strange noise problem in multilevel inverter

[kappa_am]

hi,
I designed and implement a flying capacitor multilevel inverter about one year ago. all things were ok. today I tried to run it again and test a fault ride-through strategy on it, but I saw strange thing the waveform was too noisy. I have no Idea why this is happening. The circuit does not change just some wire and terminals are added. below you can find the voltage waveform. the white-background is ok and is taken about one year ago, and the other one is new one.

Thank you for your help.


P.S: all the DC-link voltages are regulated. so the has not to be such spikes.

 

[KlausST]

Hi,

You say multilevel....

Is it possible that some levels are mixed up?

Klaus

 

[Orson Cart]

Something has changed, undo your recent little modifications and re-test...

- - - Updated - - -

Also you may be measuring differently, different load and or grounding...

 

[kappa_am]

Hi,
the level is missed up when the DC voltages are not regulated. the regulating method is ok and the capacitor voltages are well regulated.
the problem is solved when I tried this morning! it needed a one-night sleep!!!!
still I cannot figure out what the problem was. it did not work why the capacitor voltages were regulated; it needs proper firing pulses; so the circuit in my opinion was ok! something with measuring so. but there was any thing change with measuring still has not been change and it is ok now!!
magic of noise!!

 

[kappa_am]

it's back!!! exhausted.

 

[BradtheRad]

flying capacitor

This is the topology that switches the capacitor end-for-end, I believe. At any time, do you allow a component to have no connection to anything? That might create an opening for static charge to get into the act. (Example, a mosfet gate left floating could pick up ambient mains hum, causing it to assume unpredictable states.)

 

[mtwieg]

Without anything else to go on, I suspect a bad solder joint, or a cracked capacitor. Do a thorough visual inspection of every joint, and see if putting stress on the pcb has an effect. Be prepared for something to blow up if you hit it just "right."

 

[Orson Cart]

Yup, sounds like a bad connection somewhere, either on the power stage, or in the measuring...

 

[kappa_am]

I checked boards and terminals washed all sockets; nothing!!!
when the circuit works for a while the problem gets worse.
don't you think I have capacitor aging problem?

 

[Orson Cart]

if the current is high the caps can degrade due to separation from end coatings, but the fact it works sometimes, e.g. after an overnight rest, indicates something else...

 

[D.A.(Tony)Stewart]

A schematic and BOM would show more details with layout photo. Commutation noise is greatly affected by ESR of source Caps, Switched Caps and effective load impedance with transients from dead-time inductive currents.