I made a boost converter with 1 1mH inductor, a mosfet, a 100uF capacitor rated for 400v, a 20 ohm 1/2 watt resistor, and an ICM7555 cmos timer.
Using previous knowledge and wikipedia pages on the subject, I assumed that my volt6age should be brought from a 5v input to 10 volts. I assumed that the inductor effectively became a battery in series with the source. After seeing a ridiculous (and continuously rising) voltage, I became suspicious.
What is going on? I not getting 10 volts, but I am rather getting a continuously rising voltage. My mosfet is rated for 55 volts so I haven't been testing above that, but it rises steadily until I disconnect it at 50 volts. If anyone could take the time to explain what is going on, that would be great.
*NOTE : My 555 timer is connected with unknown capacitors (small) as C and a 1k as Rb and a 10k as Ra.
Circuit diagram:
EDIT : For clarification, I changed "but it rises steadily until it reaches 50 volts" to "it rises steadily until I disconnect it at 50 volts"
Best Answer
I can't even guess where you got the notion that the inductor would act like a battery. That's not at all what inductors do. Stop making stuff up. Go back and learn the basics.
What is happening is the current builds up in the inductor when the switch is on. When the switch turns off, that current will continue to flow in the short term.
A useful way to think of inductors electronically (not how the physics works) is that they give inertia to current. You apply a fixed voltage to a inductor, and the current will ramp up linearly. The slope is proportional to the voltage (how hard you push) and inversely proportional to the inductance (how much inertia the current has). When you reverse the voltage, the current will then ramp down linearly.
If you try to stop the current abruptly, it won't let you. The inductor will make as high a voltage as it takes to keep the current going immediately. This high voltage will cause the current to ramp down fast, but this still takes finite time. This is just like trying to stop a moving mass instantly. It will make a high force, which then slows down the mass quickly. The faster you try to stop the mass, the higher the force will be. That may require damaging whatever is trying to stop the mass.
Your inductor is making little squirts of current thru the diode every time you try to shut off its current. The current can't go thru the transistor anymore, so the diode is the only option. It takes whatever voltage the cap is at to allow this current. The higher this voltage, the quicker the current in the inductor will ramp down. You should be able to notice that the voltage on the cap builds up more slowly as it gets higher.
When the cap voltage gets to the maximum voltage the transistor can handle, there is now a second path for current to take. That is thru the transistor, which won't be good for it.
Again, go and learn about inductors and boost converters. There should be much out there.