I'm making a PCB for an audio amplifier + power supply (circuit here).
The power supply voltage rails with my transformer end up being about +/- 37V, and I want to generate a 5V rail from this for powering some cooling fans on the same PCB. This is pushing the limits of a typical voltage regulator (e.g. the max. input of a L7805 is 35V).
I tried a DC/DC converter (a R-78C5.0-1.0) but this produces audible noise on a speaker at the amplifier output (I put 10 uF decoupling caps at the input and output). One thing I didn't try yet is the EMC filter suggested in the datasheet here.
I found "high-voltage regulators" (e.g. an LR8), but the max. output current is 30 mA, and my fans need about 100 mA.
Is there a better way for generating 5V from 37V without producing EMC that I can hear as noise on the amplifier?
UPDATE: I'm an idiot. It turns out the noise was coming from having some signal wires too close to the transformer. I rearranged everything and now I can use the DC/DC converter without producing any audible noise at the speakers.
Best Answer
Custom regulator
Build your own linear regulator to get the flexibility you want, you won't need anything advanced here.
simulate this circuit – Schematic created using CircuitLab
Q1 is an emitter follower and R2+D1 sets the output voltage, which has to be about 0.6 to 1 V higher than what you want, because of the voltage drop through Q1's base.
R1 is optional and is just there to take some heat off of the main transistor. It may be easier to keep a power resistor cool or within limits, than it is to keep the transistor cooled. Without R1, Q1 will have to dissipate 3 watts, which requires a heat sink. If R1 is installed, Q1 should only dissipate 1 watt at most. R1 also acts as a crude current limiter, because the voltage will drop too much at high currents.
I just picked Q1 from something that seemed reasonable and existed in circuitlab. Which one you can get varies depending on where you live and where you order your components. It must be able to handle the voltage, current, and a couple of watts.
It's worth noting that no matter what you do, using a linear regulator such as this, a 7805, or an LM317, will necessarily create at least 3.2 watts of heat as losses, while delivering 0.5 watts to your fan. It does not scale well with multiple fans in parallel either, seeing how you would have to remove a lot of heat somehow.
Better solution
I noticed this:
If you have more than one fan, you should connect as many of them as you want in series, the optimal being 3 x 12 volt fans which you can connect directly to your unregulated supply.