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UC3843A: UC3843A duty cycle dropped

Milad Dowlatshahi
Prodigy 10 points
Part Number: UC3843A

Hi

I'm using a UC3843A PWM controller for a buck converter topology. for the light load condition everything is working fine, but when I increase the output power the duty cycle drops from 96% to 50%! this cause drop in the output voltage and it never gets modified. this means that the output voltage drops and FB pin remains at about 1.5V while the CS pin has the voltage about 0.4V or less. 

in other words, I use a Variac transformer to increase the input voltage gradually from zero to the nominal voltage(230V), in this case at first the duty cycle keeps value about 96% up to the input voltage of 50V, but it suddenly drops to about 50% when I increase the input voltage. I've checked the CS pin and the voltage in this pin has been less than 0.5 V (Except the noises at switching instants).

can anyone help me regarding fixing this issue? why the duty cycle drops down to 50% while the FB pin is 1.5 V and CS less than 0.5V. 

thanks

Milad

  • 0
    TI__Guru 73675 points

    Hi,

    Please provide your schematics to understand the issue.

  • 0 in reply to Hong Huang
    Prodigy 10 points

    thanks for your message, please find the schematic circuit attached.

  • 0
    Prodigy 10 points

    Please have a look at the oscilloscope waveforms. Red waveform is FB signal which is 1.6, Blue one is Output voltage which is less than the reference 80 V, Yellow represents the CS pin voltage and finally the green one shows the PWM signal. it drops from 96% to something about 50%.

  • 0 in reply to Milad Dowlatshahi
    TI__Guru 73675 points

    Hi,

    Can you only show CS and OUT - it looks CS has high noise and also CS waveform looks strange since OUT (green) stays high, CS does not show increase. But the waveforms are overlapped each other difficult to tell. So only show CS and OUT first to see if their relation ok or not.

  • 0 in reply to Hong Huang
    Prodigy 10 points

    Thanks for your message, I've got another result. the current sensor had been gone, after the replacement of INA180A3 the waveforms are shown below:

    Green: Gate-Source of the Switch

    Red: CS pin

    Blue: output DC voltage(supposed to be 80V!!)

    Yellow: Input DC Voltage

    From the above waveforms, the output should be 80 V as it is desired and designed but due to the duty cycle drop issue, the output voltage fell to 47V.

    The other thing is that, for the light load condition and no load, this converter and control circuit is working fine but when I increase the load current output and duty cycle drops down.

  • 0 in reply to Milad Dowlatshahi
    TI__Guru 73675 points

    Hi,

    It looks CS voltage step up which is not normal and it means your output inductor value too low or in saturation. The inductor current should be in a form of triangle in a buck converter. I think you need to increase the output inductance value to help help resolve this issue. Please check your design and verify the output inductor value U5 and U6.

  • 0 in reply to Hong Huang
    Prodigy 10 points

    Thanks for your message. I don't think the inductors are in the saturation region. I have tested this board for 80 V output and 6.5-ohm resistive load. It worked completely fine. but when I decrease the output resistor to 3.3 or increase the load current two times, the voltage drops and the duty cycle begins to jump to 50% when the voltage goes up from 18 V. This means that the current would be something around 5-6 Amp and it can't cause the inductor saturation while I've tested successfully at 80 VDC and 6.5 ohm load condition which draw 10-15 Amp.

  • 0 in reply to Milad Dowlatshahi
    TI__Guru 73675 points

    Hi,

    The inductor current shape is not correct. The shape should be triangle slope increase along with the pulse width increase, and triangle down when no pulse. Your waveform shows almost square. You need to find out why the shape is square. I suspect your  inductance is too low.

  • 0 in reply to Hong Huang
    Prodigy 10 points

    It is not the inductor current waveform. my converter is Buck and I monitored the negative line current Just after the rectifier and before the free-wheeling diode. it is the switch current and in buck converter input current is discontinous. 

  • 0 in reply to Milad Dowlatshahi
    TI__Guru 73675 points

    Hi,

    You labeled that as CS signal which your circuit shows it has the same loop of output inductor. I am confused of your saying. Can you check your circuit? The circuit you provided shows the current is the inductor current although INA at bottom but the loop is with output inductor.

    The inductor current shape not correct.

  • 0 in reply to Hong Huang
    Prodigy 10 points

    in the schematic circuit output current is connected to the INA at the bottom of the circuit and measures the negative side of the input current.

  • 0 in reply to Milad Dowlatshahi
    TI__Guru 73675 points

    Hi,

    Can you show INA loop to circulate current ? I see INA current same as load current, and same as U5//U6 current. Can you check the current loop of INA?