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3545 Views 1 Reply Latest reply: Apr 30, 2014 1:10 PM by davecuthbert RSS
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Apr 28, 2014 1:47 PM

DC-DC converter modules - automotive applications

I've got a project that uses a CUI  VASD1-S12-D15-SIP. I was trying to test one the other day and it quite literally exploded. I checked at my bench power supply was set for 13.5 volts rather than 12. The datasheet doesn't call out an "absolute maximum," but it does say that it only goes up to 13.2 volts, so I'll just assume that that was on me.

 

But it raises a question.

 

What would I do if I had an automotive application? Automotive power has a much wider range than that - 13.8 volts when the alternator is going and maybe as low as 11.5 or so when the engine is off.

 

I wouldn't want to use an LDO in front, because then I'd be throwing away the lower half of the usable range. Naively, I'd think putting a 13 volt zener diode across the input would be the right thing to do, but wouldn't that potentially send a *lot* of current through the diode?

 

I'd almost be inclined to use an LDO to take the input voltage down to 5 and use a VASD1-S5-D15 instead, but that just seems awfully inefficient.

 

This isn't really a problem I need to solve, I'm just trying to think how I would solve it if I did need to.

  • davecuthbert Apprentice 87 posts since
    Feb 18, 2014
    Currently Being Moderated
    Apr 30, 2014 1:10 PM (in response to nsayer)
    Re: DC-DC converter modules - automotive applications

    The 12 volt automotive power bus is a harsh environment. I recall that devices connected to the bus must survive these parameters:

     

    • 14.2 volts with the engine running
    • 24 volts for 1 minute (this is for a double battery jump start)
    • Reverse battery
    • 60 volt spikes
    • 7.2 volts cranking

    The DC-DC converter you have is specified for an input range of 10.8 to 13.2 volts and is not suitable in an automotive application unless protection is provided. A simple series pass regulator set to 12 volts would protect the converter against the 24 volt jump start and the 60 volt spikes. Protection against reverse battery can be provided with a 100 volt Schottky diode. Another solution is a Linear Technology "surge stopper IC.  The LTC4363 and LTC4356 are suitable.

     

    Normally the powered device should be designed to operate to at least 16 volts on an automotive power bus. That way the surge stopper - which is a series pass regulator - is not dissipating power except during exceptional events (24 volts for 1 minute and 60 volt spikes). I would not and have not used a shunt device like a TVS for spike protection on a power bus. This is because we don't often know what other TVS devices are on the bus and so we must assume that ours is the only one.

     

    Surge stopper document:

     

    http://cds.linear.com/docs/en/ad/LTC4366%20Ad%20Final%28P%29.pdf

     

    As your DC-DC converter is only 1 watt (1.25 watts in to the module)  the surge stopper series pass transistor can operate in the linear region all the time with the regulation voltage set to 12 volts. On a 14.2 volt automotive bus delivering 1.25 watts the series pass transistor dissipation is only 0.23 watts. A TO-220 package MOSFET without a heatsink will work.

     

    For higher power applications the surge stopper transistor should be operated fully ON and not in the linear mode. The Surge Stoppers are 2% tolerance devices plus the two 1% resistors to set the point at which they go into linear mode. That makes them 4% devices. If you don't want it going into series pass mode at 14.2 volts it must be set to 14.2 volts + 4% = 14.8 volts. The maximum it can limit at then is 14.8 volts + 4% = 15.4 volts. The power supply must be specified to survive at least 15.4 volts. Note that during a 24 volt jump start event the surge stopper will disconnect the power supply. If you need the power supply to operate during this event set the surge stopper to >24 volts and employ a power supply that will operate at 24 volts. The ideal power supply would operate from 7 to 60 volts. The next best is 7 to 28 or perhaps 10-28 volts, both with a surge stopper set to 27 volts.

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