I promised to hammer together a quick & dirty(ish) PSU for a small test CNC mill for a friend. The mill will reuse 3 vintage Sanyo Denki brush DC servomotors, specced at nominal 75VDC, ~50W, with a 24VDC brake eating a couple hundred mA each. So the PSU needs to produce 75 VDC @ max ~2A continuous and 24 VDC @ ~500mA continuous.
I am not going to make rocket science out of it, so rummaging through the parts bins i located this 230VAC to 30+30VAC ~180 VA iron transformer. This is definitely going to be the main step down component.
A quick no-load measurement revealed the voltage to be 68 VAC, accordingly the Vp of the rectified voltage will idle at ~96V or so and ~85 V at nominal load. This i consider a bit too much for the servos (actually the servo amps) so i want to drop it back to 75V. This being a servo PSU the voltage does not have to be lab quality - in fact it really does not need to be regulated as such. It just needs to be closer to 75 volts than what i get directly from the transformer after rectification.
Plan A: I could do the 3 component trick and use a resistor-zener-NPN circuit to drop the extra voltage. The dissipation however will be significant, on the close order of 30 W which is doable but a bit much. I would really like to avoid any special cooling arrangements in a smallish application like this.
Plan B: i could cobble together a simple buck circuit to shed the extra voltage with very much smaller losses. The D however will be around 0.78 and this is in the region of subharmonic instability in a trivial buck. So i need to do something about it, like a ramp compensator in the oscillator part or something. That in turn has the potential of becoming a project, which i am trying to avoid if possible.
Plan C: Locate a suitable buck IC and just apply it. I have spent a couple hours scanning DK, trying to find ms Right but so far with lukewarm success. Yes, there are chips that can be used and that will work, but my question really is, has someone done something like this recently, with a solution that a) works and b) needs a minimum amount of lab work to get there? The IC could be there but i just haven't spotted it...
I will do it one way or the other, it is just a question if someone has implemented a working solution that i could replicate. Also bright ideas are welcome, of course
