uc3843 low side Mosfet switching problem.

[codemaster11]

today i made a dc-dc converter circuit based on only controlling the duty cycle of uc3843 (switching the n-channel low side mosfet)
not a buck or boost with inductor, to maintain 14 volt at output capacitor here i used "N-channel fet" in lowside (-ve side )
mosfet source pin as the -ve input and mosfet drain pin as -ve output. there is 4.9vdc on Vref pin #5 of 3843 & 16vdc on Vcc.
but the oscillator is not starting here is my pcb layout

 

[Easy peasy]

you've appear to have the mosfet switching the ground leg - you'll have to redesign the feedback with appropriate level shifting for the 3843 to work properly in this scenario ....

 

[FvM]

I guess you have a schematic...

 

[codemaster11]

you've appear to have the mosfet switching the ground leg - you'll have to redesign the feedback with appropriate level shifting for the 3843 to work properly in this scenario ....

yes of course i want to switch the ground leg but the fet is not switching.

--- Updated Oct 20, 2020 ---

I guess you have a schematic.

no i don't have the complete schematic as the pcb layout but only the oscillator section which only works
in buck/boost or buckboost converter. i have a problem with buck boost converter with uc3843 where the 75nf75
mosfet is getting too much hot when a load of 40watt placed on output that's why i placed the mosfet in low side
to reduce heating of mosfet in low side but here the oscillator is not starting.

 

[codemaster11]

is it possible to use high side mosfet irf4905 with uc3843 & bootstrape circuit in high side.
the problem with the above pcb is the " not switching the mosfets" the feedback to
the "Vfb pin" is from the output side. no output "volts" at output side. the uc3843 is not getting any voltage
feedback.

 

[codemaster11]

the voltage of my buck boost converter drop when i put load on it also the output current disappear from input side
by giving input power from my external power supply with Vin = 30vdc and input current(dc) = 8amp the voltage drop
on output side by putting load and set voltage value appear at output by removing load, i don't know why the output
current disappear on load and voltage drop to Vout = 5vdc when set voltage is 14.4vdc here is my pcb layout

this is the same circuit as the schematic with a little difference.

 

[betwixt]

Are C8 & C9 wired correctly?

Brian.

 

[FvM]

Are C8 & C9 wired correctly?

The circuit is apparently designed as SEPIC with unusual large capacitors. We need to know the inductor spec to check if it can work.

However according to post #1, theres no inductor in the first circuit. I don't unerstand how the circuit is expected to work without an inductor? Nevertheless should the oscillator work. But I'm not particularly motivated to reengineer the missing schematic.

 

[codemaster11]

according to the schematic the C8 & C9 wired correctly in pcb layout. another point
that i use a EI33 core with air gap of approximately 0.5 mm & wind the prim & sec
both in the same direction. without load the circuit shows the set voltage adjusted through
50k trimmer but loss regulation by puting load. its a different circuit from the one in post #1
i 'm working on both of these circuits.

 

[codemaster11]

the first PCB layout of post#1 can be only used as a battery desulfator with no output voltage controller

the first circuit is same as a low side motor mosfet driver where we can't monitor the voltage across

the motor or by placing battery instead of motor as it need a resistive voltage divider.

the main problem with the 2nd pcb layout is the output voltage drop when 60watt input power while output power with

regulated voltage is 15watt.