question about temperature characteristics of transistors

• posted

I have a project that requires current limiting. The sensing has to be done on the high side and I don't want to order some special chip, so I dreamed up a circuit that just uses jellybean parts.

Icharge-->

------+-----Rs-------+------batt, 12v | | \\ PN2907 / >| x2 |< |---+----| /| | |\\ / 1M \\ | | | | gnd | | | +--------------|---- | | to comparator | +---- or op amp | | R1 R2 | | '------+-------' | gnd

Circuit operation:

the b/e junctions of the transistors will experience a differential voltage Ich*Rs. For a particular DeltaVbe=Ich*Rs, the base currents will exhibit a certain ratio and the collector currents will hew to the same ratio, at least for matching transistors. Choosing R1 and R2 in a ratio equal to the ratio that the collector currents exhibit at target charging voltage will cause equal voltage across the comparator inputs at that moment and trip it.

I haven't tried prototyping this circuit yet, I just came up with the idea this morning, so please criticize it and point out what I may have missed.

This circuit will get bolted somewhere on an internal combustion engine operating outdoors, so temperature will vary to extremes. The transistors' 2mV tempco *should* cancel; but then the circuit operation also relies on the exponential voltage/current relationship, and I haven't found any information about whether the terms of this expression exhibit a temperature dependence. Does anybody know please?

• posted

Perhaps something along the lines of...

You need to make-up for the drop in the sense resistor by biasing (adding R in emitter of one of the transistors).

Temperature IS going to be an issue. Perhaps a dual transistor package would help.

...Jim Thompson

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|  James E.Thompson, P.E.                           |    mens     |
• posted

See:

as a start.

Jon

• posted

Is is also temperature dependent. And since the voltage at some current depends on Is, that does change. Something like:

Is(T) = Is(Tnom)*(T/Tnom)^3*e^[-(Eg/k)*(1/T-1/Tnom)]

Jon

• posted

Why not add a resistor into both emitters? That will linearize the transfer function and mostly swamp the temperature mismatch. That would allow, say, 50 or 100 mV of peak drop in the sense resistor.

A little more base current wouldn't hurt. And maybe some base resistors to keep the transistors from screaming.

Note that Icharge will typically be a very nasty waveform.

John

• posted

This is starting to look like..

D from BC

• posted

A resistor in both emitters cuts the gain... only needed on the left. Run the math, you have to set a transition level somehow ;-)

The 1Meg should go bye-bye. Diode connect left-hand transistor (or right-hand) depending on signal-sex desired.

...Jim Thompson

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|  James E.Thompson, P.E.                           |    mens     |
• posted

Yep. Looks like useful "popcorn" circuits are returning, as people discover that uP's have limited capabilities ;-)

...Jim Thompson

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|  James E.Thompson, P.E.                           |    mens     |
• posted

The comparator does not trip on a ratio, it trips on a difference. This difference voltage will be something like: Is*exp(Veb/Vt)*(R1-R2*exp(-Ich*Rs/Vt)), assuming matched Is, and so you want R1-R2*exp(-IchRs/Vt)=0 or R2=R1*exp(Ich*Rs/Vt). Then the trip current at any other T will be Ich=(Vt/Rs)*LN(R2/R1) so that dIch/dT= K/(Q*Rs)*LN(R2/R1). But you selected R2/R1 to be exp(Ich*Rs/Vt) at Tnom, making dIch/dT=K/(Q*Rs)*(Ich*Rs/Vt)=Ich/Tnom or dIch/Ich=dT/Tnom. This is not what you would call insensitive to temperature.

• posted

Gosh, really?

If you don't care about DC errors.

Transition level? What's that?

His original circuit, fully symmetric, is fine.

John

• posted

[Snip ugly circuit]

I was going to glue the flat sides of a couple of TO-92 transistors together using Krazy Glue. It's amazing how well that stuff works on plastic transistors.

I'm going to study your circuit. Just two transistors. What's not to like?

In the meantime, here's something that looks better than the first circuit I posted. You guys can pick it apart if you have a mind to.

Icharge-->

------+-----Rs-------+--batt, 12v | | Re | | | | | \\ PN2907 / >| x2 |< |---+----| /| | |\\ / | \\ | | | | '-------+ | | | | | ,------------, | | | 317-based | | | | floating | out | | '------+-------' | gnd

Ilim = design limit current

Thanks everybody for the replies.

• posted

Well...Krazy glue is a good thermal insulator, does not hold up to temperature, and would make a "thick" interface between the transistors. Better yet to use a surface mount dual and layout the PCB for a thermal pad for the body to sit upon.

• posted

I need something in the largest surface mount package I can find so that I can hand solder it, that I can buy in onesies and twosies, and totally generic so that it won't get hard to find. I found a couple of npn and pnp current mirrors that would have been perfect, the BCV61 and BCV72; unfortunately, digikey has a large minimum purchase for the npn version. Also, Mouser doesn't list either of these parts, which isn't a good sign. So I'm looking for a dual npn chip and a dual pnp chip, or maybe a quad with two of each. I've never used surface mount, so I have to sort through all the different surfmount packages according to size and find out the names of the ones that are big enough. Then I can use the digikey parametric search to see what's available.

• posted

That ZDS1009 has Camenzind written all over it. It wasn't too long ago when you were bitching about it having start-up problems.

• posted

Here is a much better circuit with linear output, better control over sense transistor operating conditions, low current and low voltage operation too, patented by Analog Devices: View in a fixed-width font such as Courier.

. . -Is->

. . ----+------[Rs]------+----- . | | . [R] [R] . | | . .----+ | . | | | . | >| |< . | Q1 |-----+------| Q2 . | /| | |\\ . | | |< | . | +-----| Q3 | . | | |\\ | . >| | | | . Q4 |-------------------+ . /| | | | . | / \\ | / \\ . | | I | | | I | . Io \\ / | \\ / . | | | . | | | . '-------+--------' . | . --- . .

• posted

Nice. I'll file that one. But.. I want comparator action, not linear output (sorry, I didn't mention that before). If my circuit were to have linear output I would just end up having to add a load resistor, a voltage reference and a comparator to get what I want, an output active high when charging current exceeds Ilim, otherwise open or low output. Just for grins, I jiggered the circuit around again:

Icharge-->

------+-----Rs-------+--batt, 12v | | Re | | | | | \\ PN2907 / >| x2 |< |---+----| /| | |\\ / | \\ | | | | | | out-+ Rb | | | | | | | \\ | / \\| | |/ |---+----| /| |\\ < PN2222x2 >

| | '------+-------' | current sink I = 2 Ilim Re/Rs | gnd

• posted

Sorry, that's I = 2 Ilim Rs/Re

• posted

Good point. If it doesn't leak, how does it start?

...Jim Thompson

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|  James E.Thompson, P.E.                           |    mens     |