A common complaint we get here at Digi-Key comes from people who buy a linear voltage regulator from us and then call in, saying that they “got a bad part”. The symptoms are either that the part shuts down (no output voltage), or in some cases, it actually overheats and dies.
Most problems people have with linear regulators are not because of a bad regulator. The problem is caused by a lack of understanding of how they function. A linear voltage regulator controls the output voltage by adjusting the impedance of its internal pass transistor, which drops the excess voltage from the input until it reaches the required output level. This voltage drop, multiplied by the current passing through it, creates heat which must be removed from the internal silicon die to prevent an over-temperature condition.
Typically, problems arise when one attempts to drop too much power across the regulator, which creates more heat than it can dissipate. Even with a good sized heat sink, the device may overheat if the combination of output current and the differential between the input and output voltage is too high.
Just because an LDO says it can handle 125V on the input and supply 700mA on the output doesn’t mean it can do those at the same time.
Let’s look at the TL783, which has a Vinmax of 125V, for example. If you are trying to drop a voltage from 100V to 95V and you want the full output current of 700mA, you will need to dissipate about 5V\times700mA = 3.5W; an amount that is quite reasonable with a heat sink. If you are trying to use it to drop a 100V rail down to supply a 24V load, you’ll have to get rid of 76V \times 700mA = 53.2W, which is essentially impossible for it’s package type. Say you did want to drop a relatively high voltage to a low voltage, such as from 100V to 5V, without using a heatsink; you’d want to limit the amount of current to 10mA or so, which would give you about 0.95W to dissipate.
Therefore, if the regulator is getting too hot and not functioning as expected, the solution is to do one or more of the following:
Increase heat removal capability by using a more efficient heat sink
Increase airflow with a fan
Reduce the power dissipated inside the device by reducing the input voltage
Find an alternative solution such as a switching regulator module which can handle the conditions
Below is a link to further information on determining heat sinking requirements given a set of input and output conditions.