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EDN Access--11.06.97 Current-sense amplifier handles high voltages

-November 06, 1997


 
November 6, 1997


Current-sense amplifier handles high voltages

Andy Fewster, Maxim Integrated Products, Reading, UK

High-side current-sense amplifiers, such as IC1 in Figure 1, monitor current from a battery or dc power line in systems for which the ground-return continuity is crucial. IC1's 36V maximum operating voltage, however, excludes some applications, including traction-motor batteries and central-exchange power supplies, both of which require operation at voltages exceeding 72V. The external circuitry in Figure 1 allows IC1 to monitor current at voltages of 300V and greater. Q1 and Q2 form a current source whose compliance voltage provides the high-voltage capability. IC1 draws its 100-µA supply current from a simple floating power supply comprising C1, C2, avalanche diode D1, and the Q1-Q2 current source.

Voltage applied to R1 biases the current source and provides a ground-referenced means of shutting off the circuit. The current value (1 mA in this case) ensures regulation in D1. Q1 should have a maximum compliance of 300V, yielding 305.1V for the maximum VIN. Substituting a 33V avalanche diode for D1 boosts this maximum to 333V. IC1's output terminal provides a current source with a typical 3-mega-ohm output impedance, which generates 500 µA per ampere of current flowing through the sense terminals, RS+ and RS-. The high-voltage, pnp common-base stage Q3 routes the output current to R3. Q3 is the complement to the high-voltage npn transistor, Q1.

The result is a ground-referenced signal that is proportional to the monitored current. You select R3 to provide the desired full-scale output voltage. The only error in the output signal comes from the current-gain factor, greek alpha, lc, for a common-base stage: greek alpha, lc = greek beta, lc/(1+greek beta, lc), where greek beta, lc is the transistor's dc gain. Q1's minimum greek beta, lc is 25, yielding a minimum greek alpha, lc of 0.96. To bring the greek alpha, lc closer to unity, increase the current gain by replacing Q3 with a Darlington device. With the values in Figure 1, Q3's collector voltage changes less than 10 mV over the 10 to 300V VIN range. A similar common-base stage, Q4, translates IC1's polarity output, which indicates the direction of current flow through the sense terminals, to provide a ground-referenced logic-level Sign output. (DI #2109)

Figure 1
23DI2109
A few external passive components and high-voltage transistors multiply a current-sensing IC's operating voltage almost tenfold.


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Copyright c 1997 EDN Magazine, EDN Access . EDN is a registered trademark of Reed Properties Inc, used under license. EDN is published by Cahners Publishing Company , a unit of Reed Elsevier Inc.

 

 

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