公开(公告)号：GB2291770A

公开(公告)日：1996-01-31

申请号：GB9414906

申请日：1994-07-23

Applicant: IBM

Inventor： COYNE DAVID ,  KNOX ANDREW RAMSAY

IPC: G06F15/16 ,  G06F3/153 ,  G06F15/177 ,  G09G1/16 ,  G09G5/00

公开(公告)号：GB2298532A

公开(公告)日：1996-09-04

申请号：GB9503987

申请日：1995-02-28

Applicant: IBM

Inventor： COYNE DAVID ,  GRANT DAVID ALEXANDER ,  LISTER EWAN JAMES ,  TAYLOR LINDSAY GORDON

IPC: H04N3/185 ,  H02M3/335 ,  H04N3/18

Abstract: A switch mode power supply comprises: a switch Q4 alternately opening and closing a current path through a primary winding L1 of a transformer T1 in response to a pulse switching signal, an oscillator 110 generating the switching signal and synchronising the switching signal to a synchronisation signal HSYNC from an external source, a pulse width modulator 120 varying the pulse width of the switching signal in dependence upon a feedback signal indicative of a load on the power supply, a current limiter 150 turning off the switch Q4 to open the current path in response to current in the primary winding exceeding a threshold and compensation means 160 for varying the threshold in dependence upon the frequency of the synchronisation signal. The power supply may be used for a cathode ray tube (10, Fig 1) operated within a range of different video synchronisation frequencies. The compensation means 160 include a charge pump (C1, D3, Q1, Fig 3) providing a voltage across a capacitor (C2) dependent on the frequency of the synchronisation signal HSYNC for varying the threshold. The switch Q4 may be a bipolar transistor or FET.