Circuit Protection & TVS Diodes

[MikahB]

I'm working on a MOSFET switching circuit that will be used in light- and heavy-duty automotive applications. As such, driving power will come from either a "normal" 12V automobile system or a 24V system commonly used in heavy-duty vehicles.

I'm using two LM2940 LDR's (a 5V and a 12V) to power the IC's in the circuit. Only the drain of the MOSFET will be connected straight to incoming power.

Power comes in to the high power board from the low side of the driven load (I will not know what the driven load is), and then connects directly to vehicle ground and will be limited (via PWM duty cycle) to 30A (currently) or 50-60A (in the future).

I want this to be a very robust system and have a couple issues that I'm struggling with being new to electronics.

1) It seems, in theory, like a TVS (I've been looking at the Vishay SM8A27) across the input power and ground would do just about everything I need. Only power from the LM2940 gets to the control circuit, and it has some of its own built-in protection as well. Besides wrong polarity, high transient voltages, and high transient currents, what do I need to be concerned about? Also, would a TVS like the one mentioned above adequately protect from these concerns?

2) Anticipating that this will often be driving a motor of some sort, and not having the ability to put a flyback diode across the motor, what (else) could I do in my circuit to manage those transients, or is the TVS enough?

Thanks for your help, sorry I can't post schematics but I hope I've been clear.

 

[Boncuk]

Hi,

with a TVS you're on the right track.

However I don't see different voltage ratings on the Vishay datasheet.

Try to get a datasheet of the 1,5KExx datasheets.

1,5KExx (check out NXP semiconductors) are available for a wide range of fixed breakdown voltages and they are also available as unipolar and bipolar devices.

I've been using bipolar TVSs with a small inductor (47μH) followed by a 10μF electrolytic cap at the input of the voltage regulator to be used in MCUs or CMOS devices for automotive applications with no failures.

Boncuk

 

[ronv]

Unclamped inductive loads are problematic. Why can't you use a clamp diode or another FET to clamp the load? I don't think the avalanche rating of your fet will handle it in all cases.

At such high currents I would also worry that the 12 volt regulator will drop out due to IR losses and line inductance. Could you use 10 volts?

There are specs for load dump. Make sure your tvs can meet the spec.

 

[MikahB]

Thanks for the input and suggestions, guys.

Boncuk - I will look for thos NXP TVS diodes and do some research on a bipolar TVS versus unidirectional (which is what I thought I needed).

Ron: In many cases, I will not have access to the load being driven other than the low-side wire I'm switching, so even though sometimes I might be able to add a diode across the inductive load, it will not always (or even usually) be the case so I need to design around it if possible.

I may have been unclear about the 12V regulator - the only device getting power from the 12V LM2940 will be the UCC37322 MOSFET driver (which is rated fo 600mA continuous). I expect the LM2940 to be well under it's 1A maximum output current most of the time. The only components seeing the full current will be the MOSFET and the Allegro Current Sensor (rated to measure 50 or 100A depending on the model). One of the reasons I chose this Vishay TVS was that it's specifically designed (at least marketed) for inductive load dump protection in automotive environments. I will do some research on calculating load dump and compare against the specs of the unit.

If that was clear and you're still concerned about the 12V regulator, can you give me a little more information to research? Yes, I could use 10V instead (the UCC37322 wants between 5 and 16V input) if I needed to. My understanding about the operation of the LM2940 is that the lower output voltage I used (compared to input), the more heat I will generate which is why I chose 12V.

 

[crutschow]

........................

2) Anticipating that this will often be driving a motor of some sort, and not having the ability to put a flyback diode across the motor, what (else) could I do in my circuit to manage those transients, or is the TVS enough?

,,,,,,,,,,,,,,,,,,,,,,,

If you put a power diode from the drain of the power transistor to the battery plus power supply source (cathode to power source), that will act as a flyback diode across any inductive load. A small snubber circuit (0.1μF cap in series with 10 ohms) from the source to common should help absorb any high frequency spikes left over from line inductance.

 

[ronv]

Crutshow has the only solution I can think of but even that may not work at the frequency and current you are using. Even 5 or 10 uh in the diode line will be be a problem. Had a similar problem many years ago. Ended up having a cable made where the ground and voltage wires were together like a 2 wire flat cable to minimize the inductance. If you control that you might look at something similar. Problem is they don't go to the same place.
On the regulator my concern is that the battery may not always be the full 12.6 volts.

 

[Mosaic]

have a look at this
https://www.electro-tech-online.com/custompdfs/2012/04/an9312.pdf

What u will need to handle a genuine load dump is a 2 stage system. Stage 1 is your fast acting TVS to clamp at say about 42V, this is in parallel with a sizeable MOV perhaps 10 to 20kA rated clamping at say 40V. The TVS reacts quickly for transients and the MOV comes in a couple mS later to do the heavy clamping for a long surge. Additionally if u can place a 2 or 3 watt wirewound resistor of 2 to 5 ohms in series with the 12VDC supply it will help a great deal in limiting the current surges seen by your TVS & MOV.

Those regulators you're using should be safe to 60 V input or so.

 

[MikahB]

@crutschow: When you say to the battery plus power supply, are you referring to the B+ power for my circuit (which comes through the load, not from the battery) or another line straight to the battery? I've toyed with adding a 3rd connection (would rather not, but there are other reasons to do it too) that is a switched B+ - it would potentially give me a path back to battery that is not through the load.

@ronv: Interesting. Yes, the trouble as of now is that my two load wires go opposite directions, but maybe I can think and package my way around that - I will certainly consider it. I have some decent quality 10AWG paired cable that I work with now. I'll have to think about what that would look like on an install. Paired cable from my circuit to the load, then a single ground wire going beyond the load to ground... Hmmm

@Mosaic: Thanks for the link and the idea - I like the sound of it. I will read through the link (looks like a great paper) and do some more research. It's Friday evening and I am officially banned from work and computers until early tomorrow morning (when nobody else is up!).

Thanks guys, I will research and report back.

 

[KeepItSimpleStupid]

Take a look at this LTC4366 part: https://www.electro-tech-online.com/custompdfs/2012/04/436612fb.pdf I don't know if it could help or not.

 

[crutschow]

@crutschow: When you say to the battery plus power supply, are you referring to the B+ power for my circuit (which comes through the load, not from the battery) or another line straight to the battery? I've toyed with adding a 3rd connection (would rather not, but there are other reasons to do it too) that is a switched B+ - it would potentially give me a path back to battery that is not through the load.

I haven't heard the term B+ used since I studied tubes back in the 60's.

I don't understand. You are powering your circuit from the load? What powers the 5V and 12V regulators?