12v to 5v converter trouble and conclusion.
So i built the converter that is meant to supplement the 5v rail of my dual-PIII taking power from the 12v one. It worked... until i loaded it up. Voltage dropped and the MOSFETs got very hot. I knew that the 555 gets a bit of DC offset when driving heavy loads at high frequencies - and in my case "a bit" was a whopping 5 volts of offset... So i did a totem pole circuit, but that seemed to work even worse...
Eventually, the DC offset of the 555 wasn't the problem, the MOSFETs didn't actually care about it in this circuit. What i failed to realize at first was that i was scoping the gate drive from gate to ground, while in the buck converter the source isn't at ground potential - it's doing the pulsed DC output! When i probed from gate to source i realized what was actually going on - there simply was not enough voltage available to drive the gates. I had around 3.5 volts which is way too low even for logic level MOSFETs. Because, 12 volts (supply) minus a volt or so (the 555's output can't swing rail to rail), minus about 8 volts (since the pulse amplitude will always be higher than the smoothed DC voltage of 5 volts), gives 3 volts. Okay, in my case it wasn't that bad, but it was still bad enough.
So the best solution is still the low-tech one - two PSUs in parallel. I was actually using this method but the second PSU i had there absolutely sucked, so i picked up one of the good supplies i had lying around and will wire that up.
Now you might ask, most modern PSUs use DC-DC converters for 5v and 3.3v. How do they do it? Answer is simple. The main PWM controller is fed with 20-something volts anyway, from the auxiliary supply (the one that does the 5vsb too). The DC-DC converters switch from the 12v rail, but their controllers are actually wired to that auxiliary supply, giving enough overhead to properly drive the MOSFETs.
Of course, now someone will chime in and say that i should have used a proper PWM controller. It wouldn't have changed the equation one bit, since the supply voltage would still be 12 volts. I have used my little 555 circuit (one 555, one LM311, one TL431) in a mains powered PSU and it worked fine (still does, just that i have no use for it). And i will use it again in my upcoming split regulated supply, just that the LM311 will be replaced by a quad comparator so that protections can be implemented.
Eventually, the DC offset of the 555 wasn't the problem, the MOSFETs didn't actually care about it in this circuit. What i failed to realize at first was that i was scoping the gate drive from gate to ground, while in the buck converter the source isn't at ground potential - it's doing the pulsed DC output! When i probed from gate to source i realized what was actually going on - there simply was not enough voltage available to drive the gates. I had around 3.5 volts which is way too low even for logic level MOSFETs. Because, 12 volts (supply) minus a volt or so (the 555's output can't swing rail to rail), minus about 8 volts (since the pulse amplitude will always be higher than the smoothed DC voltage of 5 volts), gives 3 volts. Okay, in my case it wasn't that bad, but it was still bad enough.
So the best solution is still the low-tech one - two PSUs in parallel. I was actually using this method but the second PSU i had there absolutely sucked, so i picked up one of the good supplies i had lying around and will wire that up.
Now you might ask, most modern PSUs use DC-DC converters for 5v and 3.3v. How do they do it? Answer is simple. The main PWM controller is fed with 20-something volts anyway, from the auxiliary supply (the one that does the 5vsb too). The DC-DC converters switch from the 12v rail, but their controllers are actually wired to that auxiliary supply, giving enough overhead to properly drive the MOSFETs.
Of course, now someone will chime in and say that i should have used a proper PWM controller. It wouldn't have changed the equation one bit, since the supply voltage would still be 12 volts. I have used my little 555 circuit (one 555, one LM311, one TL431) in a mains powered PSU and it worked fine (still does, just that i have no use for it). And i will use it again in my upcoming split regulated supply, just that the LM311 will be replaced by a quad comparator so that protections can be implemented.
