120VDC Supply from 13.8Vdc

[Jacques St-Pierre]

Anyone can suggest a simple way to design/build a switching supply to
produce 120Vdc at 2 or 3 amps from a 13.8Vdc (car battery) with of the
shelves parts?

I thought using a cheap inverter and rectified the output, but the one I
test produce a 150Vpp square pulse, witch will result in a 150Vdc output. A
bit too high for my application. They do not appear to be adjustable to
reduce output to 120Vpp.

Bye

Jacques St-Pierre

 

[[email protected]]

Anyone can suggest a simple way to design/build a switching supply to
produce 120Vdc at 2 or 3 amps from a 13.8Vdc (car battery) with of the
shelves parts?

I thought using a cheap inverter and rectified the output, but the one I
test produce a 150Vpp square pulse, witch will result in a 150Vdc output. A
bit too high for my application. They do not appear to be adjustable to
reduce output to 120Vpp.

Bye

Jacques St-Pierre

Dangerous, but... give these a try
http://sound.westhost.com/project69.htm

 

[[email protected]]

Anyone can suggest a simple way to design/build a switching supply to
produce 120Vdc at 2 or 3 amps from a 13.8Vdc (car battery) with of the
shelves parts?

I thought using a cheap inverter and rectified the output, but the one I
test produce a 150Vpp square pulse, witch will result in a 150Vdc output. A
bit too high for my application. They do not appear to be adjustable to
reduce output to 120Vpp.

Bye

Jacques St-Pierre

yes, "these" (sorry, my finger slipped)

http://sound.westhost.com/project69.htm
http://sound.westhost.com/project89.htm

"Seriously though, the risk of severe burns and the possibility of
causing a fire in your car are very real, and should not be
underestimated. 300A from a car battery can do a vast amount of
damage in a few milliseconds - should the fuse not blow (you will use
a fuse, won't you?), then the damage can be extensive."

Enjoy

Michael

 

[Joerg]

Anyone can suggest a simple way to design/build a switching supply to
produce 120Vdc at 2 or 3 amps from a 13.8Vdc (car battery) with of the
shelves parts?

I thought using a cheap inverter and rectified the output, but the one I
test produce a 150Vpp square pulse, witch will result in a 150Vdc output. A
bit too high for my application. They do not appear to be adjustable to
reduce output to 120Vpp.

I guess you meant 150Vp, not Vpp (that would be 300Vpp). To make that
work you'd have to take a few windings off the secondary of the big
ferrite transformer in there until you get to 120VDC.

Instead of rectifying it's more efficient to disable and bypass the
bridge that creates the usual "fake" 3-step AC. Results in a little more
efficiency.

 

[Tim Williams]

I thought using a cheap inverter and rectified the output, but the one I
test produce a 150Vpp square pulse, witch will result in a 150Vdc output.
A bit too high for my application. They do not appear to be adjustable to
reduce output to 120Vpp.

Bah, trace the control circuit, they usually use a TL494. Find the feedback
resistors and insert trimmer.

Caveat: they're sensitive to turns ratio because the DC is cap-input
filtered. :-x Adding a choke after the rectifier would be a nice thing to
do for the MOSFETs.

Tim

 

[Jacques St-Pierre]

Bah, trace the control circuit, they usually use a TL494. Find the
feedback resistors and insert trimmer.

I just bought a cheap inverter (Xantrex XPower 300).

Inside, as anticipated, I found a KA7500B, with is an equivalent for TL494,
that control the DC-DC input circuit. Sadly, I did not find any schematics.

It use a transformer to produce the 150Vdc use for the output switching
stage.

I can simply use that DC, removing the output switching MOSFET.

Now, I must find a way to lower the 150Vdc to 120Vdc. Sadly, the feedback
circuit is far than simple. I was hoping to find a simple divisor on pin 1
of the chip, but not; I find a bunch of components, I will be a good work of
reverse engineering to understand how it work.

A simgle resistor return the 150Vdc and drop it to 5Vdc aprox, but as I
said, a bunch of component is locate between that 5Vdc and the pin 1 of the
KA7500B. On pin 1, no DC is present.

So at this point, this is it.

Not done yet, but with more work, I may end up with a 120Vdc supply.

Thanks for the idea.

By the way, it use a 40Amp fuse.

Bye
Jacques

 

[Joerg]

I just bought a cheap inverter (Xantrex XPower 300).

Inside, as anticipated, I found a KA7500B, with is an equivalent for TL494,
that control the DC-DC input circuit. Sadly, I did not find any schematics.

Page 2 shows a schematic of a typical inverter:
http://www.analogzone.com/dearden_042307.pdf

It use a transformer to produce the 150Vdc use for the output switching
stage.

I can simply use that DC, removing the output switching MOSFET.

Now, I must find a way to lower the 150Vdc to 120Vdc. Sadly, the feedback
circuit is far than simple. I was hoping to find a simple divisor on pin 1
of the chip, but not; I find a bunch of components, I will be a good work of
reverse engineering to understand how it work.

A simgle resistor return the 150Vdc and drop it to 5Vdc aprox, but as I
said, a bunch of component is locate between that 5Vdc and the pin 1 of the
KA7500B. On pin 1, no DC is present.

Chan ging any feedback without removing some turns off of the secondary
is going to lead to some grief. Half-bridges don't like that too much.

 

[Jacques St-Pierre]

Chan ging any feedback without removing some turns off of the secondary is
going to lead to some grief. Half-bridges don't like that too much.

Follow up:

I did change the feedback ratio. As anticipated, I was able to lower the
output voltage to 120Vdc as require, but you are right, I should remove a
few turns from secondary.

For now, it is usable to perform some tests on our target system, and isn't
not necessary to correct the transformer situation at this point.

The feedback loop is quite slow, so without removing some secondary turns,
the voltage goes around 155Vdc at power up get back to 120Vdc after a couple
of second. It will not affect my system at this point, so correcting the
transformer winding later should take care of that problem.

Thanks for your help.

Bye
Jacques

 

[Joerg]

Follow up:

I did change the feedback ratio. As anticipated, I was able to lower the
output voltage to 120Vdc as require, but you are right, I should remove a
few turns from secondary.

For now, it is usable to perform some tests on our target system, and isn't
not necessary to correct the transformer situation at this point.

The feedback loop is quite slow, so without removing some secondary turns,
the voltage goes around 155Vdc at power up get back to 120Vdc after a couple
of second. It will not affect my system at this point, so correcting the
transformer winding later should take care of that problem.

Several seconds? That's weird, it should not do that. You can reduce the
output voltage by 13.8V with an easy trick: Connect the negative side of
the output bridge rectifier to ground instead of the 13.8V rail. It
usually sits on the 13.8V rail because that saves the manufacturers a
few cents in ferrite and copper costs.