UC3843: PWM Duty Cycle

Hi Hong,

Thank you so much for your response.

We made the V FB pin to 2.5V. Now the output is not stable and oscillating from 10 - 12V.

Very high ripple in the output 

Hi,

We removed R16 and added 10K resistor in series with the capacitor. still we see the same issue.

No load condition 10 ~ 12V oscillating
Load condition, voltage down to 4 - 6V

Note : AUX voltage designed for 16V but getting 8-10V from begining 

Hi,

Did you try add some load at your converter output to see if you can get AUX voltage to close 16V?

Also can you try to did connect D11 and use an external power supply to bias VCC to see if your converter output can have about 12V?

Another thing noticed is R9 should connect to the intersection of D12 and D11 you can try this.

What is transformer turns ratios and what is your Vout ?

Hi Hong,

1. Yes, 12K Load Resistor Connected. AUX voltage nearly 12V

2. Yes it can reach 12V but output is not regulating.

3. Yes I changed connection in Hardware to connect to the intersection of D12 and D11

4. 

Fsw : 28Khz

Core : EE42 / 21 / 20

Primary ------ --------30 Turns
Auxiliary --------------3 Turns
Secondary 1--------- 8 Turns
Secondary 2--------- 8 Turns
Secondary 3--------- 8 Turns
Secondary 4--------- 8 Turns
Primary Voltage ------250 VDC
Secondary Voltage ---25 V
Secondary Current ---1 A
Auxiliary reached 12V

Input voltage up to 28V getting output voltage 12V 

Secondary Voltages reach above 50V when input voltage reaches 30V . And Secondary diodes get burnt.

Hi,

12k load is too small. Can you increase load to about 100mA to see if the output voltage can be better regulated?

You mentioned when input 28 V the output 12V, but 30V input makes 50V output. Can you keep input 28V and measure the Bode Plots and check the phase margin?

Thank you for your quick response, will increase the loads and update you 

We don't have frequency response analyzer to check BODE PLOT and PHASE MARGIN, please advise is there any alternate method?

Hi,

Noted, we are planning the same 

Now we added 120 ohms (10W) load resistor and below modifications. Now output voltage is 0V  at 150V input. Auxiliary voltage is stable at 8V. Please check and advise

If we increase up to 200V input, AUX voltage still remains same 8V

output voltage 2V

Hi,

Please provide the OUT pin waveform along with the waveform of ISENSE, RT/CT, FB and COMP when AUX is stable at 8 V.

Hi,

Please check shared waveforms

1 - OUT
2 - I_Sense
3 - V_FB
4 - Comp

1 - OUT
2 - RT_CT
3 - V_FB
4 - Comp

Hi,

CS (purple) reached over 1V to terminate the pulse so the current sense resistor value needs to be smaller.

Also, what is your transformer primary winding inductance?

Hello,

Calculated Primary Inductance 636uH. But in actual inductance in transformer 2mH

Current Sense Resistor changed 0.22E to 0.1E. Now the MOSFET is turned on completely after applying more than 30V making fuse Blow (Now added 5A fuse).

Waveform captured is at 30V input

1 - OUT
2 - I_Sense
3 - V_FB
4 - Comp

Hi,

Please retake the waveforms but change I_sense oscilloscope vertical scale to 0.1V/div.

The OUT shows full duty is because FB < 2.5V.

Hello,

Will take and send 

Can we use same transformer or need to change ?

Hello,

Okay, we will modify transformer 

Below shared waveform  0.2V/div (we couldn't capture 0.1V/div - it's beyond screen)

1 - OUT
2 - I_Sense
3 - V_FB
4 - Comp

Hi,

When OUT is high ISENSE should keep rising. But your waveform ISENSE did not. So it looks your circuit had something not correctly wired. You need to find why ISENSE did not keep rising during the time when OUT is high.

Hello,

Okay

we are arranging new transformer, once replaced will check and update you 

Thank you