Test Complex Scenarios With a Programmable Power Supply

When you start your car, the battery voltage drops drastically as the starter motor draws in a huge number of amps. The voltage then rises a little as the engine is turning. Finally, the voltage reaches a steady state as the starter turns off.

All the electronic devices in your car — the infotainment display, the indicators, and the lights — experience these rapid changes too.

So how does an engineer test the reliability of a car or other system under such complex power changes? That's where programmable power supplies enter the picture.

What is a programmable power supply?

A programmable power supply is a power source whose output voltage, output current, output power, and other electrical output characteristics can be configured or programmed at a very fine-grained level.

However, despite their sophisticated front panel controls, knobs, displays, and powerful user interface, they're not mere adjustable power supplies but far more than that. Their most unique aspect is that their power output behavior can be programmatically controlled in real-time using an external computer, as shown below.

.

Fig 1. Programmable power supplies support both sophisticated manual control and remote programmatic control

Core power supply capabilities

We first get into the core power capabilities to expect from a good programmable power supply. Since their programmability is their biggest draw, we devote an entire section to it later in this article.

Types of power sources

Programmable power systems may have different combinations of direct current (DC) and alternating current (AC) capabilities:

• A programmable DC power supply can only output DC voltages and currents of different waveforms.
• A programmable AC power supply mainly outputs AC voltages and currents of different waveforms. Most also support DC offsets to simulate DC noise. Additionally, both single-phase and three-phase AC supplies are available.
• Hybrid AC-DC supplies can output both AC/DC voltages and currents with various waveforms.

Control over voltage, current, and power

The three most basic features of a programmable power supply are precise control over the:

1. output voltage to a device under test (DUT)
2. output current supplied to a DUT
3. output power delivered to a DUT

This control is achieved through voltage and current feedback and regulation circuits.

How does voltage and current programming work in a programmable power supply?

A power supply can run in different modes:

• Constant voltage (CV): In this mode, you set a voltage level, and the power supply ensures that voltage at the DUT. If the DUT suddenly draws a high current, resulting in a voltage drop, the power supply immediately senses it and activates its voltage regulator circuit to boost the voltage.
• Constant current (CC): In this mode, you set a current level, and the power supply ensures that much current flows to the DUT by varying the voltage appropriately.
• Constant power: In this mode, you set a power level. The device varies both the voltage and current, such that the power delivered to the DUT remains constant.

In addition, they can generate transient phenomena like surges, spikes, and brownouts to test DUTs for abnormal conditions.

What are the typical voltage and current ranges?

The voltage ranges, current ranges, and power values of power supplies can vary widely depending on their intended use cases, as shown below.

Fig 2. Operating ranges of some programmable power supplies

We can see that benchtop supplies are used for low- to mid-power devices. In contrast, high-power, high-voltage DC supplies like the RP9700 series can go as high as 30-2,000 volts for testing high current applications like electric vehicle battery drain.

Display the current voltage, current, and power

Programmable power supplies support accurate readback, display, and logging of the voltage, current, and power going to the DUT. Accurate readback with high resolution is vital for test engineers to ensure that their automated tests are running as designed as well as hint at possible faults in the DUT.

Safety features

Good supplies support programmable overvoltage and overcurrent protections to protect sensitive DUTs from voltage or current surges.

The overvoltage protection circuitry operates completely independently of the voltage limit circuit, and it will shut the power supply output off if the voltage exceeds the overvoltage limit.

The overcurrent protection is part of the current limit circuit but overrides the limit and shuts the supply off if the current exceeds the overcurrent setting.

Transition speeds

Since programmable power supplies are all about simulating complex power draw profiles, they must be able to change voltages or currents quickly. High-performance power supplies have very fast transition times of the order of 50 microseconds.

Number of output channels

The number of output channels is a key aspect of a programmable power supply. More channels mean less overall costs, but each channel may have strict limits on its output power. On the other hand, high-voltage or high-power outputs may only be available in single-channel supplies.

Stacking

Fig 3. Stacking multiple power supplies

Another important feature of programmable power supplies is the ability to combine their outputs. For more voltage, stack multiple power supplies in series. For more current, connect them in parallel.

Linear vs. switched power supplies

The internal construction of a power supply also matters. Switched-mode power supplies are lightweight but since they operate at high frequencies, they may introduce noise into the output unless very carefully designed. In contrast, a linear power supply (consisting of a huge transformer and rectifiers) may be bulky but often produces low noise at the output.

Output characteristics

In a rectangular power supply, the product of its maximum current and voltage is equal to its maximum power.

An autoranging power supply has a maximum voltage and a maximum current, but it cannot output both at once. It's a good choice for applications requiring a large range of output voltages and currents within a fixed power limit.

Operating quadrants and bidirectionality

Fig 4. Voltage-current operating quadrants

We can consider the voltage-current (VI) plane as divided into four quadrants. In the first quadrant, a device is sourcing power like an electronic load. In the second, it's sinking power.

Bidirectional power supplies source power like a power supply but also switch to sinking power like an electronic load. A bidirectional supply is ideal for testing batteries and converters that both consume and produce power.

How do remote programming and monitoring capabilities enhance the usability of a programmable power supply?

The benefits of programming and automation are as follows:

• Precise control over power supply output: Precise control is crucial for many applications. Programming allows such control over aspects like the voltage, current, power, phase, and timing.
• More convenient than manual use: It's not easy to set up complex power supply waveforms manually using the knobs and controls on a regular power supply. Programmable power supplies are designed to make such use convenient.
• Easy to repeatedly replay behaviors: Automated test equipment involves repeated execution of test steps. Achieving the same states and behaviors again and again is laborious and slow when people do it. Programmability enables effortless and efficient repetition.

Programmability and automation capabilities

In the following sections, we explain the many approaches to program or automate power supplies in automated test equipment and production environments.

Built-in programmability of power sequences

The simplest way to program a power supply is by using its front panel controls and display. Good devices allow you to set up any arbitrary power supply waveform you need for your testing. For each output channel, you can program the sequence of output voltages, output currents, timings, and repetitions to create arbitrary waveforms like this.

Fig 5. Complex power supply waveform

For example, on the E36312A, the sequence programming user interface is called the output list and looks like this.

Fig 6. Onboard programming of power supply sequences

Programming power supplies with vendor software

If the onboard sequencing doesn't suffice your needs, most vendors also provide software with graphical user interfaces that you can install on a computer. These software often have many more features than the onboard firmware and enable you to easily set up, store, distribute, and replay complex power supply profiles.

For convenient connectivity from computers, power supplies often support two or more of these standard interfaces:

• local area network (LAN) over ethernet
• universal serial bus (USB)
• RS232 serial port
• general parallel interface bus (GPIB)

Fig 7. Connectivity options

Install the vendor's software on a computer, and connect it to the power supply via any of these interfaces.

For example, the Keysight Power Supply Software provides a simple, drag-and-drop, user interface for visual programming (similar to MIT's Scratch) that even non-coders can use to set up complex sequences. BenchVue's programming user interface with an example visual program is shown below.

Fig 8. Programming a power supply using PathWave BenchVue

Advanced connectivity options

If you are proficient at networking and programming, consider more advanced connectivity options to address complex requirements or organizational needs. For example, you can connect multiple power supplies over a wired or wireless local area network.

Some supplies may have additional proprietary pinout interfaces. For example, the Keysight E36300 has a digital control port that allows you to connect and synchronize multiple power supplies. You can also use it to create your own remote sensing system by wiring it up to something like an Arduino, ESP32, or Raspberry Pi, as shown below.

Fig 9. Digital control port configuration and connections

Programmatic control over the power supply

The most powerful way to control power supplies, as well as other test and measurement (T&M) instruments, is the programmatic approach.

Using programming languages like Python, MATLAB, C#, C++, and others, you can program very fine-grained behaviors into one or more power supplies.

Why would you want this? One reason is end-to-end system testing. You can programmatically control power supplies and other instruments to orchestrate complex system tests involving multiple devices.

Programmatic control involves multiple layers of software components as shown above. Let's understand what they do.

Virtual instrument software architecture (VISA)

In a complex automated test setup, a single computer may be connected to power supplies of different types via different interfaces. So, for uniform access across different connection types, the first software layer is an abstraction layer called the virtual instrument software architecture (VISA).

VISA provides a uniform programmatic interface to connect to T&M instruments and send commands to them. VISA libraries are available for various programming languages.

Fig 10. VISA libraries

For example, the PyVISA library allows Python programs to connect to programmable power supplies and other T&M instruments. If you prefer MATLAB, look at the visadev module.

You can also install a full software development kit like the Keysight IO Libraries Suite that includes all these libraries, device drivers, documentation, and examples.

Standard commands for programmable instruments (SCPI)

VISA is just a communication channel to send commands to the instrument. The actual behavior of a power supply is determined by the SCPI commands sent to it. The syntax and semantics of these commands are specified by the SCPI standard.

Typical SCPI commands look like the ones highlighted in the C code snippet below.

Fig 11. Example C code to set a power supply's voltage and measure current

In which industries and applications are programmable power supplies extensively used?

Let's look at the industries and applications that make heavy use of programmable power supplies:

• Electric vehicles and battery testing: High-power, regenerative, bidirectional programmable power supplies, like the RP9700 series, are used for characterizing battery charging and discharging in electric vehicles and photovoltaic simulations. They are also used to test the complex power draw in electric vehicles where changes are rapid and frequent.
• ATE power solutions: Rack-mount programmable power supplies are heavily used as part of automated test equipment systems in avionics test systems, automotive production testing, and inverter testing among other use cases.
• Semiconductor industry: Programmable power supplies are used for testing both analog and digital characteristics of semiconductor devices and integrated circuits.
• Defense and aerospace: Many systems such as radar and pulsed lasers draw power in non-linear, short, high-energy pulses. Similarly, avionics systems have complex power profiles. All these profiles require programmable power supplies.

Use programmable power supplies for your complex testing

In this article, we learned about the features that make programmable power supplies unique and powerful. These complex instruments are characterized by a large number of powerful capabilities and complex features.

Contact us for our expertise in selecting the right programmable power supply for your use case.

Related Posts