Minimize power losses in test racks

To deliver clean, adequate DC power from a power supply to a UUT, you must do more than simply connect two wires.

Martin Rowe, Senior Technical Editor -- Test & Measurement World, 8/1/2001

Today’s test stands often supply over 100 A DC to one or several UUTs. To deliver that much current, you must design a power-distribution network consisting of AC-DC power supplies, DC-DC power supplies, wires, connectors, switches, and perhaps bus bars and custom PCBs. Any part of the network can degrade the system’s DC power delivery. Careful design, though, can minimize voltage drops and system noise.

Why do test stands need to supply so much current? The specs for a power supply to power a 1.7-GHz Pentium 4 processor make the point. The power supply must provide over 50 A at 1.58 V to 1.75 V (Ref. 1).

With such low voltages and high currents, resistance and inductance in your power-distribution system may affect how much power your UUT receives. When a UUT requires less than 2 V, that low voltage doesn’t leave much room for losses in wires and connectors.

Fortunately, you don’t have to distribute power in its final form through a test rack. You can start with a higher-voltage bulk power supply, then use DC-DC converters to convert the power at points close to your UUTs. You can route low current through most of your power-distribution network and still deliver the power your UUTs need. You’ll minimize delivery losses because the low-voltage, high-current power travels short distances—perhaps just a foot from the DC-DC converter to the UUT.

For an equal amount of power, the higher the voltage, the less current your bulk power supply must produce, but don’t go overboard. High DC voltages result in safety hazards. Many telecom boards operate from 48 V, so that voltage has become popular as the voltage to distribute to the DC-DC converters. And you won’t get a shock if you accidentally touch wires at 48 V.

Figure 1 shows a scheme for distributing power from a bulk supply to DC-DC converters placed close to UUTs. Depending on the number of UUTs and the number of voltages required, you may need more than one DC-DC converter.

Start with specs

Before you can specify the components in a power-distribution network, you need to know how much power your test rack must deliver. Start with the UUT’s specifications and calculate the power it needs. Work backward to DC-DC converters and then to the bulk supply. Remember to include the efficiencies of your DC-DC converters when you calculate the power required from the bulk supply. To accommodate future UUTs that will need more current, you should use a bulk power supply than can provide twice the power you currently need.

Once you know how much voltage and current you need to deliver to the UUT, you can select your power-distribution components. If you need several DC-DC converters, consider using a copper bus bar to distribute power from the bulk supply. Bus bars offer two advantages over running wires to several points:

• Lower resistance and inductance. Even when carrying dozens of amperes, a 1-in. x 1/4-in. x 6-ft bus bar (less than $30) contributes negligible voltage drops. You can get bus bars from McMaster-Carr ( www.mcmaster.com) or Small Parts (www.smallparts.com).

• Easy hookup. You can drill and tap screw holes into bus bars and connect wires to the DC-DC converters or any other loads. Then, you can bolt the wires to the bus bar.

When you install bus bars, don’t forget to insulate them from the equipment rack. Use plastic hardware and be careful not to short the bars with tools when you perform maintenance on the test rack.

Figure 2. Connect power supplies to bus bars through wires capable of carrying the current. Courtesy of Datel.

You also need to connect the bulk power supply to the bus bars (for multiple UUTs) or directly to DC-DC converters (for a single UUT). Verify that the wire you plan to use can safely carry the current by reviewing the wire manufacturer’s specifications. Wire and cable manufacturers publish these specs in catalogs and online. Figure 2 shows a burn-in rack in which AWG 1 wires connect two 48-V bulk supplies to bus bars. In this application, each bulk power supply produces 48 V at 15 A. Each wire consists of seven strands and can safely carry 105 A or more, depending on its insulation material (Ref. 2).

The wires in this setup are so short that their resistance, about 0.13 V/1000 ft., proves insignificant (Ref. 3). If you need to supply high currents over dozens of feet, then you must calculate the voltage loss caused by IR drops in the wires. Measure the voltage you deliver to verify that it’s within your load’s specifications.

Because of their short length and simple routing, the wires shown in Figure 2 need little flexibility and do not need a conduit. Your application might require you to run large wires with dozens of strands through a conduit to a test fixture or card cage. Or, you may need to run several smaller, more flexible wires through a conduit. Either way, you must comply with standards for cables in a conduit. The National Electrical Code defines those standards, which you can find online from wire and cable manufacturers (Ref. 3,4).

You need to distribute the power from the bus bars or bulk-supply wires (if you don’t use bus bars) to the DC-DC converters. As a rule of thumb, you can use AWG 20 wires for current up to 8 A and AWG 14 for current up to 30 A, depending on the number of conductors in the wire and the insulation material (Ref. 2).

You can use spade lugs or ring lugs to bolt wires to bus bars, backplanes, terminal strips, chassis, and ground points. Lugs make connections easier than connecting wires directly, but you must properly attach wires to lugs. Otherwise, current in the wires and resistance in the connections may cause voltage drops, high temperatures, and wiring failures that can cause the connections to fail. Don’t use pliers to crimp lugs to wires—use a crimping tool, which you can get from most lug manufacturers or distributors. Solder the wire to the lug before you attach it to a bus bar, power supply, or test fixture. If your DC-DC converter mounts on a PCB, design a PCB to let you solder wires to it. You’ll eliminate lugs and increase reliability.

Next, you must connect the DC-DC converter outputs to the UUTs. Keep these wires as short as possible because they carry low voltages and high currents. Check wire specifications to make sure that they can safely carry the current and that wire resistances per unit length won’t degrade the voltage.

Use shielded twisted-pair wires to minimize noise pickup and emissions. Connect the shield to power ground to minimize system noise. Run a separate ground wire from each DC-DC converter output shield to power ground. If your UUTs reside in a card cage, connect the cage to power ground through a tinned copper braid, which provides a low-impedance path.

Decouple the power

Even the heavy wires or bus bars in a power-distribution system will have some impedance. Most electronic products draw changing amounts of current. Inductance impedes fast changes in current and produces voltage drops. To compensate for impedance caused by inductance, you need decoupling capacitors to supply short current bursts that keep voltages steady.

To determine how much capacitance you need, measure the voltage at the UUT with a scope and apply power to the UUT. If the voltage dips below the minimum for the UUT, you must add capacitance. The actual capacitor value requires a trial-and-error test. You can also add capacitors in parallel as needed, which also reduces the overall equivalent-series resistance and thus improves response time to changes in current. Make sure that the maximum voltage ratings of capacitors you use exceed your working voltage.

Install decoupling-capacitor networks along bus bars, close to DC-DC converter inputs and close to UUTs. If you use wires to distribute power, then use a terminal strip or design a PCB to let you install
decoupling and filter capacitors. If your DC-DC converter mounts on a PCB,
design the PCB to accommodate filter
capacitors.

Use at least two types and sizes of capacitors on bus bars. A “large” value (typically 100 mF or higher) tantalum or aluminum type capacitor can supply momentary current. The actual value you need depends on the change in current that the UUT needs and the inductance in the wires between the power supply and its load.

Add smaller (0.1 mF or 0.01 mF) ceramic capacitors to shunt high-frequency noise away from the DC-DC converter or UUT. In addition, add capacitors around your DC-DC converters to divert common-mode noise away from the converter. Figure 3 shows where to connect capacitors from each input and output line to power ground at each converter.

If you pass power through bus bars to the converters, install decoupling capacitors at a location that evenly distributes the distance from that point to all DC-DC converters. If you have two DC-DC converters, for example, mount the caps equidistant from them. Attach the bulk power supply’s remote-sense leads to the capacitor bank to minimize voltage losses. Figure 4 shows capacitors connected across bus bars with sense wires from the bulk power supply attached.

Figure 4. Install decoupling capacitors on bus bars where you connect sense wires. Courtesy of Datel.

Once they reach the steady state, DC-DC converters and decoupling capacitors can help maintain constant voltages. On startup, capacitors look like short circuits, and DC-DC converters act like capacitors. Theoretically, capacitors draw infinite current on power up. Resistance and inductance in wires, connectors, and the capacitors themselves limit that current, but it can still exceed the current capability of your power supplies. That can cause your bulk power supplies to oscillate, go into overload, or shut down. Typically, power supplies will shut down if their output current exceeds 120% to 150% of nominal current, depending on the design.

To prevent a shutdown, use a bulk power supply that lets you switch from constant-current mode to constant-voltage mode. Under computer control, set the bulk supply to provide a constant current for a few seconds. That will give capacitors and DC-DC converters a chance to charge, but you’ll limit the current flowing in the system. Then, switch the supplies to constant-voltage mode. Most DC-DC converters have a “soft start” capability. That feature slows the converter’s startup so it won’t try to draw a large amount of current.

You can also minimize the initial current draw on your bulk supply by not applying power to all the DC-DC converters at once. Under computer control, start the converters one at a time or a few at a time, depending on how many you have. Start the DC-DC converters only after the bulk supply has had a few seconds to reach full voltage. Many DC-DC converters have a “remote on-off” pin that lets you control them using a TTL-level signal. Start with all converters off, then turn them on a few seconds apart. You can use a PC-plug-in digital I/O card or other digital I/O device to control the converters.

DC power distribution on test racks requires careful design. You need sufficient DC power from your supplies and a distribution design that minimizes losses, runs reliably, and powers up without forcing your power supplies to overload. Proper selection of power-distribution components and good assembly practices will get sufficient power to your UUTs. T&MW

References

1. “VRM 9.0 DC-DC Converter Design Guidelines,” Intel, Santa Clara, CA, April 2001. p. 6. developer.intel.com/design/pentium4/guides/249205.htm.

2. “Lead Wire Current Carrying Capacity,” Belden Wire and Cable Electronics Division, Richmond, IN. bwcecom.belden.com/catalog/TechInfo/TechLeadWire.htm. Editor's Note 10/23/03: This page has moved. http://bwcecom.belden.com/Master%20Catalog%20PDF/PDFS_links%20to%20docs/03_Hook-Up%20&%20LeadWire/3.30.pdf.

3. AWG/Metric Conductor Chart, Alpha Wire and Co., Elizabeth, NJ. www.alphawire.com/pages/380.cfm.

4. You can order a copy of the National Electrical code from the National Fire Protection Association (NFPA), Quincy, MA. www.necdirect.org. 

For more information

Vicor (Andover, MA) offers several application notes on power distribution including “Powering Dynamic Loads” and “Soldering Guidelines for 2nd Generation Modules.” www.vicr.com/support/apps-info/apps-notes.htm. Editor's Note 10/23/03: This page has moved. http://www.vicr.com/documents/application_notes/an1_dynamic-loads.pdf.

Martin Rowe has a BSEE from Worcester Polytechnic Institute and an MBA from Bentley College. Before joining T&MW in 1992, he worked for 12 years as a design engineer for manufacturers of semiconductor process equipment and as an applications engineer for manufacturers of measurement and control equipment. E-mail: m.rowe@tmworld.com.