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Idea shootout: ideal diodes.
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Wimberleytech:

--- Quote from: Jay_Diddy_B on April 19, 2020, 04:47:51 pm ---Hi,

This circuit has a flaw that could cause it to fail catastrophically in some circumstances:

(Attachment Link)

If the circuit is reverse biased, Vout is positive with respect to Vin Q1 Emitter Base junction is forward biased.
Q2 Base emitter is reverse biased. If the voltage exceed the reverse breakdown voltage of Q2, Q2 will be destroyed.

(Attachment Link)

In other words this circuit can only be used applications where the maximum reverse voltage is less the reverse breakdown voltage of Q2.

Regards,
Jay_Diddy_B

--- End quote ---

A solution might be to limit the current by simply putting a resistor between the bases.
technix:

--- Quote from: magic on April 18, 2020, 05:53:18 pm ---Any problem with simply letting the hub power itself?

--- End quote ---
If I leave the wall wart in there, the hub would feed power back its uplink port (which it should not) and keep some part of my motherboard powered when it is supposed to be powered off (which is bad.) It is also ill-advised to cut VBUS as that makes the port unable to detect the uplink status which can affect enumeration. This is where this ideal diode circuit comes in, as it really should have been integrated in the hub to begin with, but somehow I have to patch it in myself.


--- Quote from: Jay_Diddy_B on April 19, 2020, 04:47:51 pm ---In other words this circuit can only be used applications where the maximum reverse voltage is less the reverse breakdown voltage of Q2.

--- End quote ---
This is protecting a 5V rail using transisotrs with a breakdown voltage of no less than 6V. Both sides also have 5.6V Zener clamps.


--- Quote from: Wimberleytech on April 19, 2020, 05:02:53 pm ---A solution might be to limit the current by simply putting a resistor between the bases.

--- End quote ---
This requires me having access to that common base, which is not the case of using a transistor pair chip pre-connected for current mirror use. The BCV62 transistor pair chip I used have the bases connected internally (Notice the different color in the schematic - maroon lines are internal to the chip, and green ones are external.)
Wimberleytech:



--- Quote from: Wimberleytech on April 19, 2020, 05:02:53 pm ---A solution might be to limit the current by simply putting a resistor between the bases.

--- End quote ---

--- Quote ---This requires me having access to that common base, which is not the case of using a transistor pair chip pre-connected for current mirror use. The BCV62 transistor pair chip I used have the bases connected internally (Notice the different color in the schematic - maroon lines are internal to the chip, and green ones are external.)

--- End quote ---

Yup...thats a problem!
exe:
I think there are plenty transistors without bases tighten together, such as https://www.tme.eu/Document/b038a6211c62764e189393900f367e18/BC807UE6327.pdf . I'd say look for transistor arrays with 6 leads, chances are parts are separated.

PS I don't know if there are any caveats related to them being on the same die. May be they are not completely isolated.
magic:

--- Quote from: technix on April 28, 2020, 05:00:14 pm ---It is also ill-advised to cut VBUS as that makes the port unable to detect the uplink status which can affect enumeration.

--- End quote ---
Yes, that's what I meant. I don't think it's going to be a problem, because the hub's PSU permanently drives VBUS to 5V anyway.


--- Quote from: exe on April 28, 2020, 05:50:42 pm ---PS I don't know if there are any caveats related to them being on the same die. May be they are not completely isolated.

--- End quote ---
Unless you pay extra for so-called "monolithic pair", you will get two separate dice in one plastic blob. Some manufacturers may promise that they came from adjacent locations on the same wafer, some even guarantee rudimentary matching (maybe 3mV Vbe offset, some tens of % on beta). NXP offers some matched duals IIRC.

In monolitic pairs, collector to collector isolation is done with reverse biased PN junctions and may break down at excessive voltage, in addition to having some parasitic capacitance.
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