120VDC Supply from 13.8Vdc

Dangerous, but... give these a try

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

"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

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.

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

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

Page 2 shows a schematic of a typical inverter:

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

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.