Laptop PSU not working on inverter

[Diver300]

I've been helping a friend to get a laptop PSU to work with his inverter. The inverter is "modified sine wave", so there are just three voltages, peak voltage forward, peak voltage reverse, and zero volts. The inverter seems to adjust the duty cycle so that the RMS voltage is 240 V. The peak voltage is very close to the peak voltage for a 240 V sine wave, but it does depend on the input voltage.

The laptop PSU is rated 110 - 240 V and it won't turn on when connected to this inverter.

It is fine on a very similar inverter, also "modified sine wave" and with a duty cycle that adjusts to keep the RMS voltage correct. This one has a slightly lower peak voltage, but the duty cycle is larger so the RMS voltage is the same.

I suspect that the laptop PSU doesn't work on the first inverter because the lower duty cycle means that the output is off for longer each cycle, and this longer off period makes the laptop PSU think the supply is intermittent.

Any other ideas?

For now I have connected the laptop PSU to the dc link voltage at 360 v dc, and it works fine. That is no surprise because all laptop PSUs that I have seen have a bridge rectifier on the input.

 

[alec_t]

Bear in mind that only 2 diodes of the bridge will conduct if the input is DC, so will have to pass a higher average current than usual.

 

[Diver300]

Bear in mind that only 2 diodes of the bridge will conduct if the input is DC, so will have to pass a higher average current than usual.

That is true to a small extent. However, the current will be constant, not peaking like it would with a sinusoid supply. Also the laptop PSU is rated down to 110 V ac, where it would be taking a lot more current than it would with 360 V dc, so at 360 V dc the current will be less than the average current for each diode at 110 V ac.

Does anyone have any theories as to why the PSU wasn't working with the "modified sine wave" from the inverter.

 

[AnalogKid]

The reduced peak voltage might not be high enough to satisfy an undervoltage lockout protection circuit. This is a built-in function of many switching power supply control chips.

ak

 

[Diver300]

The reduced peak voltage might not be high enough to satisfy an undervoltage lockout protection circuit. This is a built-in function of many switching power supply control chips.

ak

Actually, it functioned the other way round. The laptop PSU worked on an inverter with a lower peak voltage, and not on the one with a higher peak voltage. Also it worked fine when connected to the DC link voltage of the inverter with the higher peak voltage. That DC link voltage must have been slightly higher still.

Of course all those voltages changed in response to changes in the 12 V supply, but there was no change to the behaviour of the PSU, so I don't think that the voltage was marginal.

Finally, the laptop PSU was rated 110 - 240 V ac, and all the inverters were 240 V, so the peak voltages were over 300 V, so under-voltage lockout just on peak seems unlikely.

I suspect the PSU has some cunning circuit to turn off if the supply was interrupted, and it was seeing the gaps in the modified sine wave as interruptions.

 

[dr pepper]

I have the same issue with one of mine, if I try a dozen times eventually the supply will fire up, I wondered if it was the thermistor in the front end of the supply warming up or something.