About powering the UC2843 in DC-DC converter application

[belax]

Hello,

I am building a fly back DC-DC converter based on UC2843BN (8pin-version).

The DC input will be from 10V to 27 V.
The DC-out 5V.

I have question about supplying the UC2843.
If I directly connect VCC pin to DC-input, my MOSFET Gate-To-Source will exceed its maximum value of 16 Volts, when the DC-input is above 16-17 volts.

I decided to build a series emitter voltage regulator, which should keep it’s voltage around 12.4 Volts when the DC-input is grater than ~14 volts and should keep its voltage almost equal to DC-input when the DC-input less than 14 volts.

I would like to ask if this UC2843-supply scheme is stable and reliable for my application? And also, would like to ask if there another typical solution of powering UC2843, when the DC-input in the range from 10 to 27 volts.

Thank you.

 

[petarpav]

Hi. You can put zener diode for 14-15V between VT2 gate and ground.

Best Regards.

 

[belax]

Hello,

Thank you for your advise for Zener in the gate path. I will consider it as my last opportunity.

I attached the schematic of my two-outputs DC-DC converter:

It is DC-input 10-27 V.
Output 1: Non-isolated 5 V (0-300 ma), which is directly regulated by UC2845.
Output 2: Isolated 12 V (0-300 ma).

I still have questions about powering UC2845.

The transformer I will design will mostly work in CCM. In DCM it will work when the load will be close to 2 Wats.
Is my Feedback/Compenstaion network good enough (stable and not too slow) for such mode?
I have never worked with UC284x before.

Thank you.