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September 01, 2000

Load-Current Margins

Question: What Design Margin Should I Use for Load Current?

Original Question: When designing a power supply, I seem to remember that you should always design for twice the current that you think you need. For example, if I have a device that requires 250mA, I should design a power supply that can provide 500mA. Is this correct? If not, what maximum current should I use in my design at 250mA?

Answer: "Twice the current" is just a rule of thumb that has some advantages and disadvantages.

The advantages are that you have some reserve if your load requirements grow and you usually have better field reliability since the parts are not stressed as much during the life of the product.

The disadvantages are the power supply is usually bigger and more expensive.

I tend to size my design for about 20% growth, select the parts, and then use the full capability of the selected parts. This approach gives you 20% growth margin to start with and then adds further growth/reliability at no additional penalties in size and cost. This exceeding-design-requirements approach is discussed as the Design Problem in my tutorial on switching-mode power supplies.

When I buy a power supply I usually buy one that has 25% to 50% growth margin.

Another rule of thumb is to look at the efficiency curve of the power supply and place your anticipated load at the maximum efficiency point, making sure you have sufficient design margin using other rules of thumb. This minimizes the life-time-cost of a system by saving electrical power and cooling and increasing reliability.

These are just my starting ground rules and I will violate them if it makes sense. Field reliability is always my over-arching concern. Others may do it differently. Comments are always welcome.

Posted by Jerrold Foutz at September 1, 2000 09:07 AM