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| Help with Schottky datasheet thermal specs |
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| Peabody:
I would like to use a Schottky diode in a circuit where reverse leakage current could be important because the leakage current would flow into a mosfet gate and cause mischief. And I understand that leakage increases exponentially with the diode's temperature. So the question is - at the maximum current that will be flowing through the diode just before it becomes reverse-biased, how hot will the diode be. If I know that, I think I can ballpark what the leakage current would be at that moment. I'm looking at the datasheet for the 1N5820-22 diodes https://www.onsemi.com/pub/Collateral/1N5820-D.PDF and trying to understand the thermal data. Looking at Figure 6, it says that at 1 amp DC current, power dissipation is 0.4W. I assume that means the forward voltage would be 0.4V. I think Note 4 and Note 5 may have the information I want, but I'm at loss to understand them. The closest I can get is that the highest value in Note 5 is 63 degrees C per watt. Does that mean that my 0.4W dissipation would raise the temperature from 25 to 50.2 degrees (25 + (.4 x 63))? Am I anywhere near right? Well, if anyone can explain this, or point me to an explanation, that would be great. |
| Vovk_Z:
I haven't seen the datasheet now but as I know at maximum forward current (3A for 1N5820) cristal temperature can be about 100-125 C or higher (with +20..+25 C ambient). Your calculation is right for DC. If current is pulsating then diode will be hotter (as in fig.6). |
| Etesla:
Could you just put a high value resistor from the gate to the source of the mosfet to drain that leaking current away from the gate? |
| magic:
--- Quote from: Peabody on May 12, 2020, 06:45:28 pm ---I think Note 4 and Note 5 may have the information I want, but I'm at loss to understand them. The closest I can get is that the highest value in Note 5 is 63 degrees C per watt. Does that mean that my 0.4W dissipation would raise the temperature from 25 to 50.2 degrees (25 + (.4 x 63))? Am I anywhere near right? --- End quote --- Yes, that's how it works, assuming that the 1A current flows long enough to heat up the device from 25°C to 50°C. Observe that 63°C/W applies only to mounting method 2 with maximum lead length. Mounting method 1 with clipped leads is a bit more efficient and mounting method 3 shows that further improvement is possible by adding a copper plane around the diode and mounting it in a particular orientation. Beware that air near your PCB may get hotter than ambient. Note 4 shows how to model heat flow using electric equivalents: heat generation is replaced with a current source and thermal resistance is replaced with resistance, producing voltage drops which represent temperature gradients. |
| Peabody:
--- Quote from: Etesla on May 12, 2020, 08:04:07 pm ---Could you just put a high value resistor from the gate to the source of the mosfet to drain that leaking current away from the gate? --- End quote --- Yes, but it's a P-channel mosfet, so the resistor is to ground so the mosfet will turn on when the Schottky path is turned off. Leakage would raise the gate voltage, which could prevent the mosfet from turning on fully. But yes, the ultimate question is the value of that resistor to make sure leakage doesn't mess things up. I mean, I could just make it 1K, but I hope something a little less wasteful will work. I'll attach the circuit which currently calls for a regular rectifier diode. For a variety of reasons, it needs to be a Schottky. Leakage may actually not be a problem at all. If the mosfet doesn't turn on, then there isn't going to be much voltage on the cathode to leak through in the first place. Anyway, I just need the regulator to switch over to battery power from USB power with no interruption, so the Arduino being powered won't reset. It's supposed to act like a UPS. |
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