Ups pwm issue while repair

[Gaber Mohamed Boraey]

Hello everyone

While I repair 700 watt ups I find shorted micro controller number Hr7p195fgs3

Datasheet here

https://m.findic.us/doc_view/48396126

I mean with shorted “ have short circuit “ , I discovered that with voltage injection, as had 5v should be there written on pcb and when work only 1.2v , so I objected 5v and found high temperature at the controller, I’ve changed with same number from similar board, “ as we have big stock”, but not same board, and it worked fine and solved the problem

For ups check I had to measure the output with true rms Fluke Multimeter, and the reading is about 245v “ high for ups “? , right? , I thought something wrong in the board and put new board instead and the output is 230v, i had same device i checked its output and its 550vac
How you think?, bad ups board and design?

when measure frequencies at the mosfets low side and high side, pwm frequency is 50hz at two mosfets and 100hz at two others , I thought always that’s frequencies at mosfets should be Same, can anyone explain to me the switching in ups?


The output is modified squares wave by the way, not sine wave signal

 

[betwixt]

Modified sine inverters do not use PWM, they generate a DC voltage approximately equal to the average AC output then alternate its connection to the output pins, usually with a dead period between polarities. Switching at the AC output frequency is normal, why you have 100Hz and 50Hz I'm not sure but it is probably to do with the polarity swapping so that 100Hz square wave cycles swap at 50Hz rate.

Brian.

 

[Gaber Mohamed Boraey]

Modified sine inverters do not use PWM, they generate a DC voltage approximately equal to the average AC output then alternate its connection to the output pins,

Execuse me , don’t understand this part , can you clarify more?, as i always thought it’s pwm signals goes to the mosfets gates , also the controller data sheet output pins are pwm signals, and they go to gates
If generate dc voltages , where the part responsible for generating the voltage?

 

[KlausST]

Hi,

you might look up "modified sine" vs "true sine" inverters.

Klaus

 

[betwixt]

In PWM the 'M' stands for Modulation, the ratio of off to on time is constantly changed (modulated) to make the output voltage rise and fall, producing the AC waveform you need.

"Modified sine" isn't a sine wave at all but it approximates to one. Instead of following a sine curve, the output connections follow this pattern '+ -', zero '- +', zero, '+ -' and so on. It is really a square wave with the polarity being reversed on alternate half cycles at line frequency rate. It works for many applications but the sharp rise and fall of the waveform edges means some reactive loads may not work properly. For example, lighting might be fine but a motor might be very noisy and overheat.

Obviously modified sine is cheaper to manufacture, you only need to produce one DC voltage and drive the output connections across a H-Bridge driven at line frequency. There is a variant where the input voltage (12V for example) is converted to a square wave then stepped up with a transformer but these are usually called push-pull inverters and are relatively inefficient and bulky for the amount of power they produce.

Brian.