Oscillations in BJT (active load project)

[capt bullshot]

Doing so is kind of silly indeed. Power resistors can dissipate way more power than silicon devices for a given size, and dissipating power in a suitable resistor would be the preferred way for most designs.

Well, I am not so sure. If you consider the area or volume of, say, a 10W cement or woundwire resistor vs a 250W TO-247. Not to mention the clumsiness of a heatsink on the former.

One wouldn't mount the resistor on a heatsink in most cases. Resistors have way much higher surface temperatures allowed than your TO-247 transistors and do their dissipation by radiation and air convection. Of course, there are power resistors intended to mount on a heat sink, and these usually are larger size than your transistors. It's a matter of selecting the appropriate resistor. One can get open frame wire wound resistors that can dissipate 1kW in less volume than the heatsink required for the same amount of dissipation with silicon devices. One can also get transistors that can dissipate 1kW (you'd still have to be aware of the SOA). Most power resistors have way more overload tolerance than silicon.

This is an example of a transistor that can dissipate 1kW: http://wunderkis.de/pwramp3/index.html

[Jay_Diddy_B]

Hi tfm and the group,

I suggest that you look at this thread:

https://www.eevblog.com/forum/projects/dynamic-electronic-load-project/

In this thread I showed how I designed an electronic load that was stable. You should be able to apply the same techniques to your load. I suggest reading the whole thread, but you could start at reply 19.

Regards,

Jay_Diddy_B

[David Hess]

The necessary drive current is so much higher that it degrades the thermal tracking of Q4 through Q6.  This might not matter if Q4 through Q6 are part of an integrated transistor array or hybrid which is likely what HP used.

I am afraid I am slow on this one. The base currents of the power BJTs have to be significantly higher compared to the darlington/FET counterparts. But that base current is summed at the power BJTs emitters and will add a small heating effect on the same power BJTs - small compared to the much higher current through them.

Why do you say that "the current sharing steering transistors may want the same or double amount of collector current to operate properly"?

If we consider the Figure 2.82 from tAoE, the sum of the Ie currents through the auxiliary BJTs is obviously 30mA, so if one power BJT has its Ic raised (because of thermal effects) compared to the others, then the voltage drop at its emitter resistor (50mohms) gets higher. Therefore auxiliary BJTs base voltage gets higher, then they get a bigger share of the 30mA and the drop on its 220R collector resistor gets higher (and their Vce lower). Since this 220R resistor is also the base resistor for the power BJT, its Vbe gets lower and so does its Ic current.

If the above is right, then all the auxiliary BJTs do is compensate the thermal effect on any Ic (from power BJTs) differentially, compared to the others, dropping the voltage at the base of the power BJT that is ramping up its Ic/Ie current.

The higher base current from the standard power BJT (non darlington) could make it more difficult for the auxiliary BJTs to regulate the base voltage of the power BJTs, but that isn't the case again, since the higher Ib would also set a somewhat higher drop at the 220R base resistor.

The problem is thermal mismatch between Q4 though Q6 which will alter their individual Vbe at about -2mV/C.

When the change in collector current of Q4 though Q6 necessary to match the voltages across the emitter resistors of Q1 through Q3 is small, the differential heating effects on Q4 through Q6 are also small and their Vbe change due to temperature effects will be small.

If bipolar transistors are used for Q1 through Q3 instead of Darlington transistors, the collector current for Q4 through Q6 will be 10 to 100 times as great; it increases in direct proportion to the hfe of the Darlington's driver transistor which will be quite high.  So the difference in power dissipation between Q4 through Q6 also increases by 10 to 100 times degrading their Vbe match due to temperature differences by the same amount.  So the current sharing using bipolar transistors instead of Darlington transistors is much worse because it causes greater heating in Q4 through Q6.  There are various ways to combat this including lowering the collector current of the transistors used to enforce matching or lowering the thermal resistance between them to lower the temperature differences which is what integration does.  Using an integrated operational amplifier does both.

[David Hess]

It is a great idea as I mentioned but I think the reason HP was able to get away with it was by using matched transistors or possibly a transistor array.  You can certainly do that but transistor arrays are more expensive and less available and separate matched transistors will not track as well with temperature; see my answer below.  It may seem wasteful to use a 20 transistor integrated operational amplifier for each power transistor where a transistor array would be adequate for several but the former is a lot more economical than the later.

I understand your point. And agree. But we have to do a sanity check. If that holds, and HP did use transistor arrays or matched ones, well then they could simply have used matched power BJTs, mount them together (which they did anyway) and use smaller passive ballasting resistors.

Are matched power BJTs more difficult to get than small signal BJTs? I don't know.

Absolutely in the sense that matched small signal transistors can be produced simply by making them part of an integrated circuit.  Even a poor integrated circuit process and layout can achieve matching better than 5 millivolts and with some care in layout, better than 1 millivolt is easily feasible.

Integrated matched power transistors are very rare and offhand I know of no examples.  Some expensive high power integrated circuit processes support them.

Some hybrid modules used matched power transistors with the hybrid construction enforcing better thermal tracking than discrete parts mounted to a common heat sink.

BTW, did they actually measure the auxiliary BJTs in case they didn't use the arrays? I don't think so, even in not-so-large production quantities. And (if) how come they did use the arrays, being more expensive? Those have always been more expensive, right?

Many manufacturers graded discrete transistors to produce matched sets and this is sometimes still done.  Integrated transistors arrays are only more expensive if the required quantities are too low to justify their production.  But integrated transistor arrays are not always suitable to the circuit due to other factors like parasitic coupling between the transistors or to the substrate so matched discrete parts are sometimes still used.

[David Hess]

You know, that need for matched aux transistors is really bugging me. David has a point, but...

On pg 113, tAoE mentions this approach, then again on pg 213 for FETs, and then they say at a footnote that "found this cute circuit trick used in some HP (...) E3610-series linear power supplies. It’s much simpler than using individual op-amps to bias each transistor, as some MOSFET manufacturers suggest".

Matched stuff is not a cute trick, and is not much simpler than op amps, since once you replace them, you might be in trouble.

Here is a thread where Hill advises someone about this approach, to be used in a real circuit:

https://groups.google.com/forum/#!topic/sci.electronics.design/TfOGqGhjTT8

And no one mentioned the need for matched aux transistors.

Here is the schematics for the e3610a:

https://sites.fas.harvard.edu/~phys191r/Bench_Notes/A1/agilent_e3610a.pdf

On pg 15 you can see the parts list. Q4,5 are the aux BJTs that diferentially regulate the power FETs: plain 2x 2n2222a. And the current sink is just a 100k trimpot connected to -12V. (BTW, Q4 and Q5 are drawn incorrectly: emitter and collector are inverted!)

The voltage across the sense resistors reaches 3 amps * 0.6 ohms in the E3610 so 1.8 volts.  Variation of Vbe in the 2N2222A transistors will be small compared to that no matter what their thermal tracking is so there is no need to match them.  It also helps that either MOSFETs or Darlington power transistors were used so their collector currents are lower than if bipolar transistors were used.

I was only addressing Hill's (?) comment that tracking would be worse with bipolar transistors.  The increased power dissipation through the balancing transistors increases the mismatch because of thermal effects.  This may be completely acceptable if taken into account.

In the past, matched transistors or a transistor array would have been the most economical way to improve on this situation but today, using an integrated operational amplifier to enforce current sharing is by far the least expensive and highest performance solution.

[iMo]

This works fine, no oscillation with I_R5=5.00A and Vc=250V.