Dear Colleagues,

Power supplies are critical to most engineering systems. At some time or other; we are all confronted with a power supply design issue – no matter whether you are a mechanical or electronic engineering professional. Even those of us who are non-engineering professionals, will benefit from this concise set of tips which apply to you whether you are installing power supplies for a PLC control system, your pool chlorinator to designing at the electronic circuit board level.

1.  Keep Cool at All times
Power supplies generate heat and it is critical to keep this aspect under control at all times. Especially in considering the range of ambient temperatures that your system (and power supply is exposed to). Thus you need to consider where the cooling air is coming from. From a super hot desert environment or simply from passing over other hot electronic components. Watch out for obstructions in the airflow as well. Hence, maximising the cooling air flow means bearing in mind component placement, layout of the airpath flow and your inlet and outlet routes and capacity. In a control cabinet, you will place the power supply at the top of the cabinet where the hot air can easily escape (through filters).

2.  As Goldilocks says: Too Big or Too Small Doesn’t work – it must Be Just Right
Ensure your supply is not undersized to the load otherwise its operation will be erratic. Some safety features restart the power supply when it is unable to supply a load due to overloading causing problems for your systems. Alternatively, the power supplies will become permanently damaged due to the constant overload.

Providing an oversized power supply is not a good solution either. It costs more and results in inefficient operation with extra heat (and thus additional operating costs). Power supplies operate at peak efficiency at 80% to 95% of their rated output. Try and keep them at this point all the time.

3.  Voltage Drop Can Be a Hidden Killer
As we all know - Ohm’s Law states that Volt drop equates to Current x Resistance. The delivered voltage over a short piece of wire (and remember you need to calculate both to the load and the return pieces of a wire for resistance!) can be significant even with a few milli ohms of resistance of the wire, resulting in the delivered voltage lower than that required by the load. Solutions would include increasing the nominal output voltage, shortening the wire or detecting the remote voltage delivered and adjusting the output power supply voltage automatically.

4.  Mechanical Issues Are Critical
Ensure you strain relieve all your cables from your power supplies and watch out for vibrations flexing the cables and connectors and eventually cracking the insulation or copper resulting in intermittent connections. Or shorting of the open wire against the chassis or other components and wires.

Filtering of the air to keep out dust and corrosive gases/fluids is also essential.

Careful mounting of the power supply and its associated components and wiring so there is no chance it will shake loose in a high vibration environment is also a key task.

Often open-frame supplies have an exposed section with exposed components. Ensure these don’t make contact with the chassis even with flexing or vibration.

5.  Power Supplies Working Together can be Tricky
When supplies are connected in parallel; you need to ensure that if one supply fails there is sufficient redundancy to carry on providing the load without any interruptions.

6.  Electromagnetic Interference Is Always Lurking Around
Always ensure your earthing/grounding strategy is top notch. For example, there is often a serious amount of capacitively coupled noise currents for ac line-operated power supplies. Ensure the  common is grounded to the same point as the safety-ground to minimise the problems caused by these noise currents.

Thanks to Bill Lurie of N2Power, and Bill Whitlock of Jensen Transformers for a great set of suggestions in Electronic Design May 21 2013.

Watch out that you are not following Garrison Keillor's comment when working with power supply problems:  I believe in looking reality straight in the eye and denying it.

Yours in engineering learning,

Steve