Power Supplies

Highly Reliable Voltage Supply

This is an example of a transistor regulated DC power supply circuit which features high reliability through redundancy. The circuit may be considered fail-safe because if any single component fails, or if one of many multiple component failures occur, the regulated output will remain the same with none of the other elements being overstressed electrically as a result of the failure.

Zener diodes, D₃ and D₄, provide the desired regulated voltage at the emitters of transistors, Q₁ and Q₂. Diodes, D₁ and D₂, provide isolation of the two sides so that the higher of the two voltages present at the emitters will appear at the output. Thus, if one side shorts or opens, the output is unaffected by the failure. Zener diodes D₅ and D₆ are included as additional protection should D₃ or D₄ fail. Resistors, R₁ and R₂, provide the required drive to Q₁ and Q₂. Resistors, R₃ and R₄, are used to reduce the dissipation at the collectors of Q₁ and Q₂.

The value of R₁ and R₂ can be calculated by considering the drive required by Q₁ and Q₂ to supply the total current required by the load. Each side should be capable of supplying this current, and the desired amount of bias current for D₃ and D₄.

The value of R₃ and R₄ can be calculated by considering the total current supplied by each transistor, the maximum allowable dissipation at the collectors, and the desired amount of bias current for D₅ and D₆.

Typical circuit elements and component values for 12V regulator are included in the figure.