Transorb vs MOV vs National AN-1533 Circuit for Automotive 12V/24V to 5V 100mA Supply

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#1 21660146 22 Nov 2011 06:57

Pieter Kruger Pieter Kruger

Anonymous

Post #1
21660146 22 Nov 2011 06:57

I am busy designing a power supply for an automotive application, 12V or 24V input and 5V 100mA (max) output.
(Something like a TLE4270 or similar, simple linear regulator, no EMI issues.)

To accommodate protection for load dump and other over voltage conditions, I was thinking of including Transorb/MOV protection which is ok for short over voltage conditions. However something similar to: http://www.national.com/an/AN/AN-1533.pdf is fast and more suitable since it can protect against continuous over voltage.

Apart from cost, why would a Transorb/MOV only solution be considered above the national circuit? (or a combination of both?)
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#2 21660147 22 Nov 2011 09:16

Kevin Parmenter Kevin Parmenter

Anonymous

Post #2
21660147 22 Nov 2011 09:16

it is routine practice to place a load dump TVS device - usually this one http://www.onsemi.com/pub_link/Collateral/MR2535L-D.PDF
the second practice often employed is to design the circuit such that it can ride out the load dump if necessary. Place this in front of a regulator which has built in load dump protection such as this one or even an LM2931 has that I believe but this one does for sure. in other words choose an LDO that has load dump protection - ride through built in so you dont have to wonder if it will work or not. The combination of the protection and the part that can survive the load dump should be more than sufficient.

http://www.onsemi.com/PowerSolutions/product.do?id=NCV2931

place one of these self resetting circuit breakers in front off the circuit to protect it if everything goes bad at once and the TVS is drawing enough current that it pops the breaker before damage occurs.
http://www.e-t-a.com/us_thermal+M5c39ab94c7e.html

this should work great - BTW I have no connection to any of these companies - I used to work for Motorola - ON years ago and they have a very strong position in automotive and know what they are doing (motorola semi legacy)
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#3 21660148 22 Nov 2011 13:06

DAVID CUTHBERT DAVID CUTHBERT

Anonymous

Post #3
21660148 22 Nov 2011 13:06

Linear Technology makes some series pass voltage limiters for automotive load dump.

I would use these before a brute-force shunt clamp.
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#4 21660149 22 Nov 2011 14:17

Per Zackrisson Per Zackrisson

Anonymous

Post #4
21660149 22 Nov 2011 14:17

Havent done anything for automotive but my reaction on securing for continuous overvoltage is that there seems to be overkill or bad design. Voltage regulators are so cheap and made in so many specifications that there is no excuse for choosing one that cannot handle the voltage from the cars generator. But choosing a LDO(Low DropOut) regulator is not necessary you have at least seven volt to spare. Even 40 yrs old TL7805 should work fine (it drops 4 Volts) although it might be sensitive for negative voltage. But prepare for heath ((24-5)*0.1=1.9 Watts plus overvoltage might give you 3-4 Watt to cool). If you put that into a sealed box you will have an issue. With a stepdown regulator you will get 100 mWatt to consider(no problems), AND EMI issues.
I would have used a switchregulator (but I do not have all the facts)maybe not for 12Volts but def. for 24V. Good luck!
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#5 21660150 22 Nov 2011 17:15

Pieter Kruger Pieter Kruger

Anonymous

Post #5
21660150 22 Nov 2011 17:15

Thank you for your detailed replies. It is always good to get another opinion and learn from other's experience. (good or bad)

Per:
I started off with a 78L05 or LM317, but abandoned these options because of load dump, heat etc. (Nice thing about these parts is you can get them anywhere.) There are some good switching options from Linear and some of the Simple Switchers range, but I am very careful of EMI issues. The TLE4270 has good over voltage protection and happens to be a LDO.

100mA is absolute worse case, the typical current consumption would be around 70mA for 15s with an average of around 8mA. Worse case conditions are, 60'C, 30V (input) at 100mA. The unit will be in a sealed box, but with a TLE4270 in a TO263 package on a PCB heatsink, I think it is possible if the current consumption is not constant 70-100mA. (I plan to reduce the current consumption even further.)
#6 21660151 22 Nov 2011 17:23

Kevin Parmenter Kevin Parmenter

Anonymous

Post #6
21660151 22 Nov 2011 17:23

I have had the big automotive companies as customers and they put the TVS device at the input of every module for insurance. Moreover they also design the front end regulator to operate through a load dump even if the TVS was not there. these systems simply cannot fail - think antilock brake modules etc.
#7 21660152 22 Nov 2011 18:01

Pieter Kruger Pieter Kruger

Anonymous

Post #7
21660152 22 Nov 2011 18:01

I guess it also doesn't make sense to save on a relatively cheap part and risk a recall of a few million $. Having the TVS device also gives you the option to later consider a part with less protection (regulator) if you have to.
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Topic summary

The discussion addresses designing a 12V/24V automotive power supply with a 5V, 100mA output, focusing on protection against load dump and overvoltage conditions. Transorb/MOV devices provide short-duration overvoltage protection, while the National Semiconductor AN-1533 application circuit offers fast response and continuous overvoltage protection. Recommended practice includes using a load dump TVS diode such as the MR2535L-D from ON Semiconductor placed at the input, combined with a regulator featuring built-in load dump protection like the NCV2931 or TLE4270 LDO. Additional protection can be achieved with self-resetting circuit breakers to prevent damage from excessive current during fault conditions. Linear Technology’s series pass voltage limiters are suggested as a pre-regulator protection before shunt clamps. Linear regulators like the TL7805 or LM317 are generally less favored due to heat dissipation and load dump vulnerability, especially in sealed enclosures, whereas switching regulators offer efficiency but may introduce EMI concerns. Automotive-grade designs often combine TVS devices and robust regulators to ensure reliability under harsh conditions, with cost considerations secondary to system safety and recall avoidance.
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Transorb vs MOV vs National AN-1533 Circuit for Automotive 12V/24V to 5V 100mA Supply

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