An In-Depth Guide to Bench Power Supplies

No electronic circuit or device can run without power to energize it. Observing the behavior of a circuit under different power conditions is the cornerstone of every step of the product engineering cycle.

At the heart of this cycle sits the bench power supply to provide the voltages and currents that engineers need to power up their designs.

In this article, learn the essentials of bench power supplies, their characteristic features, and how they work internally.

What is a bench power supply?

A bench power supply is a power source to supply a specified direct current (DC) or DC voltage to a device under test (DUT) for observing the latter's behavior. Engineers and do-it-yourself (DIY) hobbyists frequently use bench power supplies when designing, testing, and debugging circuits and devices.

Chances are you'll find one on the lab benches of most electronics engineers and hobbyists for both professional and hobby uses. Other common names for it include benchtop DC power supply, lab power supply, and DC bench power supply.

To an engineer or hobbyist, all these names typically connote an instrument that:

• is an AC-DC converter that plugs into the mains alternating current (AC) supply and outputs a steady DC voltage or current
• has a relatively compact form factor suitable for convenient lab bench use
• has easily accessible front-panel controls to set the voltage and current
• provides banana plug sockets on the front panel to plug in the output leads
• displays the set voltage and current on an LED display or LCD display panel
• has built-in safety and protection features

How are bench power supplies different from other power supplies?

Bench power supplies are just one type of DC power supply. Let's see how they compare with other DC power instruments:

• Automated test equipment (ATE) power supplies: ATE power supplies are meant for production-scale automated testing. Unlike bench supplies, they're typically kept on racks, connected to other ATE, and remotely programmed. So their features focus more on connectivity and programmability, while bench supplies focus on usability and convenience. Plus, ATE power supplies settle much faster to keep up with the programmed sequences. That said, there are programmable bench power supplies too with relatively fast settling times.
• Source meters: Source meters are also bench instruments with power supply functionality. But unlike bench power supplies, they include the functionality of digital multimeters and electronic loads. While supplying power, they can simultaneously measure voltage or current. They can also act as electronic loads and rapidly switch between sourcing and sinking modes. Their settling times are extremely fast.
• Power analyzers: Power analyzers are also bench instruments with power supply features. But a power analyzer includes the capabilities of an oscilloscope, arbitrary waveform generator, multimeter, power meter, electronic load, and data logger. Also, while AC power analyzers are as common as DC, AC bench power supplies are relatively rare. When engineers hear "bench power supply," most assume it's DC.

Which features are important in a benchtop power supply?

Bench power supplies span a wide range of features, prices, performance capabilities, and levels of accuracy. In this section, we explain the capabilities and features found across the lineup of bench power supplies out there.

1. Adjustable output voltage and output current

The front-panel knobs enable you to set the following supply parameters:

• constant voltage to supply to the DUT within the supported voltage range
• maximum and minimum voltage range
• constant current to supply to the DUT within the supported current range
• maximum and minimum current range

In constant voltage mode, an adjustable power supply outputs the specified voltage at a steady stable level. On the digital display, it shows the set voltage and the amps being drawn at any instant.

In constant current mode, the power supply ensures that the specified current is drawn by the DUT by constantly adjusting the voltage. On its display, it shows the set current level and the volts being pushed at any instant.

The maximum and minimum levels enable DUT protection and are discussed later.

Another aspect of voltage control is the polarity. A use case for changing it is when switching a diode from forward to reverse bias. Most variable power supplies are unipolar, meaning that their positive terminals are at higher voltages than their negative terminals. For the opposite polarity, you have to manually invert the connections to the DUT. However, some power supplies are bipolar, meaning they can internally invert their polarity without rewiring the connections to the DUT.

What are autoranging bench power supplies?

An autoranging power supply automatically adjusts its voltage and current to match the requirements of the load. This allows the power supply to provide a wide range of voltage and current combinations without manual adjustments.

How do adjustable voltage and current add to the versatility of a bench power supply for various electronic applications?

Using constant voltage mode, a power supply can act like a device's wall adapter and make the DUT think it's connected to the mains.

For some use cases, constant current is necessary. For example:

• Light-emitting diodes (LEDs) and laser diodes: LEDs and semiconductor laser diodes require constant current power supply.
• Battery charging: Another use case for constant current is testing battery charging circuits. Recharging a battery involves a constant current supply followed by a constant voltage supply. Power supplies can emulate such complex sequences with features like output lists or programmability.

2. Line and load regulation

Line regulation is the ability of a bench power supply to maintain a steady output voltage even as its AC input voltage fluctuates, spikes, or includes noise and transients.

Load regulation is its ability to maintain a steady output voltage while the DUT's load and input current draw vary.

How does voltage regulation play a crucial role in the performance of a bench power supply?

Excellent line and load regulation are essential to a bench power supply's core functionality. It can maintain a constant stable voltage only if its regulation circuit can detect fast transients and rapidly compensate for them.

How are line and load regulation measured?

Load and line regulation are measured based on voltage changes away from the setpoint and specified as percentages of the setpoint value. For example, 0.01% means the supply can maintain the output voltage within ± 0.01% of the setpoint under all conditions.

3. Maximum power

The power that a bench supply can push to DUTs ranges from low to high to cater to various use cases. For example, the Keysight E36150 high-power supplies can push as much as 800 watts (W) and 60 amps for high-current applications like automotive battery testing. In contrast, the 30-W E36102B is suitable for low-power DC analog and digital circuits.

4. Stable voltage with low ripple and noise

Ripple is a periodic dip in the output voltage due to the bench supply's internal phenomena like capacitor discharging when rectifying the sinusoidal AC input voltage to a steady DC voltage.

Fig 1. Ripple in the output voltage

Noise is just random small fluctuations in the output voltage due to noise in the input line and noise picked up from ambient electromagnetic phenomena.

Good supplies must have very low ripple and noise. They are specified through the following parameters:

• maximum peak-to-peak voltage difference in millivolts
• maximum root mean square voltage of the ripple and noise signals in millivolts or microvolts
• root mean square current due to the ripple and noise in microamps or milliamps

How do ripple and noise affect electronic components under test?

Ripple and noise can result in anything from minor inaccuracies in digital signals to significant malfunctions in sensitive devices and components like:

• analog-to-digital converters
• high-fidelity audio circuits
• low-noise amplifiers
• radio frequency circuits
• digital components

In addition to low ripple and noise, most power supplies have terminals that are isolated from the chassis ground. This reduces line noise and ground loops.

5. Programmability

Programmability enables power supplies to emulate complex power sequences. A programmable power supply supports both on-device use interface features like output lists and remote programmability using programming languages.

6. Safety and protection features

Bench power supplies must have some essential safety and protection features like:

• Overvoltage protection: The maximum voltage protects the DUT from high voltages.
• Overcurrent protection: The maximum current protects the DUT from drawing too many amps. For example, this can happen if there's a short circuit.
• Temperature protection: Power supplies must detect if their internal temperature is too high due to high current draw and shut down.
• AC input spikes and surges: Power supplies must protect themselves and the DUTs from spikes and surges in the AC mains line.

Under the hood of bench power supplies

Bench power supplies can be one of two types based on their internal designs:

• linear DC power supplies
• switching DC power supplies

Linear power supplies

Fig 2. Block diagram of a linear power supply

A linear supply works as follows:

1. A step-down transformer reduces the sinusoidal mains AC voltage to a lower voltage.
2. A rectifier circuit turns that into a half-sinusoidal DC voltage.
3. Capacitive filter circuits modify the waveform to a flat DC (with some ripple).
4. Voltage and current regulator circuits ensure constant voltage or current.

Switching power supplies

A switch-mode powersupply works as follows:

1. It rectifies mains AC to low-frequency, high-voltage DC.
2. Capacitive filtering reduces the ripple and noise.
3. A switching circuit uses transistors to rapidly switch the DC on and off at a high frequency (kilohertz to megahertz), converting the DC to high-frequency AC.
4. A high-frequency step-down transformer reduces the AC voltage level.
5. The high-frequency AC is rectified back to DC.
6. The rectified DC is filtered again to further reduce ripple and noise.
7. A feedback loop is used to regulate and maintain stable output power.

What are the advantages and disadvantages of linear vs. switching bench power supplies?

The advantages of linear over switched supplies are:

• Lower noise: They have significantly less electrical noise and ripple.
• Simple design: Fewer components mean higher reliability.
• Excellent regulation: They can ensure stable output voltage under varying conditions.
• Faster transient response: Linear supplies tend to respond to transients faster.
• Less interference: They cause less electromagnetic interference.

The advantages of switching power supplies over linear ones are:

• More efficient and cost-effective: At high power levels, switched supplies are more efficient, waste less heat, and are more cost-effective due to small components.
• Lightweight and compact: The high-frequency operation enables smaller lighter transformers compared to linear supplies.
• Wider input voltage range: They can handle a wider input AC range while linear transformers must get bulkier with the voltage.

What should you look for when selecting a bench power supply?

These are some key features and aspects to consider when selecting a bench power supply:

• Power, voltage, and current ranges: These are the three basic parameters that must match your use case.
• Ripple and noise: Look for low ripple and noise, especially the maximum peak-to-peak voltage, which is the maximum variation away from the DC setpoint.
• Computer interfaces and programmability: Check if you need a programmable DC power supply and that it has the connectivity and programmability you need to emulate complex sequences.
• Voltage and current readback: Some power supplies have built-in multimeters to measure their output voltage and current. Verify that the readback accuracy is suited to your use case.
• Front or rear outputs: Front terminals are more convenient for bench use, while rear ones are better for automated use.
• Number of outputs: Check if the supply is single output, dual output, triple output, or higher. More outputs allow for more complex test sequences and save bench space.
• Safety features: Ensure that the supply has overvoltage, overcurrent, and temperature protections to prevent damage to DUTs.
• Calibration: Check the supply's calibration schedule to maintain optimal performance over time.

Bench power supply — an essential instrument for every workbench

This article gave an in-depth understanding of power supplies, their key features, how they work, and tips to select the right one for your use case.

Keysight manufactures a wide range of power supplies and instruments to cater to every industry use case. Contact us for guidance on picking the right power supplies for your labs or production floors.

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