Cheapest way to do hi side drive for offline 230VAC buckboost converter ?

[ocset]

Hello,
We are doing a single  stage , offline PFC’d 150W buckboost led driver. For the high side drive, we can either do a pulse transformer drive, or  just  use an ISH1515 isolated power block to give us high side power for a hi side FET driver. –We’ll couple up to it with a digital isolator.
Which is cheaper?
The ISH1515A  costs £5.17

ISH1515A isolated power supply, 2W
http://uk.farnell.com/xp-power/ish1515a/dc-dc-converter-15v-0-133a/dp/2484128

[ocset]

 Please find the schem...plus LTspice sim
It is 100VAC to 265VAC
fsw = 65-75khz
Constant off time
Toff = 8us

I think the ISH1515 could be a mistake since it has 20pF of pri-sec capacitance, which woudl be getting banged up and down as the switching node goes up and down...Do you agree?
I think this would be a custom flyback for the gate drive anyway
 

Do you know why such custom hi side supplies for gate drives are so uncommon?... i mean they're common for electric drives at say 10kHz and less...but not SMPS's at 100kHz or so...wonder why they're so rare?

[strawberry]

Yes you can wind low capacitance transformer with IR2153 push-pull or add additional winding to 12V auxiliary SMPS transformer, this solution could be the cheapest and simplest

I have added damping capacitors parallel to flyback SMPS MOSFET, about 12pF/1kV and it somewhat gave 1W(don't remember) loss @520Vpk and 120kHz

[jbb]

If you don't need 100% duty cycle on the high side switch (seems unlikely given the AC line voltage zero cross).

How about something with a bootstrap driver like the UCC27714?  It could drive both switches for you.