TOKYO, March 25 (Reuters) - Singapore-based hedge fund Effissimo Capital Management said on Thursday its stake in Japan’s Sanken Electric Co would increase to 19.93% as a result of a tender offer. Effissimo, which already held nearly 10% of Sanken, had planned to buy an additional 20% of the power-management chipmaker through the tender offer that closed on Wednesday, but only managed to get half of that.