[Technical Master Test Note Series]Part 3: How to choose a suitable desktop power supply?

　　

DC power supplies are standard in the test and measurement equipment that electrical engineers and circuit designers use to power and test circuit system designs. So what specific functions does it have? How can I find the right benchtop power supply for my application?

　　What is “desktop power supply” and its classification

Bench power supplies provide DC voltage to power the device under test, such as a circuit board or electronics. The desktop power supply is generally located on the engineer’s work area or workbench, so it is also called “desktop power supply”. When engineers or circuit designers need to test a device, commonly referred to as a device under test (DUT), they supply the device under test with a set voltage or current. Bench power supplies allow engineers to set and supply specific voltages to power the DUT and confirm that the device operates as intended. If it doesn’t work as expected, then debug and test again.

Despite the variety of benchtop power supplies, these instruments can be broadly divided into three broad categories: single- and multi-channel power supplies, bipolar and unipolar power supplies, linear power supplies, and switching power supplies.

Single and Multi-Channel Benchtop Power Supplies

As the name suggests, single-channel bench power supplies have only one controllable output, while multi-channel power supplies have two or more outputs. Multi-channel power supplies are typically used to develop devices that use both digital and analog or bipolar circuits.

Bipolar and Unipolar Bench Power Supplies

A unipolar power supply can only produce positive voltages. Engineers can technically switch the leads connected to the power supply to a negative voltage, while bipolar power supplies can provide both positive and negative voltages. Bipolar power supplies can handle a wider variety of power applications, but are more expensive and more complex to use, so many engineers choose unipolar benchtop power supplies for DC power applications.

Linear and Switched Bench Power Supplies

Linear power supplies provide high-precision measurements with very little noise or signal interference. But they are generally heavier, larger in size, provide less power, and are less efficient. Switch-mode power supplies are more compact and provide more power, but generally have high frequency noise and poor measurement accuracy. Switch-mode power supplies are typically used in applications where power density is an issue, such as getting much higher power in a small space, while linear power supplies are used in applications that require powering small analog circuits.

　　How to choose the right desktop power supply

By selecting the appropriate power supply and having a better understanding of its capabilities and features, engineers can run tests faster and make more accurate measurements. There are many factors to consider when purchasing a desktop power supply, the most important being:

1) Consider a programmable power supply

Manually setting voltage and current limits wastes valuable time when running long or complex tests with a benchtop power supply. Fortunately, most benchtop power supplies come with a test sequence feature that supports basic programming capabilities. Using the test sequence function, engineers can program voltage values, current limits, and the time for each setting. This provides an easy way to run complex tests with multiple predetermined output voltages and times without having to manually adjust settings on a benchtop power supply, allowing operators to spend more time getting quality measurements.

2) Choose a desktop power supply with the correct power limit

DC power supplies are usually categorized by maximum voltage and maximum current. This certainly provides important information when it comes to finding the right power source, but don’t forget to also look at the power limits. For example, the 2260B-30-72 bench power supply can deliver up to 30 V or 72 A, but its power limit is 720 W. This means that this power supply can provide 30 V, but not 72 A due to power limitations. Engineers can use the following formula to determine the generated power:

In most cases, if the power calculated with this formula is below the power limit of the bench power supply, then it will work fine.

Choose a Benchtop Power Supply with Remote Sensing Voltage Monitoring

For the most accurate voltage source, a bench power supply with a remote voltmeter or remote sensing is best. This results in a clean voltage reading on the DUT (rather than on the input terminals) as it compensates for the voltage drop across the test leads. Most standard 3 foot or so test leads have a resistance of about 50 mΩ (a pair of test leads is about 100 mΩ), so the voltage drop across the test leads can be quite large when using a low resistance DUT.

Find a desktop power supply with the appropriate response time

Response time is critical if you are running a test with rapid voltage or load changes. Response time is the time it takes for a bench power supply to ramp up (rise time) or ramp down (fall time) to the set voltage. Note that the response time often varies with load. Rise time is the time it takes for a power supply to rise from 10% of its value to 90% of its value. Fall time, in contrast, is the time it takes for the power supply to fall from 90% of its value to 10% of its value.

Transient recovery time is the length of time it takes for the power supply to return to the set level after a load is applied. If it is used as an indicator alone, it is a relatively complex parameter, so it is usually described by multiple parameters: voltage stability frequency band, transient recovery time and load current step change. For example, the load transient recovery time specification for the Keithley 2200 series bench power supplies is as follows: “Fall to within 75 mV in

　　How to use a desktop power supply

Desktop power supplies are very simple to use. These instruments connect to the DUT via leads that plug into the benchtop power panel. Engineers can use the front panel Display to set voltage or current levels to power the DUT. Most benchtop power supplies can work in two modes: constant voltage mode (CV) and constant current mode (CC).

1) Running the benchtop power supply in constant voltage (CV) and constant current (CC) modes

An important feature of benchtop power supplies is the ability to operate in constant current (CC) and constant voltage (CV) modes. In CV mode, the power supply stabilizes the output voltage according to user settings; in CC mode, the power supply stabilizes the current. Whether the power supply is in CV mode or CC mode depends on user settings and the resistance of the load. In CV mode or CC mode, the power supply has different specifications. At any point in time, the power supply stabilizes the voltage or current and matches the setting within the accuracy of the instrument.

In CV mode, the output voltage matches the voltage setting within the instrument’s accuracy specification, and the current is determined by the load impedance. In CC mode, the output current matches the current limit setting and the voltage is determined by the load impedance. These values ​​can be determined using Ohm’s Law, as follows. If you want to be more rigorous, you can include test leads in the resistor:

2) Running a benchtop power supply in parallel and series operation

If the test requires more power, multiple bench power supplies can be connected in parallel or in series to increase the available voltage or current.

Series operation: To increase the voltage, connect the positive output of one power supply to the negative output of the other, then connect the remaining positive and negative outputs to the DUT.

Parallel operation: To increase the current, connect the two positive outputs to one terminal of the DUT and the two negative outputs to the other terminal of the DUT.

When combining output, if you want to use the remote sensing function, be sure to read the power supply instruction manual. In some configurations or when using two different power supplies, remote sensing may not be available.

3) Find the best benchtop power supply for the application

The Series 2280 precision measurement DC power supplies provide up to 192W of output noise power and excellent load current measurement sensitivity. These power supplies offer up to 6µ digital resolution, 10nA current measurement resolution, and display both voltage and current graphs.

The Series 2281 Dynamic Modeling Battery Simulator integrates battery testing, battery simulation, and high-precision power functions. It can analyze the DC power consumption of the device under test, test the battery, and generate a battery model based on the battery charging process. modeling. Unique display screen shows battery status and provides precision power mode.

To learn more about benchtop power supplies, check out our benchtop power supply selection guide or browse our collection of DC power supplies. Contact a Tektronix expert to learn how to choose the right benchtop power supply for your application.