How to make good quality onion powder at home?
It's likely that your oven, even on the coolest setting, is too hot. That's why the onion burnt, and the bitterness comes from the burnt bits. 

I occasionally use my electric oven for dehydrating fruit and veg but mine goes down to 50&deg;C. I tend to preheat it to about 100&deg;C and turn it down when the food had been in for a few minutes, at which point I open the door as well.

A gas oven has another problem - burning gas produces water vapour, which makes drying harder even with ventilation (you really do need the door ajar to let the steam out). Drying on a rack (borrowed from a grill pan, or a cake cooling rack) makes a huge difference.

I also have a dehydrator, but it's not all that  much better than the oven for a small batch in winter, so I leave it stored away except when I've got a lot to dry. It wins on capacity and the fact I can run it outside - worthwhile for chillies and presumably onions.