Method of detecting switching power supply output current
A method and apparatus for detecting output current in switching power supplies forgoes the need for any resistor or other components in series with the load. A method according to the invention includes the steps of inferring the peak current through the inductor as a function of input voltage and inductor charge time, and deriving the current available to the load based upon the flyback voltage during discharge of the inductor. Although a disclosed example is based upon a buck/boost topology, other converter topologies are anticipated. In terms of apparatus, in a switching power supply of the type wherein a switching device is used to charge an inductor that discharges to a load, the invention provides devices arranged for detecting output current without the need for a resistor or other component in series with the load. In the preferred embodiment, these devices include electrical components operative to generate a signal indicative of peak inductor current, and a switch for switching the signal to provide a waveform, the integral of which is proportional to peak inductor current multiplied by the duty cycle of the discharge of the inductor. Circuitry is further used for comparing the waveform to a fixed reference so as to output a signal indicative of load current.
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This application claims priority from U.S. Provisional Patent Application Ser. No. 60/444,568, filed Feb. 3, 2003, the content of which is incorporated herein by reference.
FIELD OF THE INVENTIONThis invention relates generally to switching power supplies and, in particular, to a method of detecting output current in such supplies which uses fewer components.
BACKGROUND OF THE INVENTIONSwitching power supplies employing inductive components have been in use for some time. Although most applications require constant output voltage (usually implemented through voltage feedback), some applications require a constant output current.
Standard practice for constant current outputs involves adding a resistor in series with the load to convert the output current into a voltage which can then be used as feedback. This approach, however, imposes a tradeoff between series resistor loss and feedback voltage magnitude, or dynamic range. A need exists for a method of detecting output current in switching power supplies which uses no series components.
SUMMARY OF THE INVENTIONThis invention resides in a method and apparatus for detecting output current in switching power supplies without the need for any resistor or other components in series with the load. In broad and general terms, the technique derives the peak inductor current of a switching converter through input voltage and inductor charge time, and calculates the current integral of that peak current available to the load through the use of the flyback voltage during discharge of the inductor.
A method according to the invention more particularly includes the steps of inferring the peak current through the inductor as a function of input voltage and inductor charge time, and deriving the current available to the load based upon the flyback voltage during discharge of the inductor. Although a disclosed example is based upon a buck/boost topology, other converter topologies are anticipated.
In terms of apparatus, in a switching power supply of the type wherein a switching device is used to charge an inductor that discharges to a load, the invention provides devices arranged for detecting output current without the need for a resistor or other component in series with the load. In the preferred embodiment, these devices include electrical components operative to generate a signal indicative of peak inductor current, and a switch for switching the signal to provide a waveform, the integral of which is proportional to peak inductor current multiplied by the duty cycle of the discharge of the inductor. Circuitry is further used for comparing the waveform to a fixed reference so as to output a signal indicative of load current.
Turning now to the drawings,
Referring now to
Note that peak current in inductor 103 is a linear function of both supply voltage and switching device 102 assertion time. Note as well that this peak current linearly decreases to depletion during inductor 103 discharge. The average current delivered to load 106 is therefore half the peak inductor 103 current multiplied by the ratio of discharge time to total cycle period. Expressed mathematically, average output current can be expressed as ((V*(T205−T204))/2)*((T206−T205)/T207−T205)), where V denotes supply voltage, and Tnnn indicates absolute time at the indicated
Referring now to
Inverter 307 de-asserts switching device 314 whenever switching 302 is charging inductor 303, and asserts switching device 314 otherwise. In conjunction with resistor 313, switching device 314 then outputs a switched waveform, the integral of which is the supply voltage times the duty cycle of inductor 303 charge time. This switched waveform, being directly proportional to peak inductor charge current, is filtered by resistor 315 and capacitor 316, and buffered by voltage follower 317.
Voltage follower 317 thus outputs a filtered voltage indicative of peak inductor 303 current. This filtered voltage is then switched through resistor 318 by switching device 319 under control of comparator 323. Connected as shown, comparator 323 yields a low-switched output whenever the output of inductor 303 (at the anode of diode 304) exceeds the incoming supply voltage, and yields a high switched output otherwise. Controlled in this fashion, switching device 319 and resistor 318 then output a switched waveform, the integral of which is directly proportional to inductor 303 peak current multiplied by the duty cycle of inductor 303 discharge.
After filtering and buffering by resistor 320, capacitor 321, and voltage follower 322, this voltage, being directly indicative of output load current, is supplied to the integrator formed by resistor 310, capacitor 312, and operational amplifier 311, for comparison with the fixed reference voltage supplied by resistor 308 and zener diode 309. By the above method, power supply loop closure on output current is affected without use of any load series resistance.
Although use is shown in a buck/boost topology, it will be apparent to those of skill in the art that other converter topologies are anticipated.
Claims
1. In a switching power supply of the type wherein a switching device is used to charge an inductor that discharges to a load, a method of detecting determining a magnitude of an output current without the need for a resistor or other component in series with the load, the said method comprising the steps of:
- inferring the a peak current through the said inductor as a function of input voltage and inductor charge time; and
- deriving the a current available to the said load based upon the a flyback voltage produced by said inductor during discharge of the said inductor.
2. The method of claim 1, wherein said inductor forms part of a buck/boost topology.
3. In a switching power supply of the type wherein a switching device is used to charge an inductor that discharges to a load, an apparatus for detecting determining the magnitude of an output current without the need for a resistor or other component in series with the load, comprising:
- electrical components operative to generate a signal indicative of peak inductor current;
- a switch for switching the signal to provide a waveform, the integral of which is proportional to peak inductor current multiplied by the duty cycle of the discharge of the inductor; and
- circuitry for comparing the waveform to a fixed reference so as to output a signal indicative of load current.
4. Apparatus An apparatus for detecting determining the magnitude of an output current of a switching power supply without the need for a resistor or other component in series with the load, comprising:
- a switching device used to charge an inductor that discharges to a load in accordance with an input signal;
- electrical components operative to generate a signal indicative of peak inductor current;
- a switch for switching the signal to provide a waveform, the integral of which is proportional to peak inductor current multiplied by the duty cycle of the discharge of the inductor; and
- circuitry for comparing the waveform to a fixed reference so as to output a signal indicative of load current.
5. The apparatus of claim 4, wherein the switching device and inductor form part of a buck/boost topology.
6. An apparatus for determining a magnitude of an output current through a load driven by a power supply, comprising:
- an inductive element included in the power supply;
- a first circuit coupled to the inductive element and operable to determine a peak current through the inductive element as a function of a voltage across the inductive element and charge time of the inductive element; and
- a second circuit coupled to the first circuit and operable to determine the magnitude of the output current based upon the peak current.
7. The apparatus of claim 6, wherein the inductive element comprises an inductor that forms part of a buck/boost topology.
8. The apparatus of claim 6, wherein the first circuit is operable to provide a signal indicative of the peak current through the inductive element.
9. The apparatus of claim 6, wherein the first circuit is further operable to switch the signal to provide a waveform the integral of which is proportional to the peak current multiplied by a duty cycle of a discharge time of the inductive element.
10. The apparatus of claim 6, wherein the second circuit is operable to compare the waveform to a fixed reference voltage to provide a signal indicative of the magnitude of the output current.
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Type: Grant
Filed: Jan 24, 2008
Date of Patent: Aug 6, 2013
Assignee: JM Electronics Ltd. LLC (Wilmington, DE)
Inventor: Larry Kirn (Austin, TX)
Primary Examiner: Matthew Nguyen
Application Number: 12/019,623
International Classification: G05F 1/652 (20060101); G05F 1/656 (20060101);