Pass Transistor Driver Problems

[Kleinstein]

This 2 op-amp circuit is always going to have a delay/gap in swapping control from V to I. When the V op-amp output is controlling at around 0.5V, the I op-amp output has to come down from 3.6V to take over, not only is there a gap/delay before it does, but during this time the V op-amp(in linear mode) moves in the wrong direction and overshoots before the I amp gets down to 0.5V.

This problem can be reduced by having the local capacitive feedback at the OPs mainly from behind the diodes. Besides the down programmer this was the second change I had in the circuit. The plan is a little messy in that area and there is still some direct feedback capacitor - some OPs might need that. No need for the series resistors with these caps.
This will not fully eliminate the problem, but it can be reduced by something like a factor of 10.

[ZeTeX]

This 2 op-amp circuit is always going to have a delay/gap in swapping control from V to I. When the V op-amp output is controlling at around 0.5V, the I op-amp output has to come down from 3.6V to take over, not only is there a gap/delay before it does, but during this time the V op-amp(in linear mode) moves in the wrong direction and overshoots before the I amp gets down to 0.5V.

This problem can be reduced by having the local capacitive feedback at the OPs mainly from behind the diodes. Besides the down programmer this was the second change I had in the circuit. The plan is a little messy in that area and there is still some direct feedback capacitor - some OPs might need that. No need for the series resistors with these caps.
This will not fully eliminate the problem, but it can be reduced by something like a factor of 10.

Is moving the output caps behind the current sense resistor an option? that way the caps get charged faster when the current limit is at low value, and the voltage ramps up quickly, but I wonder if it can effect stability?
EDIT: it those, got it to osciliate with just 0.01ohm load when the caps are behind the sense resistor.

[Kleinstein]

Moving the output caps to the other side of the shunt does not help at all. Capacitance there is not helpful but more a problem.

To reduce the current overshoot one can limit the voltage at the output of the current regulation to a value just higher than needed worst case. Also making the regulation part relatively fast helps. It's the principle problem of having the current limiting first inactive not to interfere with current regulation and than having an integrating regulator to start.

[StillTrying]

Well this is where I'm up to in StillTrying to improve the 2 amp circuit's V to I crossover, don't .

The 1st graph shows the two amps/comp outputs swapping control from V to I, and the pass tr's current during this time. The swap over only takes 880ns, but then 5uS to stabilize to the set current, - and the load still has the charge in the (now 3.2uF) output cap to deal with.

[Kleinstein]

The LM339 is a comparator - this might cause trouble in a real circuit. Also 3 meg resistors are rather large, not practical for a fast real circuit.  This circuit might very well oscillate.

It can help to have the current regulation considerably faster than the voltage regulation. So the current won't go up that much until current regulation sets in.  For the cross over from current regulation to voltage regulation the capacitor in the divider and the limiting diodes can help - at least in may cases. Cross over will be slow, but at least usually coming from below.

[ZeTeX]

Well this is where I'm up to in StillTrying to improve the 2 amp circuit's V to I crossover, don't .

The 1st graph shows the two amps/comp outputs swapping control from V to I, and the pass tr's current during this time. The swap over only takes 880ns, but then 5uS to stabilize to the set current, - and the load still has the charge in the (now 3.2uF) output cap to deal with.

Could you upload your .asc file please?

[StillTrying]

Could you upload your .asc file please?

I don't think it's very good.

https://github.com/evenator/LTSpice-Libraries/blob/master/sub/LM339.sub

[ZeTeX]

Just wondering, for performance, how about using IGBT as a pass element?
As "Kleinstein" pointed out that MOSFET can be faster, but they don't work well at low currents, and they need protection and things like that, but NPN are pretty robust but slower compare to some MOSFETS, then how about combine both (IGBT) and get fast speed + robustness? do people using IGBT as a pass element? if I would be able to find IGBT model to LTspice I might try it out.

[Kleinstein]

IGBTs are usually not that fast. There are a few audio amplifiers that use IGBTs, but that is more an exception. Most IGBTs are made for switching applications, so they may have similar SOA problems as modern MOSFETs. Also most IGBTs are made for high voltage (e.g. > 600 v) as this is the area where they might make sense in switching applications. In Principle one could combine a large NPN with a N-MOSEFT to get similar properties.

For most cases my choice would be BJTs (e.g. 2SD1047) made for audio applications: they are reasonably fast and have good SOA curves, that are reliable, as they are made for analog operation.  One problem might be that good audio transistors are also candidates to find fakes, but I don't think one needs so fast ones (e.g. SD4700 or similar) unless one gets a really good layout. Fast circuits can also get sensitive to parasitic inductance and coupling.

[ZeTeX]

IGBTs are usually not that fast. There are a few audio amplifiers that use IGBTs, but that is more an exception. Most IGBTs are made for switching applications, so they may have similar SOA problems as modern MOSFETs. Also most IGBTs are made for high voltage (e.g. > 600 v) as this is the area where they might make sense in switching applications. In Principle one could combine a large NPN with a N-MOSEFT to get similar properties.

For most cases my choice would be BJTs (e.g. 2SD1047) made for audio applications: they are reasonably fast and have good SOA curves, that are reliable, as they are made for analog operation.  One problem might be that good audio transistors are also candidates to find fakes, but I don't think one needs so fast ones (e.g. SD4700 or similar) unless one gets a really good layout. Fast circuits can also get sensitive to parasitic inductance and coupling.

OK then, if IGBTs are not that fast that they are wroth over an NPN in my case then its good.
Is there any transistor that you will recommend from here?:

http://www.taydaelectronics.com/t-transistors/mj-series.html

http://www.taydaelectronics.com/t-transistors/tip-series.html

http://www.taydaelectronics.com/t-transistors/other-transistors.html

What about 2SC5200? DS: https://www.fairchildsemi.com/datasheets/2S/2SC5200.pdf
It cost about 2.7$ which is nice.

[Kleinstein]

TO247 / TO218 and similar cases are good, as you can have them an the board, so no loose cables.  TO220 is not that good for higher power - so OK for something like 40 W at most. So it might work for an 1 A supply. TO3 can be difficult to mount and often is more expensive. I personally don't like them.

The 2SC5200 is a very good audio transistor. So it is a viable choice, if from a credible source, but it's a type where you can find fakes if you get them from EBAY or a discount seller. $2,70 is not too bad, but still not cheap.

TIP3055 (cheap), TIP35 (more robust)  and TIP140 (Darlington)  are good for a low cost version, though not really fast. If you can live with something like 100 µF (per transistor) at the output, they should be fast enough.  In the simulation I even got 5 µF output cap with a 2N3055 stable, though a faster transistor might make it easier.
With these lower cost types you are less likely to find fakes.

It depends on the source, which transistors you might get for a good price.

[ZeTeX]

TO247 / TO218 and similar cases are good, as you can have them an the board, so no loose cables.  TO220 is not that good for higher power - so OK for something like 40 W at most. So it might work for an 1 A supply. TO3 can be difficult to mount and often is more expensive. I personally don't like them.

The 2SC5200 is a very good audio transistor. So it is a viable choice, if from a credible source, but it's a type where you can find fakes if you get them from EBAY or a discount seller. $2,70 is not too bad, but still not cheap.

TIP3055 (cheap), TIP35 (more robust)  and TIP140 (Darlington)  are good for a low cost version, though not really fast. If you can live with something like 100 µF (per transistor) at the output, they should be fast enough.  In the simulation I even got 5 µF output cap with a 2N3055 stable, though a faster transistor might make it easier.
With these lower cost types you are less likely to find fakes.

It depends on the source, which transistors you might get for a good price.

I will get the 2SC2500, not from ebay of course.

In the meanwhile, should I be concerned about high peak currents when the current limiting kicks in?
Here is 1ohm resistor with voltage set to 25V and current to 0.3A.

here it is only 6A~ but the lower the resistance the higher the peak current (can get to 400A+).
expect increasing the capacitance (or lowering to about 1uF when its nice a fast, so the peaks exist but they are for much shorter time), and the speed of the regulator, what can I do about it if I even should?

[T3sl4co1l]

IGBTs have higher current density, and therefore higher power density, than BJTs.  BJTs have to be specially designed to dissipate high power levels at large voltage drops (high power density).  Otherwise you get second breakdown.

IGBTs aren't rated for more than ~100s of microseconds of linear operation.  They are much more prone to 2nd breakdown for the above reason.

MOSFETs are what you need for linear.

Tim

[Kleinstein]

MOSFETs also need to be designed for linear operation, and modern ones are usually made for switching. It's rather hard and expensive to get MOSFETs designed for linear operation. They don't have 2nd breakdown, but a similar effect just with an other name. The trouble with MOSFETs is that some datasheets have wrong SOA curves - so even if there is a DC SOA curve, it might not be reliable.

BJTs for linear operation are readily available - not every type is well suitable, but many types are.

I have tested a way to limit the output of the OP that is not actively controlling the output. At least in the simulation it works reasonable well. The circuit is using a separate shunt resistor an the other side of the "GND" point to make current regulation also work with the 2 quadrant output stage (one transistor less here - still fast enough). The new part is the transistor to enable extra direct feedback when the other OP is active. Shown here only for one OP, but works for the other half too if needed.

[ZeTeX]

MOSFETs also need to be designed for linear operation, and modern ones are usually made for switching. It's rather hard and expensive to get MOSFETs designed for linear operation. They don't have 2nd breakdown, but a similar effect just with an other name. The trouble with MOSFETs is that some datasheets have wrong SOA curves - so even if there is a DC SOA curve, it might not be reliable.

BJTs for linear operation are readily available - not every type is well suitable, but many types are.

I have tested a way to limit the output of the OP that is not actively controlling the output. At least in the simulation it works reasonable well. The circuit is using a separate shunt resistor an the other side of the "GND" point to make current regulation also work with the 2 quadrant output stage (one transistor less here - still fast enough). The new part is the transistor to enable extra direct feedback when the other OP is active. Shown here only for one OP, but works for the other half too if needed.

What is the purpose of R12 and C5? Also the set_current voltage is negative, is there a way to adjust it using positive voltage?
Schematic now:

[Kleinstein]

It is possible to change the circuit to use a positive voltage to set the current. Just add a divider at the non inverting input to mix the positive set signal with the negative measured shunt voltage. It might interact a little with the measured current, but not much. So not a problem unless one wants really high resolution ( <0.1 mA range) current readings.

C12 and R5 are there to improve stability with large capacitive loads (e.g. 10 mF). They also help to reduce the bandwidth requirements for the OP. You find that part in many similar circuits, sometimes without R5. Without this cap the output impedance in the 10 Hz - 1 kHz range gets very close to a perfect inductance and thus could give very long ringing with large low ESR caps.

[ZeTeX]

It is possible to change the circuit to use a positive voltage to set the current. Just add a divider at the non inverting input to mix the positive set signal with the negative measured shunt voltage. It might interact a little with the measured current, but not much. So not a problem unless one wants really high resolution ( <0.1 mA range) current readings.

C12 and R5 are there to improve stability with large capacitive loads (e.g. 10 mF). They also help to reduce the bandwidth requirements for the OP. You find that part in many similar circuits, sometimes without R5. Without this cap the output impedance in the 10 Hz - 1 kHz range gets very close to a perfect inductance and thus could give very long ringing with large low ESR caps.

Is the connection supposed to be like this?
Like you said, I'm mixing the positive set voltage and the negative measured shunt voltage.

[Kleinstein]

There still needs to be the resistor from the inverting input to round. The set voltage is than positive.

[ZeTeX]

There still needs to be the resistor from the inverting input to round. The set voltage is than positive.

Such an obvious thing and I forgot about it, after you get the answer suddenly everything makes sense.

Anyways, I'm going to use LT1074 as a per-regulator because I'm unable to find large heat sinks cheaply.
I got the pre regulator to work:
https://i.gyazo.com/d103e9bd25098711af27f600869e5c85.png
but I need to find the correct values for the output caps & inductor.