Irvine-based Microsemi has made another acquisition, saying Monday that it has acquired Nexsem, another Orange County-based semiconductor manufacturing firm. Microsemi did not disclose terms of the deal. Nexsem makes high voltace, DC-to-DC conversion devices used for set top boxes, LCD TVs, notebooks, and other electronics products. Microsemi said the buy would complement the firm's existing power management offerings. The acquisition is the third for Microsemi in recent months; the firm bought both the Defense & Security Business of Endwave Corporation and the Space Level Power Products business of Spectrum Microwave in April.