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| [[File:Voltage regulator foldback.svg|thumb|V-I curves for voltage regulators with different overload handling: <span style="color:green">foldback</span>, <span style="color:blue">constant current limited</span>, and <span style="color:red">unlimited</span>.]]
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| '''Foldback''' is a [[current limiting]] feature (a type of [[overload protection]]) of [[power supplies]] and [[power amplifier]]s. When the load attempts to draw overcurrent from the supply, foldback reduces both the output [[voltage]] and [[Electric current|current]] to well below the normal operating limits. Under a [[short circuit]], where the output voltage has reduced to zero, the current is typically limited to a small fraction of the maximum current.
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| [[File:Voltage regulator linear foldback.svg|thumb|Power dissipation vs. load resistance for linear voltage regulators with different overload handling. Here ''V''<sub>in</sub> = 12 V, ''V''<sub>OC</sub> = 10 V, ''I''<sub>max</sub> = 1 A, ''I''<sub>SC</sub>=0.17 A. The maximum dissipation in the <span style="color:green">foldback</span> design is three times lower than in the <span style="color:blue">constant current limited</span> design.]]
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| The prime purpose of foldback current limiting in [[Linear power supply|linear power supplies]] is to keep the output [[transistor]] within its safe [[power dissipation]] limit.
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| With a linear voltage regulator, the output voltage ''V''<sub>out</sub> and output current ''I''<sub>out</sub> are maintained by simply dissipating away the surplus of input voltage ''V''<sub>in</sub>:
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| :<math>P_{\rm dissipation\, in\, regulator} = (V_{\rm in} - V_{\rm out})\times I_{\rm out}</math>
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| Under overload conditions the output voltage falls and so the difference ''V''<sub>in</sub> - ''V''<sub>out</sub> becomes larger, tending to increasing dissipation. For a simple current limit, safely handling the worst-case scenario (a short circuit) would therefore require a much larger heatsink and output transistor than would be required under normal operating conditions. Foldback partially solves this, helping to keep the normal-rated output transistor within its [[safe operating area]] under [[fault (power engineering)|fault]] and [[electrical overload|overload]] conditions. Foldback also significantly reduces the power dissipation in the ''load'' in fault conditions, which can reduce the risks of fire and heat damage.
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| Many power supplies employ [[constant current]] limiting protection; foldback goes one step further by reducing the output current limit linearly as output voltage decreases. However it adds complexity to the power supply and can trigger "lockout" conditions with non-[[Ohm's law|ohmic]] devices that draw a constant current independent of supply voltage (such as op-amps). Foldback in [[switched-mode power supplies]] is discouraged because of these disadvantages, and since the benefit of reducing the power dissipation does not apply. Despite this, foldback is still often implemented in them.<ref>
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| {{cite book
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| | title = Switchmode power supply handbook
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| | author = Keith H. Billings
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| | publisher = McGraw-Hill Professional
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| | year = 1999
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| | isbn = 978-0-07-006719-6
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| | page = 1.113
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| | url = http://books.google.com/books?id=RwAmBvRzA3EC&pg=PT115&dq=foldback+power-supply&hl=en&ei=_1JWTPTYBoH4swPkm6mmAw&sa=X&oi=book_result&ct=result&resnum=1&ved=0CDUQ6AEwAA#v=onepage&q=foldback%20power-supply&f=false
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| }}</ref>
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| ==References==
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| {{reflist}}
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| [[Category:Electrical power control]]
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