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Interference Free Switch Mode Pre-Regulator

 
I created this circuit to fix an overheating problem with a small Chinese-made stereo set. This simple solution to a perplexing problem worked so well that I felt the need to share it. This circuit can be used for other applications as well
 
The problem was that a voltage regulator had to drop the 18 volt main power supply voltage to 8 volts at 500ma to power the CD player, wasting 5 watts of power and causing a lot of heat inside the compact unit. This circuit acts as an interference-free pre-regulator to greatly reduce the power loss..
 
The output voltage of this circuit is unaffected by power line fluctuations. Load voltage fluctuation is only dependent on the on-resistance of Q2 and the value of C2 (re: ripple). The output voltage can be set so that the ripple lulls are just above the drop-out voltage of the linear regulator at maximum load for maximum energy conservation. The higher value you pick for C2, the more energy you can save and the more stable the pre-regulatorís output voltage.
 
The only down-side to this design is that an inductive kick is produced when Q2 switches off. This is absorbed by C1, is used by its load, and is therefore not wasted. If the load on C1 is not sufficient to drain off this extra charge, D9 must be added to drain some of it off, protecting C1ís connected circuitry.
 
In the case of the stereo set, the worst case condition will be at CD spin-up. The load at C2 will be maximum and the load on C1 will be minimum (no audio output).

 

Andrew R. Morris  morr3763@bellsouth.net



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