How to quickly turn on/off N-FET switching high side of 55V?

On Tue, 18 Dec 2007 09:56:49 -0600, John Fields wrote:

. . .

Snipped high side switch Rev B

Rev C:

Version 4 SHEET 1 1996 1156 WIRE 752 400 576 400 WIRE 880 400 752 400 WIRE 1104 400 880 400 WIRE 1216 400 1104 400 WIRE 1440 400 1216 400 WIRE 1216 432 1216 400 WIRE 576 480 576 400 WIRE 752 480 752 400 WIRE 880 480 880 400 WIRE 1104 480 1104 400 WIRE 1440 480 1440 400 WIRE 1216 528 1216 496 WIRE 1264 528 1216 528 WIRE 1376 528 1344 528 WIRE 752 608 752 544 WIRE 880 608 880 544 WIRE 880 608 752 608 WIRE 1648 608 880 608 WIRE 1216 624 1216 528 WIRE 1440 640 1440 576 WIRE 576 656 576 544 WIRE 624 656 576 656 WIRE 752 656 752 608 WIRE 752 656 704 656 WIRE 1648 656 1648 608 WIRE 576 704 576 656 WIRE 1104 704 1104 560 WIRE 1216 704 1216 688 WIRE 1216 704 1104 704 WIRE 208 736 96 736 WIRE 1216 736 1216 704 WIRE 1440 736 1440 720 WIRE 1600 736 1440 736 WIRE 368 752 272 752 WIRE 400 752 368 752 WIRE 512 752 480 752 WIRE 1104 768 1104 704 WIRE 1440 800 1440 736 WIRE 1504 800 1440 800 WIRE 1648 800 1648 752 WIRE 1648 800 1568 800 WIRE 96 816 96 736 WIRE 128 816 96 816 WIRE 240 816 192 816 WIRE 368 816 368 752 WIRE 368 816 320 816 WIRE 928 816 880 816 WIRE 1040 816 1008 816 WIRE 1440 848 1440 800 WIRE 1216 896 1216 800 WIRE 1264 896 1216 896 WIRE 1376 896 1344 896 WIRE 1648 912 1648 800 WIRE 96 928 96 816 WIRE 128 928 96 928 WIRE 240 928 192 928 WIRE 368 928 368 816 WIRE 368 928 320 928 WIRE 752 928 752 656 WIRE 880 928 880 816 WIRE 1216 928 1216 896 WIRE 96 960 96 928 WIRE 96 1040 96 1024 WIRE 576 1040 576 800 WIRE 576 1040 96 1040 WIRE 752 1040 752 1008 WIRE 752 1040 576 1040 WIRE 880 1040 880 1008 WIRE 880 1040 752 1040 WIRE 1104 1040 1104 864 WIRE 1104 1040 880 1040 WIRE 1216 1040 1216 992 WIRE 1216 1040 1104 1040 WIRE 1440 1040 1440 944 WIRE 1440 1040 1216 1040 WIRE 1648 1040 1648 992 WIRE 1648 1040 1440 1040 WIRE 96 1088 96 1040 FLAG 96 1088 0 SYMBOL pnp 1376 576 M180 WINDOW 0 75 83 Left 0 WINDOW 3 53 48 Left 0 SYMATTR InstName Q1 SYMATTR Value 2N5401 SYMBOL npn 1040 768 R0 WINDOW 0 71 15 Left 0 WINDOW 3 45 48 Left 0 SYMATTR InstName Q2 SYMATTR Value 2N5550 SYMBOL voltage 752 912 R0 WINDOW 3 40 81 Left 0 WINDOW 123 0 0 Left 0 WINDOW 39 0 0 Left 0 WINDOW 0 40 53 Left 0 SYMATTR Value 55 SYMATTR InstName V2 SYMBOL res 1632 896 R0 WINDOW 0 -39 37 Left 0 WINDOW 3 -41 68 Left 0 SYMATTR InstName R4 SYMATTR Value 1.8 SYMBOL voltage 880 912 R0 WINDOW 3 24 104 Invisible 0 WINDOW 123 0 0 Left 0 WINDOW 39 0 0 Left 0 WINDOW 0 39 57 Left 0 SYMATTR Value PULSE(0 5 0.02 1e-6 1e-6 0.05 0.1 1) SYMATTR InstName V3 SYMBOL res 1024 800 R90 WINDOW 0 -46 56 VBottom 0 WINDOW 3 -38 58 VTop 0 SYMATTR InstName R9 SYMATTR Value 1000 SYMBOL nmos 1600 656 R0 SYMATTR InstName M2 SYMATTR Value SUM75N06-09L SYMBOL res 1088 464 R0 SYMATTR InstName R2 SYMATTR Value 10k SYMBOL res 1360 512 R90 WINDOW 0 68 48 VBottom 0 WINDOW 3 66 47 VTop 0 SYMATTR InstName R1 SYMATTR Value 50k SYMBOL npn 1376 848 R0 WINDOW 0 72 17 Left 0 WINDOW 3 47 50 Left 0 SYMATTR InstName Q3 SYMATTR Value 2N5550 SYMBOL res 1360 880 R90 WINDOW 0 -25 58 VBottom 0 WINDOW 3 -27 56 VTop 0 SYMATTR InstName R3 SYMATTR Value 50k SYMBOL res 1424 624 R0 WINDOW 0 41 43 Left 0 WINDOW 3 34 74 Left 0 SYMATTR InstName R5 SYMATTR Value 1000 SYMBOL ind 608 672 R270 WINDOW 0 45 56 VTop 0 WINDOW 3 -8 54 VBottom 0 SYMATTR InstName L1 SYMATTR Value 5e-3 SYMBOL npn 512 704 R0 SYMATTR InstName Q4 SYMATTR Value 2N5550 SYMBOL res 496 736 R90 WINDOW 0 -11 55 VBottom 0 WINDOW 3 35 55 VTop 0 SYMATTR InstName R6 SYMATTR Value 1000 SYMBOL diode 592 544 R180 WINDOW 0 51 34 Left 0 WINDOW 3 24 0 Left 0 SYMATTR InstName D1 SYMATTR Value MURS120 SYMBOL zener 768 544 R180 WINDOW 0 63 33 Left 0 WINDOW 3 24 0 Left 0 SYMATTR InstName D2 SYMATTR Value BZX84C10L SYMBOL cap 864 480 R0 WINDOW 0 48 32 Left 0 WINDOW 3 28 61 Left 0 SYMATTR InstName C1 SYMATTR Value 10e-6 SYMBOL Digital\\\\schmitt 208 672 R0 SYMATTR InstName A1 SYMATTR SpiceLine vhigh 5 vh 1 trise 10e-9 tfall 10e-9 SYMBOL cap 80 960 R0 WINDOW 0 -44 33 Left 0 WINDOW 3 -51 64 Left 0 SYMATTR InstName C3 SYMATTR Value 1e-8 SYMBOL res 336 800 R90 WINDOW 0 -6 58 VBottom 0 WINDOW 3 36 57 VTop 0 SYMATTR InstName R11 SYMATTR Value 910 SYMBOL diode 192 800 R90 WINDOW 0 0 32 VBottom 0 WINDOW 3 32 32 VTop 0 SYMATTR InstName D5 SYMATTR Value 1N4148 SYMBOL diode 128 944 R270 WINDOW 0 32 32 VTop 0 WINDOW 3 0 32 VBottom 0 SYMATTR InstName D6 SYMATTR Value 1N4148 SYMBOL res 336 912 R90 WINDOW 0 -11 56 VBottom 0 WINDOW 3 40 59 VTop 0 SYMATTR InstName R12 SYMATTR Value 8200 SYMBOL zener 1568 784 R90 WINDOW 0 -4 32 VBottom 0 WINDOW 3 36 32 VTop 0 SYMATTR InstName D3 SYMATTR Value BZX84C15L SYMBOL zener 1232 688 R180 WINDOW 0 -41 29 Left 0 WINDOW 3 -77 -2 Left 0 SYMATTR InstName D4 SYMATTR Value DFLZ33 SYMBOL zener 1232 800 R180 WINDOW 0 -37 31 Left 0 WINDOW 3 -70 -4 Left 0 SYMATTR InstName D7 SYMATTR Value DFLZ33 SYMBOL zener 1232 496 R180 WINDOW 0 -41 29 Left 0 WINDOW 3 -69 62 Left 0 SYMATTR InstName D8 SYMATTR Value DFLZ33 SYMBOL zener 1232 992 R180 WINDOW 0 -41 29 Left 0 WINDOW 3 -77 -2 Left 0 SYMATTR InstName D9 SYMATTR Value DFLZ33 TEXT 1680 944 Left 0 ;LOAD TEXT 104 1056 Left 0 !.tran .1

You can put that current source under ground referenced logic control to keep it off under high duty conditions where Cbst works just fine. We're only talking about supplying leakage here.

Are you talking about this?

Sounds like maybe a solar power system...This is another good reason for going with a P-channel. Anyway, what's the matter with the usual MOSFET blocking switch arrangement, one ckt for each power source, we're not talking big bucks here: View in a fixed-width font such as Courier.

. . HV . | . 12V>--+-|>|-+--------+-----+-| . | | .--B npn | | | | . | | | E | | | | . | [4.7K] | +-[51]-----------|-----+ | . | | | E | | | | . | '----+--B pnp | | | | . [1K] | C === | | | . | | | |Cbst | | | . | | '-----+----------------|-----+ . | | | | | | . | |- +--+-|>|---+ | |>

. +-------||SSN1N45B | | | '--|| . | |> | | [100] |- . \\| | | | | | . 2N7000 ||-------+ -/ | === +---> out .

Oops- lost my Cbst charge-up ckt: View in a fixed-width font such as Courier.

. . HV . | . 12V>--+-|>|-+--------+-----+-| . | | .--B npn | | | | . | | | E | | | | . | [4.7K] | +-[51]-----------|-----+ | . | | | E | | | | . | '----+--B pnp | | | | . [1K] | C === | | | . | | | |Cbst | | | 5W . | | '-----+----------------|-----+--[1k]--. . | | | | | | | . | |- +--+-|>|---+ | |> --- . +-------||SSN1N45B | | | '--|| sgnd . | |> | | [100] |- . \\| | | | | | . 2N7000 ||-------+ -/ | === +---> out .

Don't need all that: View in a fixed-width font such as Courier.

. . HV . | . 12V>--+-|>|-+--------+-----+-| . | | .--B npn | | | | . | | | E | | | | . | [4.7K] | +-[51]-----------|-----+ | . | | | E | | | | . | '----+--B pnp |0.1u | | | . [1K] | C === | | | . | | | |Cbst | | | 1/2W . | | '-----+----------------|-----+--[10k]-. . | | | | | | | . | |- +--+-|>|---+ | |> --- . +-------||SSN1N45B | | | '--|| sgnd . | |> | | [100] |- . \\| | | | | | . 2N7000 ||-------+ -/ | === +---> out .

I'd like to weigh in on one thing: I actually prefer spice drawings over all other possibilities. For me, it just makes things simpler as I can really get a better feel for how things are working, as it is easy to probe. I often will convert ascii drawings into spice drawings for this purpose. I don't have access to ABSE (well, that I know about, at least) - and it's also not archived, so I don't think it's as good of a place to post schematics.

-Michael

And here's a slightly simplified version. Add the PV isolator for DC drive.

ftp://66.117.156.8/Gate_Driver.JPG

One could also use a dip dc/dc converter and the relay.

John

So the die size affects how the transistor handles transient loads, but am I right in thinking that the package (and how it is heatsinked) would contribute alot to how the transistor handled sustained loads?

Do FETs typically drive current as well from D to S as from S to D? Looking at various N-FET datasheets they don't really touch on that.

So the idea is that when both gates were high, both FETs would conduct current, but when both gates are low, you would have two diodes facing each other with both FETs off, providing no path for current?

So this is essentially my question from above - but would Rds(on) = Rsd(on)?

However, aren't MOSFETs not really rectifiers? I mean from what you're saying, it sounds like FETs can handle current in both directions.

Thanks,

-Michael

I'm trying to avoid the use of relays though - they are large and I would think that they would be susceptible to way more bouncing than I'm comfortable with.

-Michael

Hi Fred - I'm having trouble understanding your circuits. You use symbols that I really am not familiar at all with, Could you please tell me what this is:

/|\\ 10mA \\V/

Current sink, maybe? Is the "CMOS EN" the enable for the current sink?

And is this one:

| -/

A 12V zener?

Lastly, what does "cbst" mean?

Thanks, and sorry if I'm being thick. All work and no sleep makes Michael a dumb boy.

-Michael

About the size of a sugar cube. An electronic driver circuit could well be bigger.

Bounce won't matter; the initial contact hit charges the gate all the way.

John

What makes that simpler? And actually, as long as we know the MOSFETs can take fairly hefty transient peak power dissipation , I would prefer to redo the input interface and stick an integrating capacitor in there somewhere to slow the turn on/off to 15V/us range, especially now with several of these circuits driving a common power bus, preventing unrestrained slew rates from inducing transient currents into high impedance nodes. Also, I would want to at least attempt a backup interlock by connecting OUT to the gate of the 2N7000 through a resistor divider, or something, keeping the switching off while a second power source was on the power bus, but not turning itself off:-) Then there's fusing on the HV lead to consider as well as protection of the LV stuff in the event of HV component failure, possibly UVLO and maybe thermal shutdown and other things...

That's a symbol for a constant current source CCS. Could be something like this: View in a fixed-width font such as Courier.

. . . . . ~10mA . . | . | . | . |/ npn . CMOS EN>---[2.2K]--+----| BVCEO>100V . | |>

. \\| | . PN3904|----+ .

Yep- you can probably just drive it with CMOS IN/ON...

Yes- a 500mW job like the 1N5242. It clamps the voltage across the 555 voltage doubler circuit. That would be a CMOS type 555 like the TLC555 or LMC 555 configured as an astable multivibrator with the OUT pin driving a voltage doubler as shown. The doubler diodes can be relatively low voltage like the 1N914A or 1N4148. The 555 circuit should have a bypass capacitor of like 0.22u across its power/com nodes.

That is Cbst, where C is a capacitor, and 'bst' stands for *bootstrap*, Cbst enables bootstrapping where it maintains 12V VGS on the MOSFETs as their source circuits transition between GND and HV.

If you're asking if the forward and reverse-current Rds(on) values are the same, the answer is yes.

Yes. Michael, you haven't told us what's allowed during switching events. I've been assuming only one battery should be delivering current at a time, and if you further assume the current delivery should be uninterrupted, then during switching you need two batteries enabled at once, at least for a short overlap. But if simple MOSFET switches are used they'll deliver current from the higher battery to the lower, and this could be dangerously high current, up to 500A for an pair of IRF1405 FETs, looking at the datasheet for Vgs = 12V. Hence a series diode to prevent this. This diode, even if it's four 60CTQ045 sections (two diodes) in parallel, suggested earlier, will dissipate another 10.5 watts, 17W total if added to 6.3W for the MOSFET switch.

Well he could put a high current inductor ( another $10) in series with the load to coast him through a few usecs while he turns one switch off and another on, then fuse the battery feeds in case something goes awry.

Right, we don't need all that. Let's switch the entire battery + 12V, as John suggested, but let's do it with a simple BJT totem-pole stage.

,----+-------, IRF1405 | | | MOSFET (4) 60CTQ045 sections 12V / 68k | ,--||--- to +HV bus to +55V \\ | | v | | | |/ ------- | +-----| --- _|_ | |\\v | _\\_/_, 15V | | npn |_| ' | zener | | | \\ | +--|

Um, having 1/5 as many parts? Being easy to analyze?

The gate resistor controls slew rate.

More parts!

The dc/dc converter + relay is ideal for a one-off by someone who doesn't want to do a lot of design and pcb fab. You could build one in

You could even use latching relays if that would help.

John

Oops, I forgot, heavy wire!

,----+-------, IRF1405 | | | MOSFET (4) 60CTQ045 sections 12V / 68k | ,--||==== to +HV bus to +55V \\ | | v | | | | ------- | | |/ --- _|_ +-----| | _\\_/_, 15V | |\\v |_| ' | zener | | npn | \\ | +--| gate to be sure it can't engage in RF oscillation while

Not that Michael needs it necessarily, but just to point out that in general totem-pole circuits like this are attractive ways to do HV switching, but suffer from two issues concerning the pullup resistor: (1) as the output nears the V+ rail, the base- and gate-current drive decreases, slowing turn-on, and (2) the resistor creates an unpleasant high current when the output is low, and off. For example, 68k above only delivers a measly 60uA to work when ON within 4 volts of the V+ rail, but wastes 1mA and 70mW when OFF at the ground level.

My solution is to replace the pullup resistor with a current source, made from my favorite LND150 Supertex depletion-mode MOSFET. The LND150N3 has a TO-92 package (or use the LND250 in a sot-23, or LND150N8 in a sot-89 package). Every circuit designer should have a some in their stock drawers; get them from Mouser at 55 cents each.

The simple LND150 by itself gives you a 1 to 2mA current source with a 0.5 to 450-volt compliance range. Adding a resistor lets you reduce the current to 0.5mA, 0.1mA, etc.

If you want a good 1mA, etc., for MOSFET turn-ON at the full plus rail, but a reduced or zero current for the turned-OFF condition, to reduce battery consumption or power dissipation. If needed we can implement that feature with a shutoff resistor from the +5 supply:

Well of course that's going to look simplified since you collapsed all that boost generator stuff into a 12V battery symbol...and it needs to be duplicated with each driver. I found this self-boost charge pump topology for high side drive in integrated circuit form: View in a fixed-width font such as Courier.

. . HV . | . D1 D2 |- . 12.6V>----+-----|>|--+-----------+-|>|--+----- .-|| . | | | |+ | | |>

. | | [Rpu] === DRIVER--+ | . | | | C2 | | | | | . | | .-------------+-----+-----|----+ . | | | | | | | . | |+ -| | | | | . | C1=== Q2 ||---+ | | | . | | out . +-+LMC555 +------' | | . | | | | - . | | 10KHz |- IN>---' ^ . | |----------|| Q1 | . | | |> | . | | | --- . +---+ --- pgnd . | sgnd . --- . sgnd . . . Q1,Q2 SSN1N45B . . .