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Power supply +VCC -VEE turnoff using n ch mosfets?
Power supply +VCC -VEE turnoff using n ch mosfets?
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Old 9th February 2018, 06:43 PM   #11
scottjoplin is offline scottjoplin  Wales
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Old 9th February 2018, 08:33 PM   #12
Monte McGuire is offline Monte McGuire
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Power supply +VCC -VEE turnoff using n ch mosfets?
Don't use relays. MOSFETs are far more reliable. You can use two N channel enhancement mode MOSFET load switches for the power supply rails and they will both turn on and off at the same time. You want to turn both power supply rails, positive and negative, on or off, at the same time, regardless of the fault. With a relay, they will not switch at the same time, and that itself can cause problems, even if the amp circuitry is working properly.

To make a speaker load switch, you need to put two N channel enhancement MOSFETs connected back to back, with their sources connected together, one drain connected to the amp output, the other drain connected to the speaker load, and both gates connected in parallel, with the optocoupler driving the gate and source as normal, except that it's driving two MOSFETs instead of one.

Relays are tricky since the contacts can weld shut, preventing them from turning off. Every time they're used to switch current, you might burn a little spot away from the contact area, slowly increasing their on resistance. At the very least, unless you get really hard contacts, they deform every time you close the contacts, again degrading their on resistance. Finally, the time it takes to close or open the contacts is unknown and variable, depending upon a lot of silly factors. A MOSFET will simply switch the same now and 5 years from now, unless you blow it up. The on resistance can also be far far lower than a relay, often 10-100x lower resistance than most sensibly sized relays.
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Old 10th February 2018, 04:34 AM   #13
PRR is offline PRR  United States
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Quote:
Originally Posted by rhythmsandy View Post
...using a microcontroller to turn off the amp. ... doesnt work....
"Doesn't work" is not clear. What does it do? What does it not do that you would like it to do? What is the difference?

I don't like fancy parts like those opto-couplers. It can be done with simple bipolar transistors and a few resistors, if you can afford P-type and N-type Enhancement MOSFETs. (Using only N-type gets messier.)
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Last edited by PRR; 10th February 2018 at 04:37 AM.
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Old 10th February 2018, 05:39 AM   #14
Monte McGuire is offline Monte McGuire
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Power supply +VCC -VEE turnoff using n ch mosfets?
I guess it's a matter of taste. An "odd" optocoupler makes a high side switch very predictable, using only N channel MOSFETs, whereas using a circuit like you posted requires P and N channel MOSFETs to behave the same, something that will never happen with silicon: electron and hole mobilities in P+ and N+ diffusions are not symmetric. I will say that your circuit is nicely simple, but it requires a quality P channel MOSFET, of similar characteristics to an N channel device, which I will contend is just as "hen's teeth" as an optocoupler. To each his own, but thanks for posting a relatively sane and compact driving circuit for the non optocoupler version. I'll certainly consider it!

Edit: at last glance, the "war on bipolar power supplies" has forced most work on P channel enhancement MOSFETs to lag behind modern N channel trench FETs, so that's why I feel that a goofy optocoupler makes things nicer overall. But, thanks again for posting a sane, simple circuit for using N and P MOSFET load switches. I need to dig into the modern offerings and see if there are any modern P channel devices that'll work well here.

Last edited by Monte McGuire; 10th February 2018 at 05:51 AM.
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Old 10th February 2018, 10:11 PM   #15
PRR is offline PRR  United States
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Quote:
Originally Posted by Monte McGuire View Post
I guess it's a matter of taste.
...requires a quality P channel MOSFET, of similar characteristics to an N channel device, which I will contend is just as "hen's teeth"...
If we all had the same taste, we'd all buy Sanyo boom-boxes.

DigiKey (therefore any large distributor) has a selection of hen-teeth. Yes, P-type choice is pathetic next to the N-mob, and P-type is fundamentally less-good than N-type. But it is only a switch. Super-size the jaws and contacts (or die), both sides will be "perfect enough". Can we afford "super-size"?

rhythmsandy has not said how big this trolly-motor is. I picked "100+W", so dual 50V supplies, <2A average 6Apk on 8r and <4A average 12Apk on 4r, actually doing speech/music so not really working hard most of the time. So I went to DigiKey for P-type >100V >8A thru-hole and sorted on price.

The $1.06 part has a package I have not used. Taking a +3dB price range, #1 seems to meet the dartboard specs for $1.34. #2 would serve a hi-Volt amp that never saw lo-Z, $1.55.

0.2 Ohms Rds On should be ample for a 8r-4r amplifier. If not, there's another page of choices up to $12, and some may be spiffier.

An equally beefy N-type should be trivial.

Switching times will not be equal for P and N, as you say. This might favor the opto-N-plan. However the MOSFET speed is probably not the limit; with my values, the resistive gate drive against gate C will dominate. Miller Effect will be large. The P-type will tend to have more C; you can over-buy an N-type to get near-similar gate Cs. I get very-roughly 1 milliSeconds response time.

Long ago I looked at that type opto and noted 'slow' response into big C. I don't have the specs here and they may have got better.

The logic interface is, obviously, about 1.2V threshold, "TTL" (if you remember). Maybe 1.1V to 1.4V over any likely parts and temps. While I show 5V drive, actually any CMOS micro-CPU on 3.3V will swing with ample margin. There's no Schmitt so be sure the swing is quick/clean; CPU fine but a raw thermistor isn't enough. The transistor specs are mostly "any", except they must stand-off the raw supply so the 10/$1 30V parts won't do for larger power amps; 100V parts are not expensive today.
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Last edited by PRR; 10th February 2018 at 10:15 PM.
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Old 10th February 2018, 10:24 PM   #16
00940 is offline 00940  Belgium
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Originally Posted by PRR View Post
Long ago I looked at that type opto and noted 'slow' response into big C. I don't have the specs here and they may have got better.
0.2ms turn on and 0.3ms turn off into 1000pF for the tlp3906.
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Old 11th February 2018, 02:06 AM   #17
rhythmsandy is offline rhythmsandy
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Look at this one it seems working with very less dissipation with low Rds on mosfets with 7mohm Rdson.

It feels right to me but when placed in the amplifier simulation the raise time is not sharp and it takes long time to raise. Just let me know if anything can be done to this circuit.
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