How to Design an Accurate DC Power Supply

Test and measurement applications such as battery testing, electrochemical impedance spectroscopy, and semiconductor testing require accurate current and voltage output  DC  power supplies. When the ambient temperature changes to ±5°C, the current and voltage control accuracy of the device needs to be better than ±0.02% of the full scale. The 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.

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. The linear power supply can be used as an output driver for low power consumption (<5W) or low noise applications. The power operational amplifier  (op-amp) has integrated thermal protection and overcurrent protection functions, suitable for low-power applications.

Figure 1: Typical block diagram of a DC power supply 

However, due to power loss, it is challenging to use a linear output driver with higher output power, so you need to use a synchronous buck converter to achieve higher output power, and use a large filter on the output side to achieve 0.01% Full scale accuracy. For example, using a step-down converter, an accuracy of 500 µV can be achieved in the 5V output range. You also need to confirm that there are no pulse skipping and diode emulation modes in the converter that increase the output ripple at light loads. The C2000 real-time microcontroller (MCU) is ideal for precision synchronous buck converter power supplies because you can disable unwanted functions in the 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 taken into account when the system is calibrated. However, it is difficult to calibrate the offset voltage and gain drift, output noise, and gain nonlinearity of instrumentation amplifiers. 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 of the common-mode rejection ratio is relatively small, so it can be ignored.

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 resistors for 1A and 25A outputs, respectively.

Table 1: The overall unadjusted error of the current sense amplifier 

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. 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. 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 delta-sigma ADCs when the temperature changes to ±5°C. 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.

Table 2: ADC's overall unadjusted error

You can significantly reduce the error caused by the noise by increasing the oversampling rate of the ADC,  Low noise (<0.23ppmp-p), and low-temperature drift voltage reference (<2ppm/°C) (such as REF70) are sufficient to meet the needs of DC power applications. In the 0 to 1,000 hours of operation, the device only has a long-term drift of 28 ppm. In the next 1,000 hours of operation, the subsequent drift was significantly lower than 28 ppm.

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 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.

Using a diode between the constant current and constant voltage loop helps to 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.

Figure 2: Schematic diagram of constant current and constant voltage loops

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. The C2000 real-time MCU product family includes 16-bit and 12-bit ADC options.

Ⅴ. Summary

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

What is the difference between  AC  power supply and DC power supply?

AC power supply is a modern word, a proper term, referring to plugs and sockets that are used to connect to the  AC power provided by the mains so that household appliances and small portable devices can be used. A DC power supply is a device that maintains a constant voltage and current in the circuit. Such as dry batteries, accumulators, DC generators, etc.