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Overvoltage protection

P

Piotrne

Jan 1, 1970
0
Hi,

I have a question about controlling a large current in an experimental
welder. The welder should generate a short pulse of current, about
50 ms long. Additionally, it has to be regulated with a simple PWM.

Now, I have following circuit: ATMega32 microcontroller which generates
the PWM signal (2 kHz square wave, turned on for 50 ms);
then an optocoupler; then a MOSFET driver IRS2001 driving
(through a 47 Ohm resistor 47 Ohm) the gate of IRFP4468PbF.
The transistor acts as a key turning on an off the welding current
(source to ground, drain to the burner, the burner to the positive pole
of a rectifier connected to a large transformer: 300A, 50V).
Additionally, there it is also a ionizer with a filter system (designed
by someone else). The microcontroller is powered from its own power
supply, the rest of the control system (driver) is galvanically
separated and also supplied with its own power supply.


The are (at least) two problems with this circuit:
(1) the microcontroller is resetting from time to time,
(2) the transistor is damaged after a single pulse...

The problem (1) is probably caused by the ionizer. I suppose, it can be
solved by closing the circuit in a metal housing.

The problem (2) is more serious. When I turn on the transistor for
the assumed period of 50 milliseconds, it does not turn off.
I had to quickly disconnect the transformer (as the trinsistor does not
stand 300A for a long time). However, the transistor has been damaged...

I suspect that the reason was a high voltage induced at the time of
disconnecting the high current (such moments occur 2000 times/second in PWM).
The sudden power interruption can induce high voltage, which probably
caused the destruction of the transistor.

Does anyone have experience how to avoid such interference? Varistors,
filters, etc.? Experiments are a bit expensive ... The high current part
(transformer, rectifier, ionizer) is ready, I would "only" like
to switch on/off a pulse of current, with adjustable parameters
(hence the microcontroller instead of something simpler).

Regards
P.
 
T

tm

Jan 1, 1970
0
Piotrne said:
Hi,

I have a question about controlling a large current in an experimental
welder. The welder should generate a short pulse of current, about
50 ms long. Additionally, it has to be regulated with a simple PWM.

Now, I have following circuit: ATMega32 microcontroller which generates
the PWM signal (2 kHz square wave, turned on for 50 ms);
then an optocoupler; then a MOSFET driver IRS2001 driving
(through a 47 Ohm resistor 47 Ohm) the gate of IRFP4468PbF.
The transistor acts as a key turning on an off the welding current
(source to ground, drain to the burner, the burner to the positive pole
of a rectifier connected to a large transformer: 300A, 50V).
Additionally, there it is also a ionizer with a filter system (designed
by someone else). The microcontroller is powered from its own power
supply, the rest of the control system (driver) is galvanically
separated and also supplied with its own power supply.


The are (at least) two problems with this circuit:
(1) the microcontroller is resetting from time to time,
(2) the transistor is damaged after a single pulse...

The problem (1) is probably caused by the ionizer. I suppose, it can be
solved by closing the circuit in a metal housing.

The problem (2) is more serious. When I turn on the transistor for
the assumed period of 50 milliseconds, it does not turn off.
I had to quickly disconnect the transformer (as the trinsistor does not
stand 300A for a long time). However, the transistor has been damaged...

I suspect that the reason was a high voltage induced at the time of
disconnecting the high current (such moments occur 2000 times/second in
PWM).
The sudden power interruption can induce high voltage, which probably
caused the destruction of the transistor.

Does anyone have experience how to avoid such interference? Varistors,
filters, etc.? Experiments are a bit expensive ... The high current part
(transformer, rectifier, ionizer) is ready, I would "only" like
to switch on/off a pulse of current, with adjustable parameters
(hence the microcontroller instead of something simpler).

Regards
P.

You need to consider the "safe operating area" graph for that device. See
the data sheet.

You are outside of the limits.


Regards
tm
 
P

Piotrne

Jan 1, 1970
0
tm said:
You need to consider the "safe operating area" graph for that device.
See the data sheet. You are outside of the limits.

Of course that is the reason: I'm out of the limits.
But how to prevent it? The transistor is for sure able to work
for 50 ms at 300 A (being fully "on"). So, the problem may
occur at the end of the pulse. I'm not able to measure it,
because (1) it is too short, (2) it may damage my oscilloscope,
(3) each try damages a transistor (10$). And I think, that designers
of welders know how to solve the problem...

Regards
P.
 
R

Robert Baer

Jan 1, 1970
0
Piotrne said:
Hi,

I have a question about controlling a large current in an experimental
welder. The welder should generate a short pulse of current, about
50 ms long. Additionally, it has to be regulated with a simple PWM.

Now, I have following circuit: ATMega32 microcontroller which generates
the PWM signal (2 kHz square wave, turned on for 50 ms);
then an optocoupler; then a MOSFET driver IRS2001 driving
(through a 47 Ohm resistor 47 Ohm) the gate of IRFP4468PbF.
The transistor acts as a key turning on an off the welding current
(source to ground, drain to the burner, the burner to the positive pole
of a rectifier connected to a large transformer: 300A, 50V).
Additionally, there it is also a ionizer with a filter system (designed
by someone else). The microcontroller is powered from its own power
supply, the rest of the control system (driver) is galvanically
separated and also supplied with its own power supply.


The are (at least) two problems with this circuit:
(1) the microcontroller is resetting from time to time,
(2) the transistor is damaged after a single pulse...

The problem (1) is probably caused by the ionizer. I suppose, it can be
solved by closing the circuit in a metal housing.

The problem (2) is more serious. When I turn on the transistor for
the assumed period of 50 milliseconds, it does not turn off.
I had to quickly disconnect the transformer (as the trinsistor does not
stand 300A for a long time). However, the transistor has been damaged...

I suspect that the reason was a high voltage induced at the time of
disconnecting the high current (such moments occur 2000 times/second in
PWM).
The sudden power interruption can induce high voltage, which probably
caused the destruction of the transistor.

Does anyone have experience how to avoid such interference? Varistors,
filters, etc.? Experiments are a bit expensive ... The high current part
(transformer, rectifier, ionizer) is ready, I would "only" like
to switch on/off a pulse of current, with adjustable parameters
(hence the microcontroller instead of something simpler).

Regards
P.
Methinks the operation of the FET is beyond the I*I*T rating as well
as the voltage rating at transformer turnoff.
 
M

mike

Jan 1, 1970
0
Of course that is the reason: I'm out of the limits.
But how to prevent it? The transistor is for sure able to work
for 50 ms at 300 A (being fully "on"). So, the problem may
occur at the end of the pulse. I'm not able to measure it,
because (1) it is too short, (2) it may damage my oscilloscope,
(3) each try damages a transistor (10$). And I think, that designers
of welders know how to solve the problem...

Regards
P.
Not clear what you're trying to accomplish. The devil will be in the
details.

I spent several years tinkering on and off with a welder for battery
tabs. Didn't get very far.
Then I acquired a digital storage scope and a current probe.
Had it working a lot better in a day.
There's no substitute for making measurements to see what is
actually happening.

Again, depends on what you're welding, but for me, constant current
was not what I wanted. The resistance of the weldment and the contacts
and...and...and make a significant difference is how much energy ends
up where you want it.

Switching to constant energy by discharging high voltage from a cap
thru an SCR
into a transformer dramatically increased the reliability of the welds.
That's how the CD welders do it. Of course, the transformer
is an art.
 
T

tm

Jan 1, 1970
0
Piotrne said:
Of course that is the reason: I'm out of the limits.
But how to prevent it? The transistor is for sure able to work
for 50 ms at 300 A (being fully "on"). So, the problem may
occur at the end of the pulse. I'm not able to measure it,
because (1) it is too short, (2) it may damage my oscilloscope,
(3) each try damages a transistor (10$). And I think, that designers
of welders know how to solve the problem...

Regards
P.

Save some money while you are learning about power electronics :)

http://www.ebay.com/itm/IRFP4468-Or...865?pt=LH_DefaultDomain_0&hash=item5d39edf611

Less than $1.50 each. Maybe they are real.

I don't think you are turning the device fully on fast enough. It has an
input C of about 0.02 uF. Can you reduce the 47 ohms?


Good luck.
 
J

Jamie

Jan 1, 1970
0
Piotrne said:
Hi,

I have a question about controlling a large current in an experimental
welder. The welder should generate a short pulse of current, about
50 ms long. Additionally, it has to be regulated with a simple PWM.

Now, I have following circuit: ATMega32 microcontroller which generates
the PWM signal (2 kHz square wave, turned on for 50 ms);
then an optocoupler; then a MOSFET driver IRS2001 driving
(through a 47 Ohm resistor 47 Ohm) the gate of IRFP4468PbF.
The transistor acts as a key turning on an off the welding current
(source to ground, drain to the burner, the burner to the positive pole
of a rectifier connected to a large transformer: 300A, 50V).
Additionally, there it is also a ionizer with a filter system (designed
by someone else). The microcontroller is powered from its own power
supply, the rest of the control system (driver) is galvanically
separated and also supplied with its own power supply.


The are (at least) two problems with this circuit:
(1) the microcontroller is resetting from time to time,
(2) the transistor is damaged after a single pulse...

The problem (1) is probably caused by the ionizer. I suppose, it can be
solved by closing the circuit in a metal housing.

The problem (2) is more serious. When I turn on the transistor for
the assumed period of 50 milliseconds, it does not turn off.
I had to quickly disconnect the transformer (as the trinsistor does not
stand 300A for a long time). However, the transistor has been damaged...

I suspect that the reason was a high voltage induced at the time of
disconnecting the high current (such moments occur 2000 times/second in
PWM).
The sudden power interruption can induce high voltage, which probably
caused the destruction of the transistor.

Does anyone have experience how to avoid such interference? Varistors,
filters, etc.? Experiments are a bit expensive ... The high current part
(transformer, rectifier, ionizer) is ready, I would "only" like
to switch on/off a pulse of current, with adjustable parameters
(hence the microcontroller instead of something simpler).

Regards
P.

Not sure what you mean by the "Burner"? I can only assume you are
referring to the electrodes?

In any case, I am willing to bet that you are suffering from induction
kick back.

Basically you are using 50VDC, but the release of this current with
your pulse generating many more times that on the drain of your MFET..

Don't get fooled with that ZENER symbol for the foot print.. It does
not do what you may think and the current in this return pulse of HV
does not need to be high to damage the fet. Looking at the Cap rating
on the gate, I would say it would be easy for it to short through the
gate.

You may want to report in which manner the fet shorted? Gate to drain?

Source to Drain only ? etc.. This would better tell the story.

Maybe you should be considering a snubber around the FET.

P.S.

THe pulse current ratings of over 1k amps are only good for less than
400us.

Jamie
 
P

Piotrne

Jan 1, 1970
0
Jamie said:
Not sure what you mean by the "Burner"?
I can only assume you are referring to the electrodes?

Electrodes, of course (they are not called "burner" by welders?)
In any case, I am willing to bet that you are suffering from induction
kick back.

I have simulated it in PSpice (SwitcherCAD) and it shows me pulses
of about 45kV... Very bad. It also destroyed 4 drivers (IRS2001) in
2 experiments - a bit strange, because not all drivers were used.
Source to Drain only ? etc.. This would better tell the story.

Well, the transistor from the second experiment has shortened D to S only,
the first (which I did not expect to destroy and turned off the circuit
a few seconds later) is totally "thermally" destroyed.

I have replaced the driver with an IR2151 (which required to modify
the generated PWM), reduced the resistor to 10 Ohm and asked
the owner of the current source to complete it with an overvoltage
protection (varistor + capacitor). I'll report results...

Thank you
P.
 
T

tm

Jan 1, 1970
0
Piotrne said:
Electrodes, of course (they are not called "burner" by welders?)


I have simulated it in PSpice (SwitcherCAD) and it shows me pulses
of about 45kV... Very bad. It also destroyed 4 drivers (IRS2001) in
2 experiments - a bit strange, because not all drivers were used.


Well, the transistor from the second experiment has shortened D to S only,
the first (which I did not expect to destroy and turned off the circuit
a few seconds later) is totally "thermally" destroyed.

I have replaced the driver with an IR2151 (which required to modify
the generated PWM), reduced the resistor to 10 Ohm and asked
the owner of the current source to complete it with an overvoltage
protection (varistor + capacitor). I'll report results...

Thank you
P.

Is this for a TIG machine? If so, is it using HF?

Can you put up a schematic of your design somewhere?

tm
 
Jamie wrote:







Electrodes, of course (they are not called "burner" by welders?)







I have simulated it in PSpice (SwitcherCAD) and it shows me pulses

of about 45kV... Very bad. It also destroyed 4 drivers (IRS2001) in

2 experiments - a bit strange, because not all drivers were used.






Well, the transistor from the second experiment has shortened D to S only,

the first (which I did not expect to destroy and turned off the circuit

a few seconds later) is totally "thermally" destroyed.



I have replaced the driver with an IR2151 (which required to modify

the generated PWM), reduced the resistor to 10 Ohm and asked

the owner of the current source to complete it with an overvoltage

protection (varistor + capacitor). I'll report results...



Thank you

P.

How is the transformer configured? You mention just one diode, so is it just halfwave rectified making for only DC loading of the secondary? Once youget past the transient overvoltage issue, you may be confronted with a saturated core short circuit issue.
 
P

Piotrne

Jan 1, 1970
0
bloggs.fredbloggs.fred said:
How is the transformer configured? You mention just one diode, so is it just halfwave
rectified making for only DC loading of the secondary?

It is a 3-phase transformer with a fullwave rectifier consisting of 90 diodes
mounted on 6 aluminum plates (radiators). It provides a nice, slighlty
wavy voltage which does not go to 0.

The whole device should be welding small (thin) elements by melting only
their surface. Therefore, such short pulses of current should be used.

Today I have been trying to get rid of the interference caused
by the ionizer. It simply resets the ATMega32 each time when
the ionizer is turned on. It even influences separately the LCD
display (2*20 characters) and the microcontroller: sometimes
only the LCD shows strange thing, while the microcontroller
keeps working and sometimes the microcontroller resets.
Originally, the ionizer was turned on by the microcontroller
(through a small, optically isolated circuit) and I suspected,
that the distance between wires going to the ionizer and
the microcontroller are too close (of course, the ionizer
is turned on and off at the primary side, 230V).
But even after completely disconnecting these two devices
(microcontroller and ionizer) and keeping only common
power supply, the problem persisted. Finally, a filter
inserted between the mains and the ionizer removed the problem.
Unfortunately, coming closer with other wires (the high current
circuit) brought the problem back, even without turning on
the current. So, it has to be solved too...

I'll report the progress :)


Regards
P.
 
J

Jamie

Jan 1, 1970
0
Piotrne said:
Electrodes, of course (they are not called "burner" by welders?)



I have simulated it in PSpice (SwitcherCAD) and it shows me pulses
of about 45kV... Very bad. It also destroyed 4 drivers (IRS2001) in
2 experiments - a bit strange, because not all drivers were used.



Well, the transistor from the second experiment has shortened D to S only,
the first (which I did not expect to destroy and turned off the circuit
a few seconds later) is totally "thermally" destroyed.

I have replaced the driver with an IR2151 (which required to modify
the generated PWM), reduced the resistor to 10 Ohm and asked
the owner of the current source to complete it with an overvoltage
protection (varistor + capacitor). I'll report results...

Thank you
P.
is this a spot or arc welder?

Jamie
 
bloggs.fredbloggs.fred wrote:







It is a 3-phase transformer with a fullwave rectifier consisting of 90 diodes

mounted on 6 aluminum plates (radiators). It provides a nice, slighlty

wavy voltage which does not go to 0.



The whole device should be welding small (thin) elements by melting only

their surface. Therefore, such short pulses of current should be used.



Today I have been trying to get rid of the interference caused

by the ionizer. It simply resets the ATMega32 each time when

the ionizer is turned on. It even influences separately the LCD

display (2*20 characters) and the microcontroller: sometimes

only the LCD shows strange thing, while the microcontroller

keeps working and sometimes the microcontroller resets.

Originally, the ionizer was turned on by the microcontroller

(through a small, optically isolated circuit) and I suspected,

that the distance between wires going to the ionizer and

the microcontroller are too close (of course, the ionizer

is turned on and off at the primary side, 230V).

But even after completely disconnecting these two devices

(microcontroller and ionizer) and keeping only common

power supply, the problem persisted. Finally, a filter

inserted between the mains and the ionizer removed the problem.

Unfortunately, coming closer with other wires (the high current

circuit) brought the problem back, even without turning on

the current. So, it has to be solved too...



I'll report the progress :)





Regards

P.

There are industrial line transient suppression devices based on the ckt shown below. The advantage is it's a non-magnetics solution, simply parallel your rectifier with another that charges a large filter cap to peak line, then that becomes your high voltage spike clamp level:

Please view in a fixed-width font such as Courier.

..
..
..
..
..
.. ---------
.. | |
.. >----------------+--|PH1 +|------------>
.. | | | TO HIGH
.. 3PHAC >-------------+--|--|PH2 | CURRENT
.. | | | | LOAD
.. >----------+--|--|--|PH3 |
.. | | | | -|------+----->
.. | | | | | |
.. | | | --------- ---
.. | | | ///
.. | | |
.. | | |
.. | | | ---------
.. | | | | |
.. | | '--|PH1 +|-------.
.. | | | | + |
.. | '-----|PH2 | ---
.. | | | ---
.. '--------|PH3 | |
.. | -|-------+
.. | | |
.. --------- ---
.. ///
..
 
F

Fred Abse

Jan 1, 1970
0
The whole device should be welding small (thin) elements by melting only
their surface. Therefore, such short pulses of current should be used.

How do you weld anything by melting only the surface?

Sounds like the welding equivalent of a dry soldered joint,

Welding needs penetration.
 
P

Piotrne

Jan 1, 1970
0
tm said:
Is this for a TIG machine? If so, is it using HF?

It is an experimental automatic welder, following the idea
of TIG (with shielding gas - argon etc.). I don't know
the details. My task was to design a controlling circuit
which lets the user to set the duration of pulses,
PWM parameters and some delays (the process should be
repeated). Now, the controller generates all signals
(this was checked separately - without the welding part,
only turning on/off a small load). After connecting
the ionizer, the microcontroller goes crazy after
about 5 seconds of electric sparks.
Can you put up a schematic of your design somewhere?

Here is a simplified schematic (hand drawn):
http://piotrne.republika.pl/welcontroller1.jpg

The "ionizer" is a factory made device to ignite electric
arcs in welders. It should be connected through a transformer
(e.g. 24:3 windings). The secondary winding (connected
in series) adds HV pulses to the circuit. The circuit
for these pulses should be closed by R and C (just above
the ionizer on the schematic). Precise parameters are not
known. But it generates "thick" sparks about 5 mm long, at the
frequency of the mains (50 Hz). It may be about 7 kV.

And, these HV pulses are getting somehow trough the
"high current" wires to the IRFP transistor AND, through
the optocoupler (!), to the microcontroller. And resets it.
After disconnecting two wires (G and D of the transistor)
the microcontroller does not reset.

Regards
P.
 
T

tm

Jan 1, 1970
0
Piotrne said:
It is an experimental automatic welder, following the idea
of TIG (with shielding gas - argon etc.). I don't know
the details. My task was to design a controlling circuit
which lets the user to set the duration of pulses,
PWM parameters and some delays (the process should be
repeated). Now, the controller generates all signals
(this was checked separately - without the welding part,
only turning on/off a small load). After connecting
the ionizer, the microcontroller goes crazy after
about 5 seconds of electric sparks.


Here is a simplified schematic (hand drawn):
http://piotrne.republika.pl/welcontroller1.jpg

The "ionizer" is a factory made device to ignite electric
arcs in welders. It should be connected through a transformer
(e.g. 24:3 windings). The secondary winding (connected
in series) adds HV pulses to the circuit. The circuit
for these pulses should be closed by R and C (just above
the ionizer on the schematic). Precise parameters are not
known. But it generates "thick" sparks about 5 mm long, at the
frequency of the mains (50 Hz). It may be about 7 kV.

And, these HV pulses are getting somehow trough the
"high current" wires to the IRFP transistor AND, through
the optocoupler (!), to the microcontroller. And resets it.
After disconnecting two wires (G and D of the transistor)
the microcontroller does not reset.

Regards
P.

I'll add more to this later as real work invades play.

Why don't you consider using three or six triacs in the AC input to the FW
bridge? Or even as the bridge? Then you can stay away from the very high DC
currents. Also, it would use pulse transformers to drive the gates, giving
additional isolation from the HF junk.

Or even better, use an inverter tig welding power source? Most of them have
a low voltage control input that allows doing exactly what you are after.
Not only that, the pulser device is readily available OTS. Some welding
sources include the pulser built in.


Regards,
tm
 
J

josephkk

Jan 1, 1970
0
How do you weld anything by melting only the surface?

Sounds like the welding equivalent of a dry soldered joint,

Welding needs penetration.

Check into battery tab welders.

?-)
 
J

josephkk

Jan 1, 1970
0
It is an experimental automatic welder, following the idea
of TIG (with shielding gas - argon etc.). I don't know
the details. My task was to design a controlling circuit
which lets the user to set the duration of pulses,
PWM parameters and some delays (the process should be
repeated). Now, the controller generates all signals
(this was checked separately - without the welding part,
only turning on/off a small load). After connecting
the ionizer, the microcontroller goes crazy after
about 5 seconds of electric sparks.


Here is a simplified schematic (hand drawn):
http://piotrne.republika.pl/welcontroller1.jpg

The "ionizer" is a factory made device to ignite electric
arcs in welders. It should be connected through a transformer
(e.g. 24:3 windings). The secondary winding (connected
in series) adds HV pulses to the circuit. The circuit
for these pulses should be closed by R and C (just above
the ionizer on the schematic). Precise parameters are not
known. But it generates "thick" sparks about 5 mm long, at the
frequency of the mains (50 Hz). It may be about 7 kV.

And, these HV pulses are getting somehow trough the
"high current" wires to the IRFP transistor AND, through
the optocoupler (!), to the microcontroller. And resets it.
After disconnecting two wires (G and D of the transistor)
the microcontroller does not reset.

Regards
P.

From a simplistic point of view, is there some way to move the ioniser to
the other electrode? Placed on the FET switch side it will leak HV pulses
into the FET no matter what you do.

?-)
 
F

Fred Abse

Jan 1, 1970
0
Check into battery tab welders.

?-)

Still needs full penetration through the tab, partial penetration into
the terminal.

Not exactly easy to control.
 
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