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LM5121-Q1: LM5121 Boost converter with external gate driver

Part Number: LM5121-Q1
Other Parts Discussed in Thread: LM5121, LM5114

Hi,

I am planning to use LM5121 for converting input (18V to 50V) to output 55V @ 8A.

To reduce the switching FET losses,I have selected GAN FET(GS61008).

It needs a gate driver since it Vgs should not go above 7V.

If I use external isolated gate driver,SW pin of IC will be grounded.

SInce it is a adaptive dead time control,Will it effect circuit functionality rather than increased dead time?

Awaiting for your reply.

With regards,

Selvam.

  • Hi Selvam,

    I would not recommend grounding SW pin here for operation of LM5121, since the adaptive dead-time method will need to use the SW pin to function properly. Without the SW pin, I'm afraid that the part will not function as expected. I

    n this case, you could still use the 5121, but instead of having both FETs, you can implement a non-synchronous configuration by grounding the SW pin, and connect HO and BST to GND each through a 10K resistor. Afterwards, you could use the LM5114 driver to drive GaN FETs.

    The non-synchronous scheme might provide lower efficiency, even with the GaN FET, because of the diode losses. In this case, I would recommend to just use a regular MOSFET instead.

    Thanks,

    Richard

  • Hi Richard,

    Thank you for your reply.

    I will use MOSFET for HS FET with synchronous rectification.For LS(Switching FET),I will use GAN FET with LS driver.

    1.Will the above circuit works fine?

    2.Since the converter doesn't have UVLO,If I provide input voltage greater than output voltage,What will happen?

    Will the converter works without regulation i.e output voltage following the input voltage.

    With regards,

    Selvam.

  • Hi Selvam,

    Please see my responses below:

    1) It theoretically seems possible that you can drive the LS GaN FET with LM5114, and drive the high side FET with the LM5121 high side driver. You will need to build a prototype to check the adaptive dead time. If I were you, I would also build a variant with the LO FET as a MOSFET to be safe. 

    2) The LM5121 has a line UVLO feature. What you may be asking is about the bypass operation. In bypass, the high-side FET will be on for 100% , and the output will track the input. In order for bypass mode to be operational, you will need minimum 9V boost output voltage. Please see section 7.3.9 and 7.3.10 in LM5121 datasheet.

    Thanks,

    Richard