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                       SECURITIES AND EXCHANGE COMMISSION
 
                             WASHINGTON, D.C. 20549
 
                            ------------------------
 
                                   FORM 10-K/A
 
  /X/    ANNUAL REPORT PURSUANT TO SECTION 13 OR 15(d) OF THE SECURITIES
         AND EXCHANGE ACT OF 1934
 
                  FOR THE FISCAL YEAR ENDED DECEMBER 27, 1998
 
                                       OR
 
  / /    TRANSITION REPORT PURSUANT TO SECTION 13 OR 15(d) OF THE
         SECURITIES AND EXCHANGE ACT OF 1934
 
        FOR THE TRANSITION PERIOD FROM ______________ TO ______________
 
                            ------------------------
 
                          Commission File No. 0-29454
 
                                POWER-ONE, INC.
 
             (Exact name of registrant as specified in its charter)
 
<TABLE>
<S>                                  <C>
             DELAWARE                            77-0420182
  (State or other jurisdiction of             (I.R.S. Employer
  incorporation or organization)             Identification No.)
 
          740 CALLE PLANO                           93012
       CAMARILLO, CALIFORNIA                     (Zip code)
  (Address of principal executive
             offices)
</TABLE>
 
        Registrant's telephone number, including area code (805) 987-8741
 
        Securities registered pursuant to Section 12(b) of the Act: None
 
        Securities registered pursuant to Section 12(g) of the Act:
 
                         Common Stock, $0.001 par value
                                (Title of class)
 
                            ------------------------
 
    Indicate by check mark whether the registrant (1) has filed all reports
required to be filed by Section 13 or 15(d) of the Securities Exchange Act of
1934 during the preceding 12 months (or for such shorter period that the
registrant was required to file such reports), and (2) has been subject to such
filing requirements for the past 90 days. YES /X/  NO / /
 
    Indicate by check mark if disclosure of delinquent filers pursuant to Item
405 of Regulation S-K is not contained herein, and will not be contained, to the
best of registrant's knowledge, in definitive proxy or information statements
incorporated by reference in Part III of this Form 10-K or any amendment to this
Form 10-K. / /
 
    The aggregate market value of the voting stock held by non-affiliates of the
Registrant as of March 15, 1999 was approximately $83 million.
 
    As of March 15, 1999, 17,100,107 shares of the Registrant's $0.001 par value
common stock were outstanding.
 
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                                EXPLANATORY NOTE
                                ----------------

The registrant has filed this Form 10-K/A solely to amend the cover page of 
its Annual Report on Form 10-K, as filed with the Securities and Exchange 
Commission on March 29, 1999, to indicate that the registrant has disclosed 
delinquent filers pursuant to Item 405 of Regulation S-K in its definitive 
amended proxy statement, as filed with the Commission on March 31, 1999, 
incorporated by reference in Part III of its Form 10-K. Except for the cover 
page, the content of the registrant's original Form 10-K is otherwise 
unchanged.

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                                   SIGNATURES
 
    Pursuant to the requirements of Section 13 or 15(d) of the Securities Act 
of 1934, as amended, the registrant has duly caused this annual report Form 
10-K/A to be signed on its behalf by the undersigned, thereunto duly 
authorized.
 
                                POWER-ONE, INC.
 
                                By:            /s/ STEVEN J. GOLDMAN
                                     -----------------------------------------
                                                 Steven J. Goldman
                                       CHAIRMAN OF THE BOARD, CHIEF EXECUTIVE
DATE: April 6, 1999                            OFFICER AND PRESIDENT

    Pursuant to the requirements of the Securities Act of 1934, as amended, this
annual report has been signed below by the following persons on behalf of the
registrant and in the capacities indicated below.
 
    We the undersigned directors and officers of Power-One, Inc. hereby
constitute and appoint Steven J. Goldman and Eddie K. Schnopp, or any of them,
our true and lawful attorneys and agents, to do any and all acts and things in
our name and behalf in our capacities as directors and officers and to execute
any and all instruments for us and in our names in the capacities indicated
below, that said attorneys and agents, or either of them, may deem necessary or
advisable to enable said corporation to comply with the Securities and Exchange
Act of 1934, as amended, any rules, regulations, and requirements of the SEC, in
connection with this Report, including specifically, but not limited to, power
and authority to sign for us or any of us in our names and in the capacities
indicated below, any and all amendments and supplements to this Report, and we
hereby ratify and confirm all that the said attorneys and agents, or any of
them, shall do or cause to be done by virtue hereof.
 

          SIGNATURE                       TITLE                    DATE
------------------------------  --------------------------  -------------------

                                Chairman of the Board,
    /s/ STEVEN J. GOLDMAN         Chief Executive Officer
------------------------------    and President               April 6, 1999
     (Steven J. Goldman)          (Principal Executive
                                  Officer)
 
                                Sr. Vice President,
     /s/ EDDIE K. SCHNOPP         Finance, Chief Financial
------------------------------    Officer and Secretary       April 6, 1999
      (Eddie K. Schnopp)          (Principal Financial and
                                  Accounting Officer)
 
     /s/ JON E. M. JACOBY
------------------------------  Director                      April 6, 1999
      (Jon E. M. Jacoby)
 
  /s/ DR. HANSPETER BRANDLI
------------------------------  Director                      April 6, 1999
   (Dr. Hanspeter Brandli)
 
   /s/ DR. ALBERT Y. C. YU
------------------------------  Director                      April 6, 1999
    (Dr. Albert Y. C. Yu)