Linear power supply vs. switched

June 11, 2017
Heavier and larger aren’t always better

Switching dc power supplies, also known as switch-mode, and regulated linear dc supplies are the most common technologies used to produce dc power in control-system design. Unregulated linear dc supplies are an option if the load is constant and can tolerate some fluctuation in dc voltage caused by variations in the incoming ac supply.

Switching power supplies gained popularity over the past 40 years because of their high efficiency (> 80%) and small footprint. Basically, switching power supplies take the incoming ac voltage and turn it into a high-frequency 20- to 500-kHz ac voltage that is then stepped down to a lower voltage using a small transformer. The voltage is then rectified, filtered and regulated. The high efficiency translates to less heat produced and more output current in less space when compared to a linear supply. Disadvantages of switch mode power supplies include the remnants of ac noise voltage included in the dc production and electromagnetic interference (EMI) created by the switching transistors used in the supply’s circuitry.

In almost all industrial applications, this unwanted electrical noise has little or no effect. Common sensors and actuators are robust in design with respect to the supply voltage. Some applications where the undesired noise may have an effect are in audio equipment, precision measurement devices and possibly some communication systems. When using devices sensitive to dc supply voltage fluctuations in a control scheme, follow the manufacturer’s requirements. Equipment malfunctions due to electrical noise are often difficult to isolate.

Linear power supplies have been around for as long as ac power. The efficiency is much less than that of a switching supply, usually 40%-50%. The lower efficiency means more waste heat to dissipate. Regulated linear power supplies operate directly using the 50- or 60-Hz ac supply voltage. The lower frequency requires a larger step-down transformer and a larger footprint than a switching supply of the same size. Different input voltages (100-120 Vac and 200-240 Vac) require a different part number or the connection of different primary voltage taps. Linear power supplies are simpler devices than switching supplies, so their reliability can be better and the dc output voltage is free from the high-frequency noise of a switching supply.

Consider unregulated linear power supplies only when the dc load is constant. Providing power for a dc motor with a static load or a fixed set of indicating lamps are a couple of examples.

The price of components used in the electronics of a switching power supply keep coming down in price; inversely, the cost of copper in a linear supply continues to go up. For dc supplies with a small output current, linear supplies are usually less expensive. As the output supply current rating increases, the price difference begins to reverse until higher current switching supply become more cost effective than a linear supply.

The specification sheet for a dc power supply lists the necessary characteristics to choose the correct model. The efficiency rating, if listed, is directly translatable to the amount of heat generated by the device. Heat greatly affects the reliability and lifespan of a power supply. Nearly all industrial dc power supplies have a heat sink to remove excess heat from the electronics. Some also employ a fan for forced air cooling. Mount the power supply following the manufacturer's installation instructions, including allowing free air space around the heat sink portion of the enclosure and mounting in the proper horizontal or vertical angle of inclination.

AC input voltage

A range of input voltages and frequencies allow the same product offering in countries that have different electrical standards. Switching power supplies often have a wider input range than linear models. Unless specifically designed to operate with a voltage inverter, supply voltage should come from a utility or rotary generator. Inverters usually supply voltage that is not a sine wave and can quickly damage the supply’s electronics due to excessive heat.

Inrush current for a dc power supply can be many times greater than steady state current. The amount of inrush and startup time affects the current rating and trip curve of the circuit protection device. Inexperienced controls engineers often miss this fact and end up replacing the branch circuit protection device in the field after several nuisance trips.

Input EMI filters are required in some locations and are a low-cost way to protect the power supply from noise voltage generated from other sources along with protecting other devices from the noise produced by the power supply.

Other decisions

If the output terminals of a quality power supply are shorted together, it is expected that the supply does not explode, burst into flames or melt. The power supply should employ a current-limiting or shutdown feature to prevent harm to the device. Document the type of reset on the electrical drawings or equipment manual. Maintenance personnel will thank you. After an over-current event occurs, the power supply should at least provide a visual indicator that the event took place. Some products provide a digital output that can be monitored and archived in the fault history of the controlling device.

Other features a power supply may offer are remote sensing, remote control, under-voltage alarms, overheat alarms, runtime monitors and maintenance forecast outputs, as well as fieldbus and communication connections.

Remote sensing inputs have a second set of wire connected to the load. The second wires measure the voltage at the load and allow the power supply to compensate for any voltage loss from the current carrying conductor’s resistance. Remote control features may allow the power supply to be turned on and off, or they may vary the output voltage based upon an analog input level. Under-voltage alarms provide a signal or indicator when the current load placed upon the supply exceeds the upper limits, and the supply has reduced that available voltage to protect the dc circuits. Overheat alarms can indicate the supply is experiencing an excessive current draw or insufficient cooling of the enclosure. An overheat alarm may signal a need for additional cooling or start a controlled cycle stop of the equipment before the supply goes into thermal shutdown. Runtime monitors track how long the unit has been in operation. Maintenance monitors use total runtime and temperature to calculate a suggested replacement period. Fieldbus and communication capabilities allow a control or SCADA system to do in-depth monitoring of the health and performance of the power supply.

The dc power supply is the lifeblood of any control system. When choosing a power supply, consider the environment and operational parameters. Additional features beyond providing a dc voltage and current are nice if your design intends to use them, but unnecessary if not required. Simple is usually better.

ALSO READ: Why bother with energy efficiency?

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