Go Down

Topic: Multiple Automotive 12V Inputs (Read 294 times) previous topic - next topic

blues1143

Hello,

I have an issue that I was hoping you could help me with.

I have a central board that takes inputs from the car and controls other boards using I2C. The central board has 3 power inputs:

1)12V When Car Unlocks:
2)12V Live when Ignition Turned:
3)12V Live With Toggle Switch

The central board gives power to 3 other boards.

In total 20A could be drawn.

These are the different scenarios for power:

a) Only 1) on.
b) 1) and 2) on.
c) 1) and 2) and 3) on.
d) 1) and 3) on
e) Only 3) on

Current cannot be shared between the 3 power inputs on the car side. But on the boards, when more than one is available I would like power to be shared evenly.

I am guessing diodes are the right way to go. Does anyone have any suggestions as to what to use? And if so a link to one would be really helpful. The diodes at the moment will just get hot. Preferably I don't want to use a heat sink.

Thanks

Grumpy_Mike

Use a Schottky diode, they have a lower forward voltage drop and so will not get as hot.

blues1143

Can you recommend a diode that I wont need a heat sink with?

dephwyggl

I have a central board that takes inputs from the car and controls other boards using I2C. The central board has 3 power inputs:
[...]
In total 20A could be drawn.

These are the different scenarios for power:

a) Only 1) on.
b) 1) and 2) on.
c) 1) and 2) and 3) on.
d) 1) and 3) on
e) Only 3) on

Current cannot be shared between the 3 power inputs on the car side. But on the boards, when more than one is available I would like power to be shared evenly.

I am guessing diodes are the right way to go. Does anyone have any suggestions as to what to use? And if so a link to one would be really helpful. The diodes at the moment will just get hot. Preferably I don't want to use a heat sink.
Diodes or diode-like things would be the obvious solution. I've been exploring high(er) current stuff for automotive environments and encountered a similar need for a diode - except mine was for reverse polarity protection. Diodes were the obvious solution (Schottky would be best as Grumpy_Mike points out). However, I was concerned about the significant Vf of diodes (where your heat comes from), and discovered you can use MOSFETs as diodes instead. See this Infineon paper which describes using (P)MOSFETs as low-Vf diodes. At 20A and depending on the MOSFET you use, you may still need a heat sink, but you'll be able to use a significantly smaller one. :) In my case, I need 20A on the high side of my project and am using the IRF4905 (nearly exactly as outlined it the Infineon paper). If you're able to to insert something on the low side of your circuit, you can use n-channel MOSFETs - which typically will have an even lower Vf (and subsequent lower heat production).

If you go the diode route, check out Super Barrier Rectifiers from Diodes Inc. These are the lowest Vf, high current diodes I've been able to find. (They usually come in a TO-220 package in a dual arrangement, but you could double up on them to reduce Vf a little.)

PS You may also find some interesting info if you do a google search for "smart diode".

Cheers!
Dirk

CrossRoads

Take a look at some of these
https://www.digikey.com/products/en/discrete-semiconductor-products/diodes-rectifiers-single/280?k=diode&k=&pkeyword=diode&pv96=24&pv914=56&FV=fff40015%2Cfff8007f%2Cffe00118%2C1f140000&mnonly=0&newproducts=0&ColumnSort=0&page=1&stock=1&quantity=0&ptm=0&fid=0&pageSize=25
0.45Vf at 25A.
At 20A, 0.45V, part will be dissipating P = IV = 9W.  Big heat sink,  heat sink grease for the TO220, or surface mount the DPAK on a big piece of copper.

You might also look at using a P-channel MOSFET as an ideal diode.  If you can find one with a low enough Rds, it will dissipate less power. P = I^2xR, so 20A*20A*.01ohm = 4W for example.
0.0045 ohm = 1.8W dissipated at 20A
https://www.digikey.com/product-detail/en/infineon-technologies/IRF9310TRPBF/IRF9310TRPBFDKR-ND/2202251
Designing & building electrical circuits for over 25 years.  Screw Shield for Mega/Due/Uno,  Bobuino with ATMega1284P, & other '328P & '1284P creations & offerings at  my website.

tauro0221

Hi,
Since you do not want to use heat sink what about dual diode block. They come in a block with heat sink. Do you need to mount them on the board or outside of the board. I think you can get 3 diode in a module ????. I know 2 are available so if you can mounting them outside of the board you can use 2 modules  and you have one spare diode for future use.  Just a suggestion. All depend on how you want to mount them.

Grumpy_Mike

Can you recommend a diode that I wont need a heat sink with?
No. Why are you afraid of heatsinks?

You are playing with large currents here so any voltage drop is going to give you a lot of heat. This is physics.

Go Up