Problem
We wanted to use an off-the-shelf Mini-ATX motherboard in a project that must incorporate a battery backup to ensure continued operation after an AC mains power failure. The power supply had to operate from a +24VDC primary supply and cost had to be minimized. Other system features were also included on the circuit board.
Solution
We designed an ATX compatible power supply with a battery charger to keep the standby batteries fully charged. The supply functioned as an ordinary ATX supply supporting motherboard power up and software controlled power down. In addition the supply provided an isolated AC power monitoring function.
The circuit board also contained a high power analog driver circuit driven by programmable frequency synthesizers, a serial RS-232 to RS-485 converter plus RS-485 network transceivers, a LCD module interface, a relay capable of switching 250VAC, an audio amplifier speaker driver, a programmable fan driver and a few pushbutton switches. Three edges of the board were lined with connectors to other system devices. Two microprocessors controlled the on-board circuits, operating under the direction of the Mini ATX motherboard through a serial network.
The hodge-podge of different circuits and a large number of off-board connectors posed a PC board layout challenge. The numerous switching power supplies were a significant noise source and they occupied about half of the board area. The other circuits had to be arranged around the supplies, with careful ground plane and return path layout to prevent crosstalk. The 250VAC circuitry needed additional high voltage isolation. The audio speaker driver and the high power analog driver circuits were especially susceptible to interference from the switching supplies. An additional concern was suppression of electrical noise transmitted to the many connecting cables so the system would receive FCC approval.
Results
Careful power supply design, selection of components and circuit board layout gave us an inexpensive board that met all of the system requirements.