The "no blip" power supply

Bruce Desmond
Phoenix Contact Inc.
Harrisburg, Pa.

Here's what can happen when an ordinary power supply sees an inrush current as from a motor turning on: Temporary overloads can trigger the electronic protection circuits and trip the supply off. Supplies using what's called fold-back technology will cycle off, reset, and try to cycle on continuously as long as there's an overload. Clearly, inductive loads such as dc motors or solenoids won't start up properly when the power supply oscillates on and off.

In addition, there can be other problems if these supplies trip too quickly in the presence of a genuine short circuit. Fast tripping can prevent secondary fuses from blowing, not good when a potentially hazardous condition exists.

The typical way of heading off such difficulties is to specify a supply with enough capacity to handle turn-on current as part of its normal load. But this means the supply operates well below its rating for nominal output current most of the time. The expense associated with this unused capacity makes the approach uneconomical in this day and age of scarce resources.

Power supplies built with an alternative approach avoid the need to oversize the supply. They use what is sometimes dubbed a U/I characteristic (also referred to as constant-current or constant-power output), named for how the supply's output current and voltage behave during overload conditions.

The trick is in regulation circuitry, which monitors and adjusts output power. It keeps track of both the output voltage as well as the output current. It uses this information to automatically adjust the output power to compensate for excess load. During an overload, the power supply continues to provide its maximum rated output current. But it reduces its output voltage.

The amount of voltage reduction varies depending on the type of overload demand and its severity. For example, a short circuit would cause a zero voltage at the output of the supply. A minor overload would cause only a minor voltage drop at the output. This operating characteristic ensures there is always overload protection but without nuisance tripping during load inrush conditions or when peak power is required.

The U/I characteristic also helps supplies maintain their output at high temperatures. One of the greatest strengths of a true industrial-grade power supply is the ability to put out a constant nominal current at operating temperatures of 60°C (140°F). This is crucial because temperatures in control cabinets often exceed the maximum operating temperatures of general-purpose dc supplies. There are special current-handling features that target this high-temperature operation. They are sometimes referred to as Power Boost capability.

Depending on conditions, leading industrial-grade power supplies with the U/I characteristic and Power Boost can handle overloads up to 100%. Even at room temperature these capabilities come in handy. Here supplies so equipped can put out current well beyond the nominal rating of the power supply. This is especially true for applications that need only operate in ambient temperatures of 40°C (104°F).

Makers of industrial supplies post ratings for their Power Boost capabilities. These ratings at 40°C can be 50% higher than the nominal current ratings at 60°C. In other words, the supply can permanently provide 50% more current at lower operating temperatures than the nominal current rating at higher temperatures. The Power Boost ratings for these types of industrial supplies can also be met at 60°C but for a more limited period of time.

The U/I characteristic enhances Power Boost by ensuring output power is reliable at the supply's maximum current rating. This also means a supply with a sufficiently high Power Boost rating at 40°C may get by with a lower amperage rating than would otherwise be necessary to handle an application. Additionally, supplies can be sized so their full nominal current rating at 60°C matches the application's demand for maximum current. This eliminates the need to purposefully oversize supplies when handling inductive loads.

Diagnostics

Modern industrial supplies incorporate facilities for monitoring operation and diagnosing problems. Such capabilities once took the form of a few indicator lights and little more. Today built-in alarm outputs augment such visual indicators. The purpose of built-in monitoring is to give an early warning of potential load problems. The supply accomplishes this by monitoring itself for voltage drop. As a rule, an alarm triggers when the supply output voltage drops by a fixed amount. For example, the alarm may be factory set to trigger at 20 V in the case of a dc supply with a 24-V output.

Once again, the U/I characteristic can help improve such features. Circuits that implement the U/I characteristic and function monitoring can play a part generating precise and flexible diagnostic signals. The voltage monitoring and logic capability of the U/I characteristic is what comes in handy here. Alarm circuits can trigger based on a fixed percentage (say, 10%) of the adjustable output voltage as monitored by the U/I.

This capability is especially useful when applications require a voltage output to be adjusted in the field. A good example would be 24-V applications involving lengthy cable runs for various field devices. Extended cable runs incur a voltage drop over the length of the cable. It's customary to compensate by turning up the power supply voltage perhaps to 28.5 V. Here a diagnostic alarm triggered at 20 V may not warn a control system of load problems early enough to be useful.

Of course it would be better to signal the controller when the voltage output drops below 10% of 28.5 V. Power supplies incorporating the U/I characteristic and enhanced function monitoring circuitry do this. The result in this case is a diagnostic alarm trigger when voltage drops below 26 V.

Today's industrial-control systems depend more than ever on reliable dc power. The technology behind the U/I characteristic has been available for many years. But cost and component size limitations have made it impractical for industrial users until relatively recently. Many users employ the U/I characteristic in dc power supplies strictly for better overload capability. Others choose it for Power Boost and precise function monitoring. Ultimately, users of dc power supplies who fully tap into the power of the U/I characteristic reap the greatest benefits.

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