Browse Prior Art Database

Power Factor Correction Equipment for Visual Display Units

IP.com Disclosure Number: IPCOM000112473D
Original Publication Date: 1994-May-01
Included in the Prior Art Database: 2005-Mar-27
Document File: 4 page(s) / 134K

Publishing Venue

IBM

Related People

Eagle, DJ: AUTHOR

Abstract

Disclosed is the provision of a pre-regulator to correct and improve the power factor of uncorrected Visual Display Units (VDU). Existing monitors can be upgraded to follow International Standard updates. A design resulting from practical considerations is given.

This text was extracted from an ASCII text file.
This is the abbreviated version, containing approximately 46% of the total text.

Power Factor Correction Equipment for Visual Display Units

      Disclosed is the provision of a pre-regulator to correct and
improve the power factor of uncorrected Visual Display Units (VDU).
Existing monitors can be upgraded to follow International Standard
updates.  A design resulting from practical considerations is given.

      Regulations concerning the EMC performance of IT products are
in EN60555 which relates to power factor and mains current harmonics.
Power factor is a measure of how the product is drawing current from
the mains, with the ideal being a power factor (pf) of unity.
Technically, its definition is the ratio of watts to VAs drawn from
the mains.  Typical displays have a power factor of around 0.7, with
voltage and current waveforms as shown in Fig. 1.  Such current
waveforms result from simple peak rectification of the mains input
voltage to produce a DC output approximating to the peak mains input
voltage.  No current flows for at least 80% of the time and during
the conduction period a high current flows to compensate for this.
This pattern of current usage causes problems of efficiency in power
generation and distribution as the rms current flowing is typically
50% higher than that of the idealised sinusoidal current that would
be required to provide the same output power.  A sinusoidal current
waveform makes the equipment appear as a purely resistive load.
Multiples of the third harmonic in the current waveform are
particularly troublesome as in three phase generating systems these
currents will add to cause substantial losses in the common neutral
line.

      Legislation aimed to limit harmonics and improve efficiency
will require equipment to have (pf) close to unity.  Several
semiconductor companies have developed control ICs to provide pf
correction.  Future displays will need to incorporate such ICs;
PS/2* system unit power supplies are already pf corrected.  The
problem is that adding pf correction adds substantial extra circuitry
and so major reengineering the existing ranges of displays would be
required to comply with the new legislation.

      Proposed is an external unit that goes between the mains and
the monitor to provide pf correction.  The output of this unit needs
to be a DC voltage.  This is not a problem as most IBM monitors will
operate from DC voltages.  Such an arrangement is shown in Fig. 2.
The output voltage needs to be carefully selected so that the X and Y
capacitors in the monitor are not overstressed.  For a high voltage
monitor a DC voltage of 250V lies within the normal rms input voltage
of the monitor (X and Y capacitors OK) and also the DC voltage is the
same as that produced by the monitor's rectification circuit when
operated at its minimum input voltage of 180VAC hence SMPS duty cycle
stays within normal, tested range.  For a low voltage monitor a DC
voltage of 125V is similarly adequate to satisfy normal operating
requirements.  The solution is...