Isolated power supply 2

There are primary side sensing converters e.g. LT5373 that uses the flyback pulse during the off period to regulate the output voltage. They have an article on this: Primary-Side Sensing Takes Complexity out of Isolated Flyback Converter Design.

I took a DIY approach using a regular boost converter and adding my flyback pulse sensing circuit.  The flyback pulse is rectified across C2. Q1 is a current source that is used to translate the voltage to the feedback pin across resistor R2 thus regulating the voltage across C2.   The current source is also a dummy to keep the boost converter running.

D1, C2 and R3 looks similar to the snubber used in a typical flyback converter and also functions as one.

I used the LT1961 as a simulation model because of the same switching frequency to the MT3608.  Value of R2 is changed for the higher reference voltage.

I noticed negative high voltage spikes during startup in LTSpice simulation.  These spikes goes away as the secondary side load increases as voltage ramps up.

I use a clamping diode on the switch pin to ground. The BAV99 diode comes with a second diode and is used for that purpose in my design.

The output has a PI filter consists of a small ceramic capacitor, a ferrite bead and a bulk tantalum capacitor.  At1.2MHz switching frequency, the ferrite does a good job of filtering out these ripples.

Green trace: voltage across ceramic capacitor before the ferrite bead
Red trace: voltage across tantalum capacitor after the ferrite bead

This circuit isolation is limited by the  PCB spacing as well as the enamel on the magnetic wires between the primary and secondary side.  Here is the layout on a 0.8" x 0.815" (20.3mm x 20.7mm) single sided PCB.

This circuit has good line regulation but poor load regulation unlike the LT5373.  A secondary side LDO would help.  

No load voltage: 7.45V. Full load (40 ohms) voltage: 6.51V +/- 0.01V between input voltage of 12V to 18V.  The wall wart that this module is replacing has a no load voltage 7.8V and 6.74V at 40 ohms load.