Electrical – Transient power supply with LM317 and LM2576

failurelm317powerpower supplyraspberry pi

I have a power supply to power an 8W LED board and a Raspberry Pi.

enter image description here

I have used a 24VAC (12-0-12V) 2A transformer to feed the bridge. I have also used a LM317 to provide a voltage of approximately 15.8V for the LEDs. The output is also used by the LM2576 to provide the 5V for the RPi. The V+ socket is connected to the LED PCB through a switch and the return to ground is an unbroken connection. The Rpi is always ON. The LEDs consume 500mA at 15.8V and the RPi consumes a maximum of less than 1A at 5V (measured using an ammeter.)

When I power ON, with the switch connecting power to the LEDs (full load,) the circuit works without a problem. However, when I switch off the LEDs after power ON, soon after a few seconds the RPi also goes down. The 5V output is now 2.3V and not sufficient for the RPi. The V+ reads 2.8V to 3.3V.
Even if I power on without the switch connecting power to the LEDs, the RPi soon shutdowns due to low voltage.

This is really puzzling for me. How can a power supply work fine at full load but fail at lesser loads? Has anyone had such issues before and if so how did you solve it?

Best Answer

Connect the buck converter output to C1, not to the output of LM317. This will reduce current through LM317 and reduce power wasted as head in LM317 too.

Also add ceramic decoupling caps on the input of the buck converter. Something like two 1-10µF MLCCs of appropriate voltage, placed and routed for low inductance. You also need at least one MLCC on the output, like 10µF 6.3V.

enter image description here