I am working on a pulsed induction linear motor (aka multi-stage coil gun) as a hobby project. I'm more of an electronics kind of guy so this is an opportunity for me to become more familiar with power electronics.
The first component I need to build is a 12V->500V booster circuit to charger a capacitor bank. I found the schematic below here.
I modeled the circuit as well as I could in LTSpice and I'm seeing the expected voltages. However, the currents I'm seeing through the inductor & MOSFET are way over the max ratings. The inductor is rated for 4A (continuous) and the MOSFET is rated for 13.5A (continuous), yet both are seeing currents of 15A RMS. I am well aware how dangerous things get at voltages this high so I am very skeptical about this schematic.
I don't have the exact components in my library but I tried matching everything as closely as possible. I set the series resistance of the inductor to the datasheet value and chose a MOSFET with similar drain-source resistance (ON). I included the series resistance of the 12V battery as well.
So my questions are:
- Am I being overly cautious?
- Am I forgetting to model something?
- If the schematic is wrong, what is the best way to reduce the current? MOSFET with higher series resistance (which I'm guessing results in higher power dissipation), larger inductor?
Best Answer
You want the resistance of the inductor and MOSFET to be as low as reasonable to minimized loss. The current should be limited by the on-time of the MOSFET (not the resistance).
From the basic relationship \$ V_L = L \frac{dI_L}{dt} \$. For constant \$ V_L \$, \$ \Delta t = \frac{L \Delta I_L}{V_L} \$.
Plug in peak current \$ \Delta I_L = 4A, V_L = 12V, L = 130uH \rightarrow \Delta t = 43us\$.
So you need to set the 555 to have the MOSFET on-time to be less than 43us (leave some margin).