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UC1825: Help with Output signal

Prodigy 30 points

Replies: 4

Views: 57

Part Number: UC1825

Hello Sir,

I am currently designing forward converter using UC1901(Isolated feedback generator IC) and UC1825 (PWM generator IC) in Conventional Voltage mode. UC1825 configuration is as shown below where PGND= Power GND or Primary GND, INV is feedback signal generated by UC1901 which is isolated by 1:1 transformer followed by envelop circuit and TRG is going to GATE terminal of MOSFET. This simulation is done on Capture CIS or Pspice. Out A and Out B, both stay low for this configuration, i.e. no pulse is generated. I can not find what is wrong with it, I need help with it. Please let me know where i am wrong as soon as possible. Generated Out A pin signal waveform by simulation is shown below,

Thank You!!

  • Hello Harsh,

    This looks like the controller is well past its internal shutdown threshold. Pin 9 ILIM/SD has an absolute maximum voltage of 6V, hence connecting it with the AUX of the transformer is not a very good idea. Normally you can connect this pin with the Vref pin that would provide a constant of 5V and use a resistor divider to set the voltage on pin 9 to be less than the Shutdown threshold of 1.4V as mentioned in the datasheet.

    I hope this helps.

    Regards,

    Sonal

  • In reply to Sonal Singh:

    Hello Sonal,

    Thanks for drawing out my attention to it. I do not want to use shutdown at all for now, so i have connected it to PGND. It did work but it doesn't matter what i connect to Pin RT and CT for oscillator frequency, it operates at 1MHz frequency only. I am having "WARNING(ORNET-1018) Connection to unmodeled pin" at pin RT,CT and Clock.

    Thanks again!

    Harsh

  • In reply to Harsh Shilu:

    Hello Harsh,

    That sounds weird to me, attached is my TINA simulation for UC1825 please cross-check this schematic with your own. I noticed that you are trying to run this controller in voltage mode so I tried to replicate that bit as well. I do not see the issue with the RT,CT pin as you reported.SLUM653.TSC

    I hope this helps.

    Regards,

    Sonal

  • In reply to Sonal Singh:

    Hello Sonal,

    I am beginner and i do not know about TINI-TI. I will check that. I am using TINA TI Spice Model SLUM654. It shows me like this 

    Oscillator is working at 1MHz frequency only. You can check RT and CT are not set for that frequency.

    Thanks. I will check TINI-TI as soon as possible

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