uc3843 low side Mosfet switching problem.

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codemaster11

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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
 

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 ....
 

Easy peasy said:
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 ....
Click to expand...
yes of course i want to switch the ground leg but the fet is not switching.
--- Updated ---

FvM said:
I guess you have a schematic.
Click to expand...
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.
 
Last edited:

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.
 

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 said:
Are C8 & C9 wired correctly?
Click to expand...
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.
 

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.
 

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.
 

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