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[SOLVED] Regarding UC3844 IC used for PWM switching

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MD_SHAHRUKH

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Hello everyone,
My question is related to the circuit below.
Capture1.PNG
This is a reference image for the problem which I am facing here. I wanted to know the behaviour of the UC3844 IC in the circuit. How the ISENSE PIN gets input as feedback to control the output pulse to the MOSFET. I am designing a welding system for which the reference circuit is all about. I am understanding that the output pin can give constant frequency using the RT/CT pin and variable duty cycle using the ISENSE pin.

How the duty cycle is changing with changing ISENSE value? I have referred to the UC3844 datasheet where the internal diagram shows that the PWM output turns off if i will give 1V to ISENSE pin, then how it is a current sensing pin?

Please quote me If I have missed something. or provided some insufficient data

Thank you.
 

kindly show the entire schematic

the missing stuff to the right likely has a connection for Isense
that will aid in answering your question
 

It appears that the schematic either:
- has a drawing error
- they are operating the 3844, which is a current mode controller, in voltage mode.

Incorrect in either case.

EDIT: Indeed, you require to show us the complete schematic.
 

Hi,

The Isense pin senses current indirectly. The current that you want to sense has to be made to flow through a resistor of suitable value, called current-sense resistor. This resistor has to be appropriately sized so that the voltage drop across this resistor (being Ipeak*Rsense) is 1V or a little below 1V.

- - - Updated - - -

Usually for flyback, you have the sense resistor between the MOSFET switch source terminal and primary GND.

- - - Updated - - -

Like has been stated, your schematic requires review.
 

Thank you for your reply.
Here is the extended schematic.

Capture3.PNG

called current-sense resistor. This resistor has to be appropriately sized so that the voltage drop across this resistor (being Ipeak*Rsense) is 1V or a little below 1V.
I can relate to this explanation. How to control the output current from the TR1 transformer.
Please correct me here, I am guessing that the ISENSE pin is getting 1V whenever the current transformer is sensing the current exceeding the limiting value which will be set by a potentiometer here and the combination of the resistors in the image/schematic before the ISENSE out. So, the MOSFET will be be switched at a constant frequency of 42kHz (calculated using f=1.72/(RT+CT)). And the MOSFET will be turned ON and OFF again but not because of the switching frequency but now because of Current transformer feedback. And this continuous current sensing leading to controlling the constant current output delivery.?
am I correct here?

Another thing which I found in the datasheet of UC3844 is that the duty cycle can be varied. If this is happening, then how the UC3844 is varying the duty cycle? Can you support me this question with graphical answers, like how the current is changing at ISENSE and how it varying the PWM duty cycle?

Thank you.
 

Thank you for your reply.
Here is the extended schematic.

View attachment 156669
...
So, the MOSFET will be be switched at a constant frequency of 42kHz (calculated using f=1.72/(RT+CT))
The UC3844 has a maximum duty cycle clamped at 50%, so if your switching frequency is 42kHz, then your oscillator frequency should be set a 2*42kHz = 84kHz. Hope you considered that.

...
And the MOSFET will be turned ON and OFF again but not because of the switching frequency but now because of Current transformer feedback. And this continuous current sensing leading to controlling the constant current output delivery.?
am I correct here?
The sensed current alongside the voltage at the COMP pin sets the duty cycle by switching the MOSFET off and the switching frequency sets the switching period by turning the MOSFET back on.

Another thing which I found in the datasheet of UC3844 is that the duty cycle can be varied. If this is happening, then how the UC3844 is varying the duty cycle? Can you support me this question with graphical answers, like how the current is changing at ISENSE and how it varying the PWM duty cycle?

Thank you.

The voltage feedback loop produces a voltage (let's call it Vb) at one terminal of the comparator. This voltage serves as a reference for comparing the voltage across Rsense. As the Rsense voltage rises from 0V, it gets to a point where it is equal to Vb and so the comparator changes state and the MOSFET is switched off. It is important to note that the Vsense rises at a constant rate. When there is less load demand, Vb decreases. When Vb decreases, then Vsense equals Vb sooner and so the MOSFET is switched off sooner. This decreases the duty cycle. As more load is demanded, Vb increases and so it takes longer for Vsense to equal Vb and so duty cycle is increased.
 

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