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'[EE]: SMPS circuit (updated)'
2002\09\05@120823 by Roman Black

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I was recently asked to modify my (almost popular)
2-transistor +5v buck regulator:
http://www.romanblack.com/smps.htm

to add short circuit protection to the simple buck
regulator circuit. The person also wanted a higher
supply voltage and higher output voltage of 20v
at about 100mA. I thought the result was worth posting
to the list. :o)

Adding current limiting to my original 2-tran circuit
cost only one NPN transistor, one R and one C.
The circuit should now regulate voltage via switching
and ALSO regulate max current via switching. Simply
put, both >V and >I will cause the main PNP switcher
to turn off, so even driving into a short circuit or
overload condition the thing will still be switching
efficiently and supplying the (correctly limited)
current.

To allow input voltages up to 70v means using a darl
PNP for the switcher, to reduce base current and losses
on Rb and Rz. The top 2 transistors need to be rated at
least 100v (pref 150v). The new circuit is shown below.
:o)
-Roman



battery
+22v to ------*--------------------*-----*------
+70v in.      |                    |     |
             |                    |     |
             |                    R     |
             Rz                   |     |
             |                    |     E
             |      ,-----Rb------*----B   PNP Darl
             |      |                   C
             |      C                   |
         ,---*-----B                    |
         |   |      E-------------------|-------,
         |   |         NPN              |       |              Load
         |   |                          *---L---*---R---*----- +20v
         |   |                          |       |       |       (typ
20mA
         |   |                          |       |       |        to
100mA)
         | Zener                        _       |       |
         |   |  21v                     ^       C       C
         |   |                          |       |       |
         |   |                          |       |       |
         C   |                          |       |       |      Load
    NPN   B--*--------------------*-----*-------*-------*----- GND
         E                        |     |
         |                        C     Rsense
         |                        |     |  0.6v = current limit
  GND  --*------------------------*-----'          (150mA)
battery

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