This is a tough one intelgal. My own advice is to say that you have had Depression. There is no need to go into detail. I would say a couple of things about this...
1.) Most people will be much more understanding than you might anticipate.
2.) Remember that 1 in 4 people will have a mental health problem at some point in their lives, and so a lot of people will either have experience themselves or know someone who has a MH issue.
Now I say this but I know that it is a very tough thing to do and can still remember (though it is over 20 years ago

) how difficult I found it the first time I went back to work after being off with Depression. Even now I am not always open with people about myself at least at first (although I always intend to be!). So if you find the prospect unbearable and it is placing additional burdens on you then I would suggest trying to avoid the topic by saying you'd rather not talk about it. I think actively lying would be likely to place even more strain on you.
I imagine your manager/s at least must know from your sick notes what has been wrong? It is a bit difficult to give specific advice without knowing more about where you work. A good employer should be offering you support and helping you with the return to work process.
Nick.