Electronic – Designing a MOSFET circuit for low pass filtered PWM operation and with saftey considerations

What are you trying to accomplish with PWM? Do you want to convert the voltage efficiently? You can't do that without an inductor:

Can a charge-pump be 100% efficient, given ideal components?

If you do add an inductor, then you have a buck converter. You can roll your own, or buy them as complete modules.

Or is efficiency not as much of a concern as simplicity? If your load won't require more than \$25mA\$, then we aren't talking about a whole lot of power. At worst:

\$25mA \cdot 300V = 7.5W \$

is dissipated, either in the load, or in something dropping the excess voltage. The share of that between the load and the something else is determined by the voltage required by your load. A TO-220 can dissipate \$7.5W\$ with a heatsink, and around \$2W\$ without.

If you can deal with the excess heat and reduced battery life, then what you want is a linear regulator, which will be simpler, cheaper, better regulated, and more reliable than any inductorless 555 PWM scheme, while not being any less efficient.

There are many ways to make linear regulators, enough to merit another question, but it would be hard to get simpler than this:

^{simulate this circuit – Schematic created using CircuitLab}

Regulation is poor and could be improved with an error amplifier, but it's hard to get simpler. It will be just as efficient as your 555 circuit, and at \$2.5mA\$, how efficient do you need to be?

If you want to switch a big current, at a small voltage, it's easy, just whack plenty of volts on the gate to make sure it's fully on (in this case 10V is where it is specified, so 10 or 12V would be a good voltage to pick).

If you want to control a big current, at a small voltage (for example, to get 100A +/- 5% into a small load resistance), you'll want to close a control loop around the MOSFET with an error amplifier. Either a low-value shunt resistor or a hall-sensor could be used to detect the current. That way the amplifier will be responsible for controlling the gate voltage (which will lie somewhere between the threshold voltage and 5 or 10 volts) and will alter the gate voltage to maintain the current as the MOSFET heats. The amplifier should have a supply of perhaps 12V to allow it to drive the gate fully on.

The concept is illustrated here:

Typically \$R_g\$ would be about 100 ohms, R would be perhaps 1K, and C might be 10nF. Rs will determine your transfer function \$I_{out}\$=\$ V_{in}\over R_s\$, and must be rated to not overheat with the maximum possible load current.

A suitable resistance might be something like \$1m\Omega\$ or \$500\mu\Omega\$. Care needs to be taken in the layout at such currents (Rs will need a Kelvin connection).

An approximate calculation for C is 0.2 * Cin, assuming Ro is 100 ohms, Rg is 100 ohms and R is 1K. So the values I show will be stable for loads of up to 50nF at the MOSFET gate. That should be okay for your purposes \$C_{ISS}\$ = 31nF typical (no maximum given), since you state very low voltage, but for a high voltage Miller capacitance will add to the \$C_{ISS}\$ and you might want to increase R a bit if the step response overshoots at all. When the loop is operating properly, the MOSFET gain will reduce the effect of \$C_{ISS}\$, but it's better to have it stable under all conditions.

The loop compensation reduces the frequency response into the audio range-- if you need to drive a huge 200A MOSFET at many kHz, an ordinary op-amp is going to need some help driving the gate.