The plasma display technology is relatively new and has generated a whole new set of requirements for regulated power supplies. A plasma display will typically require a power supply which generates highly regulated 200 volt D.C. and 5 volt D.C. outputs. The supply voltages are generally isolated from other voltages within the computer and the power supply must generally be current limited and short circuit protected. An additional requirement of the power supply used with a plasma display is that the 200 volt D.C. output must be enabled only after other computer operational voltages have stabilized. If other operational voltages fail for any reason, the 200 volt D.C. voltage for the plasma display must be disabled. This sequencing of the 200 volt D.C. output is required to prevent damage to the plasma display.
An adjustable three terminal regulator, due to its not having a ground end, can regulate high voltages as long as its differential voltage rating is not exceeded. In Linear Technology Data Book, 1986, at pages 71-72, a circuit is described which will protect the regulator from a short circuit condition but, as stated in the reference, such a scheme for high voltage regulation is limited by the power dissipation capabilities of the device in series with the regulator.
With an input voltage of 250 volts and a typical current of 200 milliamps, the power dissipated is approximately 50 watts. Should a short circuit condition exist for an extended period, which may occasionally occur, an excessive amount of heat is generated requiring an unacceptably large power transistor and heat sink.
An attempt to reduce the power dissipation by causing the current limit circuitry to oscillate causes the load change to be reflected to other outputs, causing high ripple content at a frequency approximating the current limit oscillations.
Further, providing an input voltage to the regulator that will not exceed the safe operating levels of the regulator becomes a concern due to the voltage overshoot caused by transformer leakage inductance. Left unimpeded, this inductance can allow the input voltage to vary by as much as 30%.
The plasma display technology further requires a sequencing or timing control of the 200 volt D.C. output to the display to prevent possible damage o the display. The display voltages must be enabled only after all other computer operational voltages have been established and have stabilized at their normal level. In the event one of those operational voltages fail, the display voltages must be disabled. This sequencing or timing requirement requires coordination of the display voltages with other operational voltages within the computer.