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EDA & Development

Measuring power supply performance

Robert Green looks at what you can look forward to and what you can do now with current generation power supplies


he test and measurement industry both helps to create new technology and benefits from technological advances. Certainly the revolution in wireless technology and new semiconductor technologies, which leads to new, more capable chipsets, are just two examples of major industry drivers. Green energy initiatives for electric vehicles, solar energy conversion devices, lighting and display technology are additional examples. Even the modest power supply is impacted by these technology drivers. New power supplies will become

improved simulators of batteries so that manufacturers of electric vehicles and portable, battery-operated devices can characterise the performance of their devices under controlled but very realistic operating conditions. Other specialised power supplies will more accurately emulate the I-V output characteristics of solar panels so that inverter designers can develop more efficient circuitry. More capable chipsets will enable power supplies to have more advanced measurement performance providing more accuracy, more resolution, and higher measurement speed. Most likely, high performance power supply measurement circuits will soon be as capable as 5 ∏- and 6 ∏-digit DMMs. The advancement in the families of DSP chips and FPGAs will lead to new power supplies that employ more sophisticated, digital control loop technology to speed up response to load changes while minimising overshoot and undershoot and maintaining stability.

36 December 2012/January 2013

New power supplies will offer the LAN LXI interface as the primary interface. The GPIB interface bus, the mainstay for the industry since the 1970’s, will finally start to lose its loyal users. Initially, IT departments refused to permit LAN-based test systems for fear of potential risks to corporate networks. Now, network security technology has substantially improved. Switches and controllers have become much more intelligent; and, there are a number of ways to secure a network. For example, a set of addresses for a test system can be linked only to a quarantined network. The LAN LXI interface enables remote access to a test system which is especially valuable when the test engineer is in a different location than the manufacturing location. The LXI instruments also have embedded web hosting so that the instrument can be controlled from its web page. The most dramatic improvement in next- generation instruments, including power supplies, will be in display technology. The display technology in smart phones and tablets will shortly find its way even into power supplies. Thin film transistor (TFT) displays will make instruments easier to use and provide the user with much more information than just output voltage and current. A glance at the display will show measurements, computations, and the state of the instrument. The displays will be able to show numerous parameters simultaneously on the screen to allow fast setup of the supply without having to wade through numerous screens. Many of the next generation power supplies will have the capability to be controlled by smart phone and tablet applications through a wireless link. Look for significant increases in performance and usability in next generation power supplies. While the next generation of power supplies will help make your test development and your accumulation of measurements much easier, you have more than enough issues to address today.

Insufficient capacity

The most significant decision is ensuring that sufficient power is available to energise your DUT. While this is obvious, it is

Components in Electronics

important to be aware that different types of power supplies have different power envelopes. One type of power supply has a rectangular power envelope in which any current can be supplied to the load at any voltage level. This is certainly the most versatile power envelope, and it is the easiest to understand on a power supply data sheet. An alternative version of this type of supply is one that has multiple rectangular power envelopes representing

the DUT where the power leads are connected. The sensing circuits measure the voltage at the DUT so that the supply can compensate for any voltage drop in the test leads (See Figure 1). If you are considering a multiple channel power supply, consider having remote sensing on all channels. You need to consider the effect of the wiring that has resistance, RLead, determined by the length of the lead, the conductivity of the conductor material, and the geometry of the conductor. Without remote sensing, the voltage at the load is: VLoad=VProgrammed – 2*VLead = VProgrammed – 2*ILoad*RLead. If the load requires high current, then ILoad is high and VLead can easily be a few tenths of a volt, especially if the power supply leads are long, as can be the case in an automated test rack. A voltage at the load could easily be 80mV to 160mV lower than the

FIgure 1

multiple voltage ranges. Just make sure you review the literature since the maximum current that the supply can output will not be available at the maximum voltage. Some supplies have a hyperbolic power envelope, a more continuous transition than a multiple range power supply. High power output supplies tend to have the multi-range or hyperbolic envelope. Be cognizant of what your application requires, so that the supply you select delivers the required power at the levels of voltage and current at which you will need to test.

Maximising output accuracy If tight control of voltage at the load is essential for research experimentation, device characterisation or production testing, then a careful review of the power supply’s output accuracy and read-back specifications are important. However, that accuracy can be compromised if the supply is controlling the voltage at its output terminals. What you need is feedback control right at the DUT. That means your supply should include sense connections (remote sensing) that can be connected to

desired voltage (with 2A to 4A flowing through a five foot length of 0.004 /foot, 16-gauge wire). The remote sensing technique solves the

problem of voltage drop in the leads by extending the power supply feedback loop

FIgure 2

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