LM3478: Doubts on the selection of Mosfet based on Webench reference design

Ishan Panchal

Part Number: LM3478
Other Parts Discussed in Thread: CSD19538Q3A

Hi There,

I have a reference circuit for LM3478 from Webench based on inputs. It needs to operate in range of Vin = 15 - 32 VDC and Vout=24VDC at 0.6A. The design is okay for me but the mosfet (CSD19538Q3A) in that design is rated at 100V/15A, this ratings seems way more than the required operating specs. So can I just select a mosfet (SI2392BDS-T1-GE3) around 100VDC/2A ?

I need to select a fet which is small, easily available and have pin to pin alternates available, the suggested mosfet has a package that won't be that easily available.

Please let me know.

Thanks..!!!

over 1 year ago

0 Moritz over 1 year ago

TI__Genius 12435 points

Hi Ishan,

Thanks for reaching out.

The current rating of the selected MOSFET should be selected with respect to the eak current fowing trogh the MOSFET (+ some margin).

This would be the average input current plus half the ripple current. Depending on the swiching frequency and the used inductor 2A might be not enough.

But i also think that 15A is more than needed.

Best regards,

Moritz

0 Ishan Panchal over 1 year ago in reply to Moritz

Intellectual 640 points

Hi Moritz,

Yes, I understand your point..!!!

So based on my application of running a solenoid coil rated at 24VDC/0.5A, what would be the best fit for mosfet in terms of voltage and current.

Please let me know.

Thanks..!!!

0 Moritz over 1 year ago in reply to Ishan Panchal

TI__Genius 12435 points

Hi Ishan,

Can you share your schematic please?

Best regards,

Moritz

0 Ishan Panchal over 1 year ago in reply to Moritz

Intellectual 640 points

Hi Moritz,

Please find attached snap of schematic for buck-boost circuitry. Input will be 18-32VDC. Please let me know if need additional info.

Thanks..!!!

0 Moritz over 1 year ago in reply to Ishan Panchal

TI__Genius 12435 points

Hi Ishan,

Please have a look at this application note for designing SEPIC converters:

https://www.ti.com/lit/an/slyt309/slyt309.pdf?ts=1696258567533&ref_url=https%253A%252F%252Fwww.google.com%252F

There is a good description on how to select a proper MOSFET.

Best regards,

Moritz

0 Ishan Panchal over 1 year ago in reply to Moritz

Intellectual 640 points

Hi Moritz,

I went through the app-note where it shows calculations on selection of mosfet and also shows the design example. Based on calculation, it seems that for my design the peak current for mosfet is 1.1A and power dissipation is 575mW. Based on the design example I noticed that mosfet of nearly 3X rating is chosen and its also written that output diode should be able to handle the same amount of peak current as Mosfet.

Now if I go back to Webench design, the suggested output diode is rated at 2A but suggested Fet is rated at 15A. In conclusion I can choose a mosfet rated around 2-3A and voltage rating of at least 60V.

If you have any further thoughts on my component selection, please let me know.

Thanks..!!!

0 Moritz over 1 year ago in reply to Ishan Panchal

TI__Genius 12435 points

Hi Ishan,

From my calculation the peak current through the MOSFET will be around 2A. So i would recommend a margin when selecting it.

For the Diode the peak current is the same but RMS current is about 1A.

Best regards,

Moritz

0 Ishan Panchal over 1 year ago in reply to Moritz

Intellectual 640 points

Hi Moritz,

Thanks for the final comments, I will surely choose the mosfet and diode with a margin.

Thank You..!!!

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LM3478: Doubts on the selection of Mosfet based on Webench reference design