Problem with boost converter

[qwwe]

Hi, I've designed a boost converter with UC3842.
You see the electrical circuit below.
The problem is that the output should be set to 21 volts, but the output goes to much higher values.
Circuit Profile:
VIN (min)=9v
VIN(max)=18v
VIN(nom)=12v
VOUT=21v
Iout(max)=0.6A
fsw=344KHZ

At all, feedback has no effect on the output of the circuit.
This circuit is closed in the Multisim software
Thank you for helping me find a defective circuit.

 

[Harald Kapp]

Is this a problem in the simulation or in the real circuit?

Where is that circuit from?
The datasheet states on page 5 that Vref needs to be bypassed to ground with a 100 nF capacitor as close as possible to the pin.
The resistors R4, R5 and R6 seem to be correctly calculated for Vfb=2.5 V at 21 V output voltage.
It looks like the connection between R7/C4 and the base of Q2 is missing (cf. datasheet page 19) - although the connecting dots are hard to read at the resolution of your picture. Also the emitter of Q2 seems to lead nowhere but to XSC1. It should be connected to the Isense input of U1.

There may be more loose ends. Check all connections (compare to figure 25 in the datasheet).

 

[davenn]

Thank you for helping me find a defective circuit.

I agree with @Harald Kapp , your circuit has many problems. You should have followed the circuit in the datasheet.
Your circuit looks nothing like it.

Redo your circuit as per the datasheet schematic using the parts and the values that they do .... get it working and once working
THEN change the component values that are needed to be changed to give the required output voltage


I also have to say, this is a VERY complex switching circuit, there are many much easier chips to use that have a far lower external component count
and I would suggest that you look at some better designs

 

[qwwe]

The problems you mentioned come back to the poor quality of my image now, with a larger image and zoom.

I have closed this circuit but the current is a lot of power supply and the output sometimes stays on 21 volts and sometimes goes up
You said that there are better ICs if you can name it. Thanks

 

[davenn]

I have closed this circuit but the current is a lot of power supply and the output sometimes stays on 21 volts and sometimes goes up

you understand this this still looks nothing like the datasheet example ?
and that is probably the main reason why it isn't working properly.
you have no isolated feedback loop for voltage monitoring
you are not using the output inductors as shown in the datasheet

anyway, boost and buck convertors are a few $$ on eBay
eg
https://www.ebay.com.au/itm/DC-DC-B...-Module/122698377266?var=&hash=item1c9164a832

https://nl.aliexpress.com/item/XL60...Ebay&msclkid=846a80e3a46b178f0698dbcb3a5a2773


The LM2577 is a common switching chip ... see the second link

look how tiny the boards are in both links and the small number of components

 

[qwwe]

An example in a datasheet is an example of a isolation flyback converter since the input is ac=265v and the output is dc=12v .
But in my circuit, the input was ultimately 18 volts and the output was 21 volts so I did not need to isolate it.
Meanwhile, my circuit is a Boost converter( dc to dc ).