Electronics > Beginners
Transformer voltage for dual power supply (+12V / -12v)
David Hess:
--- Quote from: hli on September 01, 2018, 08:06:08 pm ---
--- Quote from: David Hess on September 01, 2018, 12:40:02 pm ---How does the datasheet claim it should not happen while including SOA data in both the specifications and curves?
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To quote the datasheet:
Like any of the IC power regulators, the LT1083 has safe area protection. The safe area protection decreases the current limit as input-to-output voltage increases and keeps the power transistor inside a safe operating region for all values of input-to-output voltage. The LT1083 protection is designed to provide some output current at all values of input-to-output voltage up to the device breakdown.
When power is first turned on, as the input voltage rises, the output follows the input, allowing the regulator to start up into very heavy loads.
I understand this as "you can start into a heavy load and it should work".
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The exact same thing applies to the LM317 and LM337 and it is discussed in some old application notes. They can start into heavy loads as the input voltage rises but an output short can result in the regulator latching off at high input-to-output voltages.
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--- Quote from: David Hess on September 01, 2018, 12:40:02 pm ---The difference is about 7.5 versus 15 volts but the LM317 has almost twice the junction-to-case thermal resistance making up for this. Under the same conditions, the current limit curves are actually better for the LT1083/4/5. The guaranteed tested curves are lower but I suspect this reflects a more conservative attitude because the LM317 datasheet does not even include them.
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From what I can see the LM317 actually does not enforce its SOA. It will limit over-current, and otherwise will reduce output current only when it gets hot. This means that you can, short-term, violate its SOA (or stated otherwise: it has no defined SOA)
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I know the LM317 enforces its SOA at low temperatures but it may be not as picky as the improved Linear Technology parts which have their current limit trimmed during production.
--- Quote ---It might be that I dug my own hole, though, since I have an additional capacitor at the ADJ pin - this might slow down the rise of the output voltage just enough to exhibit this problem in the right circumstances. I did not experiment with this too much, since I had a LM350 at hand which behaves like the LM317 in this regard.
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A partial solution is to use a higher current part but this comes with the disadvantage that I mentioned of raising the current limit which may not be desirable. Another solution is to add an input cascode transistor to absorb excessive input-to-output voltage. And another solution is to use an input transistor to boost the output current of the regulator. Of course adding external power transistors comes with the risk of damaging the power transistor under adverse conditions if care is not taken with the circuit design.
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