Enough current to turn on 3 mosfets? - Page 2 - diyAudio
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Old 8th July 2014, 10:22 PM   #11
SGK is offline SGK
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I think this fixes the error in the VOM1271 parts.
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Old 8th July 2014, 10:50 PM   #12
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Going back to posts 6 and 8, does this work?

PGO goes high showing 5V on gate of Q4 which allows current to flow to the three VOM1271 which in turn activate the main switches Q1-3. Probably short a resistor or three...
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Old 9th July 2014, 06:27 AM   #13
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Quote:
Originally Posted by SGK View Post
Going back to posts 6 and 8, does this work?

PGO goes high showing 5V on gate of Q4 which allows current to flow to the three VOM1271 which in turn activate the main switches Q1-3. Probably short a resistor or three...
The first circuit can work ,but you'll need a limiting resistor in series with led's.
If the total forward voltage of the leds is 3*1.4V=4.2V this resistor should be (5-4.2)/0.01=80 ohm or higher
I doubt if Q4 will open in the second schematic as the gate source voltage is very low .

Cheers ,

Rens
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Old 9th July 2014, 06:42 AM   #14
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...and see that this Vgs should be enough to provide for at least 5A of Ids for each of the 12V and 5V rails? Vgs of 3V3 is not depicted, however? Perhaps there are mosfets more applicable to this application?
Hi Steve
There seems to be a misconception here that no one has picked up on.
You don't need to worry about a Vgs of 3.3 V
The photovoltaic coupler "floats: the Vgs.
So the 3.3 V rail will still have the full 7 or so volts of drive.
So you should be fine on that issue.
Unless it's me that needs another coffee.

As pointed out, the IRF540 with 77 milliohm Rds is not ideal.
The latest from TI has a sub 1 milliohm Rds, seems remarkable to me.

Best wishes
David

Last edited by Dave Zan; 9th July 2014 at 06:48 AM.
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Old 9th July 2014, 10:59 AM   #15
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Hi David, yes my original concern re the 3V3 rail was misguided as I was confusing Vds and Vgs. Learning here but making some sloppy mistakes in thinking while I am at it (as particularly evidenced by my inability last night to multiply 1.6 by 3!). However, I think the recommendation regarding the IRFB3077 is still appropriate. (Less Vgs to get them kicking.) Am I right to think of Rds as like inertia: the lower the Rds the more responsive the switch? The IRFB3077 has typical Rds of 2.8 milliohm.
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Old 9th July 2014, 12:06 PM   #16
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... the IRFB3077 is still appropriate. (Less Vgs to get them kicking.) Am I right to think of Rds as like inertia: the lower the Rds the more responsive the switch? The IRFB3077 has typical Rds of 2.8 milliohm.
We all make mistakes, no problem, I was just concerned that no one had pointed it out.
Rds is more like friction than inertia, it's close to a simple ohmic resistance so the usual Ohms law calculation applies.
A couple of amps across 2.8 milliohm is only about 5 mV so the IRFB3077 or similar will be fine.
1 milliohm is probably lower than many ordinary switch contacts, which I find remarkable.

Best wishes
David
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Old 9th July 2014, 03:56 PM   #17
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Originally Posted by doctordata View Post
The first circuit can work ,but you'll need a limiting resistor in series with led's.
If the total forward voltage of the leds is 3*1.4V=4.2V this resistor should be (5-4.2)/0.01=80 ohm or higher
I doubt if Q4 will open in the second schematic as the gate source voltage is very low .

Cheers ,

Rens
Hi Rens

Isn't the constraining factor the amount of current PGO can sink when held low? That's 10mA. As a result the series resistor R14 needs to be 500R (5/0.01). When PGO is held high then the current across the 3 VOMs is a mere circa (5-3x1.4)/500 = 0.16mA. (In fact it is lower to the tune of the current drawn by the mobo through PGO i.e.120uA to 200uA.)

Hence I think I'm still lacking a good deal of current. Or am I missing something obvious again?

Regards

Steve
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Old 9th July 2014, 07:45 PM   #18
AndrewT is online now AndrewT  Scotland
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the mosfet does not need current to pass.

It only needs current to charge up the gate capacitance as it turns on.
Once turned on no current passes into the gate.

The SPEED of turn on is limited by how fast you can charge up the gate. That fast charging needs more current. But then the current stops.
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Old 9th July 2014, 07:56 PM   #19
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Interesting. Now how about focusing on the current needs of the VOM1271 which drive the voltage on the gates of the mosFETs and the dependence of their output voltage on the current thought their LED? This is what we are talking about.
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Old 10th July 2014, 12:49 AM   #20
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When PGO is held high then the current across the 3 VOMs is a mere circa (5-3x1.4)/500 = 1.6mA. (In fact it is lower to the tune of the current drawn by the mobo through PGO i.e.120uA to 200uA.)
corrected

still not enough current through the VOM diode to its Voc > Vth of the IRFB3077.



Hmm…maybe if I cascade the rail turn-on. PGO turns on the 5V rail and the 5V rail turns on the other VOMs…something for tomorrow...
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