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AC-DC Converters

SANGHEETHA A P
Prodigy 60 points
Other Parts Discussed in Thread: UCC28740EVM-525

I am looking for a  solution to generate a 5V/0.1A output out of mains supply (500V-1500V) using a AC/DC non-isolated converter. Could you please suggest the circuit with schematic and BOM.

Regards,

Sangheetha A P

  • Prodigy 60 points

    Is it possible to extend the input voltage range of UCC28740EVM-525 Schematic to 500V to 1500V ??

  • in reply to SANGHEETHA A P
    TI__Expert 4015 points

    It is probably possible, but all of the power stage components will need much higher voltage rating, as will the bulk capactior, output diode, bias doide and maybe others.

    A very large turns ratio (close to 100:1) would probably be required on the transformer and a clamp may be required on the Vs pin. The maximum negative current from the Vs pin is ~ 2mA so if the negative winding will draw more current than this, a clamp would be required.

    Hope this helps.

  • in reply to Billy Long
    Prodigy 60 points
    Hi,
    Thank you for the reply. I am having a doubt again. The frequency range for the input voltage range is specified as 47-63 HZ for UCC28740EVM-525. But i need to build a AC DC converter with
    i) a variable frequency range of below 10 HZ. (Will this increase the size of the circuit?)
    ii) negative to positive wide voltage range
  • in reply to SANGHEETHA A P
    TI__Prodigy 250 points
    plz tell more about the AC source used. also it is isolated design but you mentioned NOn-isolated to be designed.
    For 1500V DC rating , 1800V to 2000V rated MOSFET/BIPOLAR or IGBT will be requried. As ratio is huge, the SW freq should be 20-30kHz only.
  • in reply to SANGHEETHA A P
    TI__Expert 4015 points

    The lower the AC frequency the more bulk capacitance will be required to keep the minimum bulk voltage above the desired level. The IC can only tolerate a range of approximately 4:1 on the bulk cap without requiring a clamp on the Vs pin. I would recommend a minimum bulk voltage of at least 600V in your case. The bulk cap should be sized for this based on the input frequency. 

    I'm not sure what you mean by negative to positive wide voltage range. Do you mean it's an AC input?

  • in reply to Billy Long
    Prodigy 60 points
    Thank you.
    Ya, meant the AC input voltage only (for example -500 V to 1500 V). Is it possible to achieve this wide input voltage range?
    For this range, is it ok to choose the minimum bulk voltage of 600 V as you mentioned earlier?
    And one more thing, how the rating of MOSFET be chosen for this input voltage range?
  • in reply to NarendraKumar
    Prodigy 60 points
    Hi,
    AC voltage source spefications are
    amplitude:-500 V to 1500 V
    Frequency :10HZ.
  • in reply to SANGHEETHA A P
    TI__Expert 4015 points

    I don't understand your input voltage specification. Are these RMS voltages. RMS voltages are positive by definition since they are the root of a sqaure.

    AC voltage by definition swings both positive and negative.

    MosFET rating is generally chosen as peak AC voltage + reflected voltage + leakage inductance spike + margin.

  • in reply to Billy Long
    Prodigy 60 points

    The input AC voltage (-500 V to 1500 V), in that the negative voltage is just a phase shift i.e., the voltage signal does not starts from zero origin. The limit of amplitude of the voltage signal in the positive side is 1500 V and in the negative side is 500 V.

  • in reply to SANGHEETHA A P
    TI__Expert 4015 points
    Do you mean DC shift not phase shift. If the input voltage is +/- 1000V from a DC offset of 500V, then it is only a single input voltage of magnitude 2000V and the components should be sized accordingly.
  • in reply to Billy Long
    Prodigy 60 points

    If the input voltage of magnitude 2000 V is chosen, how should the minimum bulk voltage be chosen without clamping on the Vs pin?