Hello, we are using the LM2596SX-5.0 to go from 24VAC to 5VDC. We recently got some circuit boards made and begun testing. We have 50% of our devices fail on initial power up. We are powering these with a 24VAC output transformer. The 24VAC goes through a bridge rectifier, and paired up with some capacitors it creates approximately 30 volts DC (with ripple), which goes into the LM2596.
On the failed devices, I am noticing that the high 30VDC input voltage is being output on the output pin of the LM2596. So basically 30VDC is ending up on the 5V rail and starts smoking other components on the circuit board.
I took an ohm measurement between the input and output pin on a failed board, and I see 75 ohms. (component still installed on PCB).
Here is the schematic and PCB layout centered around the LM2596SX-5.0. This is a low power device... less than 200mA. All of the capacitors are surface mount ceramic capacitors.
I have 2 additional concerns:
1. I probed one of the circuit boards that appeared to be functioning properly, and I noticed that the output voltage from the LM2596SX-5.0 was only 4 volts instead of 5. The circuit still worked because that 4V went into a 3.3V linear voltage regulator... but this is still concerning. I have not verified that every device is like this, but at least 1 of them was.
2. On several of the circuit boards that were working, we disconnected power, then re-connected power. Upon re-connecting the power, the LM2596SX-5.0 immediately smoked and failed. I have no idea what could of caused this since it was previously working just fine?
Thank you for your time, and any help with this is greatly appreciated!
