Increasing reverse voltage across diode with temperature rise - Page 1
Forum section: electronics / beginners

author: logictom
author_rank: Unknown
timestamp: May 06, 2014, 08:39:59 am
content: I have a circuit that can be powered via 12V or will partially power from USB, the circuitry in question is shown in the image attached.5V1 receives power from a switch mode powered via 12V and VCCUSB via the 5V USB supply. VBus goes to a microcontroller input that detects USB power to allow USB functionality.The problem I am having is that when powered from 12V it works as expected until I start to heat the device. As I heat I get a rising voltage at VCCBUS, this eventually rises to the threshold input voltage of the micro and it thinks the USB has been plugged in causing the main functionality to stop.I have read about using diode as temperature sensors but only in forward bias. I want to understand the problem and also how to resolve the issue.

author: han
author_rank: Frequent Contributor
timestamp: May 06, 2014, 08:56:26 am
content: Is it silicon or schotty diode?I have try heating schotty diode and the reverse voltage breakdown is decrease.maybe simple schematic will help to trouble shoot

author: logictom
author_rank: Unknown
timestamp: May 06, 2014, 10:37:47 am
content: Quote from: han on May 06, 2014, 08:56:26 amThanks for you reply Han. I had originally added a schematic but it must have lost it when it timed out on posting - have now added it to the original post.I have replaced D4 with a BAT750TA (link) it is a schottky diode.

author: Andy Watson
author_rank: Super Contributor
timestamp: May 06, 2014, 11:02:11 am
content: What have you got to load VCCUSB down? Is there more to the circuit? At the moment it looks like the only load on VCCUSB is the CMOS input of the micro, and the leakage of the decoupling caps. What you are seeing is the reverse leakage through the diode which will increase with temperature.

author: ConKbot
author_rank: Super Contributor
timestamp: May 06, 2014, 11:12:06 am
content: The spec that is bothering you here is the reverse current vs temperature, not the reverse voltage. See figure 2 in the BAT750 datasheet. at 25C and 5V reverse voltage, there is 10uA leaking though the diode, at 75C that's upped to ~125uAIf you can find a schottky diode with a lower leakage current at temperature, it should improve your situation. If not, quite a few companies make power supply ORing controllers/selectors that will select between power sources, but not let current leak backwards. Check out the 'power managment' sections of TI/Maxim/linear/analog/fairchild, (or maybe digikey/mouser since you probably only care whats in stock and available anyway...)

author: Rufus
author_rank: Super Contributor
timestamp: May 06, 2014, 12:13:05 pm
content: Quote from: logictom on May 06, 2014, 08:39:59 amJust diode leakage which increases with temperature and schottky diodes are leaky to start with.Put a resistive load on VCCUSB to GND so the leakage has somewhere to go.

author: T3sl4co1l
author_rank: Super Contributor
timestamp: May 06, 2014, 09:13:16 pm
content: Also, why connect a supply object to a port symbol? That's usually a compile warning, and isn't useful (you can use the supply globally already) unless you have strict hierarchy enabled (supplies connect sheet level only).Tim

author: lewis
author_rank: Frequent Contributor
timestamp: May 06, 2014, 09:42:51 pm
content: Quote from: Rufus on May 06, 2014, 12:13:05 pm^^ This. 10K should do it, 4.7K is better. Also, the 33pF caps are too low for any useful decoupling, try 100nF, 1uF or 10uF depending on your application (with X7R dielectric).

author: logictom
author_rank: Unknown
timestamp: May 07, 2014, 12:39:08 pm
content: Thanks for all the feedback.Quote from: Andy Watson on May 06, 2014, 11:02:11 amThat's correct, only the caps and micro input are connected to VCCUSB.Quote from: ConKbot on May 06, 2014, 11:12:06 amI managed to skip over that graph even after looking over it a few times. I will check what other diodes are available and look into the controllers you mentioned, thanks.Quote from: lewis on May 06, 2014, 09:42:51 pmWill hook up a load and report back. There is a Telit module on the board and after a design review they suggested adding the 33pF caps 'to avoid harmonic generation issues'.Quote from: T3sl4co1l on May 06, 2014, 09:13:16 pmWill fix up that and replace with only supply objects.Thanks.