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Browse Prior Art Database

Technique for Current Detection in Power Converters

IP.com Disclosure Number: IPCOM000113948D
Original Publication Date: 1994-Oct-01
Included in the Prior Art Database: 2005-Mar-27
Document File: 4 page(s) / 103K

Publishing Venue

IBM

Related People

Keidl, SD: AUTHOR

Abstract

Control schemes in switching power converters and Pulse Width Modulation (PWM) motor drives typically use load current as a controlled variable. A method of deriving a wide bandwidth and very low ripple representation of this current by using an analog switch, band limited integrator, and multi-pole filter combination is described. The analog switch-integrator portion gives a measure of average converter load current independent of duty cycle and, therefore, input and output voltages, and the bandlimited integrator-filter portion allows the system to realize a wide bandwidth.

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Technique for Current Detection in Power Converters

      Control schemes in switching power converters and Pulse Width
Modulation (PWM) motor drives typically use load current as a
controlled variable.  A method of deriving a wide bandwidth and very
low ripple representation of this current  by using an analog switch,
band limited integrator, and multi-pole filter combination is
described.  The analog switch-integrator portion gives a measure of
average converter load current independent of duty cycle and,
therefore, input and output voltages, and the bandlimited
integrator-filter portion allows the system to realize a wide
bandwidth.

      An implementation of this approach in a DC-to-DC converter is
shown in the Figure.  The current sense circuit portion consists of
an analog bi-directional switch, S1, a band limited integrator,R1,C1,
and an amplifier/buffer/filter,A1,L,C,R.  The transformer sensed
current waveform Va, is connected to the analog switch which is
turned on only during the output inductor L1, charge time.  During
this time a voltage representing the output inductor charge current
is applied to the band limited integrator R1 and C1.  The average
value of this waveform, Vb in the figure, represents the average load
current, Iout.  Iout for the example converter shown, is equal to the
average output inductor L1 current.  At the end of the output
inductor charge time (PWM switch on time) the analog switch is turned
off.  Therefore, during this time the band limited integrator can not
charge or discharge since the source of charge is removed.  This
eliminates all integrator dependence on the PWM duty cycle.  The
charge on the integration capacitor is a true representation of
average output current.

      A band limited integrator and filter combination is used
because a pure integrator would unnecessarily restrict the bandwidth
of the derived current signal.  Even a band limited integrator alone
would result in a bandwidth too small for most current mode control
applications.  Since there are no unwanted frequencies in the derived
current signa...