* Qatar Solar to invest 35 million euros, become shareholder
* Existing shareholders will own 5 percent of group
* SolarWorld shares down 16 percent (Adds details on debt deal)
FRANKFURT, June 18 (Reuters) - SolarWorld still needs the backing of some bondholders for a wide-ranging debt restructuring that would make Qatar a major new shareholder in the company.
SolarWorld, once Germany's largest solar energy company and 27.84 percent owned by its chief executive and founder, Frank Asbeck, said on Tuesday it had already secured the support for the plans from some creditors.
But holders of its two convertible bonds , with a combined volume of 550 million euros ($734 million), have not yet agreed to the plans.
They will meet on July 8 and 9, respectively, to appoint a joint representative and subsequently decide on the plan.
Western manufacturers of solar power equipment, particularly in Germany, have come under intense pressure from overcapacity in the solar industry, which has forced many big players such as Q-Cells and Solon, to file for insolvency.
SolarWorld, long immune to the sector's crisis, in January warned creditors of a debt restructuring.
Under the debt restructuring deal, Qatar Solar will invest 35 million euros in SolarWorld, becoming a 29-percent shareholder in the group, while existing shareholders will end up with 5 percent.
Qatar Solar is owned by the Qatar Foundation and owns 70 percent in Qatar Solar Technologies (QSTec), a joint venture with SolarWorld, which owns 29 percent.
As part of the deal, Asbeck will invest 10 million euros of private funds, giving him a 19.5 percent stake.
SolarWorld shares were down more than 16 percent by 1402 GMT. ($1 = 0.7492 euros) (Reporting by Christoph Steitz and Anneli Palmen. Editing by Jane Merriman)