Switching power supply selection pointers

The linear power supplies of the past are being replaced with today’s switching type that are smaller, lighter, and more cost effective, but require several additional considerations for their specification.


Over the years I have had to provide consulting/engineering for PLC and DCS control panel hardware that literally cannot be shutdown. One item most often over looked is the switching power supply. What is best suited for application? The linear power supplies of the past are being replaced with today’s switching type that are smaller, lighter, and more cost effective, but require several additional considerations for their specification.

Power supplies for safety applications typically require multiple power supplies for redundancy. This ganging of several power supplies can have added hardware outside the power supply such as diode auctioning to the DC Bus. Manufacturers have now gone the mile to provide diode auctioning and or circuitry that would allow the connection of power supplies together directly to the DC buss with no voltage drop issues.

It is common to operate power supplies in an enclosure or area where the temperature will be elevated above the normally rated current capacity of the power supply. A number of manufacturers provide a de-rating curve of the current capacity of the supply at these elevated temperatures.  Consider the de-rated power supply curve value for your anticipated operating temperature as your available output limit and then  “up size” your power supply as required to compensate for this temperature de-rating. Consider failure of cooling components in your design for the reduced or worst case cooling condition. 

The power supply will produce heat in an enclosure.  Use the heat dissipation curve or value from the power supply manufacturer. Heat will kill electronics. Your power supply is the life blood of your PLC, DCS, HMI, and field devices.  I recommended performing a heat contribution summary of the enclosure components and a dissipation analysis considering whether this enclosure is inside or outside, sealed, with or without sun shade, and next to a heat source (boiler, radiant heat).  Provide cooling solutions if required to reduce the heat to satisfy the power supply and electronics in the enclosure, typically to be less than the lowest rated temperature maximum of any electronic device in the enclosure.  

Enclosure wiring, fuses, and electrical components can produce voltage drop from the power supplies to the field devices. Yes, fuses of small mA size have voltage drop. The combined voltage drop within a cabinet can consume several volts so appropriately size wiring, fuse selection, and components to minimize voltage drop. In cases where this is not an option, select power supplies that can be adjusted to compensate for the voltage drop within the cabinet.  An example is a 24 VDC power supply that can be adjusted between 24 and 28 VDC. Additionally, manufacturers have power supplies that are stated to be 24VDC and are actually outputting better than 25.5 volts. On adjustable supplies, note the rated output current can change. Refer to the de-rating curve.     

Before you turn up the supply voltage, check that the field devices can withstand the adjusted voltage increase. An example of this is protection barriers, MOV’s, transorbs, zenor diodes and instruments. Lessons learned have shown that some surge protection components will start regulating and will eventually fail after a period of time or load down the supply. 

Fusing of power supplies has been notable in some cases. Some power supplies have equipment fuses inside the power supply. Some power supplies have self-protected input to the power supply (no serviceable parts), and output of the supply (no serviceable parts). Consult your manufacturer and local code authority in that you may require a branch or supplemental rated protection device. Be aware of the Short Circuit Current Rating (SCCR) of the enclosure where the power supply will go and components to include power supplies. The output of the common switching supply has protection in several different ways as follows:

  1. The output supply will “crowbar” or shutdown when short circuited or over-current is reached. Reset is by recycle of the primary power.  Alarm relay contacts can be optional.  
  2. The output supply will maintain the voltage until the maximum current is reached and crowbar or shutdown. The supply will return to operation after a short period of time. 

The output supply will maintain the voltage until the maximum current is reached and basically laydown or vary to zero volts based on the over current load. 

Overcurrent and short circuit protection are recommended on the output of the power supply. It is ever so important that the proper size and type of fuse be used for the loads to avoid a crowbar, shutdown, or self-limit of the supply. Your process may survive that one field device fault, but loss of the total power supply solution is not an option. 

Solenoids and relays have different steady state and inrush current requirements. This inrush can be multiple times above the sealed value.  Linear power supplies of the past were transformer based and could support load inrush typically 400 percent. Load inrush of a typical switching supply is usually limited to 5 percent. Take the time to summarize the inrush and steady state values and size the switching power supply to within the rated power supply capacity. 

Leave yourself some room for spare capacity, otherwise, those last minute changes in the field will place your power supply capacity in jeopardy.   

Label the fuse holder area, power supply, and schematics with the proper type and size fuse.   

Torque the power supply component screws and lugs to manufacturers torque values. You would be surprised at how many power-related troubleshooting activities end with a loose wire.  

Surges to a power supply output from solenoids, relays, or devices can blow fuses or render the power supply off-line for a brief moment or longer. Lessons learned have been to check all of the surge producing field devices and apply the proper surge protection.

A reliable control system requires with a dependable power supply solution. With these few points, a switching power supply can be integrated in applications with great success. 

This post was written by Dan Auringer, P.E., PMP. Dan is a Principle Engineer at Maverick Technologies, a leading automation solutions provider offering industrial automation, strategic manufacturing, and enterprise integration services for the process industries. Maverick delivers expertise and consulting in a wide variety of area including industrial automation controls, distributed control systems, manufacturing execution systems, operational strategy, business process optimization and more.

Maverick Technologies is a CSIA member as of 9/8/2015

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