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I was working on the UCC3895 PWM generator but am having a problem getting a complete duty cycle up to 100. I have used the following diagram for my circuit: enter image description here

I wanted to use the UCC3895 to get a PWM wave so that I could give it to an H-bridge. For now, I haven't added the TL431 for the constant reference voltage and have used a simple open-loop configuration of the IC. But the results I received were not better. At a voltage of 1.16V on the EAP I was able to get a small duty cycle shown and after increasing the voltage for this pin to 5V at max, the duty cycle remained the same small amount shown below, all four outputs showed similar behavior. I want to achieve a linear duty cycle of pwm by changing the voltage at the EAP but am not able to achieve it. Kindly help me with this and correct any mistakes there. enter image description here

This is the voltage on EAP after which the duty cycle didn't change: enter image description here

The yellow diagram below shows the signal I received on the EAN pin while the other one is on CT. enter image description here

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  • \$\begingroup\$ Try to measure using scope the voltage difference between OUT A and OUT D vs pot seting. \$\endgroup\$
    – G36
    Commented Aug 23, 2023 at 17:32
  • \$\begingroup\$ The channel D is giving almost zero voltage , can see small spikes on the oscilloscopr, and the voltage of the OUTA is shown above. The voltage remains same after 1.16V as explained above \$\endgroup\$
    – kam1212
    Commented Aug 23, 2023 at 17:47
  • \$\begingroup\$ You can set trigger to Ch1, 1-2V, falling edge, to stabilize the display. Why is your EAN sometimes showing pulses? That seems important to fix. \$\endgroup\$ Commented Aug 23, 2023 at 18:52
  • \$\begingroup\$ Yes the EAN is showing these pulses and I myself think thats the issue because this is just the output of a buffer according to the internal diagram \$\endgroup\$
    – kam1212
    Commented Aug 23, 2023 at 19:15
  • \$\begingroup\$ @TimWilliams dont know why am I getting such as I have checked that my EAP is a DC straight line but EAN isnt. I got this type of wave as I increased the voltage on the EAP to 5V. I am not having any good simulator to simulate this as most of them are inaccurate. \$\endgroup\$
    – kam1212
    Commented Aug 23, 2023 at 19:18

1 Answer 1

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It is hard to help from a distance via the internet because we do not see the real circuit.

Also, do not forget that the voltage at EAP should be kept below 3.6 V for proper operation. This circuit is not a classic "PWM generator" but a phase shift PWM. What you should see at the output is almost 50% duty square wave. The only thing that is changing vs. "error voltage" is the phase shift between the outputs, not the duty cycle.

enter image description here

Only the phase shift is changing.

enter image description here

Try to build this version:

https://www.electronics-lab.com/wp-content/uploads/2021/12/Schematic-4.pdf

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  • \$\begingroup\$ Why does this circuit have the two pins of the potentiometer at EAP shorted out? Is this right or wrong? Secondly, I do not want to use the ADS and CS pins therefore I have shorted them to ground. The rest of the circuit seems to be the same as My one. Ill buy another IC and test with it to ensure the IC is working well. \$\endgroup\$
    – kam1212
    Commented Aug 24, 2023 at 14:26
  • \$\begingroup\$ In this case, the pot is working as a variable resistor. Not the best solution in this case. The better one is to use VRef --->20k resistor-->50K POT---> GND. So that we stay below 3.6V. tinyurl.com/2axsmf87 \$\endgroup\$
    – G36
    Commented Aug 24, 2023 at 15:57
  • \$\begingroup\$ so it means from 0 to 3.6 we will have an approximate phase shift from 0 to 180 degrees. The duty cycle is then related to the phase angles. Secondly the question I asked previously, I dont want to use the ADS ans CS and want to ground this, will this create any effect or not? \$\endgroup\$
    – kam1212
    Commented Aug 24, 2023 at 16:37

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