EEVblog Electronics Community Forum
Electronics => Projects, Designs, and Technical Stuff => Topic started by: german77 on April 06, 2018, 04:55:53 pm
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I made a circuit with an ideal diode to protect from monkeys faults. It must handle up to 6A continuous. It works fine in the right direction, but if I put power in the wrong direction(not polarity) it stills lets current trough but at a lower voltage 15.2v.
I make sure that everything is connected properly, even tested each component to see if it's fine. I measured the voltage at the gate and source and they are both 24v exactly. In theory for me that's well powered off as gs voltage it's around -2v.
To make things more interesting the 15.2v are as solid as a rock, consistent between different components and values. Tested up to 1.3A with not significant drop on the voltage until the magic smoke scapes, the mosfet heats up as is taking 11w.
If someone can tell me what I did wrong, I would appreciate it very much.
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Found the source of the problem. I removed the mosfet from the circuit. And the 15.2v were still present. Only t16 is connected to de input, so it must be the source of the problem, but I still not know what's going on. The transistor works like a diode even confirmed the 0.7v and open voltages with a multimeter. The break down voltage must be around 80v.
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B-E breakdown is only about 7V. This circuit only works at low voltages.
Here's a faster diode, but it's intended for common-ground applications. You'll need a floating 5V supply (maybe a DC-DC converter or charge pump?) to power it:
(https://www.seventransistorlabs.com/Images/Synchronous_Rectifier.png)
This uses inverted BJTs so the breakdown voltage can be high, unfortunately the gain is low, needing more parts to get good performance.
Tim
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It's not easier to help the transistor with an extra diode? It will block the reverse current, be low cost and have almost no difference in functionality.
The other transistor shouldn't need that diode because the mosfet will be on. Please correct me if I'm wrong.
Edit: Works fantastic, with no downsides. Thank you for finding the problem, I should be more careful reading those datasheets.
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It works in the reverse direction, sure... now try reversing it. ;)
You need matched diodes to do that, so the offset is still ~0V.
A zener from MOSFET gate to source is also a good idea, so as not to exceed Vgs(max).
Tim
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Note that Linear makes pretty nice ideal diode controllers that would make your design fool-proof, such as the LT4351 or even the LT4320 which is a full-bridge controller.