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Help with diode dissipation - Arduino UPS
Peabody:
I'm working on adding a load sharing circuit to this 18650 charger and boost converter shield:
https://www.aliexpress.com/item/32870411748.html
The shield consists of a lithium ion charger and both 3.3V and 5V regulators. But the USB input does not directly power the load. Instead, the load is powered by the charger and the battery, which has certain drawbacks when using it as a UPS. So I want to add a load sharing circuit that will allow the USB power source to power the load and charge the battery independently at the same time when it's plugged in, then have the circuit automatically switch to battery power when USB is not connected. The additional circuit consists of the mosfet, diode and resistor in the middle of this schematic:
I think as a practical matter this device is good for about 1A. So I'm depending on the very low RDSon of the mosfet to prevent any thermal issues so long as the battery is powering the load. But when USB is plugged in, the mosfet will be off, and the entire load will be sourced through the 1N4001 diode.
I rigged up a test circuit with parts at hand, and the closest I could get was about 760mA. The diode Vf at that current was up to a full volt, but it worked fine and the Vf stayed steady. But it was quite hot to the touch. The situation is made more prickly by the fact that the diode would be soldered to a tiny SOT23-to-SIP adapter board that's less than a centimeter square, so very little heat would be dissipated through that.
So the question is whether I can leave the diode as is, or should I change it to a 3A diode, or even a Schottky. My understanding is that the same heat would be generated in a 3A diode, but the package and leads are considerably larger, so air cooling is more effective. A Schottky of course would have a much lower Vf, and would generate less heat, but I would prefer to avoid any leakage issues if I can, and, well, I don't have one on hand.
Anyway, I just wondered if anyone here has experience running 1A through 1N400x series diodes, and if so, how did they handle that 1W.
David Hess:
--- Quote from: Peabody on April 21, 2020, 11:53:58 pm ---Anyway, I just wondered if anyone here has experience running 1A through 1N400x series diodes, and if so, how did they handle that 1W.
--- End quote ---
Rectifier diodes are common rated by forward current and not power dissipation. For the 1N400x series, this is 1 amp average forward current at an ambient temperature of 75C so you will be fine.
Still, it would not be a bad idea to use a higher current diode. While not idea, 2 diodes in parallel would also improve the situation until you get get a better part.
A higher current diode will have a lower forward voltage drop at the same current which will lower the power dissipation. You might also look for diodes with higher conductivity for lower forward voltage drop.
Vovk_Z:
Why use usual silicon diode instead of Shottky? High leakage current is only a problem if Shottky become hot (and/or voltage is high enough). But it seems to me that is not a case.
Peabody:
--- Quote from: Vovk_Z on April 22, 2020, 09:48:55 pm ---Why use usual silicon diode instead of Shottky? High leakage current is only a problem if Shottky become hot (and/or voltage is high enough). But it seems to me that is not a case.
--- End quote ---
In my circuit (see above), if there is too much leakage, it can raise the voltage on the mosfet gate and reduce or shut off current flowing through the mosfet. So I have to make sure that doesn't happen. I just don't have any experience with Schottky diodes in this kind of situation. I don't know how much leakage current I should plan for. If USB power has been supplying current to the load through the diode, it may be hot. Then if that is turned off, the battery may not kick in properly if there's too much leakage current. So that's why I would prefer not to deal with leakage current if it's not necessary.
jackthomson43:
Instead of using 1N4007, you should try Schottky Diode. That's a better option.
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