I appreciate your comments ! Thank you
September update: Finished the debugging of layout. Fixed a lot of simple mistakes. Reached the design goal of 0.5% accuracy in 3 of 4 ranges. The lowest impedance range is too sensitive to layout because of copper traces / probes ohmic resistance. Currently reconsidering use of several parts, replacing them with higher cost parts. About 10% of schematics changed. I will publish it ASAP.
To answer the comment about mixing the 555 chip into the middle of high cost parts.
I honestly tried to find the most advanced, contemporary, easy to use up-converter and made a lot of reading about available converter chips. There is a lot of interesting, sometimes simple and sometimes sophisticated ways to generate multiples of voltage rails with integrated DC-DC converters. At the moment with all of my knowledge, there is no single chip converter which complies to my requirements.
DC-DC power supply requirements:
- The conversion must start with control inputs being unpowered
- The conversion must stop when control inputs are triggered
- After certain period, the conversion must restart no matter what control inputs do (for example software errors, poor programming, etc)
- Converter must be "stateless". There should be no latched states, requiring software resets (for example after the overloads, thermal shutdowns)
- Converter must be popular, very well known, understood to let even unexperienced users to gain usable knowledge
- There should be multiple schematics adapted by community to allow easy upgrades, contributions to schematics from community
- Converted must have zero activity (even internal generators must stay completely) shut when required
- Converter schematics must be not tied to any advanced magnetics: transformers with calibrated gaps, ferrites with more than 500 KHz requirements.
- Magnetics for converter must be easy to implement, replace, calculate. So consequently several complex to calculate or tune designs are not acceptable: inductors with high DC bias, autotransformers energy gaps and with pulse width ratios, coupled inductors designed for super high efficiency at 1 MHz, etc.
- Diodes must be a common schotky type, not a specifically rare ones
- The design must be crude, able to start and stop in a worst supply, load conditions
- Efficiency is nice to have but is not critical. Some costly parts can be avoided since users are not interested in super high efficiency.
- Efficient board space is nice to have, but not critical
- Other details: At 3-4 watt, it should feed 6 DC rails (current prototype runs at 4.85..5.15V with current 0.89..1.25A).
- Most important requirement 1: it has to output multiple voltages.
- Most important requirement 2: it must be super easy to understand for starter.
- Most important requirement 3: It must not self destruct with 0 load conditions. Who knows what users will do during the build.
At the end I finalized my choice on 555. (At least I, myself can explain every single stage and part, why it was chosen, how to debug it, how it works).
Other details about what is being changed in power supply and what lessons learned:
- The worst mistake was choosing the particular P-MOSFET. Fixed the mistake by replacing it with "logic gate level" P-MOSFET. The lesson learned is: look at actual curves in datasheets and realize that at worst gate voltages MOSFETs behave as current limiters, not as milliohm resistors. To get to milliohm range at >1A at 4V use the "logic voltage gate" MOSFET. The cost is correlated to gate voltage capability, has to pay for it, but there is good reason why it costs so.
- The PNP current limiter from Art of Electronics. It is useful only when MOSFET control is ideal - high gate to source voltage. In situation with poorly chosen MOSFET the PNP part can be removed completely with no effect on work of the circuit. I still decided to keep PNP after upgrading to ideal MOSFET. It is yet to be seen if it helps. The thought is to protect random unpredictable external supplies from overloading. The absolute limit must be 1.1..1.2A, or else the USB supplies (even ones rated for 2A) will act up. I do not want to force users to start suspecting that their USB wall-warts are at fault (even when they most likely will be at fault if device starts consuming 2A).
- Interesting limitation of 555 chip: The reset input must not exceed 555 VCC rail even few millivolts. So I had to move reset input to blue LED with guaranteed 3V level < 4.25V VCC.
- Another correction (attached schematics has to be changed): power stage with transformer (mid tap) has to be supplied from before 1N4007 diode.
- Another improvements: Add jumpers in several test points (voltage controls, for oscilloscope, multimeter probing etc), add low ohmic resistors as current test points for approximate current measurements.
- Other good find: At low power levels and relaxed efficiency requirements - copper wire of transformer coils works as a free low-ohm high power resistor. It is OK to treat it as resistor for up to 0.25W with no problems.
BTW: 1N4007 may look like another cheap offence to surrounding high cost parts. The reason to choose 1N4007 (not even lower 1N400X) is that I needed indestructible piece of very low doped silicon to have the simple isolating DC switch with leakage current at very low nA level.