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LM5113: LM5113 Maximum Bootstrap Capacitor

Part Number: LM5113

Dear Madam or Sir

 

We are currently working on an inverter using your LM5113 gate driver

 

Since our power semiconductors have a significant gate leakage current, a big bootstrap capacitor of roughly 500uF would be necessary to assure proper functioning.

 

In the data sheet section 8.2.2.2 no information about a maximum bootstrap capacitor is stated.

 

Do you know of any practical limitation to the bootstrap cap?

 

Thank you very much in advance for your reply

 

Best Regards

 

David Menzi

  • David,

    Thank you for your interest in the LM5113 gate driver. There is no particular limitation on the value of the bootstrap capacitor. However, there are some factors to consider to ensure proper operation:

    a. I recommend still having one or two smaller capacitors for high-frequency currents. Perhaps one at 0.1-0.22uF and a 1-2.2uF along with your bulk cap. Place the smaller capacitors closer to the driver HB and HS pins.

    b. What kind of power devices are you driving? A part number would be helpful. I ask just in case we can offer a more suitable driver for your needs. Note that the LM5113 was designed specifically to drive enhancement mode GaN FETs. If GaN is not a requirement for your application, there is more flexibility in the type of driver you can use.

    c. Note that such a large cap will require quite a bit of energy and time to charge up, depending on the low-side duty cycle.

    d. What was your calculation that arrived at the 500uF requirement? 

    I hope this post helps answer your question. If it has, please press the "Verify Answer" button below.

    - Daniel

  • Daniel,

    Thank you so much for your fast reply!

    Concerning your answers:


    b. Due to current and voltage ratings of our converter we are using GaN-Chips (EPC2032). The LM5113 is therefore necessary.


    d. We just derived a simple simulation model for the bootstrap capacitor voltage over a few mains periods for a given PWM signal. The 500 uF resulted from the requirement, that the gate-source voltage of the upper switch should not fall below 10% of the nominal value.


    Due to spacing issues (500uF would be challenging to place close to the gate drive circuit), we will probably go for an external schottky diode to supply our bootstrap capacitor, leading to a better charging performance. Do you have any experience with such an approach for the LM5113?

    Best regards

    David
  • David,

    Thank you for the additional information. Yes, we have run this device using an external diode to charge the bootstrap supply. TI's recommendation is to use a resistor in series if using an external diode. The reason for the resistor is to prevent overcharging the bootstrap supply when the SW swings negative during the dead times.

    The SW only swings negative if the low-side goes into third-quadrant conduction. If this converter only ever works in a boost operation and the current never reverses through the inductor, then the SW will never swing negative and you can eliminate the resistor because there is no risk of overcharging. If instead at some point the current reverses through the inductor (maybe at light load), then put the resistor in to be safe.

    For best performance, choose a small, low-capacitance diode in series with a resistor of 50-500 ohms.

    - Daniel
  • Daniel,

    Than you again very much for your help!

    The operation of the converter is actually in all four quadrants, therefore the OV-protection needs to be considered.

    Sadly adding a series resistor would again significantly increase the minimum bootstrap capacitance s.t. our voltage requirements are being fulfilled.

    Shouldn't the internal clamping mechanism of the LM5113 be able to limit the bootstrap voltage anyway? Or could OV be prevented by adding a clamping circuit with a Z-diode directly  in parallel to the bootstrap capacitor?

    I'm asking this because we are as well discussing an external charging circuit for the bootstrap capacitor where again we would need to assure, that the voltage does not exceed its limits.

    Best regards

    David

  • David,

    Although the datasheet refers to it as a clamp for simplicity, the HB voltage is actually internally regulated. If VHB –VHS is at or below the desired window, the integrated boot diode is allowed to conduct. If it is above the window, the diode is switched out of the circuit to keep the HB voltage from going too high. This feature only controls the conduction of the internal bootstrap diode.

    Yes, a zener diode can be used in the way you propose to prevent overvoltage of the bootstrap supply when an external boot diode is used.

    I hope this post answers your questions. If it has, please press the "Verify Answer" button below.

    - Daniel