Page:Gametronics Proceedings.djvu/83

 8.)Power Supply Failure Due To Overload

This failure is quite common on video monitor supplies which have no current limiting and on "in house" low cost power supplies that have basic current limiting or no current limiting. The problem is frequently caused during test or servicing by accidently shorting the supply lines. It may also occur due to a chip failure or other load short. The result is catastrophic. The series pass transistor fails shorted due to excess current if no limiting is used or due to excess dissipation when the entire voltage at 110 to 130% of rated current appears across it. The high current clears the external short (or the tester removes the short) and the full unregulated upstream voltage then appears on the output busses destroying at least several chips on the game board. This not only increases the cost of reapirrepair [sic] bu tbut [sic] the multiple fault makes diagnosis and repair more difficult and may greatly increase down time if only one replacement module is ordered and could result in a second failure.

This problem is practically unheard of in good quality O.E.M. power supplies since they all employ foldback current limiting, as the current attempts to increase beyond about 120-130% of load, the output voltage falls. As the output voltage falls, the foldback circuit reduces the output current until at short circuit, the output current is less than half the rated current. As a result, the dissipation on the series pass transistor during short circuit is actually less than the dissipation at normal full load and the power supply is undamaged. The solution to overload failures is to employ foldback current limiting with a foldback ratio of at least 2:1 on 5 volt supplies and 3:1 or more on higher voltage supplies.

We have pretty well covered the field reliability problems that can be encountered other than the video display and coin box problems and broken controls. Now I would like to cover the design considerations for games power supplies.

The prime problem for any power supply is heat. Due to the increase of chemical activity with heat, capacitor life will increase 50% for every 10%/°C. rise above rated temperature; similar life shortening occurs in the transformer insulation. Power transistors also have a life limiting function due to thermal cycling which causes fatigue failure of internal connections due to the different thermal coefficient of silicon and the metallic elements. A 25°C. temperature rise can half the thermal cycle life.

The larger the heat sink or chassis area, the longer the life and higher the reliability of the supply in general. However, the cost increases proportionately so there are some practical limitations. UL allows a maximum average chassis temperature rise of 65°C. above a 25°C. ambient. This establishes a practical minimum size chassis or heat sink. A 65 C. rise corresponds to 0.6 Watts/