Power supply design is a rarely touched subject these days.
The design is shared to show the general idea and not as a copy/paste cloning example as your requirements are going to be very different than mine. Also the parts are very old and there are better/cheaper/higher specs parts by now.
This was one of my first Eagle CAD projects. It was an very old design for a low specs 500MHz VIA Mini-ITX motherboard with the parts I had on hand. i.e. $0 budget for me.
Note: The input is 12V regulated as it is fed from a 12V brick. Do not connect this to a "12V" car battery directly as its voltage is rarely at 12V +/- 5%.
There was some funny power sequencing issues with that MB which locks up the Ethernet port. I connected a 5V fan to the 3.3V rail (see bright brown 4 pin connector in picture) and that cures it. I would probably design it differently these days.
If you have to buy new parts for this, you are better off buying a supply e.g. pico itx supply. There are Chinese ones too.
The 12V input is switched by a P-MOSFET to the +12V rail. At idle, R4 pulls the gate of Q1 high which keeps Q1 off. Q2 is a NPN transistor with its base biased at 3V with R1 & R2 voltage divider. This is arranged in a Cascode configuration and serves as a voltage translation between the logic level control ATX on/off control from the motherboard and the 12V level. Its emitter is connected to a logic level active low ATX on/off control signal. When the signal is pulled to logic '0', Q2 conducts and pulling gate of Q1 low via R5. When the signal is above ~2.5V or so, Q2 is off.
The voltage divider R4/R5 sets the MOSFET gate voltage at about "-10V" (relative to its source) turning the P-MOSFET on. C4 is used as a feedback and together with the Thévenin equivalence resistance of the divider controls the rise time of the +12V rail. This controls the inrush current if you have a lot of capacitance bulk decoupling caps at the output.
See here for inrush control article:
A high current switch cap inverter (Microchip part) is used to provide a -12V rail. Hardly any new boards uses it.
U2 is a TI (PowerTrans) POL (Point of Load) non-isolated DC/DC module that convert the 12V input to 5V. It is always on to power the 5V Standby rail in the ATX supply. A N-MOSFET Q4 is used to switch the output to the main +5V rail. It is controlled by the switched +12V output rail. R3/C5 controls the inrush current.
No comments:
Post a Comment
Note: Only a member of this blog may post a comment.