UC3845B startup issue

[SATENDER]

Dear all,

I am making a SMPS to operate from PV voltage (35-120V) to 24V@1A . circuit is not working because VCC is not developing even though startup resistance is reduced. No VCC , there is no Vref = 5V and hence no opertation. I have attached my schematic as well pcb layout. Please share your feedback on layout as well.

 

Thanks a lot

 

[AScha.3]

Hi,

as you have R25 , 5k, to get some voltage to start at all - what voltage you measure at 24V in at VCC ?

0.5 V ? ...then maybe D7 is rotated or mirrored . 

[SATENDER]

my mistake, i am geeting round 7.25v @vcc. vref= 2.6v. 

[AScha.3]

So 7.2 V is not far from 8.5 V = starting voltage ;

try :

1. use bigger C15 , try 10uF . (0,1u is too small, chip cannot work)
2. if not starting, remove D7 (maybe wrong, bad, ...)
3. if not starting, lower R25 ; try 2k . (but something wrong here, should need not so big start current.)