Electronic – Controlling heater with PWM through MOSFET

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?

The MOSFET takes no significant current from the output pin and the 2k2 in series with the LED will not pull the voltage down. Q2 should be a digital type (turn on 5V). The 10K could be lowered to discharge the gate-source charge more quickly and I would add a (snubber) diode across the fan just in case of spikes when it is turned off.

It works!