elaborate in detail, it will be useful for me to grasp.
My design output is DC and it is SMPS type.

I did not find yet the Linear Technology reference.
Anyway, any power supply has set point and feedback, or should be.
Suppose a configuration with “voltage” set point.
Add a current measurement circuit  and a comparator with a threshold set point (current set point) that when the current exceed (current set point) will reduce the voltage set point. That is the basic idea.
A linear simple conceptual implementation can be made by OP AMP (as a comparator) driving an open collector transistor that shunt the voltage set point.
On SMPS the power stage (pulse generation) cut off for one or more cycles may be easy to do and simple to balance (loop stability).

[tAhm1D]

Hi DTiziano,

Your Idea is to Power Control. But I want output current control, keeping the output voltage constant for proper Arc Welding. I have formulated an Idea to do that with the help of Saturable Core Reactor made with Ferrite Torroid and it will be placed after the Main Transformer and before making DC, therefore before the Rectifiers. Hope to post the schematic soon. More Ideas are welcome.

[Taner]

Hi tAhm1D,
I can't understand what is the exactly electrical schematics of your SMPS welding machine.
What type is your switching power supply circuits: Push-Pull (with two power MOSFETs and one center-tapped transformer), Half bridge (with two transistors, two center tapped capacitors and one transformer) or Full bridge (with four transistors and one transformer). Depending on circuits you can use a ready PWM IC's (TL494, SG3524 or SG3525) or some digital switchmode regulator made it with some MCU's (PIC or AVR).
For output current feedback you can use one of the known solutions: current sense resistor, current transformer or Hall-effect sensor. Choice is your.
And you must maintain a constant output current not a voltage. And this can be made with PWM control of the output voltage (by the switchmode power supply circuit).
There is a one paper related with current measurement and current sensors in SMPS:
http://www.4shared.com/file/79637554/be589d7c/Current_Sensing_Solutions.html

Best regards.

[DTiziano]

Hi DTiziano,

Your Idea is to Power Control. But I want output current control, keeping the output voltage constant for proper Arc Welding. I have formulated an Idea to do that with the help of Saturable Core Reactor made with Ferrite Torroid and it will be placed after the Main Transformer and before making DC, therefore before the Rectifiers. Hope to post the schematic soon. More Ideas are welcome.

 

Hi tAhm1D
If the arc characteristic is stable you do not need any current control, constant voltage is ok and produce constant current or vice versa.
I am’t a welding expert, but quite sure they change, even only because the user change the torch distance, the equivalent result is:
- at constant voltage, the current change
- at constant current, the voltage change

I do not know any other condition.
The saturable solution you mention, should use a variable impedance device in series to the load (but for AC line only), and by changing the impedance  change the voltage on the load and so the current.
Tell me if anything wrong in my arguments.

[tAhm1D]

Hi Taner and DTiziano,
I appreciate both of you for your kind effort to help me. I am also not an expert in welding. But I am interested in the project as it will help ultimately in numerous fields in Electronics in controlling current. Let me clarify you all regarding the Modalities of my Project.

It is smps type. First, I will rectify the Line Voltage to DC and then convert it in required voltage by Full Bridge Topology and before again converting it to DC, I will channel this 100khz Ac through saturable core transformer/inductor to ensure required amount of current and then it will be converted to Dc and will be supplied.
For proper Arching, min voltage is required, it should be 40-45v for using any size of welding Rod,but with the reduction of the size of welding rod for welding less sized material, less amount of current is required. That is why, I want to keep the voltage constant to around 40v and current should be changed from 50-150A according to requirement.
Yes, saturable reactor is variable impedence device, a Ferrite Torroid Core which will have the winding of the main wire and at the same time different wiring will be there with Dc controlled supply,which will saturate the core according to requirement to control the flow of current.

Hope, I could make you understand my modality. I am at the final stage of preparation, and it will take some time to finalize.
Your all positive suggestions will be accepted with gratitude.

[okk]

I think I can be of assistance.

I assume that you wish to build a welding power source suitable for MMA (stick electrode) welding. If this is correct the power source will have to provide a roughly constant current output characteristic (I will expand on this later).

For ease of arc striking a no load voltage voltage of >50 volts would be preferable, the arc running voltage will be dependent on a number of factors (I will also expand on this).

My main concern is with the operating frequency of 100Khz.

Unless you intend to make a complex resonant ZVS or ZVC system you will have to limit the frequency to 20-30Khz for an IGBT design. Any higher than this then you will have to use power MOSFET's.

I will post some more information over the weekend

[tAhm1D]

Hi okk,
Yes, you are correct. I intend to use Power Mosfet in ISOTOP Package from ST Microelectronics if I intend to use 100khz Freq or IGBT, if I use 20-30khz. You have understood my concept clearly. I will be waiting for your reply.

[tAhm1D]

Hi All,
I am almost at the final stage of my Project. Here, I have used the Saturable Core Reactor,to control the Output Current Flow. The principle of which I tried to show with the following schematic.

         

Controlling the AC current across the lamp L with the DC current,
regulated by variable resistor R. B- battery, G - AC source.

Still I am facing some problems to finally control the output current. Further suggestions are invited to overcome the problem.

[okk]

Sorry for the delay in replying until now tAhm1D.

With an output current of 150 amps you will easily be able to weld with 3.2mm electrodes and you will just have enough current to weld with 4.0mm electrodes.

The power source will have to provide enough voltage to maintain an arc at the maximum output current. According to specification EN60 974-1 the required voltage at 150 amps will be 26 volts.

This is given by: -

V=0.04 X I_max +20 (for MMA only)

It is possible to to weld with lower output voltages but it becomes more difficult to maintain the welding arc. 

Maximum output power will =  3900 watts.

With an overall conversion efficiency of 80-85% the primary inverter will have to be able to provide about 4500 watts of power (a lot!).

Have you thought of using a half bridge design? I have been involved in 3 phase half bridge designs producing more than 25 kilowatts out. A half bridge design will certainly be easier to implement (at least that's my experience).

One more question, I assume the input voltage will be 230 volts single phase?

[tAhm1D]

Hi okk,
I have studied your reply and found it useful. I am thinking about 230v or 380v Ac input or both. I have planned to use Full Bridge Topology but after your suggestion, will think regarding half bridge also. I have decided to put no load voltage= 45v and with load according to requirement. If you kindly provide me voltage and current requirement for Arc Welding (MMA) according to specification EN60 974-1 or source,from where I can collect the data, it will be of great help for me. I am just calculating basing on datas provided by an Expert welder. Hope for your further help.

[tAhm1D]

Hi Okk,
I have considered using half-bridge topology, but there are some disadvantages to using half-bridge.
Opposed to full-bridge where peak current is 1.4 times nominal current, in half-bridge topology peak current is 2.4 times nominal current. So here the MOSFETs will have to handle greater current.
But the main disadvantage I find is that, half-bridge topology relies greatly on the input bulk capacitors as opposed to full-bridge where  the circuit relies on all the MOSFETs. Thus I find it is riskier to use Big Capacitors at such high powers and MOSFETs are safer and cheaper as well.
So my decision stands at using full-bridge topology. I will be looking forward to your suggestion.

[tAhm1D]

Hi,
Though I am highly inclined to Saturable Core Reactor method to use in Current controlling, I am also getting some knowledge regarding the latest mode, that is Hall Effect Sensors. From bbarney's Link in this Post regarding Allegro Microsystem and the beautiful Book on Hall Effect Sensor posted by good friend ravenfeather in the link:http://www.sonsivri.com/forum/index.php?topic=20093.0  helped me to formulate a concrete Idea regarding current control through Hall Effect Sensor. I am working on it and hope to get more ideas/suggestions in this regard, so that I can utilize those in my Arc welding Project.

[abcsantosh]

Hi tAhm1D, I am also working on inverter power supply for tig/mma welding. Currently I have built the prototype smps of 1500W. Today, I am going to test it and verify all design parameters. I have not yet implemented the linear current control for simplicity. But, sooner or later I am going to work on it. For prototype I am using fix output current control using current sense transformer. The circuit is comprising of UC3879 main control, Ir2110 driver and center tap output configuration. On this center tap winding, I have placed the current transformer. In future, I am planning to change the CT's fix gain to variable gain using Opamp, to achieve variable current control. Yet I have not implemented it, but I thought it is the best method to control output current. As for current sense purpose you can use any method appropriate for your application. But I think, CT is best for large current output.