Exhibit 3.3

                                VICOR CORPORATION
                           Certificate of Amendment of
                      Restated Certificate of Incorporation

      Vicor Corporation, a corporation organized and existing under and by
virtue of the General Corporation Law of the State of Delaware, does hereby
certify:

      FIRST: That the Board of Directors of said Corporation, at a meeting duly
called and held, adopted resolutions setting forth a proposed amendment to the
Restated Certificate of Incorporation of said Corporation, declaring said
amendment to be advisable and submitting said amendment for consideration by the
stockholders of said corporation. The resolution setting forth the proposed
amendment is as follows:

      RESOLVED:   That the first paragraph of Article FOURTH of the Restated
                  Certificate of Incorporation of the Corporation be amended so
                  that, as amended said first paragraph of Article FOURTH shall
                  be and read as follows:

                  "FOURTH: The total number of shares of capital stock which the
                  Corporation shall have authority to issue shall be thirty-six
                  million (36,000,000) shares, of which twenty-six million five
                  hundred thousand (26,500,000) shall be Common Stock, par value
                  $.01 per share ("Common Stock"), eight million five hundred
                  thousand (8,500,000) shall be Class B Common Stock, par value
                  $.01 per share ("Class B Common Stock") and one million
                  (1,000,000) shall be Preferred Stock, par value $.01 per
                  share, issuable in series ("Preferred Stock")."

      SECOND: That thereafter a meeting of stockholders of the Corporation was
duly held pursuant to notice duly given and that a vote was taken on the
proposed amendment; and that at such meeting the necessary number of shares
required by statute and by the Restated Certificate of Incorporation were voted
in favor of said amendment.

      THIRD: That said amendment was duly adopted in accordance with the
provisions of Section 242 of the General Corporation Law of the State of
Delaware.

      IN WITNESS WHEREOF, the Corporation has caused this certificate to be
signed by Patrizio Vinciarelli, its President, and attested by Thomas A. St.
Germain, its Secretary, this 10th day of May, 1991.

                                      VICOR CORPORATION

                                      By: /s/ Patrizio Vinciarelli
                                          --------------------------------
                                          Patrizio Vinciarelli,
                                          President

ATTEST:

/s/ Thomas A. St. Germain
---------------------------------
Thomas A. St. Germain,  Secretary
[Corporate Seal]


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