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Active Common-Mode Filter for Power Supplies

IP.com Disclosure Number: IPCOM000113850D
Original Publication Date: 1994-Oct-01
Included in the Prior Art Database: 2005-Mar-27
Document File: 2 page(s) / 48K

Publishing Venue

IBM

Related People

Lindfors, EA: AUTHOR [+2]

Abstract

Disclosed is the use of an active integrated circuit in a common-mode power supply filter. Instead of actually filtering the noise, the circuit samples the common-mode noise current and generates an equal and opposite current at the point of measurement. Since the two currents are equal and opposite, they cancel each other.

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Active Common-Mode Filter for Power Supplies

      Disclosed is the use of an active integrated circuit in a
common-mode power supply filter.  Instead of actually filtering the
noise, the circuit samples the common-mode noise current and
generates an equal and opposite current at the point of measurement.
Since the two currents are equal and opposite, they cancel each
other.

      The Figure is a schematic representation of the circuit, with a
resistance 10 representing a load corresponding to the input Line
Impedance Stabilization Network (LISN), a capacitance 12 representing
the parasitic capacitance of the power switching transformer, and a
voltage source 14 representing the noise source of the power supply,
which, during normal operation produces a corresponding current
through parasitic capacitance 12.  This current is converted into a
voltage by sensing resistor 16.  The voltage developed across
resistor 16 is then integrated by an active integrator composed of a
first operational amplifier 18, a resistor 20, and a capacitor 22.
The output of this active integrator is then applied to the positive
input terminal 23 of a second operational amplifier 24, which, along
with resistors 26 and 28, forms a unity-gain buffer amplifier used to
apply the voltage at positive voltage at positive input terminal 23
to a capacitor 30.

      As a result of the phase shifts through amplifiers 18 and 24,
with the correct choice of component values for resistors 16 and 20...