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UCC28722: revised Non-Isolated SMPS for smart meter (Application Report SLUA721)

CHAMKHI HAITHEM
Prodigy 110 points
Part Number: UCC28722


Hi,

I'm designing my power supply for my single phase smart meter based on this Application Report SLUA721 and i have some questions:

1/ I'm wondering why in this Application Report they designed the power supply using the maximum voltage as line to line voltage and not as line to neutral despite that this power supply aims a Single Phase Smart Meters which mean the power should be drawn from a phase to line voltage.

For example the voltage of the capacitors diodes, varicap ... was chosen considering the voltage level equal to 415 VAC +20% which is equal to 500 VAC

so considering that i'm designing my power supply for a Single Phase Smart Meters which mean the voltage should be at maximum equal to 240v. Can i revise some components voltage like varistor, VCE for the NPN transistor to a minimum value of 240 + 20% which is equal to 288v?

2/ in continuation of the previous question, can i replace the switching bipolar transistor used in the documents KSC5026MOS which the Voltage - Collector Emitter Breakdown equal to 800V to this one KSE13003H2ASTU which the Voltage - Collector Emitter Breakdown equal to 400V ?

3/ i was reading this document  'Basic Calculation of a Buck Converter's Power Stage ' and i want to know, please, where the equation number 12 came from ? 



 

thanks,

  • 0
    TI__Genius 9790 points
    Thanks for your interest in TI here. I've contacted the appropriate product group. You should hear from them soon.
  • 0 in reply to Yunsheng Qu
    TI__Expert 3860 points

    Hi,

    Thanks for your interest in this design. Your doubts are very relevant to those who try to design a power supply based on this. I will try to address your questions to the best of my understanding.

    1. This design was originally done to address an energy meter power supply that required withstand of line to line voltage (to survive accidental connection between two of the phases instead of between one phase and neutral). If your specifications do not include this requirement, there is no problem in revising the ratings of the components used in this design.
    2. For input voltage of 288Vac, the DC input will be 407V. Therefore, 400V rating is not sufficient for the transistor.
    3. This comes from the basic charge equation Q=C*V=I*t. The change in charge (and hence voltage) in the output capacitor is due to the change in inductor current. The change in capacitor current is DIL/2 in the positive and negative directions. The peak to peak change in output voltage (output ripple) happens during the positive portion of the capacitor current (swing from negative to positive) and during the negative portion (swing from positive to negative). So calculating the area of either of these portions will give the swing in charge and hence output ripple.
    4. The area of the positive (or negative) portion of capacitor current is given by:

    DQ = CO*DVO =  ½*T/2 *DIL/2

    CO = DIL/(8*fS*DVO)

    Regards,

    Salil