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Pre-Charge Circuit for Capacitor

Hi, I have an accumulator (battery bank of 300 V DC) and have a pre-charge resistor connected in series to it. I would like somebody to suggest me a circuit that will enable the 300 V Supply charge the DC Link (350 farad) through pre-charge resistor upon a 12 V Input to the circuit. And after 3-4 seconds (by the time DC Link reaches 90% SOC) it can open the relay to which the pre-charge resistor is connected thereby disabling that path and also send a signal to activate isolation relay to have direct current flowing from 300 V path (the path without the pre-charge resistor). Turning ON and OFF of the inverter happens frequently while testing. A discharge circuit to dissipate the energy in DC Link whenever 12 V is turned OFF would also be helpful. 

  • Hello Sri, 

    Thank you for describing your design requirements on E2E.  To be sure we understand the requirements and which part(s) of this circuit you need designed, I'm hoping you can provide a block diagram or functional diagram describing the various blocks and requirements.  Which devices have you already selected and which ones need to be determined?  For example, do you have a 300V power supply, a 12V power supply, or do these need to be designed as well?  How much current is required for the pre-charge and have you selected the resistor value? 

    You also describe "turning ON and OFF the inverter" but I'm not clear on how exactly this inverter is used in this design. 

    Any and all detail about the various functions will be of great help in further defining this design. 

    Thank you!

    ~Leonard    

  • Hello Sri,
    Please provide an update/answers so we can move forward with this inquiry. We will be glad to assist, pending further clarity and definition of what you are wanting to design. Thanks!
    ~Leonard
  • Hi, in the below schematic (2 inverters), the 2 contactors (12 V for coil activation) , 2 NO relays (12 V for activation) , the 12 V input to two inverters all have same ground.

    Each resistor used is 600 Ohm. DC Link Capacitance is approx 350-370 microfarad. Initially, the 300 V bus is not active because the contactors are open. As a result the precharge resistor is connected from positive to negative of a contactor via Normally Open Relay. The pin going to 1 should first supply the 12 V for the NO relay coil upon it receiving the 12 V input. And the precharge gets activated. After 3 seconds (when Capacitor's state of charge is 90%), the pin 1 should stop giving 12 V input to NO relay and should give 12 V input to contactor via pin 3. If, after 3 seconds, the capacitor's SOC is not 90%, the circuit shouldn't activate.

    What we need is the the circuit to which pins 1 and 3 are going and we will provide 12 V input to it.
    Please let me know if you need any information. Once again, many thanks for your positive response.
  • Hi Sri,

    If my understanding is correct, then you are looking for two different circuits here (associated with pins 1 and 3). For pin 1, you need a switching circuit that will connect the 12V supply to the NO relay coil once the 12V supply is applied. After 3 seconds, the capacitor will be charged to 90% and the switch on pin 1 will turn off (open). At this point, another switch on pin 3 will be activated and 12V will be applied to pin 3. If after 3 seconds the capacitor is not charged to 90%, then the rest of the circuit is not meant to activate. Is my understanding correct? Also, how much inrush current are you expecting from the capacitor banks during charging?

    Thanks,

    Alek Kaknevicius

  • Hello Alek,
    Many thanks for your reply. Yes, after 3 seconds the circuit should automatically turn off the 12 V supply from pin 1 and send it through pin 3 to activate contactor. The resistor selected for the pre-charge is 600 Ohms and it can charge the Capacitor to 100% voltage by 3 seconds. If it didn't charge to 90% by 3 seconds, then rest of the circuit shouldn't activate. The Capacitance of the capacitor is 500 microfarad. Initial Current going through the capacitor during the charging is 0.5 Amps (max).
    Please let me know if you need more information.
  • Hi Sri,

    Please have a look at TPS25940A device. The dVdT can be programmed for 3 seconds rise time. Please refer the app report highlighting the use of TPS25940A device for charging large capacitor and let me know if you have any questions. www.ti.com/.../slva920.pdf

    Best Regards,
    Rakesh
  • Hello Rakesh,
    Many thanks for your suggestion. I noticed the linear dv/dt rise for the capacitor and controlled current rate at which it is charging capacitor. But the article you sent mentioned this device can work for applications powered from 2.7 V to 18 V. Can this work for 300 V DC Battery input and charge the capacitor to 300 V?
    Thanks,
    Sri
  • Hi Sri,

    We don't have solution to work at 300V DC.

    Best Regards,
    Rakesh