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BJT cutoff specs

I'm trying to understand the valid operating characteristics of BJTs but specifically the Fairchild FJPF5027. My circuit is like a resistive voltage divider with the high side resistor replaced by the BJT and being driven by another voltage divider to program the base current. The low side resistance, the "load", is from 100kOhms to

10MOhms or even higher.

The FJPF has a collector current and emitter current cutoff of 10uA. This means that if the load is around 10MOhms with the voltage of 100V then the maximum current is 10uA and the transistor will not pass any current. It is in the "off state"? What this means is that even for lower loads such as 50MOhms will not be driven?

Are there solutions to this? I thought about adding a parallel resistance to the main load that will always drive the BJT out of cutoff. Say 100kOhm in parallel which will give a maximum collector current of 1mA. Unfortunately this ends up requiring the resistor to potentially dissipate 20W. I can increase the resistance by a factor of 10 to make this more reasonable yet then my "drive" current is only

100uA.

I want the BJT to be "on" and not operate close to the cutoff so that the load ends up being cut off from any power. I also want the BJT to work somewhat in the linear region. Do I have the wrong BJT? What exactly is the operation of the BJT near the cutoff? Is it sharp or smooth? i.e., Will 15uA "bias" the BJT so that it will function in the linear region?

More generally if I am working with low power high voltage designs do I need to look for specific BJT's? e.g., Will high power high voltage bjt's create such problems as stated above?

Also, is it difficult to replace bjt's with mosfets excluding the gate driving differences? From what I can tell mosfets do not have a minimum drain current needed to operate. If I use a mosfet instead of a bjt will it work better in this situation. The goal here is to have a programmable voltage divider circuit but the load needs to be on the load side.

Reply to
James Rollins
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On May 17, 12:18=A0pm, James Rollins wrote: [....]

All MOSFETs have a source to drain leakage current. This sets a minimum current for them. At low amounts of gate bias, their characteristics vary quite a bit from unit to unit.

Reply to
MooseFET

It would help if you posted a schematic.

"Collector cutoff current" is OFF-state leakage, collector-to-base leakage in this case. It's probably not as high as 10 uA in real life... that's just a value they can easily test for. If you are suggesting that there is some minimum "on" current, no, there isn't.

This is maybe a big transistor to use to switch your apparently light loads. Bigger transistors do have more off-state leakage.

Up to 400 volts, an optoisolator can be a very handy switch, easy to drive because the output transistor is, well, isolated.

John

Reply to
John Larkin

Hello James,

The Icbo is just the collector base leakage current. It does not mean that the transistor doesn=92t function below that current. Your transistor will function in its linear region below 10uA.

I don't know the number of units that has to be made, but this leakage current (at 100V Vcb) and low junction temperature will be extremely low. Just try to measure it for some devices, you will probably see nA range values.

You should design your driver in such away that you can shunt Icb to the emitter (or below). When the cb leakage current goes into the base, it will be amplified by HFE.

Best regards,

Wim PA3DJS

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Reply to
wimabctel

But how well will it function? Is 10uA the "knee" in the graphs?

Reply to
James Rollins

As I said, it is simply a resistive voltage divider with the high side resistor replaced with the bjt. I'm not sure how much clearer a schematic would make that.

How well will the bjt behave near the cutoff?

I've looked for high voltage bjt's with lower cutoff but they all seem to be 10uA. Maybe this is a somewhat generic number.

Reply to
James Rollins

[...]

The main purpose of a schematic is to give a compact, unambiguous portrayal of how things are hooked up. It also indicates component values used.

...

For the FJPF5027 the 10uA figures you are looking at are in the "max" column of the Electrical Characteristics chart. Max I_CBO < 10 uA is a guarantee that collector-base leakage current (with V_CB = 800V, E open, T_C=25°C) won't exceed 10 uA. If you look at transistor specs that show typical I_CBO, it may be 1% or so of the guaranteed max. Also, as others have suggested, leakage at only 100V will be a lot less than at the 800V of the spec. Have you tried measuring I_CBO or I_CEO for the transistors you plan to use, just as a sanity check?

--
jiw
Reply to
James Waldby

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Hello James,

What "knee" do you mean?

At low Ic you will have a low HFE (lower then published in the datasheet), as Phil also mentioned.

How you will bias your voltage dividing transistor and why do you take a 800V/3A device? Can you mention something more about the application?

Best regards,

Wim PA3DJS

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remove a,b and c when using PM

Reply to
wimabctel

Much clearer.

John

Reply to
John Larkin

I'm surprised that John L. didn't mention it but the free LTSpice has become a sort of lingua franca around here for exchanging schematics. It's (not surprisingly) rather vendor specific but a lot better than hand-waving.

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Rich Webb     Norfolk, VA
Reply to
Rich Webb

You can also hand-scribble it and post to any of the free picture-hosting sites, or to a.b.s.e.

The OP sounds a bit confused about how transistors work, and about the terminology, so his schematic might have all sorts of problems. I've seen some whoppers.

Agree, LTspice is about the only universal, free schematic format. It's also a great simulator. It must be seriously hurting the for-big-bucks simulator companies.

John

Reply to
John Larkin

Yep. He needs to go over to "BASICS".

Not really. It lacks in sophisticated post processing of the data. If it could match PSpice's Probe, it'd be a winner.

And PSpice is an order of magnitude cheaper than the "for-big-bucks" simulators.

...Jim Thompson

--
| James E.Thompson, P.E.                           |    mens     |
| Analog Innovations, Inc.                         |     et      |
| Analog/Mixed-Signal ASIC\'s and Discrete Systems  |    manus    |
| Phoenix, Arizona  85048    Skype: Contacts Only  |             |
| Voice:(480)460-2350  Fax: Available upon request |  Brass Rat  |
| E-mail Icon at http://www.analog-innovations.com |    1962     |
             
 Stormy on the East Coast today... due to Bush\'s failed policies.
Reply to
Jim Thompson

Hi Jim, Don't know if you noticed, but Altium recently lowered the price on their stuff to just $4K. That should drive EMA crazy... 8-)

Charlie

Reply to
Charlie E.

It sure is enjoyable to have the EMA guys calling, nearly weeping, trying to sell me something ;-)

...Jim Thompson

--
| James E.Thompson, P.E.                           |    mens     |
| Analog Innovations, Inc.                         |     et      |
| Analog/Mixed-Signal ASIC\'s and Discrete Systems  |    manus    |
| Phoenix, Arizona  85048    Skype: Contacts Only  |             |
| Voice:(480)460-2350  Fax: Available upon request |  Brass Rat  |
| E-mail Icon at http://www.analog-innovations.com |    1962     |
             
 I love to cook with wine     Sometimes I even put it in the food
Reply to
Jim Thompson

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