boostbuckconverter
I am looking for a Buck-Boost converter design that can take an input of 85 or 90 vrms to 265 Vrms. I would like the output to be in the range of 112-115V (DC) and current at least 150mA (dc). The overall power output at least 17Watts
Please Help I’ve been searching for a few days now and cannot find.
*Found a few by Fairchild however they will not work due to size of the transformer *
This is very nearly the description of the first half of a common PC power supply. You could learn how they work, and then use many of the parts from one. But they are not simple.
What that part of the circuit does is rectify the input to a high voltage DC. The resulting DC is then run through a switching supply with isolating transformer to get the desired voltages. Regulation is done by looking at the output voltage and using it to control the switching system upstream.
It is rather involved, but this may be necessary in order to get the wide input range you seek.
The diagram you're building from is basically wrong in a number of critical respects.
Firstly, the positive output of the boost is on the lower rail. This makes it much harder to follow. The return path from the boost is then not to battery ground but to battery positive! What you've actually built is a strange sort of resonant tank circuit; that's consistent with the observation of a large no-load voltage but no ability to drive actual loads.
You've also used a PNP symbol for an NPN transistor. There's no base resistor so you're potentially sinking a large current from the PWM pin.
Here is a more conventional boost design: http://reibot.org/2011/08/07/intro-to-boost-converter/ which uses a MOSFET, which is probably going to be required to achieve worthwhile efficiency.
Try this instead:
simulate this circuit – Schematic created using CircuitLab
That works in the simulator (try "time domain", time step 0.00001s for 0.01s or 0.1s). Ignore the part numbers, they're circuitlab defaults.
Edit for the record: the TIP120 was another problem, Calin switched to MOSFETs which worked properly. I'm still skeptical about achieving 5V @ 500ma from this as that implies >1A input from 2.1V (NiMH?) batteries through a breadboard, but Calin now has it basically working.
Best Answer
In general, this sort of "universal" AC-input power supply is going to be done as two separate stages: a boost converter (optionally including power factor correction) that creates a loosely-regulated 400VDC bus, and then a buck regulator that converts that to your output voltage, using a transformer to achieve isolation from the mains.
As an alternative, you might consider purchasing an off-the-shelf switching power supply rated at, say, 24V @ 1A (this would give you your isolation and most of your required certifications) and then use a relatively simple boost regulator to convert this to 112-115V @ 150 mA.