Voltage creep way beyond turns ratio

Thread Starter

MGPERF

Joined Jun 8, 2021
64
Hello, I made a crude inverter without voltage regulation. When there is no load I often get voltages over twice what the voltage should be according to the transformer turns ratio. Any idea what could be going on?

Thanks,
Jon
 

MisterBill2

Joined Jan 23, 2018
10,602
Yes, I am aware of the excess voltage problem. What happens is that there are spikes in the primary current that are vary narrow, but also higher than the supply voltage. With no loading these spikes are passed through the transformer, stepped up, and also rectified and the result is a much higher voltage on the capacitors.

This problem has been a source of grief in inverter high voltage power supplies for quite a while. So you are not alone here.

The solution is not simple and usually requires better magnetic coupling in the transformer, in addition to more effective spike prevention in the driving circuits. So it is by no means a small task.

There have been some solutions published, I did not follow them because I do not normally design such supplies.
 

Thread Starter

MGPERF

Joined Jun 8, 2021
64
That makes a ton of sense! It will eventually blow past the SCR which is the only real problem with it other than safety. I will put a 1 KV rated SCR on the device that should honestly fix the issues. I am also going to add a bleeder resistor. It takes several seconds for the capacitor (4.7uf) to saturate with 800 volts.
 
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MisterBill2

Joined Jan 23, 2018
10,602
That makes a ton of sense! It will eventually blow past the SCR which is the only real problem with it other than safety. I will put a 1 KV rated SCR on the device that should honestly fix the issues. I am also going to add a bleeder resistor. It takes several seconds for the capacitor (4.7uf) to saturate with 800 volts.
I suggest monitoring closely the first time. A 2000 volt supply, while experimenting, went up to over 3000 volts no load. Capacitor breakdown failure happens quickly.
 

Thread Starter

MGPERF

Joined Jun 8, 2021
64
Thanks, would decreasing the frequency help reduce the voltage spikes? I am running at 200 khz (by accident just realized that now) however I could turn it down to 20 khz easily by swapping out a resistor. My power supply is very redundant and be plenty powerful enough at 20 khz.
 
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MisterBill2

Joined Jan 23, 2018
10,602
No telling what the effect will be. I have read and reread the one article about an inverter type high voltage supply and it described both the cause of those spikes and that designer's solution, which, among other things, was a transformer core with a lot more material than the normal calculations require, and much better magnetic coupling between the primary and the secondary, which evidently reduces the leakage inductance. That designer blames leakage inductance as a major source of the spikes, the explanation given did not enlighten me fully. But now you have some more clues, and directions to examine.

Does your design use a standard transformer or one of your creation? If you have created it then it will be simpler to change.
One useful concept, which Motorola has used since the era of tube radios, is instead of a single high voltage winding and high voltage diodes, three separate windings, each with it's own rectifiers and capacitors, all in series to provide the high voltage. Four windings, each producing about 450 volts DC, in series, allows the use of lower voltage rated parts and makes it more reliable and less expensive as well. Much less stress on the diodes and capacitors and a much bigger realm to select from.
 

Ian0

Joined Aug 7, 2020
4,169
An inductor on the output between the rectifier and the capacitor should deal with them (provided that it doesn't have too much self-capacitance, which is often the case with high-voltage high-inductance components with lots of turns of fine wire).
I'd agree with @MisterBill2 on the Motorola design.

The spikes are flyback pulses cause by poor coupling in the transformer. I presume you have two primary windings. Are they wound bifilar, or concentric, or one either side of the secondary?
A H-bridge circuit might work better.
 

MisterBill2

Joined Jan 23, 2018
10,602
An inductor on the output between the rectifier and the capacitor should deal with them (provided that it doesn't have too much self-capacitance, which is often the case with high-voltage high-inductance components with lots of turns of fine wire).
I'd agree with @MisterBill2 on the Motorola design.

The spikes are flyback pulses cause by poor coupling in the transformer. I presume you have two primary windings. Are they wound bifilar, or concentric, or one either side of the secondary?
A H-bridge circuit might work better.
Leakage inductance and poor coupling between windings are the two sides of the same coin! That may be the motivation for splitting the secondary into multiple sections. (see post #7) and also for adding quite a bit more material to the core, so that more of the magnetic flux passes through all of the secondary. We can be sure that there is a benefit if somebody who knows goes to the extra effort and expense to make a better transformer.
 

Thread Starter

MGPERF

Joined Jun 8, 2021
64
I just have a center tapped EE42 ferrite core transformer. Unfortunately I shorted that and have to rewind it. By turning down the frequency I was able to get the voltage spikes down to manageable levels. I will try adding an inductor next.
 
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