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Power supply choices for automotive project

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tdeyle

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Okay. I am designing a device that will adjust the signals read from a map sensor from a vehicle's engine.

I had some trouble using a 7805 linear regulator with another project, (iPhone charger), the 7805 kept getting hot and going into shutdown. This, I believed to be caused by the fact that the 7805 had to dissipate 4-7W of power through it's case. I did not want to put a heatsink on it since it would be too large.

I solved this problem by purchasing a cheap USB power adapter that put out 5V. I noticed that the internals were of a "switching" variety.

So I set out to design a power supply around a switching regulator for my automotive design.

It is all designed, the gerbers are sent to the fab house, should be in Monday. So I had a thought today to crack open a device that measures and logs different sensors in my car, to see what that designer used for a power supply. Great timing! Should have waited on those gerbers. They used a linear regulator, in d2pak form. I have never noticed this device get excessively warm where the regulator was. I believe this was possibly due to the fact that the copper pour of the pcb is being used as a heatsink.

Due to the fact that I require 12V, analog and digital 5v, 3.3v and -5v sources, it would be more cost effective to use a linear regulator than a switching variety. My question is, if I use linear VRs, attached to the copper layer they should be fine for dissipation, no? Has anyone used linear regulators in a vehicle environment without thermal shutdown?
 

Diver300

Well-Known Member
Most Helpful Member
The power dissipation in a linear regulator is a function of the difference between the input and output voltage and the current taken.

The temperature rise is a function of the power dissipation and the heatsink.

The only place that the choice of regulator comes into this is that it will form part of the heatsink.

Use a switching regulator. If you add windings to the coil, you can get the other voltages from the main regulator, with maybe a low drop regulator for each of the other voltages.

A linear regulator will be only 25% efficient at 12V in and 3.3 V out, so you will get 3 times as much heat in your regulator as in your circuit. -5V is only possible with some sort of switcher.
 

Willbe

New Member
vehicle power

Watch out for the load dump transient.
 

tdeyle

New Member
@3V0: I have seen alot of hobby circuits use the 7805 for automotive applications, keyword there is "hobby". I am looking for something more robust, as you say for that environment.

@Diver300: I am aware of the dissipation factor and where it comes from.

@Willbe: I have a ST RBO40-40G protecting against load dump and other overvoltage transients, along with reverse voltage issues.

What I was wondering is if anyone has used a linear regulator in one of their projects without thermal shutdown, and if so, what current was being drawn, what was the heatsink they were using, etc.

I have decided to use a switching regulator up front, after the RBO, to regulate the voltage at 12V. The regulator, LTC3780, operates from 4-24V, able to bring the voltage up if it drops below 12V.

After that, I will decide to use either the linear or switching regulators based on the amount of current being drawn by the parts. If the amount is below 250mA, then I don't see an issue with using a TO-263 soldered to the copper.

Like I said, I have seen this in the device that is meant for the automotive environment, and from what I have seen inside, it does not look like there are heavy current hogs inside that device.

Thank you all for your input.
 
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