hey all need your biggg help here:

phenomenon: to realize an adjustable module PWM: UC3842 using switching power IC，open loop system; Vf pin was connected with Potentiometer adjustment feedback voltage, compensation pin was not connected. the result was not as i expected. i can indeed change the duty cycle via potentiometer outside, but in a very small arrange and not accurate enough, take D=60%, and a small move, it would turn to D=40% or 30% for example.
my speculation: if the compensation pin was disconnected, which means the op-amp is working in open cycle and linear range is very small and has a really big slope. then i should insert a resistor between voltage feedback pin and compensation pin to let the op-amp run in closed loop. is that okay?

if you have any theory and method, please give a hand here. thanks.

and there is one more thing: there are two frequencies named CH1 and freq1 on the oscilloscope, but i noticed occasionally that there's another frequency shown on the top-right place, what does that mean?

thanks again.

 

hey all need your biggg help here:

phenomenon: to realize an adjustable module PWM: UC3842 using switching power IC，open loop system; Vf pin was connected with Potentiometer adjustment feedback voltage, compensation pin was not connected. the result was not as i expected. i can indeed change the duty cycle via potentiometer outside, but in a very small arrange and not accurate enough, take D=60%, and a small move, it would turn to D=40% or 30% for example.
my speculation: if the compensation pin was disconnected, which means the op-amp is working in open cycle and linear range is very small and has a really big slope. then i should insert a resistor between voltage feedback pin and compensation pin to let the op-amp run in closed loop. is that okay?

if you have any theory and method, please give a hand here. thanks.

and there is one more thing: there are two frequencies named CH1 and freq1 on the oscilloscope, but i noticed occasionally that there's another frequency shown on the top-right place, what does that mean?

thanks again.

They pretty much jump from min to max duty cycle if you operate them open loop.

The 3842 was the most common PSU chip in most VGA monitors, there are various schematic/service manual archives where you can download a pile of tried and tested example circuits.

They almost always needed isolated outputs, so regulation feedback was via an optocoupler. The LED section is usually in series with a TL431 programmable Zener - that is fed by a voltage divider across the rail that's most important to regulate accurately. The opto transistor acts on the 3842 control pin. Using an opto whether you need isolation or not, probably gives the lower component count.

 

The Uc series chips use 2 methods of voltage feedback.
MOD NOTE, BOTH CIRCUIT EXAMPLES USE TRANSFORMERLESS SUPPLIES.


1) uses a Mirror winding on the transformer of the output to apply voltage on the feedback pin.



2) uses a Tl431 zener and opto-coupler on the isolated output..

 

The Uc series chips use 2 methods of voltage feedback.
MOD NOTE, BOTH CIRCUIT EXAMPLES USE TRANSFORMERLESS SUPPLIES.


1) uses a Mirror winding on the transformer of the output to apply voltage on the feedback pin.

View attachment 119251

2) uses a Tl431 zener and opto-coupler on the isolated output..

View attachment 119252

Primary side sensing is fairly common on laptop power-bricks because the laptop has its own internal regulators and the main input voltage isn't all that critical.

Most computer PSUs directly sense the 5V output for the best accuracy they can get. Generally; any 12V rails rely on tight coupling of the secondaries for acceptable accuracy. One ATX PSU I hand traced, used a single TL431 with the 3.3 & 5V rails summed into it with a resistor network.