You have to transfer your SBD to STEEM, then from STEEM you can power up as much or as little as you want. Hope that helps, (p.s. I think it takes a few days for SBD to transfer to STEEM). I have done it a few times.

Go to your wallet, click on the Steem Dollars, and hit "Convert to STEEM" and convert as much of your SBD (steem dolloars) into STEEM, the STEEM can be used to Power Up.