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Why are electric vehilcels having 15V auxiliary power bus?

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treez

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Hello,
We have just been asked to do a 300W SMPS for electric vehicle, input is the vehicle battery, which is 300-410VDC, and the output is to be 15V. Why is the output so low in voltage? The only reason petrol cars have a 12V system is because that’s the typical voltage of a lead acid battery with a few cells.
Why aren’t we being asked to provide a more practical voltage output like 42VDC?
 

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15V is a better starting point for the supplies to the lighting, instruments and control kit. You can use LDO regulators. With 42V their power dissipation would be too much, so SMPS would have to be used with their complexity. Plus a massive amount of 12V ancillaries, bulbs, fans, relays. .. .
Frank
 
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treez

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15V is a better starting point for the supplies to the lighting, instruments and control kit. You can use LDO regulators. With 42V their power dissipation would be too much, so SMPS would have to be used with their complexity. Plus a massive amount of 12V ancillaries, bulbs, fans, relays

..thanks, but there’s no need to use linear regulators…with electric cars , everythings going to be designed new..and can use 42V.
If not 42V, then why not at least 24v for the electric vehicle service voltage bus? (ie not the “power train bus” which is 300v plus)
 

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..with electric cars , everythings going to be designed new..and can use 42V.

Not really. When you are designing a vehicle, cost is paramount. As such, even on a brand new design, one tries to keep as close as possible to readily available devices.
I can tell you that we build electronic assemblies for heavy duty construction machines (28V battery), and many of those machines have a 28 to 12V supply to run the main cabin panel and accessories.

But I do agree with you, a 24/28V bus would have been better. Less copper.
 
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42V has been suggested for modern cars (also with combustion engine) since long, there are many articles about it in electronics magazines. But most concepts admit to keep the "legacy" 14V bus and introduce a second 42V bus for high power loads, e.g. fans or electrical heaters. There would be another, probably bidirectional DC/DC converter connecting both busses.

28V is the truck standard, but as all car electronic systems are designed for 14V now, you won't get rid of the voltage level easily.
 

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treez

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surely things like car radios could instead use truck radios, which run on 24v. ECU wouldn't use 12v anyway, it would need say 3v3. LED lights certainly don't need 12v. Windscreen wipers could use truck windscreen wipers. So things could easily go to 24v , surely. I cannot think of a single ancillary that absolutely needs 12v. The bus could surely be made 24v, at least?
 

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Agree with the technical issue. But again, the overriding factor is cost.

The key to low cost is HIGH VOLUME MANUFACTURING.

The 12 volt bus is ubiquitous and its products are manufactured in the hundreds of millions of units per year. From the cost of manufacturing standpoint, you can't beat that.
 

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