We are having a problem in some of our circuits that use the LT8300 controller. See schematic below.
Our application has a Vin of 24 - 26.4V and is to produce a 10V output. We use a Wurth 100uH 1:1 coupled inductor (744281101) for a transformer. Rfb is 102k. We have a diode snubber consisting of a BAV20W diode with its cathode connected to the cathode of a 110V zener (MMSZ5272BT1G) and the series combination is connected across the transformer primary. A 1nF cap is connected from the transformer Vin primary line to the negative output line of the secondary. The secondary connects to a BAT40W schottky with a 0.47uF filter and a 10k dead load. The external load can be as high as 15mA.
The problem first manifests with the output measuring about 7v instead of 10V. After removing the external load (two LT comparators), the controller chip appears to lose regulation control and allows the output to go to about 100V. I suspect that the 110V zener has some influence into allowing that level.
Some scope traces are shown next. The gold trace is measuring the switching node to 24V return. The green trace is measuring the switching node to Vin (24VIN). The first image is the entire envelope of the circuit's operation.
The next image is the switching at the very beginning of the waveform envelope.
The next image is further into the switching envelope.
And finally we get to the highest switching voltages at the end of the envelope.
Any help on what is happening and what is likely causing it would be appreciated.
