automotive power supply circuit sanity check - Page 1
Forum section: electronics / projects

author: ToBeFrank
author_rank: Regular Contributor
timestamp: November 29, 2011, 09:38:01 pm
content: I've designed my power supply for automotive use as follows. I think I've done my calculations correct and I should be ok as far as transients and load dumps, but I'm no expert. The max load from my circuit will be around 300mA. The fuse has a max voltage of 60V. I'm a little unsure if the zener power rating is sufficient. It supports a continuous current of 264mA and a maximum surge current of 5.5A. Do you guys see any problems?

author: Psi
author_rank: Super Contributor
timestamp: November 29, 2011, 11:58:38 pm
content: It will work fine, but it might be a good idea to add a TVS diode.TVS diodes are specially designed to absorb super high energy for a very short period.Also the bi-directional TVS diodes cost very little more than the uni-directional ones and will protect against any negative high voltage spikes better than the polarity diode.

author: NiHaoMike
author_rank: Super Contributor
timestamp: November 30, 2011, 03:28:44 pm
content: You may want a larger input capacitor and use a PTC instead of a resistor.

author: ToBeFrank
author_rank: Regular Contributor
timestamp: November 30, 2011, 03:40:49 pm
content: Quote from: Psi on November 29, 2011, 11:58:38 pmThanks, I'll look into it.

author: ToBeFrank
author_rank: Regular Contributor
timestamp: November 30, 2011, 03:42:28 pm
content: Quote from: NiHaoMike on November 30, 2011, 03:28:44 pmI used the wrong schematic symbol for F3. It is actually a PTC resettable fuse. Can you explain why I would need more input capacitance? I've googled trying to find how to calculate how much I would need, but I didn't find anything concrete.

author: slateraptor
author_rank: Frequent Contributor
timestamp: November 30, 2011, 04:41:05 pm
content: What's the PN on that Zener?

author: ToBeFrank
author_rank: Regular Contributor
timestamp: November 30, 2011, 04:43:41 pm
content: Quote from: slateraptor on November 30, 2011, 04:41:05 pmhttp://search.digikey.com/us/en/products/SMBJ5355B-TP/SMBJ5355B-TPMSCT-ND/1636221

author: slateraptor
author_rank: Frequent Contributor
timestamp: November 30, 2011, 04:53:41 pm
content: I'm confused as to why you would shunt an 18V Zener with a 12VDC nominal input.EDIT: ...unless it's your intention to have the input hang out before the knee and the Zener is there to clamp the input in the case of transient spikes?

author: Balaur
author_rank: Unknown
timestamp: November 30, 2011, 05:02:17 pm
content: Quote from: slateraptor on November 30, 2011, 04:53:41 pmIt's for limiting all transient spikes at a maximal(ish) value of 18V. This way, the 78xx regulator is kept inside nominal electrical ratings.Cheers,Dan

author: slateraptor
author_rank: Frequent Contributor
timestamp: November 30, 2011, 05:05:52 pm
content: Yeah thanks. I looked at it a bit harder and realized that it's a voltage clamp.

author: Psi
author_rank: Super Contributor
timestamp: November 30, 2011, 11:48:43 pm
content: One other note on TVS diodes. Don't go too low, voltage wise, for your TVS.You want to avoid your TVS being the lowest voltage TVS diode in the whole car.Otherwise it may activate first and try to take on all the spike itself.Ideally you want to share it around a bit.18-22V is fine. But i wouldn't go much lower than 18V

author: ToBeFrank
author_rank: Regular Contributor
timestamp: December 01, 2011, 05:01:30 am
content: Here is my updated design. The TVS diode I've chosen has a breakdown voltage of 22.2 to 25.5V and a max clamping voltage of 32.4V. The 7808 I've chosen has a max input voltage of 36V so I'm good there. It's also below my electrolytics' rating of 35V. Any problems with the updated design?

author: PeteH
author_rank: Unknown
timestamp: December 01, 2011, 06:58:35 am
content: I am a big fan of using the series resistor (present in your original schematic); not only does it help the linear regulator in terms of thermal dissipation, but creates a low-pass filter and in-rush limiter with your electrolytic network.If you're using a linear regulator anyway; at 300mA you can add a ~3 ohm resistor "Rx" (up to 270mW taken from your linear regulator)...Effective drop-out:Vdropout = 8+Rx*0.3+Vdo(PS1at0.3A) +Vf(D2at0.3A)+Vd(F3at0.3A), adjust accordingly.It also helps reduce transient clamping currents; adding it before the TVS, it will reduce the operating knee and force the system to take a larger portion of the fault current.

author: kripton2035
author_rank: Super Contributor
timestamp: December 01, 2011, 11:36:20 am
content: just for curiosity, how does a smps dc dc converter comport in front of "high voltages" transients like the ones you can find in an automotive environment ?wouldnt it be simpler to use one here ? wouldnt it dissipate less power ?

author: PeteH
author_rank: Unknown
timestamp: December 01, 2011, 01:50:03 pm
content: A switching converter would be a better choice in this application; all depends on cost. Converters with high switching speeds will be able to ride through an automotive transient with little change in output; as long as the converter's duty cycle does not go below it's minimum (unlikely). Linear tech. has a ton of high transient (automotive) input switching regulators. You can also find a lot of high Vin linear regulators which would also be capable of dropping the transient for short amounts of time.

author: ToBeFrank
author_rank: Regular Contributor
timestamp: December 01, 2011, 03:19:18 pm
content: Quote from: PeteH on December 01, 2011, 06:58:35 amThe trip current of my fuse is 1A so during a load dump when the TVS is conducting, in the worst case, wouldn't that require a 3W resistor? At any rate, the reason I removed the series resistor is because with the increased size of the TVS, I ran out of board space.

author: ToBeFrank
author_rank: Regular Contributor
timestamp: December 01, 2011, 03:22:47 pm
content: Quote from: PeteH on December 01, 2011, 01:50:03 pmUnfortunately, there are less options because I need 8V output. However, the LM2937 would work great for me. The datasheet says the following:QuoteDo I still need all my protection circuitry if the regulator itself does the protection? The regulator is expensive so eliminating the other expensive protection components would offset the cost increase.

author: ToBeFrank
author_rank: Regular Contributor
timestamp: December 01, 2011, 03:25:58 pm
content: Quote from: ToBeFrank on December 01, 2011, 03:22:47 pmQuoting myself here... that's a dumb question. Of course I do to protect my electrolytic filtering caps. But I only need the TVS to clamp the voltage, right? So another question is: with the protection I have in place, why would I choose this expensive regulator over the cheaper 7808 (that supports up to 35V max input)?

author: qno
author_rank: Frequent Contributor
timestamp: December 02, 2011, 12:09:58 pm
content: For automotive use check out ISO 7637-2 (2011).http://caxapa.ru/thumbs/281680/ISO_7637-2_2004.pdf