BJT totem pole questions

[MK14]

It's a helluva big project :-)  That currently are put a bit on hold when it comes to any practical tests since I'm in the middle of a move from Malaysia back to Dubai...

Well, the compete plans are not drawn out yet, I'm building/planning the major blocks first and then time will tell how to glue them together. It's a bottom up version. I could of course have done it the other way, started with all my blocks in standard TTL ICs and then made the glue in discrete, followed by replacing the chips with insane amounts of PCBs of discretes...

I currently hope to make it a single cycle processor (https://en.wikibooks.org/wiki/Microprocessor_Design/Single_Cycle_Processors) where all info required by one instruction is already there in the 11 bits of code.  Then microcode is basically not really necessary and can be replaced by a simple lookup table to enable the correct muxes and other bits and bobs in the modules.  At least this is what I hope :-)

The machine will use quite a lot of power when fully operational.  For instance the 16 bit program counter module (with an integrated single level stack) is pulling about 4 amps at 2.5 volts....  The Code ram modules (16 words each with two pcs of 4-to-16 decoders) are about 3 amps. And I plan to have 128 of them

We are completely back on topic then!

Because 128 (Code RAM modules) X 3 Amps = 387 Amps + other stuff.

So you will either need a MEGA super duper, fast FET driver, to drive the power FET(S), to make the MASSIVE SMPS.

Or alternatively you will have to bid, on an old pre-historic super Mainframe computer.

I.e. You may have to get hold of an old Cray1 SuperComputer to *Power* your "new", DTL computer.

I.e. You raid/steal the massive power supplies from an old supercomputer, just to get/make a PSU big enough.

(On a more serious note, You can buy large industrial metal framed (or similar), single low voltage SMPS, quite easily, but you may need quite a few of them, by the sound of it).

When/if you finish it, you could send it to Dave, for a (non-destructive) tear-down and interview session. I originally typed this as a joke. But on reflection, it is not such a crazy idea. But I think the computer will weigh and be physically, a tiny little bit too much, to travel as hand luggage (joke).

4 Amps for *JUST* the 16 bit PC counter, sounds rather HUGE. But I later realized, you have gone for VERY low value resistors, for MAX speed, but terrible power consumption.

The room is also going to get very hot. Your design reminds me a bit of the bottom end (simple) PIC micros. They are sort of Harvard with this sort of combined word thing ("all in one", word).

[matseng]

At 2.5 volts even 384 amps isn't a really huge amount of power.  It's less than 1000 watts - about what a decent gamer PC is using. So if I can get the entire system running at 2 KW or less I consider that an success.

But distributing 2.5 volts from a single point, or even a few points, is not reasonable. It would require a nasty amount of really thick wires and/or copper busbars.  Then I rather cheat a bit by scattering many cheap 4/5 amp buck smsps around the system as needed and then power them by a few of the common 12V 200/300 watt PSUs usually used for LED lightning.

If I then some day would go full retro (or full retard) I could special order a bigass toroid transformer and make a 2.5 regulator of or a hundred or so 2n3055's and a equally bigass heatsink. :-) But with the losses there I'd probably need another separately fused mains circuit.

[MK14]

At 2.5 volts even 384 amps isn't a really huge amount of power.  It's less than 1000 watts - about what a decent gamer PC is using. So if I can get the entire system running at 2 KW or less I consider that an success.

But distributing 2.5 volts from a single point, or even a few points, is not reasonable. It would require a nasty amount of really thick wires and/or copper busbars.  Then I rather cheat a bit by scattering many cheap 4/5 amp buck smsps around the system as needed and then power them by a few of the common 12V 200/300 watt PSUs usually used for LED lightning.

If I then some day would go full retro (or full retard) I could special order a bigass toroid transformer and make a 2.5 regulator of or a hundred or so 2n3055's and a equally bigass heatsink. :-) But with the losses there I'd probably need another separately fused mains circuit.

I agree.
The 384 Amps sounded like tons of current/power. I did not pay enough attention to the voltage.

I think some really old supercomputer/mainframes, e.g. Cray1 supercomputer.

Would have had trouble making such a huge power supply.

So it used a pair of GIANT motors, one an alternator, the other a motor, mechanically linked.
By careful control of the rotational speed, the voltage could be regulated.
In those days (around 1976 I think), such a huge current (it was something crazy like tens of thousands of amps, maybe more, I'm not sure though), would have been difficult to do with just the semiconductors of the time.

But maybe it could have been done with semiconductors then as well.

So a 2600F 2.5V UltraCapacitor, will power your entire new computer build for ages.
Or hundreds of milli-seconds, whichever comes first.

The thing about a linear power supply, is that the 2.5V, final regulated voltage, would still need to be a fair bit over the 2.5V, so the regulators can work. Also the bridge rectifiers would need extra voltage.
So it could end up being 8V * 384A = 3072W + transformer losses = Get money back from your computer build by renting out "Sauna Rooms".
3072W * 1.1 (transformer losses) = The biggest and best sauna in your area! (Joke).

[Ian.M]

Hmm.  Getting regulated smoothed low voltage DC efficiently with '60s technology will be *DIFFICULT*. I have visions of a massive multi-disk homopolar generator with Mercury wetted brushes driven by a big universal motor with a Heath-Robinson governor system for motor speed and generator field coil current.

[MK14]

Hmm.  Getting regulated smoothed low voltage DC efficiently with '60s technology will be *DIFFICULT*. I have visions of a massive multi-disk homopolar generator with Mercury wetted brushes driven by a big universal motor with a Heath-Robinson governor system for motor speed and generator field coil current.

That's similar to what I had (approximately) envisaged.
In its day, the Cray 1, was an incredibly powerful computer.
I would have loved to have met Seymour Cray.

Source: http://research.microsoft.com/en-us/um/people/gbell/craytalk/sld062.htm

The Cray 1 had two of these motor-generators to generate the 3 phase 400 cycle power that was fed to the power supplies located around the base of the computer. Seymour claimed it was the world’s most expensive “love bench”.
The other unit was a large refrigeration unit that pumped Freon into the computer cold plates. Heat was transferred to chilled water that could be used to heat the building.

[step_s]

Hi again guys.
Thanks for all of the great answers. It seems like you have had fun trying to figure this one out xD

But looking at the circuits, they seem to be quite complex for the tiny project I'm currently on.
So i have been scouting around the web for solutions and came up with some stuff I wanted to hear your opinions on (Sorry for the long links):

1: A CMOS configuration with two smaller MOSFETS driving the bigger one, where the gate capacitance of the smaller MOSFETS is 1/5 or lower, than the big. This would mean a pullup resistor setup, with only 1/5 of the loss. Found this one very cheap.
http://www.newark.com/nxp/pmgd290ucea/mosfet-transistor-np-channel-725/dp/12X8769?ost=PMGD290UCEA&selectedCategoryId=&categoryName=All+Categories&categoryNameResp=All%2BCategories

2: A mosfet driver. . . Yes, normaly they are expensive, but I did manage to spot a cheap one that would be able to drive a synchronous buck converter setup, or just a single MOSFET.
http://www.newark.com/on-semiconductor/ncp5901dr2g/mosfet-driv-high-low-side-soic/dp/42Y0833
The datasheet does confuse me a bit, since it seems to be tri-state, and figure 5 is quite strange to me. . .
It seems this driver needs around 3.4V logic input as high, so I would have to drive the MCU at a higher voltage for it to work though.

Please let me hear your thoughts

[Simon]

Your first one is not a mosfet driver but a 2 channel mosfet. Unfortunately farnell are a mess when it comes to categorizing things.

[Zero999]

What about a CMOS level shifter such as the CD40109B or CD4504B before the emitter followers?

[Simon]

isn't a gate not verry good as it's not a hard enough drive ?

[Zero999]

Yes a logic gate would be slow. The idea was to use it to drive a pair of emitter followers. I suppose I should post a schematic.

[Ian.M]

While that would work nicely from 12V, with a typical MOSFET with a gate threshold voltsage >3V, delivering at least 10.5V swing at the gate,   the O.P. wants to use 5V, so wont see more than 3.6V swing at the gate, and is already concerned about the OFF state level.

Unless one is designing for volume production, its generally more cost-effective to bite the bullet and pay for an appropriately specified MOSFET driver IC.

[chris_leyson]

Building a bjt driver for 5V threshold mosfets is going to be hard work. I would level translate your 2.8V logic levels to 5V with a jfet or mosfet level translator, sorry this is the best link I could find, https://learn.sparkfun.com/tutorials/bi-directional-logic-level-converter-hookup-guide. I've used this I^2C level translation from 3V3 to 5V0 they work really well.

Anyway, use a small mosfet to translate to 5V, buffer your 5V logic signal with a hex or octal HC or AHC buffer IC. If your hex or octal buffers can deliver 20mA then you've got 120mA or 160mA gate drive, fast rise and fall times, enough for a 5V threshold mosfet.

[Ian.M]

That's over-complex.  74ACT logic is fast,relatively high output drive capability and has TTL compatible inputs so is happy with logic 1 levels right down to 2V.   I'm thinking 74ACT245, with /OE and DIR strapped low, and all the B pins paralleled as inputs, and A pins paralleled as outputs.  One gate will drive to within 0.5V of the rails with a 25mA load, will settle to within mV of the rails when loaded less than 50uA and it can deliver more like 75mA in the center third of the swing.  Times eight, that's 200mA of fast drive to 5V levels and is going to be a lot better than can easily be done with discretes.

[Jay_Diddy_B]

Hi,

Has the OP told us the part number of the MOSFET that he wants to drive?

Some MOSFETs with low gate charge are easier to drive than others.

Regards,

Jay_Diddy_B

[Ian.M]

In Reply #6: SI2301

See http://www.vishay.com/docs/70627/70627.pdf for datasheet.

That's P channel with a gate threshold of only 0.45V, and a max gate charge of 10nC, which he wants to switch at up to 500KHz.  To do that with reasonable losses is going to
need a couple of hundred mA gate drive.

[Simon]

Oh, that small. Does ot need a driver ? won't the 40mA drive of a micro do it ? if not it needs a proper gate driver just so that another mosfet can fully pull the gate high to get under the 0.45V of the gate theshold.

[Ian.M]

I was figuring it as: half period of 500KHz is 1us.   For switching time not to exceed 10% of the period, to charge/discharge 10nC in 100ns needs 100mA. That's a fairly high proportion of the period, and it would probably be better to aim for 5%., requiring 200mA.   It looks like my suggestion of an octal 74ACT buffer with all sections paralleled would probably work.

[Jay_Diddy_B]

Hi,

In the original post the OP said:

"The MC is switching at 2kHz" which is 500us not 500 kHz.

The OP shows a circuit with an N channel MOSFET, in the first post.

In reply 6, the Si2301DS is P channel.


I think we need clarification of the requirements, 2 kHz is a lot easier than 500kHz.

Regards,

Jay_Diddy_B

[chris_leyson]

I went back and read the original post, 2kHz switching, so a simple level shifter will be fast enough.

[Ian.M]

The O.P admitted 2KHz was a mistake:

@dom0
Ye, I'm sorry that i have said 2kHz in my test setup, when what I'm currently talking about is the implementation.
The SI2301 is sitting in a buck converter setup, where it will have to switch at around 250kHz-500kHz, and if the 10nC on the gate needs to be drained fast enough, a simple pullup resistor on the gate will have to be around 200ohms, and cause a lot of power loss in the system.
So the CMOS would drive the SI2301 and the microcontroller would drive the CMOS
Hope it makes sense.

[chris_leyson]

Thanks Ian.M

Chris