Almost any kind of power relay will maintain its state for practical purposes at 50g, but the coil power will be a bit hefty compared to a MOSFET, not too much more, maybe 2x. You can save yourself a lot of circuit design by going with a P-channel, then the drive is a simple TTL-> HV current source-> V+ referenced MOSFET gate. Digikey has these in stock for $5.40:
All of that is just so ugly, I think I'm going to get sick...
Okay how about this, a traditional bootstrap augmented with a SCVM, simple capacitive voltage multiplier, activated by HV output, to sustain gate drive. The SCVM could be anything, cheapo logic schmitt self-oscillating or cheapo CVM IC or anything... View in a fixed-width font such as Courier.
. . . . . HV . | . .------+-|-|>|---| r |--------|| | . | v | | ||> | . | r | === | | . | | |Cbst | | . +----+ | | | . | '------+---------------+---> out . | | | | . | | ----- - . | | | | ^ . CMOS IN >---' | |SCVM | | . Dz -/ | | | . ^ ----- --- . | | pgnd . +-------' . | . [Rb] . | . --- . sgnd . .
Guess that should look more like this: View in a fixed-width font such as Courier.
. . . . . HV . | . .------+-|-|>|---| r |--------|| | . | v | | ||> | . | r | === | | . | | |Cbst | | . +----+ | | | . | '------+-----+---------+---> out . | | | | | . | | ----- - . | | | | ^ . CMOS IN >---' | |SCVM | | . Dz -/ | | | . ^ ----- --- . | | pgnd . +-------' . | . [Rb] . | . --- . sgnd . .
A bit more explicit: View in a fixed-width font such as Courier.
. . . . . HV . | . 12V>-|>|------+------+-| | . | r | === | | . | | |Cbst | | . +----+ | | | . | '------+-----+---------+---> out . | | | | | . | | ----- - . | | | | ^ . CMOS IN >---' | |SCVM | | . Dz -/ | | | . ^ ----- --- . | | pgnd . +-------' . | . [Rb] . | . --- . sgnd . .
I didn't answer this, sorry. You can estimate relative MOSFET dies sizes by comparing power dissipation ratings and gate capacitance Ciss.
The switching time of a cmos gate, typically under 10ns, is basically zero compared to the several 3 to 5us delay in the optoisolators, and the 10 to 15us time to charge the MOSFET gates. So one before the other has no impact. Generally one likes to draw a circuit including necessary driving pieces, etc., so as not to make excessive demands on the signa; source, etc. That's why an inverter would be customarily shown at the input to start things off.
Well, even so, are you thinking well about the bigger picture? I mean, is this just a bunch of batteries to gang up and help run a motor? What's wrong with just using a diode-OR on each battery? Is it necessary to isolate the OFF batteries? If so, then you'll need two MOSFETs back-back (source-to-source) to insure the ON battery doesn't work to charge the discharged ones. What about fuses, current-limiting, status indicators, etc. Does the motor have a PWM modulator or something, you don't just always give it the full juice, do you? What about current monitoring, etc.?
All that said, if your batteries are all tied together and share the ground system, there's a MUCH simpler approach that can be used. We can dispense with all the optically-isolated stuff. Like I said, generally it's more fun on usenet s.e.d. to talk about whatever interests us most at the time, as conversation goes, rather than what may actually be best for a project.
Took me 2.5 hours to get home! But my Prius was an ideal car for the task - it's great at stop and go traffic - the gas engine shuts down so you can inch forward on batteries. I can get up to 35mph before the gas engine kicks in, if I'm accelerating slowly. (And if the battery is well charged, and if the heater is off, and if it's not so cold the software insists on protecting the battery. That's a lot of ifs.)
What's SCVM?
I think the "Cbst" capacitor power idea was covered, "Cbst" would have to hold enough charge to run the "drvr" for half an hour or more. Maybe if "drvr" is a micropower circuit, and if "Cbst" is big enough.
Wouldn't power dissipation ratings also be highly dependent on the package that the FET was in?
Ah - that makes perfect sense.
So this board is a power distribution board. It carries power to a number of other boards that run various motors through all sorts of current loops. It also powers some digital systems that take in the high voltage and step it down to something more usable.
The reason I want be able to switch on and off the power sources is that I want to be able to protect them from undervoltage and overcurrent situations. They are li-polys, so if I drain them too much or if I pull too much current they die. If I just used diodes I would lose that protection. I also want it high side switched as we have had problems due to low side switching - with fun ground loop problems like through external VGA cables and stuff like that. Also, the hope was that using FETs would give me a bit more efficiency, though I suppose that is not a huge gain. Note that there will be 3 power sources - two batteries and wall power. The wall power input does not need undervoltage protection, obviously (except for it being too low to power anything - at about 9VDC).
I don't understand why I would need two mosfets, or even how that would work... I mean once the FET is off (VGS < VT)current can't flow through it, or at least that was my understanding. Errr wait - intrnisic body diode... Argh. It has a 1.3V forward voltage according to the IRF1407PbF datasheet. I see your point now. What happens to a FET when it is reverse biased? It looks like the body diode turns on at about -1.3V and then can handle a ton of current, but what about between VDS = -1.3V - 0? I'm assuming it's just off. That's not really relevant, but now I'm really curious as I've never really heard the properties of N-FETs discussed with regards to VDS < 0. But why put two FETs source to source? To me it'd make more sense to put a schottky diode in series with the FET. I'd probably put it on the drain side of the FET just so as to increase VGS a little. I mean - if you put two FETs source to source wouldn't you see a 1.3V drop across one of them?
As for current limiting and indicators and whatnot - I am planning on putting a 5V buck converter that powers all the logic switching these FETs, as well as a high side shunt resistor with a high side current sense amplifier. A comparator will watch the output of the high side current sense amplifier and drive an OR gate that is driving the FET driver. I haven't figured out what all indicators are needed - but that is planned. Probably one to show if a battery has been turned off for undervoltage, and another to indicate which power source is being used. Maybe a couple latched LEDs to show if there were any transient problems (ie a surge in current).
The batteries all share a common ground. What simpler system were you thinking of? I would really like to keep it high side switched, if that was your idea.
Ah - I did better than you! Took me a mere 1:45 to get home (typically ~25 minutes). I heard that the highways were completely backed up so I went back roads the entire way, and it worked out fairly well. It was clear sailing from Watertown to Cambridge - almost no cars on the road whatsoever.
The general concept is path splitting, using separate, paralleled gain stages for the DC and AC parts of the signal. We're working on a laser driver, dc to 15 GHz, that uses split paths. The tricky part here is the final combining circuit, and getting the step response really flat in the crossover region. We may just digitize the dc component, run it through a fir filter, and dac it into the combiner; then all we'll need is a tuning algorithm, which might finally be a use for my deconvolution program, which was originally designed to beautify the step response of a fast-but-ugly TDR.
John
I only drew 2 inverters to enforce solid drive to both led's. If the incoming logic level is known to be sufficiently stiff, a single inverter will do.
Except that, as drawn, it just doesn't work any more.
There's no need to consider delays in the led drive; the phototransistors are orders of magnitude slower than the gate delays driving the led's.
John
Oops - all the snow went straight to my head. :o Resistor should be on other side of B optoisolator.
I had it in my head that phototransistors were fairly slow, hence the confusion. Thanks for clearing it up!
-Michael
SCVM is a multiplier used to maintain the gate charge in the big one. Everything is the same. A simplified drawing assuming minimum HV at 10V or more would be as shown: View in a fixed-width font such as Courier.
. . HV . | . 12V>--+-|>|-+--------+-----+-| | . | '----+--B pnp | | | . [1K] | C === | | . | | | |Cbst | | . | | '-----+------+---------------+---> out . | | | | | | . | |- | +-|>|-----+ - . +-------|| SSN1N45BTA| | | ^ . | |> | | [100] | . \\| | | | | | . 2N7000 ||-------+ -/ | === --- .
I'm not impressed with those numbers or your techniques...very boring stuff.
This stuff keeps me amused, and paid.
What amuses you?
John
No. At least not very much. You'll find that dies mounted in TO-220 packages have the same thermal resistance as the same dies mounted in a much larger TO-247 package. And as the smaller D2-PAK package. It's the die's area footprint that matters.
They conduct current either way if the gate voltage is high enough over the source. If the reverse-direction current is high enough the drop across Rds(on) will increase to the point where the anti-parallel intrinsic diode also conducts.
To make a fully-bidirectional four-quadrant switch, that is completely OFF in one state and completely ON in the other.
When the FETs are both ON you get 2*Rds(on), not 1.3 volts.
You may be better off with a diode - assuming you want to switch from one source to the other without a power break, you'll need a diode-OR action. An ON/OFF two-MOSFET switch could conduct huge currents from a high-voltage battery into a low one.
Before leaving the topic, it's worth noting that MOSFETs as active rectifiers are better and cheaper than Schottky diodes. For example, a 60CTQ045 dual 45V Schottky will drop about 500mV at 30A, whereas an IRF1405 55V MOSFET will drop only 210mV at 30A (both warmed up to Tj = 100C). The diode costs $2.78 qty 100, vs $2.64 for the MOSFET.
I'll answer later. Meanwhile look at Fred's ASCII drawing, as a start. I'd change a few things, but the basic idea is to use a capacitor to deliver the MOSFET operating voltages, and transistor level-shift and driver stages.
Note, 0.5V at 30A for a 60CTQ045 dual Schottky is 15W of heat that has to be dissipated, not a pretty sight! Two 60CTQ045 (all four sections in parallel) lowers the voltage drop to about 0.3V at moderate Tj, and lowers the power dissipation to 9 watts total, and to 4.5W in each of the TO-220 packages, which is much more sensible.
I'm not sure how useful it is to post Spice analysis files, it tends to stop or slow down the conversation, because one can't quickly read it on the spot.
WRT your circuit, John. Whew! A 65-volt gate swing! That's bold, and assumes the MOSFET's source will follow fast enough to keep Vgs below the 20-volt limit. Well, maybe, but I'd at least want a protective gate zener.
I liked Fred's circuit better, but the current sink + zener scheme as part of the flying gate-voltage generator bothered me.
Also, neither circuit addresses Michael's issues, which involves connecting two batteries and a third power source, to the 30A output power bus. The sad power-dissipating 30A diode-OR issue and its wiring, etc., is still unsettled.
--- I like ASCII as much as anyone, but when stuff starts getting complex to the point where timing is an issue or an ASCII drawing would just be too busy, I prefer to either run a simulation and post the schematic list or post a PDF to abse. In any case, how much bother could it be, if one is interested, to post the thing into LTSPICE and print out the schematic?
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--- LOL, it's an N channel enhancement mode MOSFET high side driver switching a 55 volt source into a grounded load!
How else would you propose to do it?
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--- Easy enough to do. My intent with the "second cut" wasn't to present a fully fleshed out circuit, but rather to illustrate a way to use a boost converter to generate the regulated gate drive voltage and have it ride on the positive supply voltage as it (the supply voltage) varied.
BTW, did you notice how the current used to drive the Zener and establish the gate drive voltage is returned to the battery instead of being wasted? Kinda slick, I thought...
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--- Oh, well...
---
--- IMO, gett "The batteries all share a comm "I'll answer later."
I thought you were going to take that one on. No?
-- JF
Sorry, but it probably limits the conversation to a few -- you, me, anybody else? And look at the delay. You may not have had any answer at all.
No Sir. Check out the rail-rail current of your pnp and npn pullup and pulldown transistors, ughh!
The driver's currents should be referenced to the MOSFET's source. That's where the capacitance is.
In the noise level, compared to the mayhem underway.
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