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Old 10th July 2014, 03:45 AM   #21
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SGK,
The IRFB3077 mosfet has a high input capacitance (~9nF) so it will need a reasonable amount of current to charge this gate capacitance up to get the mosfet to turn on. The vom1271 will produce sufficient voltage (~8.5v if the datasheet is to be believed), but its effective source resistance is very dependent on the LED current. I would drive the vom1271s LEDs (in series) at 20mA to 25mA of current. See attached diagram for my suggestion.

Paul Bysouth
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Old 10th July 2014, 04:12 AM   #22
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Quote:
Originally Posted by PaulBysouth View Post
SGK,
The IRFB3077 mosfet has a high input capacitance (~9nF) so it will need a reasonable amount of current to charge this gate capacitance up to get the mosfet to turn on. The vom1271 will produce sufficient voltage (~8.5v if the datasheet is to be believed), but its effective source resistance is very dependent on the LED current. I would drive the vom1271s LEDs (in series) at 20mA to 25mA of current. See attached diagram for my suggestion.

Paul Bysouth
+100%
I was just reading the TPS3510 datasheet and came to the same conclusion.

Cheers ,

Rens
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Old 10th July 2014, 04:42 PM   #23
SGK is offline SGK
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Paul/Rens

Thanks for this.

Perhaps I am mistaken but I don't think PGO can be pulled up from the 12V supply. The max recommended voltage in the datasheet for PGO is 7V and the ATX spec calls for a 5V (TTL compatible) signal to the motherboard (from PGO). The TPS3510 can sink up to 10mA maximum through PGO when held low.

The TN2510 has a low Vgs(th) of 2V which would seem to be good. I need to track down a Spice model for the TN2510. If V1 is 5V and R1 500 ohms (R3 removed) Vgs is 5V, as Andrew has noted little current is needed to charge the gate, your use of R2 and R4 means Vds is still 5V... It seems all ok, no? Is my logic/analysis correct?

Regards

Steve

PS: I keep thinking about how to have PSON be the driver of the rail switches but it would seem that using this as the trigger (PSON pulled low by the motherboard) would require more complex TTL programming.
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Old 10th July 2014, 04:48 PM   #24
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Since there is only one "ground" for the entire circuit, and the photovoltaic couplers are not being used for galvanic isolation of two independent subsystems with different "grounds" ...

It seems to me you could save a lot of money on parts by building a straightforward, DC coupled signal booster. Just a logic inverter with level shifting. Use the 5V digital signal "PGO" on TPS3510 pin#8, to create a 12V digital signal "12V_PGO_inverse" with a very low output impedance / very strong output drive.

Then you use "12V_PGO_inverse" to drive the gates of three Pchannel MOSFET pass transistors. You're only passing 5.0 amperes of current; for that, you can fit the very low-cost SMD device IRF9317. Its dissipation will be IxIxR = 5x5x0.007 = 175 milliwatts, well within the device ratings even when the ambient air is 70 degrees C. And it's in the SO-8 gullwing package which is quite easily hand-soldered; there's no ground-slug beneath the package body that requires oven-reflow soldering.

You don't need to pay for expensive photovoltaic drivers, you don't need to pay for expensive 1 milliohm FETs, and you don't need to drive high capacitance from a scrawny TPS3510 5V digital output pin.

In this arrangement, the weakest link will be the 3.3V pass transistor, since it has the least Vgs. If that frightens you, and it shouldn't (see Figure 1 of the IRF9317 datasheet), just connect two (or more) pass transistors in parallel. This reduces Rds_on by a factor of N, which reduces the voltage drop across the composite switch by the same factor of N.

Or, if you absolutely insist upon using big, expensive, thru-hole Nchannel pass transistors ("high side switches"), you could buy high side switch driver ICs for a lot less money than photovoltaic isolators. The Micrel MIC5014, with on-chip charge pump, is one possibility. Price is 50% less than VOM1271 photovoltaic isolator AND it was purpose-built by expert designers, to drive a big whomping MOSFET.
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Old 10th July 2014, 06:29 PM   #25
SGK is offline SGK
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Quote:
Originally Posted by Mark Johnson View Post
Since there is only one "ground" for the entire circuit, and the photovoltaic couplers are not being used for galvanic isolation of two independent subsystems with different "grounds" ...
Oh, this had been needling my conscience for awhile. But I ignored it, unsure and thinking I was in sight of a workable solution even if it was inefficient and not cost effective.

Quote:
Originally Posted by Mark Johnson View Post
It seems to me you could save a lot of money on parts by building a straightforward, DC coupled signal booster. Just a logic inverter with level shifting. Use the 5V digital signal "PGO" on TPS3510 pin#8, to create a 12V digital signal "12V_PGO_inverse" with a very low output impedance / very strong output drive.

Then you use "12V_PGO_inverse" to drive the gates of three Pchannel MOSFET pass transistors. You're only passing 5.0 amperes of current; for that, you can fit the very low-cost SMD device IRF9317. Its dissipation will be IxIxR = 5x5x0.007 = 175 milliwatts, well within the device ratings even when the ambient air is 70 degrees C. And it's in the SO-8 gullwing package which is quite easily hand-soldered; there's no ground-slug beneath the package body that requires oven-reflow soldering.

You don't need to pay for expensive photovoltaic drivers, you don't need to pay for expensive 1 milliohm FETs, and you don't need to drive high capacitance from a scrawny TPS3510 5V digital output pin.

In this arrangement, the weakest link will be the 3.3V pass transistor, since it has the least Vgs. If that frightens you, and it shouldn't (see Figure 1 of the IRF9317 datasheet), just connect two (or more) pass transistors in parallel. This reduces Rds_on by a factor of N, which reduces the voltage drop across the composite switch by the same factor of N.
I put this in the same basket as my "PS" in my last post. I don't know where to start but I will begin investigating.

Quote:
Originally Posted by Mark Johnson View Post
Or, if you absolutely insist upon using big, expensive, thru-hole Nchannel pass transistors ("high side switches"), you could buy high side switch driver ICs for a lot less money than photovoltaic isolators. The Micrel MIC5014, with on-chip charge pump, is one possibility. Price is 50% less than VOM1271 photovoltaic isolator AND it was purpose-built by expert designers, to drive a big whomping MOSFET.
Maybe this is an interim solution.

This is exhausting...but I am learning with the help of you guys. much appreciated.

Cheers

Steve
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Old 10th July 2014, 09:27 PM   #26
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I had about an hour this evening to do a bit more reading and came across this article Using MOSFETs in Load Switch Applications. Fig 3 would seem informative with EN replaced by PGO. What might be a suitable small-signal NMOS transistor for Q1 and what would be the disadvantages of this versus your proposal of a DC coupled signal booster?

I note also the article's comments regarding the impact of Rds(on) on output voltage and its load dependency, and the noted disadvantage of P-channel MOSFETs in this regard. Is it worth worrying about this?

With respect to the common ground, I went back to Piisami's circuit which provided the original inspiration for my suggested use of VOM1271. Isn't the same point applicable here i.e. common ground for +ve, -ve and the circuit providing the control voltage?

(I realise this is likely even more exhausting for you guys to teach me this stuff…)
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Old 10th July 2014, 10:20 PM   #27
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If you discard photovoltaic optoisolators, and if you confine yourself to BJTs, JFETs, MOSFETs, resistors, capacitors, diodes, ICs, and inductors, these are native elements in LTSPICE. Which means that you can try out your circuit design in simulation and see, with your own eyes, exactly how well it performs. If adequate, rejoice. If not adequate, redesign and resimulate.

It's probably a good idea to decide upon your important objectives, and your "that's plenty good enough for me" stopping criteria, lest you waste a lot of time needlessly polishing a coprolite. I suspect that "at max load current, input to output delta-V shall be less than WXYZ microvolts" might be one of your objectives.
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Old 11th July 2014, 08:10 AM   #28
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Agreed and thank you. I would like to understand better the common ground point and why the decision to use photovoltaic MOSFET drivers in Piisami's circuit. Sounds like there were other objectives driving that decision which may not be applicable here. So that may well have been a wild goose chase albeit a learning experience.

On p versus n mosfets, I at least have a better appreciation of the impact of Rds(on) now. I have voltage rails available and so if there is a n equivalent of the p channel MOSFET you mentioned but with appreciably lower Rds(on) then maybe that's worth using.

For Q1 in the paper I linked to, i will for now explore using the transistor Paul used unless there are better suggestions.

I will also try to learn more about your doc coupled signal boosters. Thanks again.
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Old 11th July 2014, 01:57 PM   #29
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Quote:
Originally Posted by SGK View Post
I would like to understand better ...why the decision to use photovoltaic MOSFET drivers in Piisami's circuit.
For power amplifier circuits like Piisami's, there is a need to break supply lines of both polarities simultaneously, and with fairly substantial currents and volts.
The desire to reduce losses at that current level tends to push towards N Fets in both supply lines. The supply rails are above the 30V limit of the MICREL 5014 (is this easy to work-around?).
And you have some handy extra rails.
So trade-offs aren't quite the same.
On the other hand, I don't know where Mark Johnson has his prices from, perhaps better than my on-line lazy choice.
Mouser shows the Micrel 5014 as just a little more expensive than the VOM1271 rather than cheaper. And "expensive photovoltaic drivers" ? Mouser price is less than $4 each. So photovoltaic is hardly a bad solution, even if you need three.
Probably more a case of choose what you find convenient.

Best wishes
David

Last edited by Dave Zan; 11th July 2014 at 02:18 PM.
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Old 11th July 2014, 03:07 PM   #30
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Dave, you're right, I made a mistake. Through carelessness, I incorrectly compared the qty=1 price, against the qty=2500 price (!). Sorry for the confusion.
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