Browse Prior Art Database

Standby Power Supply for Electronic Equipment

IP.com Disclosure Number: IPCOM000108357D
Original Publication Date: 1992-May-01
Included in the Prior Art Database: 2005-Mar-22
Document File: 3 page(s) / 134K

Publishing Venue

IBM

Related People

Eagle, DJ: AUTHOR [+4]

Abstract

Described is a circuit that provides a standby power supply for electronic equipment efficiently and cost effectively. Standby supplies commonly used in televisions (TVs) and other types of equipment also occur in computer displays. The disclosed concept involves varying the effective inductance of a switched mode power supply (SMPS) transformer to achieve a wide range of normal/standby powers.

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Standby Power Supply for Electronic Equipment

       Described is a circuit that provides a standby power
supply for electronic equipment efficiently and cost effectively.
Standby supplies commonly used in televisions (TVs) and other types
of equipment also occur in computer displays. The disclosed concept
involves varying the effective inductance of a switched mode power
supply (SMPS) transformer to achieve a wide range of normal/standby
powers.

      Much domestic electronic equipment has a 'standby' mode where
the unit is effectively powered off but its remote control receiver
is active;  the complete unit may be powered up via remote control.
Clock and other functions may also be active during the standby time,
but while in standby, power requirement would typically be less than
ten percent of the normal operating condition's power.  A PC could
possibly have some logic active, effectively the remote control
receiver and still provide reduced power to the CRT heater to speed
up the transition from standby to full operation.

      The problem is how to supply a few watts needed in standby mode
yet still be able to satisfy full load requirements during normal
operation.  Most TV receivers, displays (VDU) and similar equipment
use SMPSs.  Under worst-case conditions the SMPS will typically
operate at a duty cycle of around 50%.  When supplying a standby load
of, say, 5% of full load, it will only require a duty cycle of 11%.
Achieving stability over this wide range as well as a wide range of
input voltage power supplies (90V - 265 V) imposes more stringent
conditions.  A 50% duty cycle for full load at 90 V mains input
corresponds to around 17% duty cycle at 265 V mains input.  In
standby condition where the load is only 2% of the full load, at
maximum mains input voltage, the duty cycle of the SMPS would be less
than 3%.  An SMPS with a duty cycle range of 2 - 50% presents serious
problems because most SMPS controllers cannot handle such a range.
Existing solutions commonly used are the single standard flyback
converter with the previously described range problems;  two SMPSs
with change over switch, or one SMPS for main power and a linear
regulator for standby.

      The proposed circuit has advantages over the above solutions,
where a wide range input voltage and sizeable output currents are
required.  The new solution is cost effective, simple, with few extra
components and energy efficient.  The idea is to use an existing SMPS
but to modify its performance during standby to allow it to operate
with reduced power output.  Assumed is that the power supply runs at
the same frequency in both standby and normal modes. Most power
supplies free-run at a frequency 10-20% away from their 'locked up'
frequency.  Also the design point of the supply duty...