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18W offline flyback with large secondary capacitor.

We are currently looking to design a 18W offline flyback as follows:-


Currently I have done the design  and have chosen to use UCC28C40D , and another design with UC3843.

-However, my boss believes the component count is too high and says we need a different solution.

-Please can I speak with Texas staff  about our requirements?




Vin = 85-265VAC


Vout1 = 36V     +/-2V;            13W

Vout2 = 5V,      +/-0.25V;         4W

Vout3 = 3.1V,   +/- 0.3V;         0.6W


(Vout1,2,3 are fully SELV isolated outputs….not isolated from each other though)


Efficiency = 85%


Standby Power =  <500Mw.

Frequency = 50-150KHz.


Skip-cycle during light load.


Capacitance on 36V Rail = 30mF.

(…..this is for energy storage for when there is a mains power cut when the customer is  running  fluid from the tanks….it  closes the  fluid flow off with the stepper motor valves)

-this 30mF  capacitance should  be charged up in  ~ 1 second after power_ON……but we may allow this to extend up to say 3 seconds.










Notes on Standby:-

Our standby requirement is interesting, because during standby, our 5V & 3.1V  rail is actually loaded with 170mW……..

….-this is due to the RF transceiver, which must keep listening for an ON command from the remote   device during standby.)


High  energy storage capacitance:

Most PWM controllers go into hiccup mode when  charging up our big secondary capacitance. –and so are not useful to us



Anyway,  myself and the R&D Engineering Manager, were wondering if you may have any suggestions to suit our requirement?





……..please note that LT1243  is pin for pin compatible with our intended device, (UCC28C40D) from Texas


Here is the simulation file in LTSpice:-


 my phone number is UK 07798 851534