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 necessity on 5 volt supplies since the most common failure was shorting of the series pass transistor, which could more than double the voltage on the output buss destroying many load IC's.

Today, well designed highly reliable supplies such as those produced by Adtech Power have mean time between failures of 75,000 to 100,000 hours. Even the dual supplies exceed 50,000 hours. Field failure experience shows much less than 17 a failure rates on the well proven standard supplies and under 0.17, in most video game applications. Also the cost of the load IC's has dropped considerably. In order to protect the load against even a 1% failure 100 overvoltage protectors would have to be purchased for each protection. Since the OVP costs from $5 to over $15 each depending on supply size, it is seldom economically justified unless we are protecting an extremely expensive circuit. There are some extenuating circumstances however. If multiple supplies of the same polarity are used and there is a likelihood of a higher voltage supply being tied to the 5V buss, an OVP may be justified. If a very low cost and crude power supply with a very low MTBF (under 10,000 hours) is used, OVP may be necessary although it would make more sense to put the cost of the OVP into building a more reliable power supply. If a new unproven power supply design is used, it would be wise to use overvoltage protection on early runs until a field failure history is established. If the repair cost of the board to be protected exceeds $500 (excluding the service call that would be necessary to replace or repair the failed supply) an OVP would generally be justified.

I have tried to cover the main factors that affect life and reliability of the power supply and the overall system. The rest of the power supply design will be found in the classical engineering texts. One strong caution however, all diodes are not the same. Surge current ratings can vary 5:1. All electrolytic capacitors are not the same; for a given capacitance and voltage, ripple current ratings can vary by more than 4 to 1 among different types and manufacturers. Be sure your components are well specified in all important parameters at the anticipated temperatures and know your vendors. In some cases, the selection of a good, reputable and reliable manufacturer will be more important than the adequacy of your design. Bargains in the critical areas can be extremely costly in terms of rework, field failures, lost sales, and reputation.

The last consideration is the build or buy decision. If you are going to build well over 10,000 supplies and have the necessary engineering expertise and manpower and the time, facilities, and equipment to thoroughly test the reliability of the supply under adverse conditions before it goes into production, it will usually be more economical to build your own supply. Be sure however that you really consider all costs when you make that decision. These