Test and measurement applications such as battery testing, electrochemical impedance spectroscopy andsemiconductor testing require accurate current and voltage output DC power supplies. The accuracy of the device's current and voltage control needs to be better than ±0.02 per cent of full scale at ambient temperaturevariations of ±5°C. Accuracy is highly dependent on the temperature drift of the current sense resistors andamplifiers. In this article, you will learn how different components affect system accuracy and how to select theright components for your precision DC power supply design.

Output Driver

Figure 1 shows a block diagram of a power supply, including output drivers, current and voltage sense circuits, control loops, analogue-to-digital converters (ADCs) and digital-to-analogue converters (DACs). The choice of output driver depends on the output accuracy, noise and power level. Linear power supplies can be used as output drivers for low-power (5W) or low-noise applications. Power operational amplifiers (op amps) have integrated thermal and overcurrent protection for low-power applications.

 Figure 1: Typical block diagram of a DC power supply

However, using a linear output driver with higher output power is challenging due to the power losses that can occur, so you need to use a synchronous buck converter to achieve higher output power, with a large filter at the output to achieve 0.01% full-scale accuracy. For example, with a buck converter, you can achieve 500µV accuracy over the 5V output range. You also need to make sure that there are no pulse jumps and diode emulation modes in the converter that increase output ripple at light loads. the C2000™ real-time microcontroller (MCU) is well suited to precision synchronous buck converter power supplies because you can disable unwanted features in software.

Current and Voltage Sensing

High-precision current shunt resistors and low-drift instrumentation amplifiers measure output current. The instrumentation amplifier's input offset voltage error and gain error are not a problem because they are taken into account in system calibration. However, the instrumentation amplifier's offset voltage and gain drift, output noise, and gain nonlinearity are difficult to calibrate and should be considered when selecting a current sense amplifier.

Equation 1 calculates the overall unregulated error of the current sense amplifier, as shown in Table 1. The common mode rejection ratio error is relatively small, so it can be ignored.

The INA188 has the smallest error of the amplifiers listed in the table. Errors are calculated using a ±5°C temperature change with 100mΩ and 1mΩ current resistors selected for 1A and 25A outputs, respectively.

Table 1: Overall unadjusted error of the current sense amplifier

ou can use a differential amplifier or instrumentation amplifier to monitor load voltages very accurately. The amplifier senses the output voltage and ground of both loads, eliminating errors due to any voltage drop in the cable. System calibration adjusts the amplifier's offset voltage and gain error, leaving only the input temperature drift. You can calculate drift in parts per million by dividing the temperature drift by the full-scale voltage. For example, for 2.5V full-scale and a temperature drift of 1µV/°C, the drift would be 0.4ppm/°C. If you need a lower output voltage drift, you can choose a zero-drift op amp (such as the OPA188), which has a maximum input temperature drift of 85nV/°C. However, precision op amps with a 1µV/°C temperature drift are sufficient for most applications.

This ADC

Adjust the ADC offset voltage and gain error during system calibration. errors caused by ADC drift and nonlinearity are difficult to calibrate. Table 2 compares the errors of three different high-precision Δ-sigma ADCs for a temperature change of ±5°C. Of the ADCs listed in the table, the ADS131M02 has the smallest error. The error calculations do not include the ADC's output noise and voltage reference error.

Table 2: Overall Unadjusted Error of ADCs

You can significantly reduce the error due to noise by increasing the oversampling rate of the ADC. The low noise (0.23 ppm-p) and low-temperature-drift voltage reference (2 ppm/°C) (e.g., REF70) are adequate for DC power applications. During 0 to 1,000 hours of operation, the device has a long-term drift of only 28ppm. Over the next 1,000 hours of operation, the subsequent drift is significantly less than 28ppm.

Control Loop

Figure 2 shows the analogue control loop for the power supply. Even if you don't need a constant-current output, retaining the constant-current loop will help with short-circuit protection. The constant current loop will limit the output current by reducing the output voltage, and the current limit is programmable through the IREF setting.

Using a diode between the constant current and constant voltage loops will help with constant voltage to constant current conversion and vice versa. Multiplexer-friendly op amps are suitable for constant current and constant voltage loops to avoid short circuits between amplifier inputs during open loop operation. The op-amp may generate differential voltages greater than 0.7V at its input pins when any control loop is open loop. Non-multiplexer friendly op amps have reverse shunt diodes at the input pins that do not allow the differential voltage to exceed the diode drop. As a result, non-multiplexer friendly op amps increase the bias current of the amplifier and may cause the device to self-heat and degrade system accuracy as this current interacts with the source impedance.

 Figure 2: Constant-Current and Constant-Voltage Loop Schematic

You can also implement control loops in the digital domain within the C2000 real-time MCU, where high-resolution pulse-width modulators, precision ADCs, and other analogue peripherals can help reduce the total number of components and bill-of-materials. the C2000 real-time MCU product family includes 16- and 12-bit ADC options.

Conclusion

When designing a DC power supply for test and measurement applications, temperature drift and noise specifications should be considered. Accuracy of less than 0.01% can be achieved if low drift amplifier and ADC products are selected.

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