Modeling Control Systems in LTSPICE

2019

The AC analysis functionality of LTSPICE is a convenient, accurate, and intuitative way of modeling control systems. This is especially convenient for designing compensation networks for switched mode power supplies.

Typical USB Lighter

Linear Regulator

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Overview of Driver Board

Full Assembly

The source of the noise was the fact that the device switched in the audible range. The unit I measured was around 16kHz. Above 10kHz, you run into the problem of older engineers not being able to hear switching noise and not realizing it is an issue. I haven't had the chance to do this yet, but I plan to replace the micro controller with something that is pin compatible so that I can bring the switching frequency up to 20KHz. I'll also increase the duty cycle to maintain the same peak current as I currently see to get a bit more power out of the unit.

MOSFET Gate to Source Voltage After Switch Pressed

MOSFET Drain to Source Voltage

Input Current From Battery

LTSPICE Simulation

The inductance of the primary of the flyback transformer can be calculated from the slope of the current waveform. It turned out to be around 22uH. The inductance of the secondary can be calculated as well, but other measurements would be required, so I put a guess that results in an open circuit voltage on the output of 20kV in my simulation. I selected a MOSFET with a suitably low turn on voltage, but since LTSPICE doesn't model avalanche on MOSFETs, I had to throw a Zener diode in parallel to get the correct clamping on the switched node.

LTSPICE Schematic

Simulated Waveforms