How to Hold All Linear Power Supply Indicators (I)

Power Specifications

At first glance, a variable DC power supply device seems simple. But it is actually an extremely complex, accurate and durable high-load electronic device. Whether it is resistive, inductive, capacitive, low impedance, high impedance, steady state or variable load, the power supply device must reliably provide stable, precise and clean voltage and current. Therefore, we should choose the appropriate power supply requirements according to the application and thoroughly understand the power supply indicators.

Linear power supply indicators

First, let's talk about linear power supply specifications. Linear power supplies are very durable, accurate, and provide low noise power. Their simple, direct feedback mechanism provides excellent load regulation and overall stability. Figure 1 shows a simplified block diagram of a linear power supply.

There are many specifications for linear power supplies [1], but they can be logically divided into three categories: accuracy and resolution, stability, and AC characteristics. We will introduce the important specifications belonging to each of these three categories.

Most DC power supplies have two operating modes. In constant voltage (CV) mode, the power supply adjusts the output voltage based on the user settings. In constant current (CC) mode, the power supply regulates the current. Whether the power supply is in CV mode or CC mode depends not only on the user settings but also on the load resistance [2]. Different specifications apply to the power supply in CV mode and CC mode.

Accuracy and resolution specifications

At any given time, the power supply regulates the voltage or current and matches it to the setting within the accuracy of the instrument.

• In CV mode, the output voltage matches the voltage setting within the instrument accuracy specification. The current is determined by the load impedance.
• In CC mode, the output current matches the current limit setting. The voltage is determined by the load impedance.

Historically, DC power supply users used potentiometers to set the output voltage or current. Today, microprocessors receive input from a user interface or a remote interface. A digital-to-analog converter (DAC[3]) receives the digital setting, converts it to an analog value and uses it as a reference for the analog regulator. The value of the setting resolution and accuracy is determined by the quality of the conversion and the regulation process.

Voltage and current settings (sometimes called limits or settings) have resolution and accuracy specifications associated with them, respectively. The resolution of these settings determines the smallest increment that the output can be adjusted, and the accuracy describes how closely the output value conforms to international standards. Setting and readback specifications should be considered separately [4]. Good readback accuracy does not necessarily mean good setting accuracy.

Most DC power supplies provide built-in meters to measure voltage and current. These meters measure the voltage and current provided by the power supply output. Since the meters read the voltage and current returned to the power supply, the meter measurements are often referred to as readback values. Most professional power supplies contain digital meters using analog-to-digital converters and these internal instrument specifications are similar to those of a digital multimeter. The power supply displays the meter values ​​on the front panel and sends them through its remote interface (if configured).

How to set the accuracy?

Setup accuracy determines how close the adjustment parameters are to the theoretical values ​​defined by international standards. The uncertainty of the power supply output is mainly determined by the DAC error terms (including quantization errors). Setup accuracy is tested by measuring the adjustment variables with a traceable, precision measurement system connected to the output of the power supply. Setup accuracy is:

±( % of setting value + offset )

For example, the voltage setting accuracy specification for the Keithley 2200-32-3 power supply [5] is ±(0.03% + 3mV). Therefore, when the output is set to 5V, the output value uncertainty is (5V)(0.0003 + 3mV), or 4.5mV. Current setting accuracy is similarly specified and calculated.

Teach you how to hold the resolution!

The setting resolution is the minimum setting change option for voltage or current on the power supply. Sometimes, this parameter is called programming resolution. The resolution specification limits the number of discrete levels that can be set. Usually, the number of discrete levels is determined by the number of user interface bits and the number of DAC bits. A DAC with more bits can more finely control its output and provide a clearer reference value for loop control. However, offset and gain error correction can reduce the resolution to less than the number of bits provided by the DAC.

Changing the resolution with a single-step setting will not always result in a corresponding change in output. However, the setting accuracy specification governs the relationship between the setting and the output, and the instrument should be calibrated within this tolerance.

The setting resolution can be expressed in absolute units or as a percentage of full scale. For example, the voltage setting resolution of the Keithley 2200[6]-32-3 is 1mV and the current setting resolution is 0.1mA.