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Original question:

UCC27714: MISSING DIODE 2-SWITCHFORWARD CONVERTER

UCC27714: C_Boot charging circuit

Felix Elsaesser1
Prodigy 60 points
Part Number: UCC27714


Hello,

We are using the UCC27714 for a 2-Switch Flyback, in our first iteration we did not implement the circuit recomended in Figure 49 to improve charging of the C_Boot capacitor. For our second iteration we want to do that. Unfortunatly we are not sure how the circuit should work. Our simulations show that if we implement the circuit like it is in Figure 49 a high current around 5 A is runnung through R_Boot when LO is turned on. If we implement an additional resistor as recomended by Derek Payne in https://e2e.ti.com/support/power-management/f/196/t/703960?UCC27714-MISSING-DIODE-2-SWITCHFORWARD-CONVERTER the n-Fet connected to HS will not be turned off properly and HV Voltage is connected over Q1, and the additional Diode and n-Fet to Ground resulting in a very high current around 300 A, which will destroy the circuit.

If we simulate the second suggestion and replace the limiting resistor with a p-Fet we still get a current around 6 A through R_Boot.

Can you recommend a circuit which will work with 100kHz to 180 kHz switching frequency?

Best regards

Felix Elsäßer

  • 0
    TI__Guru**** 189401 points
    Hi, Felix, Our office is closed today for a US holiday. My colleague will get back with you tomorrow when he is back in the office.
  • 0
    TI__Genius 13445 points

    Hello Felix,

    Thanks for reaching out.

    You are correct that FIgure 49 will induce load current flow through the boot resistor and boot cap which will certainly damage the circuit. 

    As for the 2 resistors solutions, there will still be load current flow.

    An N-Ch FET (controlled by LO, with adequate current/voltage ratings) between VDD and HB might work, this should solve the current limitation through the previous boot resistor while not impacting the high-side FET referenced to the HS.     

    Please confirm if this addresses your inquiry.

    Regards,

    -Mamadou

  • 0 in reply to Mamadou Diallo
    Prodigy 60 points

    Thank you for your response,

    unfortuantly I was not able to confirm your suggestion with my simulation. I am not sure how you would place the N-Fet exactly and wich other components you would use or remove. Could you draw you suggestion into Figure 49 ?

    Regars,

    Felix

  • 0 in reply to Felix Elsaesser1
    TI__Mastermind 23215 points

    Hello Felix,

    I have edited the datasheet drawing with the different resistor arrangement as discussed in the previous E2E forum. I would suggest this approach to limit the current in the boot resistance, so separate the boot resistance from the pull up resistance on the FET gate to source.

    As mentioned earlier, the added pullup resistance can be a higher value of 1K ohms to limit the pullup current.

    Confirm if this addresses your questions, or you can post additional questions on this thread.

    Regards,