A 20V to 10V DC-DC converter based on a 5V switching power supply sawed out of
a settop box PCB. The original SMPS has a fixed 5V LM2596 switching regulator.
The feedback path of the LM2596 contains 2 internal resistors to set the
output voltage to 5V. Inserting a 10K resistor between pin 4 and the output
220u capacitor increases the output voltage to approx. 10V.
See the LM2596 datasheet for details.
To get overvoltage protection (mini laptops are not that cheap) a lowdrop
lineair voltage regulator, build with a TL431 and a power FET, follows the
switching regulator. Voltage drop across the FET is below 0.5V @ 1 Amp.
Test results
Iout A
Iin A
Vin V
Vreg V
Vout V
Effeciency %
0.5
0.28
19.8
10.0
9.9
89
1.0
0.55
19.7
10.0
9.7
90
1.5
0.83
19.7
9.9
9.6
89
2.0
1.11
19.5
9.9
9.4
87
Test setup with laptop power supply and
DC dummyload
Test data
At 2 Amps the regulator gets warm, but not hot.
After half an hour @ 2Amps the output voltage drops to 9.2V
Archive containing the Xcircuit file (encapsulated postscript).
