Building a DC Boost Converter

Hello All,
So I have created so far a boost converter which is driven by my arduino's pwm. I am trying to keep a steady output voltage when various loads are attached.

I am using a single cell 3.7-4.2v 18650 type of battery, a mosfet two capacitors, a shottky diode and have tried a wide range of inductors from 0.06uH to 0.110uH. I can get a lot of volts out this thing but the voltage is always reduced to almost nothing when I attach a load.

I am attempting to power a small heating element. These elements range in ohms from 0.02 up to 1.8. My ideal output voltage would be 5V and I would like 10 amps. I cant figure out whether I need to do some type of load isolation, need more boost or what.

While this isn't my first electronics project I am still pretty new to all of this and would really appreciate some help.

Thank You, John

my parts are like this:

my arduino is actually a pro trinket 3.3v and it runs as 8MHz. So, as I understand it it's running at 100KHz.

I was using diode DO204AL but have pulled a much larger one out of an old power supply thinking that might help. it did not.
The inductor is 33uH and I've have tried 8uH and 110uH. The capacitors are electrolytic . C1 is 16v 10uF and C2 was the same but am now using 35V 220uF.

The mosfet that I'm using now is STP27N3LH5 I was using RLB8721

A few bits for you:

First, A high current boost converter is very difficult circuit to build. I have wired a similar circuit in typical prototype fashion, and due to the large loop it didn't work much at all. When it was moved to a PCB with a proper layout, it worked fine. If you are not using a PCB, keep the Q1, schottky, C2 loop as small as possible. I'd recommend doing an internet search to learn more about boost topology and the high dI/dt current loop.

Second, the power level you are looking for is pretty high. 5V across 0.02 ohms is 1250 Watts. I don't think your battery can put out that much power. You battery might be able to put out 100 Watts or more, but it would pull the voltage way down and would require a more complex circuit. The 1.8 ohm heater would draw a more reasonable 14 Watts. If you have a 0.5 ohm heater, that would draw the 10A at 5V that you mentioned, and be 50 Watts. It's also possible that the resistance of the heating element goes way up when it gets hot, to maybe the one that is 0.02 ohms at room temp will no be quite so bad after it warms up.

The mosfet datasheet shows the Rds on specification with 10V Vgs. You are only using 3.3V, and I'm not sure how well the arduino can drive the gate. I'd recommend some type of logic level mosfet that is more fully enhanced at a low voltage. Another solution would be to use a small boost circuit to generate 10V at a very low current, and then use that to power a mosfet driver that could fully enhance the larger mosfet.

The electrolytics will probably not be sufficient. The output capacitor goes from charging to discharging very high currents at very high frequencies. You will probably see large voltage variation and premature wearing out. Normally, capacitors with very low parasitic resistance are needed such as ceramic or tantalum.

One thing you might try is just hooking up the various loads to the battery without turning on the PWM. Then you can see how the battery handles the load on its own. If the battery gets pulled way down already, it won't be a good power source for the boost circuit, and you may need multiple batteries in parallel or a larger battery type.

I hope that helps you get closer to a working circuit. Good luck.

When looking at this sort of problem, you MUST do the maths.
You say that you want 5V at 10Amps, but when you look at the range of load resistances, this does not add up. The ampere hour and maximum current capacity of the battery do not stack up either. Roughly speaking, it provides a few ampere hours, and I would not recommend discharge at more than 2C, or say 4 amps, which will result in voltage sag, battery heating, and no more than half an hours running. Again, you must do the maths. V=ir and w=iv are essential ingredients for solving this type of problem.
The boost converter you have shown is just a basic model, in practice you need a controller providing feedback if you want to maintain regulation on the output for changes in source voltage and load resistance. A boost converter would be doable by a hobbyist at the higher load resistance, but not at the lower one. There is a lot of info on the WWW on designing boost converters, you need to do some reading, and work through the maths of some existing designs, as Abe said, the PCB design is also quite critical, for example,
http://www.ti.com/lit/an/slva372c/slva372c.pdf
http://www.simonbramble.co.uk/dc_dc_converter...gn/boost_converter/boost_converter_design.htm

Cheers,
Richard