Key Characteristics of Programmable DC Power Supplies
Programmable DC power supplies are used in virtually every product lab and production test environment. These devices are tested under various power supply conditions, causing vendors and suppliers to require flexible control of voltage, current, and power with a high level of precision and resolution. A sufficient amount of built-in analog and digital interfaces (such as USB and ethernet) offer users more test agility, remote access, and system upgrades. The production test environment calls for the use of multiple channels for parallel testing capabilities, speeding up the automated test process while saving on test equipment cost.
This whitepaper dives into the various characteristics of programmable power supplies and how these may benefit users in the modern R&D and production test environment working with transistors, ICs, and op amp circuits as well as solar batteries, rechargeable batteries, and fuel cells. It also discusses how the PWX power supplies offered by Kikusui meet the needs of these applications.
Basic requirements of programmable DC power supplies
A programmable DC power supply simply allows for the remote control of the output voltage of the supply with some kind of control signal. This can be with an analog signal controlled by a knob or by a computer with a serial interface (such as RS232, GPIB, or even USB). Modern programmable DC power supplies offer much more beyond the standard knob and variable voltage/current levels. Outside of the standard power and maximum voltage specifications, programmable DC power supplies can grow in complexity to support more modern test cases.
Using one piece of equipment to test one device is not particularly efficient, especially in automated test scenarios. Multi-channel pieces of equipment allow the control of many different channels where multiple devices can be tested at the same time in production with one PC (Figure 1). This can be further optimized with a virtual bus where a singular PC can control multiple power supplies (or multiple PCs can control multiple power supplies).
Figure 1: A bus connection can use either an RS-232 connection or USB for communication with a PC where the slave units are controlled at the same time the master unit is controlled. Image courtesy of Kikusui.
Remote control of equipment
The remote control of test equipment is highly desirable in production tests or facilities that engage in large amounts of automated tests. It is critical to not just remotely make adjustments to the testing speed and routine, but also to monitor the test results. A test can run all day with defunct data due to some small mishap. This testing downtime can waste precious time. Power supplies with dedicated LAN interfaces that are LXI-compliant allow for network-based remote control and monitoring to optimize test design as well as improve testing efficiency and accuracy. This way, a user can control and monitor a power supply from a browser on a PC, smartphone, or tablet.
Figure 2: Sample LAN network configuration with remotely controlled racks of power supplies. Image courtesy of Kikusui.
Using a singular multi-range power supply to test multiple DUTs or one DUT with multiple voltage and current settings
Power supplies will typically follow a “square” power profile where the rated power is only delivered at the maximum voltage and current (Figure 3). In instances where the user has a device under test (DUT) that has an input voltage less than the rated voltage of the power supply and an input current that is less than the rated current of the power supply, the square power profile will suffice. However, this is given that the rated power (I*V) of the DUT is both consistent in operating conditions and far less than that of the conventional rectangular output power supply.
Figure 3: Conventional power supply with a square power profile. Image courtesy of Kikusui.
There are, however, many cases where the DUT will draw constant power under different input conditions. DUTs such as DC motor drives, regulated DC-DC supplies, and data center switches will all operate under a wide input voltage range. This means these devices will also pull more current in the lower half of their voltage range. This is often where the square power profile will fall short. So, for example, a DUT may have a voltage range from 12 V up to 42 V with a current range from 8 A to 28 A for a total power rating of 336 W (see Figure 4).
To test this DUT, a single-ranged unit such as the PAS60-18 (a unit with a square power profile) might seem more than sufficient: the PAS60-18 has a power rating of 1.08 kW with a maximum voltage rating of 60 volts. However, this means that the maximum current offered by this unit would be 18 amps. This falls short for our DUT as it will need to be tested up to 28 amps. One potential solution would be to parallel two PAS60-18 units to double the current rating to 36 amps, but this increases test complexity, cost, and space.
At 750 watts, a single multi-range PWX750ML offers less than half the rated power of two PAS60-18 units (2.16 kW) but can actually cover this testing range with a voltage range from 26.8 V to 80 V and a current range up to 28 amps. This characteristic also allows the PWX750ML to test multiple DUTs with the same equipment, saving on lab cost and space.
Figure 4: Example with four different DUTs covering a voltage range from 12 V to 42 V and current range up to 28 A. The multi-range PWX750ML covers all of these DUTs, or a singular DUT that covers this range. Image courtesy of Kikusui.
Simulating battery behavior with a variable internal resistance
Some DUTs will change in internal resistance. Fuel cells and batteries will both exhibit this type of behavior where the voltage output will decrease as a result of an increasing internal resistance. These devices are found everywhere from power conditioners and power tools to fuel cell vehicles and avionics.
With a conventional constant voltage power supply, the only solution to simulate this feature would be to connect external circuits that adjust the voltage based on a change in current. However, changing the resistance value internally is much more desirable as it saves on test bench space, cost, and complexity by removing the need to design external jigs for all the different batteries that may be simulated (Figure 5).