How to Design an Accurate DC Power Supply

Topics Covered in This Article:

Ⅰ. Output Driver            

Ⅱ. Current and Voltage Sensing            

Ⅲ. Analog-to-Digital Converter (ADC)            

Ⅳ. Control Loop            

Ⅴ. Summary

Application Areas: Test and measurement applications such as battery testing, electrochemical impedance spectroscopy (EIS), and semiconductor testing require accurate current and voltage output DC power supplies. When the ambient temperature changes by ±5°C, the current and voltage control accuracy of the device needs to be better than ±0.02% of the full scale. This accuracy largely depends on the temperature drift of the current sense resistor and amplifier.

In this article, you will learn how different components affect system accuracy and how to select suitable components for the design of precision DC power supplies. We will explore the complete design process from output drivers to control loops.

Ⅰ. Output Driver

Figure 1 is a block diagram of a power supply, including output drivers, current and voltage sensing circuits, control loops, analog-to-digital converters (ADC), and digital-to-analog converters (DAC). The choice of output driver depends on the output accuracy, noise, and power level.

Figure 1: Typical Block Diagram of a DC Power Supply

Low-Power Applications (<5W): Linear power supplies can be used as output drivers for low power consumption or low noise applications. Power operational amplifiers (op-amps) have integrated thermal protection and overcurrent protection functions, making them suitable for low-power applications.

High-Power Applications: Due to power dissipation issues, it is challenging to use a linear output driver at higher output power levels. Therefore, a synchronous buck converter is needed to achieve higher output power, with a large filter on the output side to achieve 0.01% full-scale accuracy. For example, using a buck converter, an accuracy of 500 µV can be achieved in the 5V output range.

Important Note: You need to confirm that there are no pulse-skipping mode and diode emulation mode in the converter that would increase the output ripple at light loads. Texas Instruments' C2000 real-time microcontroller (MCU) is ideal for precision synchronous buck converter power supplies because you can disable unwanted functions in software.

Ⅱ. Current and Voltage Sensing

High-precision current shunt resistors and low-drift instrumentation amplifiers can measure output current. The input offset voltage error and gain error of the instrumentation amplifier are not a problem because these two errors are accounted for during system calibration. However, offset voltage and gain drift, output noise, and gain nonlinearity are difficult to calibrate. These errors should be considered when selecting a current sense amplifier.

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

Table 1: Overall Unadjusted Error of Current Sense Amplifiers

Among the amplifiers listed in the table, INA188 has the smallest error. The error calculation uses a temperature change of ±5°C and selects 100mΩ and 1mΩ current sense resistors for 1A and 25A outputs, respectively.

Voltage Monitoring: You can use a differential amplifier or instrumentation amplifier to monitor the load voltage very accurately. The amplifier senses the output voltage and ground of the two loads, eliminating errors due to any voltage drop in the cable. System calibration will adjust the offset voltage and gain error of the amplifier, leaving only the input temperature drift.

You can divide the temperature drift by the full-scale voltage and calculate the drift in parts per million (ppm). For example, for 2.5V full scale and 1µV/°C temperature drift, the drift will be 0.4ppm/°C. If you need lower output voltage drift, you can choose a zero-drift operational amplifier (such as OPA188), which has a maximum input temperature drift of 85nV/°C. However, a precision operational amplifier with a temperature drift of 1µV/°C is sufficient for most applications.

Ⅲ. Analog-to-Digital Converter (ADC)

The ADC offset voltage and gain error are adjusted during system calibration. Errors caused by ADC drift and nonlinearity are difficult to calibrate. Table 2 compares the errors of three different high-precision delta-sigma ADCs when the temperature changes by ±5°C.

Table 2: Overall Unadjusted Error of ADCs

Among the ADCs listed in the table, ADS131M02 has the smallest error. The error calculation does not include ADC output noise and voltage reference errors.

Noise Optimization: You can significantly reduce the error caused by noise by increasing the oversampling rate of the ADC. Low noise (<0.23ppm peak-to-peak) and low-temperature drift voltage references (<2ppm/°C) (such as REF70) are sufficient to meet the needs of DC power applications.

Long-Term Stability: During 0 to 1,000 hours of operation, the REF70 only has a long-term drift of 28 ppm. In the subsequent 1,000 hours of operation, the drift is significantly lower than 28 ppm, ensuring long-term stability.

Ⅳ. Control Loop

Figure 2 shows the analog control loop of the power supply. Even if you don't need a constant current output, keeping the constant current loop will help with short-circuit protection. The constant current loop limits the output current by reducing the output voltage, and the current limit can be programmed through the IREF setting.

Figure 2: Schematic Diagram of Constant Current and Constant Voltage Loops

Loop Switching Mechanism: Using a diode between the constant current and constant voltage loops helps achieve constant voltage to constant current conversion and vice versa. Multiplexer-friendly operational amplifiers are suitable for constant current and constant voltage loops, avoiding short circuits between amplifier inputs during open-loop operation.

When any control loop is in an open-loop state, the operational amplifier may generate a differential voltage greater than 0.7V at its input pins. Non-multiplexer-friendly operational amplifiers have anti-parallel diodes at the input pins, and the differential voltage is not allowed to exceed the diode voltage drop. Therefore, a non-multiplexer-friendly operational amplifier will increase the amplifier's bias current, which may cause the device to self-heat and reduce system accuracy when the current interacts with the source impedance.

Digital Control Implementation: You can also implement a control loop in the digital domain within the C2000 real-time MCU. The high-resolution pulse-width modulator, precision ADC, and other analog peripherals of the C2000 real-time MCU can help reduce the total number of components and bill of materials costs. The C2000 real-time MCU product family includes 16-bit and 12-bit ADC options, providing flexible design solutions.

Ⅴ. Summary

When designing DC power supplies for test and measurement applications, temperature drift and noise specifications should be considered. By choosing low-drift amplifiers and ADC products, you can achieve an accuracy of less than 0.01%.

Key Takeaways:

• Choose the appropriate output driver: Use linear power supplies for low-power applications and synchronous buck converters for high-power applications

• Use high-precision current sensing components: Zero-drift instrumentation amplifiers like INA188 provide optimal performance

• Select low-noise, low-drift ADCs: Delta-sigma ADCs such as ADS131M02 ensure accurate measurements

• Implement appropriate control loops: Ensure smooth transitions between constant current and constant voltage modes

• Consider long-term stability: Use low-drift voltage references like REF70

Difference Between AC Power Supply and DC Power Supply

AC Power Supply: An AC power supply refers to plugs and sockets that are used to connect to the AC power provided by the mains, enabling household appliances and small portable devices to operate. The voltage and current direction of AC power changes periodically.

DC Power Supply: A DC power supply is a device that maintains a constant voltage and current in the circuit, such as dry batteries, accumulators, and DC generators. DC power supplies are crucial in modern electronic devices because most semiconductor devices require stable DC voltage to function properly.

Modern Trends (2025): With the development of renewable energy and electric vehicles, high-precision DC power supplies are becoming increasingly important in energy storage systems, battery management systems, and power electronics testing. Advanced technologies such as four-wire Kelvin sensing and fuzzy adaptive PID control algorithms are being widely adopted to further improve accuracy and response speed.

Last Updated: October 2025 | Revised and Expanded from 2021 Original Article