If you have a related question, please click the "Ask a related question" button in the top right corner. The newly created question will be automatically linked to this question.

• TI Thinks Resolved

# LM5050-1-Q1: MOSFET selection for up to 40 A current

Part Number: LM5050-1-Q1

Hello team,

We are evaluating LM5050-Q1 for reverse polarity protection. It is for Auto application with the continuous current up to 40A. The nominal input voltage from the battery is 24V.

Can you help us selecting an appropriate N-Channel MOSFET for this application? I am considering CSD18540Q5B (TI), STH272N6F7-6AG (ST), STH265N6F6-2AG (ST),

Thank you very much and best regards,

John

• HI

I am looking into this and will get back on Monday.

Regards,

Kari.

• In reply to Karikalan Selvaraj:

Hi

For 24V automotive application, we recommend to use 80V rated MOSFETs along with LM5050 or LM74700.

Note that CSD18540Q5B is not automotive qualified.

Other two ST MOSFETs are only 60V rated.

MOSFET selection and TVS selection for 24V battery protection is discussed in datasheet of LM74700 (page 17 and 18, section 10.1.1.4 and Figure 21.)

Other parameters of MOSFET look fine, except Vds rating of the selected 3 MOSFETs.

Regards,

Kari.

• In reply to Karikalan Selvaraj:

Hi Kari,
Thank you very much for your quick reply. I will change the MOSFET with Vds of 80 V.
I am still struggling with the power dissipation calculation. The lowest R_DS(ON) of the two ST MOSFETs has R_DS(ON) = 1.5 mOhms at 25 degree C and about 2.8 mOhms at 175 degree C. Thus, the worst case of the conduction loss could be as high as P_cond = I^2 * R_DS(ON) = 40^2 * 0.0028 = 4.5 W. Assuming a typical automotive application with ambient temperature of 100 degree C, the thermal resistance from junction to ambient = (175 - 100)/ (40 x 40 x 0.0028) = 17 C/W, which is well below the thermal resistance specified in the MOSFET data sheet. Could you please tell anything wrong in my calculation? That is why I am concerned whether it is feasible for using with LM74700-Q1 or LM5050-Q1 in reverse polarity protection.
Another concern is that the MOSFET has a typical gate threshold voltage of 3.5 V, higher than the required 2 V for LM74700-Q1. Does this mean that I should use LM5050-Q1?
I really enjoyed reading your report, Chapter 5 Reverse Polarity Protection, 11 Ways to Protect Your Power Path, Design Tips and Tradeoffs Using It’s Power Switches. It was well written and very informative.
1. Ideal diode controller LM5050-Q1 has been cited in several TI design references. But it is not listed at Table 6, Ideal Diode Controller Examples. Could you please comment on it?
2. I don’t see much difference between LM5050-Q1 and LM74700-Q1. Could you please point out the key differences between these two ideal diode controller. Both are AEC-Q100 qualified. Both have similar input voltage range, 3.2 ~ 65 V for LM74700-Q1 and 1 ~ 75 V for LM5050. Two differences between LM74700-Q1 and LM5050-Q1 I notices are a) gate threshold voltage Vth, and b) R_DS(ON). LM74700-Q1 limits the max gate threshold voltage to 2 V while LM5050-Q1 limits to 5 V.  LM5050-Q1 data sheet (page 15) suggests that R_DS(ON) be no more than 100 mV at the nominal load current while LM74700-Q1 data sheet (page 16) suggest that R_DS(ON) be no more than 50 mV at the nominal load current. For Auto application with nominal input voltage of 12 V or 24 V application, which is better choice for reverse polarity protection? Since the data sheet of LM74700-Q1 has applications for 12 V battery and 24 V battery and the data sheet of LM5050-Q1 has applications for 12/24/48 V battery, may I conclude that LM74700-Q1 is good for low current applications using 12 V and 24 V battery systems while LM5050-Q1 is better for high current applications in 12/24/48 V battery system?
3. Another common method for reverse polarity protection is using a MOSFET and a BJT. Could you please advise why this method is not listed in your report? Could you also please comment on pros and cons of using a MOSFET and a BJT compared with using an Ideal Diode for reverse polarity protection? May I say that both methods are similar except that a MOSFET and a BJT method requires a charge pump gate driver while an ideal diode using LM74700-Q1 or LM5050-Q1 has charge pump gate driver, thus no external power is needed.
4. Could you please comment on all five reverse polarity protection methods regarding meeting ISO 16750-2? May I say that Methods 1) using an ideal diode, 2) a MOSFET and a BJT, and 3) using eFuse all meet ISO 16750-2 requirements, while methods 4) using Schottky diodes and 5) discrete MOSFETs cannot meet ISO 16750-2 requirements?
Sorry for so many questions, thank you very much for your help and I look forward to hearing from you,
John
• In reply to John Pan:

Hi

I acknowlege your response and would need more time to get back for the above questions.

Is next week okay with you?

Let us know if you need a speedy response.

Regards,

Kari.

• In reply to Karikalan Selvaraj:

Hi Kari,

Thanks,

John

• In reply to John Pan:

Hi

Power dissipation calculation is based on the RTHJ-PCB of the MOSFETs, for the ST parts you indicated, it is 35C/W when mounted on a 1 sq inch PCB area with 2 ounce copper thickness.

To base the calculation, we need to know the PCB board temperature near DRAIN pad of the MOSFETs.

If the PCB board temperature near the DRAIN pads of the MOSFET is 100C, based on the power dissipation calculation of 4.5W worst case, Junction temperature would rise above 100C PCB temperature by 4.5W * 35C/W = 157.5C, that is TJ will be at 257.5C.

This design will not work unless you increase the number of MOSFETs to dissipate more power or add a heat sink.

Regards,

Kari.