78xx Regulator with Vin < xx

[Jester]

I'm designing an automotive power supply. The nominal voltage required is 5Vdc and I will likely be using a MCP16322 as the buck converter. This Buck converter is rated for 24V in Max.

Assuming a worst case 28Vdc in (car being boosted by a tow-truck), I need to have some sort of voltage limiting in front of the buck converter.

I'm thinking of using a 7824 (good for +40V in, and is less expensive than a resistor+zener). Under normal conditions the input to the 7824 will be 12-14Vdc, so losses should be minimal. Is there anything wrong with this approach?

Also what would be a suitable transorb for load dump spikes at the input of the 7824?

 

[xenos]

You could use:
https://www.onsemi.com/pub_link/Collateral/LM2576-D.PDF
that is capable up to 40V (in normal version and 60V in high voltage version).

The 78xx will not work correctly when you have low input voltage.
The 24Vdc voltage if reached, may damage other electronic parts of the car. Are you sure it can happen?

 

[KlausST]

Hi,

There are hundreds of buck converters wich can handle more than 24V input voltage.


There is no need for
* adding more parts
* wasting more power
* lowering performance
* decreasing reliability

Klaus

 

[schmitt trigger]

I agree with Klaus. Use an automotive specific device.
Another key parameter you may have not considered is ultra low standby current.

 

[Easy peasy]

Often a series input fet is used to limit the max volts seen by the converter - with a P -fet this is pretty easy to do... and it saturates to a low on drop under normal Vin.

 

[Jester]

Re: 78xx Regulator with Vin &amp;lt; xx

I agree with Klaus. Use an automotive specific device.
Another key parameter you may have not considered is ultra low standby current.

Standby current is not an issue this device will be off when the auto is off.

- - - Updated - - -

If I use a 7812 instead of a 7824 (LM7812's are pretty darn cheap and reliable) with a nominal input voltage of 14V the 7812 losses are minimal.

Another issue is the LM2576 with it's large inductor and capacitor requires about 2-3x board space (original switcher was 1MHz, the new one is 50kHz)

With the car running, 7812 would likely be running in regulation, only dissipates significant power when being boosted so not an issue. The only real issue I see with this is when the car is not running, the input to the 7812 would be slightly below minimum required for regulation, however the 5V switcher will operate just fine with say 10V in (I don't need regulation just voltage limiting while being boosted). I'm just not sure if the 7812 would be unstable with low voltage in?

- - - Updated - - -

You could use:
https://www.onsemi.com/pub_link/Collateral/LM2576-D.PDF
that is capable up to 40V (in normal version and 60V in high voltage version).

The 78xx will not work correctly when you have low input voltage.
The 24Vdc voltage if reached, may damage other electronic parts of the car. Are you sure it can happen?

I realize the 78xx will not regulate with Vin < Vreg, however for this situation this is not an issue as the downstream 5V buck regulator will work fine from say 10-24V in.

In general automotive electronics should withstand 28V for a few minutes because this can occur when the car is boosted via a tow-truck (they often boost with 2 batteries in series for added oomph).

 

[Jester]

Re: 78xx Regulator with Vin &lt; xx

I setup an experiment with a 7815 (TO-220 package), 12Vin, 0.5A load, and an alligator clip for a heatsink, 100nF in and out.

Vout = 9.82V
7815 temperature is 39°C
Output is perfect.
This is the old school type of regulator with the NPN pass transistor and it appears to be perfectly happy at room temp.

The low voltage (max 24V in) buck converter is $2 less than the 40V in version and has an integrated catch diode and O/P transistor, so even with the ($0.5), 7815 this appears to be a smaller and less expensive solution than the high voltage switcher.

As long as the 7815 behaves when warm and cold this seems like a better solution.

- - - Updated - - -

Often a series input fet is used to limit the max volts seen by the converter - with a P -fet this is pretty easy to do... and it saturates to a low on drop under normal Vin.

Circuit example?