POWER SUPPLY APPARATUS
FIG2 is a schematic circuit diagram of a power supply apparatus according to a second example embodiment
FIG2 shows a schematic circuit diagram of a power supply apparatus 200 according to a second example embodiment. The power supply apparatus 200 is connected to a first power source 1, and a second power source 2. In common with the first embodiment above, the power supply apparatus 200 comprises a transformer 210 with first and secondary primary windings P1, P2 connected to the first and second power supplies 1,2 respectively, and a secondary winding S that serves to generate an output. The transformer 210 is provided with primary switching elements Q1,Q2, input rectifiers D3,D4, an output rectifier D7 and an output filter R13,C2. The operation of the primary switching elements takes place under the control of the controller. The power supply apparatus 200 comprises a voltage detection circuit 221 that includes a voltage reference circuit U3. The voltage reference circuit U3 in this embodiment is provided as a Zener shunt regulator, and is arranged to provide a reference voltage for detection of the first power source 1, a voltage shut down circuit 222 and an under voltage lock circuit 223. In addition, drive circuits D1,D2 are provided for the primary switching elements, as integrated circuits. Suitable devices are available from Texas Instruments in their UCx84x family of current-mode PWM controller devices.
schematic circuit diagram
210
transformer
221
voltage detection circuit
223
voltage lock circuit
222
voltage shut down circuit
2
second power source
3
andFigure
200
power supply apparatus
['1. A power supply apparatus, comprising: a transformer comprising a first primary winding for connection to a first power source and a second primary winding for connection to a second power source, and a secondary winding for delivering power therefrom as the output of the power supply apparatus; a controller for receiving voltage inputs from the first power source and the second power source, and in response to the received voltage inputs control supply from the first power source to the first primary winding and from the second power source to the second primary winding, such that: if the first power source and second power source are available the controller operates to allow power to be supplied to the first primary winding from the first power source and operates to shut off supply of power to the second primary winding from the second power source.', '4. The power supply apparatus of any preceding claim, wherein the controller comprises a voltage detection circuit arranged to receive power from the first power  supply, to detect power from the first power source, and in response to operate to allow supply of power to the first primary winding from the first power according to a drive circuit for the first primary winding.', '5. The power supply apparatus of any preceding claim, wherein the controller comprises a the voltage shut down circuit operable to shut off the supply of power to the second primary winding from the second power source and arranged to selectively provide a shut-down signal to a drive circuit for the second primary winding.', '8. The power supply apparatus of any preceding claim, wherein the controller further comprises an under voltage lock circuit arranged to detect an under-threshold voltage, and in response to prevent the controller from allowing power to be supplied to the first primary winding from the first power source.']