TPS24742 - Selecting MOSFET for Design

Benjamin Moore

Other Parts Discussed in Thread: TPS24742, CSD17573Q5B, CSD16415Q5, TPS24742EVM-667, CSD17559Q5

Hello Team,

I'm working on a design where I believe it will be Hotswap then ORing. I'm looking at using the TPS24742 in this design, but I need help selecting the proper MOSFET for my application. Does anyone have a recommendation?

I looked at the CSD17573Q5B and I'm using the design calculator for the TPS24742 to see if I might have any design issues.

Originally I had the CSD16415Q5, but it has a low gate to source voltage of 16/-12. I want something with 20V so the gate charge pump can't damage the FET. My output is 12V @ 20A.

I really appreciate the help!

Best Regards,

Ben

over 7 years ago

0 Benjamin Moore over 7 years ago

TI__Mastermind 29085 points

It looks like with the design calculator the CSD17573Q5B gives me a red SOA. Perhaps I'm reading some values off of the datasheet wrong. For instance, it is not clear to me what the exact values that should be entered here are:

I'm using the following graph from the datasheet to figure this out, but it isn't too straightforward.

Best Regards,

Ben

0 Darwin Fernandez over 7 years ago in reply to Benjamin Moore

TI__Mastermind 40100 points

Hi Ben,

The TPS24742 EVM uses the CSD16415 in a 12V/45A solution. I don't see why it wouldn't work for the 20A solution. For the excel sheet, you must use your expected maximum input voltage (13V perhaps?), then for each timing spec on the SOA graph you associate a specific current. This is what should be inputted in the excel.

0 Alex Triano over 7 years ago in reply to Darwin Fernandez

TI__Genius 12380 points

Hi Ben,

If you have any questions about the design tool, we have a series of brief video tutorials to explain each step of the tool. I highly recommend checking them out at www.ti.com/hotswap --> "Support & Training". In this case, watching the Video specifically for Step 3 would be applicable.

Thanks,
Alex

0 Benjamin Moore over 7 years ago in reply to Alex Triano

TI__Mastermind 29085 points

Thanks Darwin, Alex,

I'm going to try out the TPS24742 EVM (TPS24742EVM-667) and see if I think it will work for my application. After looking more at the TPS24742 datasheet, I don't think I should have a problem with the CSD16415.

However, do you know of any 30V FETs (with max Vgs of approx. 20V) that could do the job as an alternate? I was thinking the CSD17559Q5 might work.

Also, are there limits how high the power limit can be set? I would like the design to be sound even in the case of voltage spikes or when going from 1A to 20A load during start-up.

Best Regards,
Ben

0 Alex Triano over 7 years ago in reply to Benjamin Moore

TI__Genius 12380 points

Hi Ben,

Attached is a presentation for overall 12V hot swap design. The TPS24742 includes ORing as well, but the hot swap portion will be the same.

So for 12V, 20A, see slide 9 in particular. Something like CSD17573 or CSD17570 could be a good choice. You would need to crunch the numbers using that tool.

2_4_16_Hot Swap Presentation.pdf

Thanks,

Alex

0 Benjamin Moore over 7 years ago in reply to Alex Triano

TI__Mastermind 29085 points

Thanks, Alex.

I had gone through the TPS24742 design calculator with the CSD17573 and ended up with a "high risk" based on the SOA. That was the original reason I was seeking clarification on using the tool.

I'm getting better results based on the CSD17559, unless I'm entering something wrong.

I have attached my screenshots for the point on the SOA curves that I'm using as a reference and my populated design sheets.

CSD17573:

/cfs-file/__key/communityserver-discussions-components-files/196/5226.TPS2474x_5F00_DesignCalculator_5F00_CSD17573Q5B.xlsx

CSD17559:

/cfs-file/__key/communityserver-discussions-components-files/196/6835.TPS2474x-Design-Calculator_5F00_CSD17559Q5.xlsx

Regards,

Ben

0 Alex Triano over 7 years ago in reply to Benjamin Moore

TI__Genius 12380 points

Hi Ben,

You did an awesome job with the tool, it looked great to me!

You're right, the CSD17573 is a bit marginal and the CSD17559 would be better. In that same 12V hot swap presentation, you can check out slide 4 which shows some MOSFET SOA values for different part numbers in a convenient table. So yes the CSD17559 would be great if they accept its cost. It would have plenty of design margin (it is less overkill than the CSD16415 but still may be over-designing). Some slightly cheaper FETs like CSD17556 or CSD17570 may be enough to get the job done too with enough margin (and they would have lower Rds-on, so more power savings).

When it comes to hot swap, you usually want to pick a FET strong enough to survive with sufficient margin, and then optimize for power savings/cost.

Thanks,
Alex

0 Benjamin Moore over 7 years ago in reply to Alex Triano

TI__Mastermind 29085 points

Perfect!

Thanks, again. The video and slides were quite helpful!

Best,
Ben

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TPS24742 - Selecting MOSFET for Design