What is the Isolation strength for LT8304 isolated micropower fly back converter?

The external transformer that will isolate the primary side from the secondary side (output) will determine the isolation strength. Please see recommended transformers on pg 13 of the datasheet. For example, if planning to use 13324-T087 from Sumida, the isolation rating would be 1500Vrms.

Why is the output of the LT8304 out of regulation and unstable at No Load?

The LT8304 has a minimum load requirement to ensure accurate output voltage is being feedback to the controller. The LT8304 samples the isolated output voltage from the primary-side flyback pulse waveform, which occurs when the primary switch turns off and the secondary winding conducts current. To sample the output voltage, the LT8304 must turn on and off for a minimum duration and at a minimum frequency. Even during light load conditions, the LT8304 delivers a minimum amount of energy to ensure accurate output voltage information. This minimum energy delivery establishes a minimum load requirement. The LT8304 generally requires less than 0.5% of its full load power as the minimum load. When designing the circuit, ensure to verify the minimum load requirement in real applications.

Is it possible to design a flyback converter to achieve a 1000V output from a low input voltage (4-28V)?

Designing a flyback converter with a high-voltage output presents unique challenges. For a detailed guide on creating a 1000 V output, no-opto isolated flyback converter, refer to the following technical article:

Why does the LT8304-1 sometimes restart the soft-start cycle multiple times during power-up?

During the LT8304-1’s internal 11 ms soft-start period, the controller checks whether the voltage across the external ground-referred reference resistor (RREF) exceeds a threshold—typically 0.6 V, which corresponds to approximately 60% of the target output voltage. If this condition is not met by the end of the soft-start timer, the device automatically initiates a new soft-start cycle. This behavior is expected under certain conditions, such as: Output short circuits, where the voltage cannot rise due to a fault. Excessive output capacitance, which can delay the voltage ramp-up, preventing the threshold from being reached in time. As a result, the LT8304-1 may perform multiple soft-start attempts until the output voltage reaches the required level.