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Optocoupler extends high-side current sensor to 1 kV
Roger Griswold, Maxim Integrated Products, Sunnyvale, CA -- EDN, 3/1/2001
The task of sensing dc current at high voltage is often problematic. Most high-side current-sensing ICs available off the shelf are good only to 30 or 40V. Combining an optocoupler with such an IC yields a sensing circuit in which the only limitation of the high-side voltage is the optocoupler's standoff voltage (Figure 1).
A precision, high-side current-sense amplifier, IC1, and a high-linearity analog optocoupler, IC3, extend the high-side working voltage to 1000V dc. IC3 supports a continuous 1000V dc. Its UL rating is 500V rms for 1 minute, and its transient surge rating is 8000V dc for 10 seconds. You should follow all proper safety precautions when working with high voltage.
The circuit has a floating section and a grounded section, each requiring a local low-voltage supply. The floating section detects load current and drives the high-voltage side of the optocoupler. The grounded section monitors the optocoupler's low-voltage side and outputs a voltage proportional to the high-side load current. IC3 has a feedback photodiode on the high-voltage side that virtually eliminates the LED's nonlinearity and drift characteristics. In addition, IC3's two closely matched photodiodes ensure a linear transfer function across the isolation barrier.
During operation, the load current passes through shunt R1 and produces a small voltage. IC1 monitors this voltage and outputs a proportional current of 10 mA/V. This proportional output current routes through R2, which produces a voltage proportional to the main load current. The rest of the circuit generates a copy of the voltage across R2 but on the low-voltage side of the optocoupler. IC2 monitors the voltage across R2 and drives the optocoupler's LED via Q1. The LED generates light that impinges equally on the high- and low-side photodiodes. IC4 monitors the low-side photodiode and outputs a voltage proportional to the high-side load current. A graph shows the output voltage as a function of shunt current (Figure 2).
If R3 and R4 are equal, the overall transfer function is:
Three parameters let you modify the circuit to monitor other maximum load currents and output a different voltage range. The maximum IC1 output current is 1.5 mA, so the maximum allowed shunt voltage is 150 mV. Also, the maximum allowed photodiode current is 50 µA. Choose an R1 value that produces 150 mV at the maximum load current that the circuit monitors. Then, choose an R2 value that produces the desired corresponding maximum output voltage at 1.5 mA. Match R3 and R4, and choose a value that allows less than 50 µA through the photodiode at the maximum desired output voltage, or
The circuit output then faithfully reproduces the voltage across R2. The MAX4162 op amp is a good choice for this circuit because of its input-bias current of 1 pA, its rail-to-rail input and output swings, and its ability to operate from one 9V battery. With R1=150 m? and R=3.32 k?, the output voltage for ISHUNT=1A is 4.80V using the given transfer function. Experimental results at ISHUNT=1.00A give VOUT=4.84V with an error less than 1%.
