SMPS with Battery Backup and NiMH Charger

I designed this circuit for the same reason that i designed the SMPS Boost regulator with the MCP1640 To gain experience in SMPS technology. This time i use the high power LM2595 buck regulator. The circuit accepts any AC voltage from 8 to 45 Volts.
This wide input voltage range is a remarkable feature! Plus, it can provide up to 1 ampere of current at 5 volts! Imagine what wold take to design a linear power supply to provide 5V 1A power from 45 volts! A huge power dissipation of 40 Watts! Your soldering iron provides probably less heat! The LM2595 has an efficiency which varies from 75% to 95%. Typically, the previously descried system has an efficiency of 80%, which means that the system will dissipate about 1 Watt of heat…
SMPS with Battery Backup and NiMH Charger

The AC input is rectified through the B1 bridge rectifier. A U LC filter decouples the circuit from the input (C4,L1,C1) and smooth’s the input voltage. The R1-R2 voltage divider along with the charge capacitor C3 perform a delay-startup. This part may be omitted if the delay-startup is not necessary. The heart of the circuit is the LM2595 with the catch diode D1, the inductor L2 and the filtering capacitor C2. This is where the step-down is done.
I’ve included several outputs. X1-6,7 and 8 are the PSU outputs. X1-6 is the ground terminal. X1-8 provides a rectified and filtered output of the raw AC input. Remember to calculate the RMS to peak conversion. If for example the AC input is 24VAC, the output at this terminal will be 34 Volts DC. The output X1-7 provides 5V to power any external circuit needed.
This circuit features also a trickle battery charger and a power failure output. The trickle charger is a simple¬†transistor constant current driver. R4 and D2 sets the base voltage. R4 gets power from the output of the SMPS chip, therefore the output of the zener diode is quite stable. T1 controls the current through the batteries. The batteries are connected at X1-2 (positive) and X1-1 (negative). D3 blocks any current flow from the batteries back to the LM2595 when the power is off. R3 sets the charging current. I’ve set my trickle charger to about 30mA, that is C/100 for my 3000mAh batteries (look below). Finally, a protective fuse ensures that the charging current will not increase due to some malfunction of failure of the current driver.
The Power Failure output (X1-5) is actually a transistor switch which is LOW when the SMPS operates, and is pulled HIGH (to the battery level) through R6 when the SMPS is not working (when there is no power). This pairs perfectly with the MCP1640 Boost regulator

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