Three common mistakes when designing power supplies

When I posed this question to companies that design power supplies, I thought that I was going to receive varying answers. Surprisingly, these companies tend to agree on the biggest mistakes that engineers/designers make in designing in-plant power supplies. For the purpose of this article , I have used  specifications from a common ATX12V Power Supply Design Guide, as examples to each topic.

1.)  Over or under specifying their total power requirements.

Avoiding this mistake will not only prevent short circuits, but it prevents the other circuitry from being destroyed. Power supplies should be designed to accommodate any increased +12 VDC currents. Under normal or overload conditions, no output shall continuously provide more than 240 VA under any conditions of load including output short circuit, per the requirement of UL 1950/CSA 950 / EN 60950/IEC 950.

2.) Not considering start-up and peak power loads required by motors, pumps, solenoids, etc.

When designing power supplies, accommodations should be made for continuous operation. A power supply shall be capable of supplying full-rated output power over two input voltage ranges rated 100-127 V ac and 200-240 Vac rms nominal. The correct input range for use in a given environment may be either switch-selectable or auto-ranging. The power supply shall automatically recover from an ac power loss. The power supply must be able to start up under peak loading at 90 Vac.

Parameter	Minimum	Nominal+	Maximum	Unit
Vin (115 VAC)	90	115	135	VAC rms
Vin (230 VAC)	180	230	265	VAC rms
Vin Frequency	47	—	63	Hz

 3.) Not fully considering environmental aspects such as dust, humidity, altitude, ambient temperatures, etc.

 All power supplies need ventilation. It is the designer’s choice of a power supply cooling solution, which depends, in part, on the targeted end-use system application(s). At a minimum, the power supply design must ensure its own reliable and safe operation.

These are some examples of what designers aim for:

Temperature

-Operating ambient +10 °C to +50 °C (At full load, with a maximum temperature rate of change of 5 °C/10 minutes, but no more than 10 °C/hr.)

-Non-operating ambient -40 °C to +70 °C (Maximum temperature rate of change of 20 °C/hr.)

Thermal Shock (Shipping)

Non-operating -40 °C to +70 °C, 15 °C/min ≤ dT/dt ≤ 30 °C/min, Tested for 50 cycles; Duration of exposure to temperature extremes for each half cycle shall be 30 minutes.

Humidity

-Operating To 85% relative humidity (non-condensing)

-Non-operating To 95% relative humidity (non-condensing)

-Note: 95% RH is achieved with a dry bulb temperature of 55 °C and a wet bulb temperature of 54 °C.

Altitude

-Operating To 10,000 ft

-Non-operating To 50,000 ft

Mechanical Shock

Non-operating 50 g, trapezoidal input; velocity change ≥ 170 in/s.

Random Vibration

Non-operating 0.01 gÇ/Hz at 5 Hz, sloping to 0.02 gÇ/Hz at 20 Hz, and maintaining 0.02 gÇ/Hz from 20 Hz to 500 Hz. The area under the PSD curve is 3.13 gRMS. The duration shall be 10 minutes per axis for all three axes on all samples.

Acoustics

For power supplies designed for low noise, the following provides some general guidance.

Guidelines Sound Power: The power supply assembly shall not produce a declared sound power level greater than 4.0 BA. Sound power determination is to be performed at 43C, 50% of maximum rated load, at sea level. This test point is chosen to represent the environment seen inside a typical system at the idle acoustic test condition, with the 43C being derived from the standard ambient assumption of 23C, with 20C added for the temperature rise within the system (what is typically seen by the The declared sound power level shall be measured according to ISO 7779 and reported according to ISO 9296.

Special thanks to Mel Bergman, TDK-Lambda

ATX12V Power Supply Design Guide,