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UC3843 30W Boost problems
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ali_asadzadeh:
Hi,
I have designed a boost based on the UC3843, it's the first time I'm using this chip, I have separated the power input which is 15V and the aux power supply which is 12, to make things easier for now, every thing works as expected, the oscillator runs, the ref output is stable, and the output would be regulated to 51V,But when I try to get  some load from the circuit, the voltage drops completely! Do you have any Idea?

strawberry:
How much is it some load
That MOSFET is 3ohm, too high for some load
15V is missing capacitor
ali_asadzadeh:

--- Quote ---How much is it some load
--- End quote ---
I have tried a 120 ohm resistor


--- Quote ---That MOSFET is 3ohm, too high for some load
--- End quote ---
I know and I have used it deliberately, because it's super cheap and I want it to be able to handle 400V at least.



--- Quote ---15V is missing capacitor
--- End quote ---
How much cap should I use? the input comes from a DC power supply, also I wanted to use a 400V input cap too, so the input cap can not got any higher than 47uF because of it's size!
SiliconWizard:
I'd take a look at the ISENSE pin while the circuit is running under load.
spec:
HI ali_asadzadeh,

A few obvious points:

Decouple the 15V supply line with two 100nF X7R disc ceramic capacitors in parallel.

What is the part number of D1? It needs to be a high-speed, low-loss, Schottky type, of at least 2A

What is the part number of L1. It needs to be an HF high-current type, of at least 1A.

What type of capacitor is C1. It needs to be an HF, low ESR type.

Decouple the 50V supply line with two 100nF X7R disc ceramic capacitors in parallel.

Change R9 to 4K7

Change C6 to 4n7 (HF low ESR type)

(the new values for R9/C6 are intended to give 50KHz oscillator frequency)

Remove R8 and its wiring.

https://www.onsemi.com/pub/Collateral/UC3842B-D.PDF

https://www.vishay.com/docs/91059/91059.pdf
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