I am just looking for some idea's on 24V automotive systems. I have a regulator that I would like to use (LM3488) which has a max input of 40V. To protect against load dump and transients I was thinking of a PolyFuse/TVS arrangement on the input but if the system is to work on 24V I would have to use a 30V TVS which clamps at around 48V, a little high for the input of my regulator! Any ideas?
If you really want to clamp the peaks to less than 40 volts, you might use a 36 volt, 3 watt zener. But with a +-5% tolerance, this means that it might clamp as low as 34.2 or as high as 37.8 volts. The only energy in a load dump is that stored in the inductance of the wiring. There is also a possibility that the alternator will produce its full output into the battery till the regulator recovers, and the worst case for this would be when the vehicle is operating with the battery disconnected.
I don't know what TVS you're using but a 1N6281A/1.5KE27A has a nominal clamp voltage of 25.7 min - 28.4 max, and a "Maximum Clamping Voltage at Ippm", which is spec'd at 40.0A, of 37.5V. It's called a "Transzorb":
What???!!! The load dump scenario is a catastrophic loss of battery connection, the alternator pumps the whole car load up to whatever 60V with allowable 500ms rundown.
I believe a decent electrical system should have a load dump clamp somewhere in the system. There is quite a bit of energy in a load dump so it takes a sizeable device to handle it. I believe alternators can be purchased with such a clamp. If you can determine that there is a clamp in your system then you only have to find out what the maximum clamp voltage will be, and deal with that.
However, the clamp does not need to protect the transformer and fet with the same protection that the chip needs. The 3 watt zener is good for something like 10 watts for a half second starting from cool, so the peak current needs to be limited to something like 1/4 amp. If the chip runs okay with a 270 ohm resistor between the battery and its positive supply line, a 100 volt, 1/2 second pulse will not destroy the zener. The resistor must withstand the 20 watt pulse for that time, also, but I don't think it is so hard to cover that base. The bypass capacitor across the chip's supply also helps with big, brief pulses.
That's what a load dump is. For an official version of a load dump transient, see: ISO 7637-2, Test Pulses 5a and 5b. There is a lot of energy in a load dump event.
Best regards, Spehro Pefhany
--
"it\'s the network..." "The Journey is the reward"
speff@interlog.com Info for manufacturers: http://www.trexon.com
Yeah, a load dump is a nasty thing. In either case; clamp or disconnect, the OP should determine whether a load dump handler (of some sort) is already present in the system.
Imagine 1963-1964, my alternator regulators were blowing apart like sticks of dynamite for no "apparent" reason. Then I bought a storage 'scope... a _very_ expensive item in those days ;-)
...Jim Thompson
--
| James E.Thompson, P.E. | mens |
| Analog Innovations, Inc. | et |
According to SAE J1211 (1978), pages 19 and 20, the load dump transient has this characteristic:
V = 106*exp(-t/.188) + 12
It is caused by the load on the alternator being suddenly reduced. Worst case is caused by disconnecting a discharged battery when the alternator is operated at full load (often initiated by a defective battery terminal connection).
It is not the highest voltage mentioned in the document. There are inductive switching (-286V) and mutual coupling (214V) transients but their time constants are about 1ms.
Note that SAE J1211 concerns itself with 12V systems. There is also good information on other conditions within the automotive environment (temperatures encountered in various areas, shock, vibration, dampness, etc.)
For anyone contemplating designing equipment for automotive equipment, I would recommend visiting the SAE site for documents. I think they have more up to date information on 12V systems than does J1211, and documents for 24 and 48V systems. The documents are not free.
But it doesn't need any where near 800 mA, does it? My point is that you can put quite a bit of resistance between the chip and its protection to limit the current while connecting the fly back inductor/transformer much more directly to the battery line.
You're welcome, but don't get that excited. You still have to add the current consumed to drive the gate capacitance. But I think excluding the power section makes a low cost zener (1 to 3 watt surface mount) look like a practical clamp. About $0.25 in quantity.
Thanks for the feedback thus far. I was thinking of using a 5KP30A originally and the regulator will be running at around 800mA. Im not sure how much energy transzorb may need to dissipate, could I get away with using a lower power one to get a lower max clamping voltage? I like John's idea, "A shunt clamp using a transistor and zener following the PolyFuse" and have also found this site
formatting link
which show a circuit I may be able to use to disconnect the load during transients and spikes. This seems like a good way to go to me, has anyone tried any of these circuits or have any feedback about them?
ElectronDepot website is not affiliated with any of the manufacturers or service providers discussed here.
All logos and trade names are the property of their respective owners.