zener diode calculations?

Question

how do you find the worst case for keeping a zenor diode in breakdown. Which is the worst case below.
case 1: 11.2volts input, current 0A
case 2: 11.2volts input, current 100mA
case 3: 14 volts input, current 0A
case 4: 14 volts input, current 100 mA.

Also to calculate the max current throught the diode without exceeding its max power dissipation, Would i use the zenor voltage x the maximum regulator current. On the data sheet they list a test current and a maximum regulator current, which one do i use or what is the difference?

Answer 1

The worst case for maintaining zener voltage comes when the supply voltage is minimum and the draw current is maximum. The series resistor must be low enough to supply the required zener current under those conditions. The power dissipation is the zener voltage times the actual operating current through the zener. The maximum power dissipation occurs when the zener is carrying its maximum current. This in turn occurs when the supply voltage is maximum and the load current is minimum. The test current is what is used to guarantee the specified zener voltage. Designers will normally require that the minimum current is somewhat more than the test current to ensure good regulation. The maximum current represents the maximum safe current that the diode can handle. Theoretically operation at above maximum current will destroy the diode, but the specs are usually conservative. Maximum current is to be avoided in any good design. This is not necessarily related to the maximum power dissipation; excess power dissipation destroys the diode by overheating, and you can exceed max power for short periods. Excess current breaks down the junction, and can destroy the device instantly.

Answer 2

Use the voltage X test current.
Test current is nominal useable current.
Max regulated current is just what it sounds like.
And...will lead to an over-heat failure.