MOSFET linear regulator circuit

[VEGETA]

Yup it has done the magic after spending the night trying and measuring rather that the obvious choice of playing League of Legends! (do you play it?)

Currently, the IRFP250N is the choice that I will go for it but someone suggested this mosfet: http://www.digikey.com/product-detail/en/fairchild-on-semiconductor/FDB33N25TM/FDB33N25TMCT-ND/1923068

I think I examined it and worked in simulation. It has the advantage of SMD version at least, I still don't know if it is good enough or not.

If you increase R_sense of the SEPIC converter, it will reduce the maximum output current allowed. We are already near the edge, now it supports 2A and it is stable as you see. The coupled inductors are 10uH each so I guess it is OK even for 20A spikes. you mean by 4.7uH inductors the coupled ones in the SMPS circuit or the output filtering ones?

I remember 4.7uH didn't work nice and I had to use 10uH to allow more current with the 5mR sense resistor... so getting back to 4.7uH will be bad I guess.

As for the oscillation in the start up, I did notice it but didn't give it importance since it is eventually gone and the output became clean. Should we try to kill it or just leave it as it is?

My interest is to know if my previous analysis and posts are right or not?

What should we do next?

[VEGETA]

"""However this should be relatively easy to fix (use the divider form the shunt, instead of from GND)"""

?

[Kleinstein]

The 10 µH inductors you linked before had ratings for the saturation current of something like 10 A. So it will be a big problem if the current goes higher than this, as the current will than go up way to fast. The peak current has to be limited to less than the saturation current.

Also the DC resistance is not that low - so there will be quite some power loss at the inductors. As a 3rd point, there will be magnetic loss in the inductors. At such a high frequency the maximum useful magnetization is usually reduced (but rare peaks at higher current still allowed), as otherwise the inductor would overheat from the loss. This could mean the useful current may be only at half the saturation current - so even 2 of those inductors in parallel might not be enough. The loss rating on inductors are not that simple and I have not looked in detail on the data-sheets. With the 10 µH inductor there is also more "ringing" after a load change. So in this case the compensation would still need a little more tweaking (double C5 to 20 nF). The simulation does not even look bad with only 3 µH inductance.

A larger inductor is also physical larger and more expensive. This is also true for the filters. One might get away with 1 µH there if 2-5 µF ceramic caps are used.

I know the power is somewhat limited by the peak current, but so far is looks like we could go slightly down with the peak current. Under normal steady state operation something like 15 A is good enough, even for 2 A at 20 V.

For the output rating, the hard part is high current at high voltage. So limiting the peak current at high voltage would make some sense. At lower voltage, even more than 2A are not a problem. So maybe 1.5 A at 20 V / 2 A for  < 15 V and 3 A for < 10 V, maybe even 5 A at < 5 V, might be a kind of compromise. For this type of SMPS it is just natural to have a power limit, and not a fixed current at all voltages.

I think the SMD MOSFET (FDB33N25TMCT) should also be OK. I have some doubt in the higher voltage part of the SOA curve shown in the DS (not sure this is not just reflecting the Ptot curve without case for thermal stability). But as the voltage is quite low here, this fet should still be ok.

The previous analysis on the feedback and for the "10-50 kHz" is OK. For those oscillations there seem to be two types: the ones shown in your simulation plots, with a rather non sine waveform. These somehow involve reaching some hard limits and thus likely appears during start up when the current is very high. There is also a slower more sine like version, after a step in the load current. For the current configuration this is damped enough (at least with 5 µH, not so well with 10 µH).

[VEGETA]

I've set inductor settings to 15A saturation current and above, then I chose 4.7uH and this was the result:

http://www.digikey.com/products/en/inductors-coils-chokes/arrays-signal-transformers/73?k=&pkeyword=&pv1097=248&FV=d300001%2C1f140000%2Cffe00049%2C1d7c000a%2C1d7c000b%2C1d7c0011%2C1d7c0012%2C1d7c00bf%2C1d7c00c5%2C1d7c00d9%2C1d7c00dd%2C1d7c0019%2C1d7c011f%2C1d7c0035%2C1d7c0038%2C1d7c0050&mnonly=0&newproducts=0&ColumnSort=1000011&page=1&stock=1&quantity=1&ptm=0&fid=0&pageSize=25

As I told you the specs are out of question, I can search for suitable parts all day long otherwise xD.

Here is what I did:

1- changed the mosfet to FDB33N25.
2- made soft-start capacitor 20nF instead of 10nF (practically I will put 2x 10nF in parallel for consolidation).
3- changed R_sense to 7mR instead of 5mR. This will surely help reduce the maximum peak switch current.
4- changed filtering inductors to 2.2uF because of point 5 below.
5- coupled inductor is 3.3uH now (+21A saturation current and +10A rated current!). I tried 4.7uH (16A saturation current) and worked well too.

the 3.3uH is here: http://www.digikey.com/product-detail/en/bourns-inc/SRF1280A-3R3Y/SRF1280A-3R3YCT-ND/5031136
new 7mR sense resistor: http://www.digikey.com/product-detail/en/rohm-semiconductor/PMR100HZPFU7L00/RHM.007AUCT-ND/2094559
2.2uH filtering inductor (suggestion): http://www.digikey.com/product-detail/en/laird-signal-integrity-products/MGV10042R2M-10/240-2935-1-ND/5269990  << this has +25A saturation current.

for 2A output current, the max peak is about 11A or so, call it 12A. For the new circuit of 3.3uH and 7mR the current waveform shows around 10A peak when it settles on 2A max output, and around 17.5A before during ramping up. so 10A maximum peak current while the inductor of 3.3uH allows a maximum peak saturation of more than 21A... this is the half saturation current you looked for isn't it?    filtering inductors are the same too, +25A saturation if it is necessary, if not then it is ok to pick for the rated current only.

here is an image of the current in 7mR:





and this is the image for the current in L1 and L2 (coupled inductor):



settles around 9.5A and 6.5A.

So I guess this is the solution, right?

[Kleinstein]

The 3.3 µH inductor should work ok. In theory power loss could be still an issue. But this is hard to tell. So it needs a prototype.

For saturation the relevant current is the sum of the two current. As they are often out off phase this will usually not make a big difference.

The output filter inductors do not need such a high current rating. Here the current will no be much above 2 A (maybe 3A) and occasional saturation would not be a big problem (output noise spikes). So these inductors could be  smaller form factor - more the DC resistance is important.

[VEGETA]

So can we put another coupled inductor in parallel with it? to solve saturation issue. Off phase will help right? I will try to choose another filter inductors which has very small resistance with 3A or more capability.

So besides saturation current issue, can we saw the circuit is good and ready?

I will "try" to make a PCB using CircuitMaker (hopefully before next month) but my laptop is wooden! I am gonna buy a new one in the last of this month anyway. A PCB will be only this stage not the full PSU, so no LCD, battery,... etc. Just the circuit shown here. I will make the "Isolated" supply a battery pack while the input is something else like a laptop charger or maybe another battery... since it is isolated.

[MarkF]

I haven't read through entire thread but offering the power supply design Peter Oakes did as a reference. It's similar to your design.

[Kleinstein]

Having a second inductor in parallel could be a little tricky with coupled inductors, if they are not perfectly 1:1. Normally they should at this low inductance, as there will be something like 5 or 10 turns - so easy to get exactly the same number of turns.  If one wants one could have the input side directly in parallel and have the coupling capacitor, inductor half to ground and the diode separate.

The peak current through the diode is rather high anyway. So splitting the current to two smaller diodes might not be so bad for thermal reasons.

Having the option to use a second inductor is a good idea.
Now the frequency is rather high - this helps with small inductors and easier ripple filtering, but is also makes it more sensitive to the layout and parasitic effects. As I don't have much experience with SMPS, I would prefer a lower frequency for the beginning and thus more like 2 of the 10 µH inductors in parallel.

There may be still a few points of BOM reduction at a few points. I am not so sure the 100 Ohms in parallel to the filter inductors are really needed. The input side might need a little more capacitance for EMI reasons. Also a fuse might be a good idea - though not that easy at 10-20 A. I would guess a under-voltage lockout is a good idea too - I don't know if the SMPS chip is good enough for this, or if more is needed.

In the linear part, there is still the trade of question on how fast the current limit needs to react. A very fast current limit cause larger voltage drop on transients, when close to the limit. There is still the question if an extra fast current limit is needed - this could be rather simple, like just one more small transistor. As the SMPS part can deliver more than 2 A at lower voltage, one could allow more than 2 A for lower voltage settings in the linear part too.

The circuit has 3 main logical parts: the SMPS, the linear regulator part and control / display part with µC/LCD and ADCs/DAC. For a first try one could use simple pots instead of digital control.

The DCDC converter is just a small part - so I would still have place for it on the board (e.g. near linear regulator) some of the DCDC blocks have a good chance to work directly from an 6-8 V battery as well - so one might get away without a regulator before the DCDC. Having a regulator just for the DCDC block is somewhat strange - the efficient way would be an isolated SMPS like a small flyback converter or a Royer converter.

With the SMPS part there is still a chance it will not work well the first try due to layout or similar issues.
If there is sufficient cooling (e.g. IRFP250), the rest would also work without the SMPS part.

[VEGETA]

Having a second inductor in parallel could be a little tricky with coupled inductors, if they are not perfectly 1:1. Normally they should at this low inductance, as there will be something like 5 or 10 turns - so easy to get exactly the same number of turns.  If one wants one could have the input side directly in parallel and have the coupling capacitor, inductor half to ground and the diode separate.

The peak current through the diode is rather high anyway. So splitting the current to two smaller diodes might not be so bad for thermal reasons.

So you suggest putting 2 of the same diodes in parallel to each other, as well as, 2 of the same coupled inductors in parallel too? Like L1 || L2 and L2 || L3 while both of them are 3.3uH? splitting the current helps for sure but I hope it won't do other problems. The inductor is cheap and around 1.5$, 2 of them is not a problem. No reason to believe they are not identical because they are the same part.

Now the frequency is rather high - this helps with small inductors and easier ripple filtering, but is also makes it more sensitive to the layout and parasitic effects. As I don't have much experience with SMPS, I would prefer a lower frequency for the beginning and thus more like 2 of the 10 µH inductors in parallel.

Well, I don't like the idea of returning to use 10uH L1 and L2 since the overall saturation current is high and it will cost a lot to get one to suit it, that is why 3.3uH one is nice since it is cheap and can tolerate saturation currents. Unless you want to put 2 10uH coupled inductors in parallel.

Also, I believe 500KHz is still considered slow in SMPS world. Some ICs allow 2 MHz or even 3 MHz of operation. As for layout, I will try to do it like the datasheet suggests and why not read about it more.

There may be still a few points of BOM reduction at a few points. I am not so sure the 100 Ohms in parallel to the filter inductors are really needed. The input side might need a little more capacitance for EMI reasons. Also a fuse might be a good idea - though not that easy at 10-20 A. I would guess a under-voltage lockout is a good idea too - I don't know if the SMPS chip is good enough for this, or if more is needed.

the additions must include output enable and short circuit protection, as well as, a down programmer if it is necessary. output enable can be added by putting a transistor as a switch from the "+P" voltage to the linear mosfet gate right? no need for the current mirror circuit. I don't know about short circuit protection, but maybe the type of circuit that you posted before can work... the transistor using the current monitor shunt resistor, so it can provide 2 jobs. I don't know anything about under voltage lockout.

what do you mean by the IC being good enough? this IC LT3757A is a must here for many reasons. the A version is to be used not the normal one.

remind me why we put the parallel 100R?

In the linear part, there is still the trade of question on how fast the current limit needs to react. A very fast current limit cause larger voltage drop on transients, when close to the limit. There is still the question if an extra fast current limit is needed - this could be rather simple, like just one more small transistor. As the SMPS part can deliver more than 2 A at lower voltage, one could allow more than 2 A for lower voltage settings in the linear part too.

faster CC is better I guess, but can we actually manipulate it? the faster current limit (short circuit protection) is good idea as I said before. Well, I'd like to keep the design simple in terms of specs with 2A maximum current at all times. My concept is to use 18650 batteries as the source, like putting 2 parallel and 2 series which will be 8.4v @ 6AH approximately. So drawing 2A is around 3 hours without a charger which is nice. increasing the specs will make it a non-practical design which will eat the batteries so fast.

The circuit has 3 main logical parts: the SMPS, the linear regulator part and control / display part with µC/LCD and ADCs/DAC. For a first try one could use simple pots instead of digital control.

Hmm I must include the op-amps in this (didn't decide on which one yet -> LT1014 is the first to my mind). Yes POTs can be used but I must get a 2.048v voltage reference anyway. I will get some pots to do it as you suggested.

The DCDC converter is just a small part - so I would still have place for it on the board (e.g. near linear regulator) some of the DCDC blocks have a good chance to work directly from an 6-8 V battery as well - so one might get away without a regulator before the DCDC. Having a regulator just for the DCDC block is somewhat strange - the efficient way would be an isolated SMPS like a small flyback converter or a Royer converter.

I will try to get one, the requirements are 5-9v input to 12v output while delivering something like 100mA or so. Help me out searching for one. I know that it is not efficient to get a boost for this DC-DC converter but it won't be pricey.

Anyway, this is what I think a suitable part is: http://www.digikey.com/product-detail/en/cui-inc/PCN2-S5-S12-S/102-3929-ND/6181024

it's only drawback is the input is 4.5-5.5v only so my solution is one of these:

1- from a 5v regulator which I think will exist in the design << referenced to "battery" side of course.
2- from a simple resistor divider, this is bad because of the battery pack voltage won't be the same thus it will get lower than 4.5v. so forget it. (not to mention power).
3- connect a 5.1v zener diode before the input. I think it is the best choice since the max output current is around 167mA @ 12v thus 2W of power. I think this can work.

so it is either 1 or 3. what to choose?

With the SMPS part there is still a chance it will not work well the first try due to layout or similar issues.
If there is sufficient cooling (e.g. IRFP250), the rest would also work without the SMPS part.

what about " the rest would also work without the SMPS part"? do you mean supplying it with 6v and asks for 5v?

I don't believe SMPS has any good reason not to work ^_^. What stuff in layout can affect it?

[Kleinstein]

A lower frequency would likely need a higher inductance, like 4.7 µH. 2 of the 10 µH inductors in parallel would be 5 µH and twice the saturation current. For the inductors one could use them directly in parallel or alternatively have the second coil, that goes to out- separate, so with it's own capacitor and its own diode. The second version would also provide current sharing for the diodes.

A low frequency SMPS is more like 20-50 kHz. With an external MOSFET 500 kHz is already quite fast. The really high frequencies SMPS are controllers often have the power stage integrated. The critical parts in the layout are the different current paths that change - they need to be low area. Also ground currents can be tricky, as at 500 kHz there is no such thing as a low impedance ground. The "reverse rovery"/capacity discharge current from the diode can include current spikes in the ns range and thus frequency components up to the 100s MHz range - this can cause surprising resonances. So there is some room for surprises.

There is already a two step short circuit protection: the CC mode regulation and the current limit at the SMPS part, thought this might give something like 15 A at 1 V.  I would suggest a second fast acting current limit that would act at something like 5 A, just with a small transistor to parallel the CC mode regulation.  When having a second fast current limit, it may not be that desirable to have the current limit extremely fast - it really depends on what the user prefers. There is an option to make it a little slower if needed with just a minor change (e.g. move one resistor).

Most SMPS chips include a protection against to low a input supply and will turn off in this case. I am not sure if this function could also be used to protect the batteries from to low a voltage. There is a small chance it could, but I am not sure. So one might need an extra one or could be lucky and get away without.

Increasing the specs would be only at low voltage, like < 10 V.  The SMPS is mainly power limited, so is the battery. At 2 A and 20 V the battery will only last about 1 hour, just like 4 A at 8-9 V.

For the OPs I would tend towards an TLC274 for a low cost option and maybe OPA4177 as an accurate version. One might still use 2 dual OPs as well. The LT1014 is rather slow and expensive. It somewhat depends on the voltage of the DC/DC converter.

A down programmer part is possible, but it should also be included in the output enable part. For this reason the current mirror version might be the better choice. Otherwise just pulling the gate towards GND would be enough. The down programmer version I have in mind would however deliver current that is not measured with the shunt - so the current reading would not include it. Software could compensate a constant current version.

For the DCDC converter, a small flyback converter would be the logical solution. It just needs the suitable "transformer". Having an extra regulator before the DCDC will waste quite some power and thus might not be good if the µC /LCD needs a lot of current. If you don't mind winding a small transformer, a royer converter would be also easy - it like a DCDC converter the old style: jelly bean parts (except for the transformer - e.g. small ring core), low noise and a fixed ratio, but free to choose via transformer windings. So something like 6-8 V to 9-13 V would be OK.

[VEGETA]

A lower frequency would likely need a higher inductance, like 4.7 µH. 2 of the 10 µH inductors in parallel would be 5 µH and twice the saturation current. For the inductors one could use them directly in parallel or alternatively have the second coil, that goes to out- separate, so with it's own capacitor and its own diode. The second version would also provide current sharing for the diodes.

if it won't affect stability I have no problem with it, not to mention speed. So should we revert back to 300 KHz? In the schematic it shows the coupling capacitor has "x2", is it 2 of it in parallel? this means we must put 2 to each second inductor which means a total of 4x 4.7uF caps.

A low frequency SMPS is more like 20-50 kHz. With an external MOSFET 500 kHz is already quite fast. The really high frequencies SMPS are controllers often have the power stage integrated. The critical parts in the layout are the different current paths that change - they need to be low area. Also ground currents can be tricky, as at 500 kHz there is no such thing as a low impedance ground. The "reverse rovery"/capacity discharge current from the diode can include current spikes in the ns range and thus frequency components up to the 100s MHz range - this can cause surprising resonances. So there is some room for surprises.

So get back to 300 KHz? you mentioned 4 layers board is good for high speed, but it is not preferred here. what could we do?

There is already a two step short circuit protection: the CC mode regulation and the current limit at the SMPS part, thought this might give something like 15 A at 1 V.  I would suggest a second fast acting current limit that would act at something like 5 A, just with a small transistor to parallel the CC mode regulation.  When having a second fast current limit, it may not be that desirable to have the current limit extremely fast - it really depends on what the user prefers. There is an option to make it a little slower if needed with just a minor change (e.g. move one resistor).

So a transistor with the shunt resistor is ok here? is our current CC fast or too fast?

Most SMPS chips include a protection against to low a input supply and will turn off in this case. I am not sure if this function could also be used to protect the batteries from to low a voltage. There is a small chance it could, but I am not sure. So one might need an extra one or could be lucky and get away without.

I guess that is another issue, as I will design (I am have to) a battery protection and charging circuits since it is Li-Ion battery pack. So that could solve it if the SMPS IC cannot.

OP-AMPS

this is yours: http://www.digikey.com/product-detail/en/analog-devices-inc/OP4177ARZ/OP4177ARZ-ND/820357

8$ is too much, plus is there any linear.com part instead?

Increasing the specs would be only at low voltage, like < 10 V.  The SMPS is mainly power limited, so is the battery. At 2 A and 20 V the battery will only last about 1 hour, just like 4 A at 8-9 V.

As I told you, no need for such high currents. 20v @ 2A is only one hour with 8.4v/6AH battery pack?

A down programmer part is possible, but it should also be included in the output enable part. For this reason the current mirror version might be the better choice. Otherwise just pulling the gate towards GND would be enough. The down programmer version I have in mind would however deliver current that is not measured with the shunt - so the current reading would not include it. Software could compensate a constant current version.

I don't really understand the goal of down-programmer so much and why it is good for power supplies. Combining it with the current mirror output enable adds more complexity unless you have something in mind already.

For the DCDC converter, a small flyback converter would be the logical solution. It just needs the suitable "transformer". Having an extra regulator before the DCDC will waste quite some power and thus might not be good if the µC /LCD needs a lot of current. If you don't mind winding a small transformer, a royer converter would be also easy - it like a DCDC converter the old style: jelly bean parts (except for the transformer - e.g. small ring core), low noise and a fixed ratio, but free to choose via transformer windings. So something like 6-8 V to 9-13 V would be OK.

The reason I hate this is that I don't know how to design such circuits, plus the need for a transformer which will be pricey with the other ICs. For now the DC-DC module works fine... this maybe an enhancement when everything else is working nice.

[VEGETA]

I couldn't get a suitable Linear.com part, maybe LT1884 was nice but the output had some weird little spikes. Do you know a linear.com op-amp to use here? it is ok if it is only dual channel. OP4177 is massive 8$ so out of question already (is its accuracy gonna affect much?) and TLC274 seems good 4 op-amps in one cheap package with 2.2 MHz but I fear its accuracy is going to ruin what we want.

[VEGETA]

I started a project in CM for the prototype board, but now I should search for SMD version of our parts. I replaced the PNP with BC807-40 due to this reason, simulation was ok but the drop voltage is now a little bit less than 1v.

Although there as some nasty stuff in the ramping up stage when using the new PNP:



I don't know what that is but it eventually settles up. Is it ok or should we search for a new SMD PNP? I choose BC807-40 because it is in LTSpice already and it has SMD package (SOT23-3).

[Kleinstein]

For the OPs it really depends on the source where you get them. Another possibly good type (more on the cheap side) would be the TLV4171 - could be slightly better than the TLC274. The specified dirft is not that bad at all, and due to the switcher up front the supply will not be super low noise anyway. One might use a better OP for the slightly more critical part of current sensing, so two dual OPs instead of a quad.
I have not found much at LT - they are not so much in low cost general purpose parts, more higher price good quality.

The circuit is not critical to the OPs performance, so there is no need to limit the parts to those where you have special models for.

The current limiting in the last linear regulator is relatively fast. So it could just work without an extra fast current limit. However this fast limit also means there will be a not so good response to transients that go close the to set current limit - a slower current limit would allow a short current peak to go higher and this way it can be faster back to the set voltage. In this case the extra fast current limit is likely needed.

I would at least plan for 2 inductors in parallel, so there would be the option to go for a lower clock like 200-300 kHz if we need to. It also leaves some room for the case a higher current is needed.

The down programmer is a circuit part to allow the output voltage to be regulated down too. So a kind of limited 2 quadrant output. It depends on the application if you need it or not - it can help a little with a fast voltage regulation. The minimum version would be a constant current sink, maybe with control from the µC, so it can be turned off together with the normal output disable. The constant current sink is rather simple.

The under voltage lockout of the LT3757 might be good enough - only still need to turn off the DCDC converter to.

2 A at 20 V is a power of 40 W. With something like 80% efficiency this would need something like 50 W from the battery and thus something like 7 A at 7.2 V. The battery will be 8.4 V only when full. On average it is more like 7.2 V. So it might not even last a full hour at full output power.

[VEGETA]

For the OPs it really depends on the source where you get them.

Digikey only for now.

One might use a better OP for the slightly more critical part of current sensing, so two dual OPs instead of a quad.

Ok, like what? I got the idea of using a nice dual op-amp for CV and CC while the rest are with TLC274 or so. Just what op-amp should it be? I searched linear.com parts because they are in LTSpice, but they are kinda pricey. Actually, some of them didn't show a good output at all in simulation. How do I know a certain op-amp is suitable or not?

The current limiting in the last linear regulator is relatively fast. So it could just work without an extra fast current limit. However this fast limit also means there will be a not so good response to transients that go close the to set current limit - a slower current limit would allow a short current peak to go higher and this way it can be faster back to the set voltage. In this case the extra fast current limit is likely needed.

Should we do it now or make the prototype without it?

I would at least plan for 2 inductors in parallel, so there would be the option to go for a lower clock like 200-300 kHz if we need to. It also leaves some room for the case a higher current is needed.

2 inductors of 10uH each @ 300 KHz?

The down programmer is a circuit part to allow the output voltage to be regulated down too. So a kind of limited 2 quadrant output. It depends on the application if you need it or not - it can help a little with a fast voltage regulation. The minimum version would be a constant current sink, maybe with control from the µC, so it can be turned off together with the normal output disable. The constant current sink is rather simple.

you mean faster transition from high to low voltages?

include it now or after first prototype board?

The under voltage lockout of the LT3757 might be good enough - only still need to turn off the DCDC converter to.

maybe they are the same result anyway.

2 A at 20 V is a power of 40 W. With something like 80% efficiency this would need something like 50 W from the battery and thus something like 7 A at 7.2 V. The battery will be 8.4 V only when full. On average it is more like 7.2 V. So it might not even last a full hour at full output power.

that is the responsibility of the user. also, the design has a wall adapter of at least 2A (ready ones, won't make it myself) for charging purposes. So if he wants such high powers continuously, he must connect the charger to charge the batteries and share current too.

the other option is to go for 6 of 18650 li-ion batteries. either 3 parallel x 2 series (8.4v @ 9AH) or 3 series x 2 parallel (12.6v @ 6AH). Fort now, let's keep it like it is.

[Kleinstein]

The requirement for the OPs are not really high. Something like 1 MHz (maybe 500 kHz) is fast enough, and no need to have single supply or rail to rail IO. The supply would need to go up to about 12 V - so the modern 2-5.5 V OPs are not a real option.
Just avoid the LM358 (and related) as they have this nasty cross over delay.

Something like the TLC272 / TLC277 (slightly better) should be well good enough. I would not go much cheaper/older than the LM1458. This is one thing one can change later if one really needs high accuracy and has decided on a good DAC/ADC. So I would consider the TLC272(or 4) good enough for the first board.

With the speed of current limiting, you have to decide. One can slow it down a little with moving the path to the anti windup transistors base to the other side of the diode. The extra fast limit is just a small transistor, that limits the drop over the shunt to 0.5-0.6 V. So I would include the extra transistor (it does not really hurt) and maybe the option for the second resistor position.

Similar the current sink to bring the voltage down again faster is not a big effort and nothing special. So one can include it from the beginning. There is still the option not to populate.

[VEGETA]

OK, I will choose TLC277 dual op-amp for CV and CC for now, this one: http://www.digikey.com/product-detail/en/texas-instruments/TLC277CDR/296-26747-1-ND/2255142

What I want to know is how to determine "quality" increase of the op-amp in the design. Or namely how op-amp1 is better quality here than op-amp2? My guess would be the minimum voltage that it can detect and the speed which it runs by - 2.2 MHZ (according to digikey) for the TLC277 I guess, right? although it says 0.3 MHz GWP in datasheet.

I am willing to use 16-bit ADC/DAC (not a very great pricey one, just one that works) so a good op-amp is a must here. I wanna know how to choose one and how to differ between them.

What I came up with is the following choices:

1- LT1678, it is a good high quality choice with somehow reasonable (not really xD) price of 5$. there is the 79 version of 4 op-amps but no need.
2- OPA2180IDR, price is 2.87$.
3- MAX44245ASD+  -> 4$ but has 4 op-amps in it.

I leave this topic for discussion later on, for now we'll get normal opamps.

Now the rest is 2 opamps, one for CV\CC indication and the other is for voltage monitoring. I guess TLC272 can work here for now: http://www.digikey.com/product-detail/en/texas-instruments/TLC272CDR/296-1310-1-ND/404948

or maybe get the 4 opamps version if I need more. If not, then maybe a precision high quality 4 op-amp package can be used for component consolidation.

TL;DR:

For now, TLC277 for CV\CC and TLC272 for the other 2.

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I don't see why we should slow the CC part, if you remember, it was THE problem of the past typologies xD. Thus, adding an extra current limit which acts only when short circuit or violent events happen is the best option. Dunno how to do it now so I will leave it to the rest... down-programmer is the same too.

___

I feel very confident now since the major stuff are good enough in this main circuit, the rest is somehow manageable. I will "try" to make a PCB with CM in this laptop, so I hope I will make some progress before getting the new laptop in the last of this month.

___

BTW, I made a newer version with these changes:

1- parallel 10uH coupled inductors.
2- changed the shottky diode to MBRB2545CT because it has SMD version (which is important)... circuit functions better now, or maybe I am not seeing correctly.
3- Q2 is now BC807-40 for SMD version.
4-  switching frequency is now 300 KHz.
5- due to 4, filtering inductors are 4.7uH again.
6- Q3 is now BC807-40 for the smd version.

drop voltage is now less than 1v (0.94v @20v) due to the change of Q2. Notice that the negative supply of it's base is not precise too so there should be some compensation or just leave it as it is. I picked a divider resistor of 1k with 1.5k due to standard values. the negative isolated supply is not precise too since it doesn't give 9v and -3 exactly.

from now on, anything must be SMD except for what is impossible to get SMD for.

[Kleinstein]

The main important properties for the OP are offset drift and to a lesser extend low frequency (e.g. 0.1-10Hz) noise. For voltage regulation the the contribution to output drift would be something like 10 times that of the OP. So an OP with a 2 µV/K drift would cause 20 µV/K for the output voltage. Normal (not extra expensive) resistors are at about 100 ppm/K and thus at 1 V output would cause something like 100 µV/K. So for the initial version the LTC272(4) should be well good enough for the voltage regulation. If really high precision is wanted, one would need to use better resistors (R1 and R4) and a good reference too.

For the current regulation, the voltage at the OP is smaller (e.g. 200 mV for full scale) - still the TLC272 would cause about as much error as the use of normal 100 ppm/K resistors. With a 0.1 Ohms shunt, 1 µV of drift gives something 10 µA for the set current. The more critical part is usually the shunt, as the shunt will also get hot from the current and thus can have a significant temperature change. Still a few mA of drift neat full scale ouput is usually not that bad.

The diodes specified are rather big one, especially with two diodes. The peak current for both diode might reach 20 A, but the average will be only 2 A. So somewhat smaller diodes should be better.

R22/R34/R36 can be simplified to two resistors (e.g. 10 K and 15 K).

Even with 300 kHz I don't think L3 and L4 need to be so large, 2,2 µH could still be enough.

[VEGETA]

this TLC272 has an offset of 290uV (per temperature?): http://www.digikey.com/product-detail/en/texas-instruments/TLC272BCDR/296-26742-1-ND/2255137

while this one TLC277 has 250uV: http://www.digikey.com/product-detail/en/texas-instruments/TLC277CDR/296-26747-1-ND/2255142

TLC272 is 1.45$ while TLC277 is 2.16$ and both has 2 op-amps.

this TLC274 has 390uV which is a lot more than the 2 op-amps in 272 version: http://www.digikey.com/product-detail/en/texas-instruments/TLC274BIDR/296-1312-1-ND/276580  (TLC279 is quite the same with 4 ops: http://www.digikey.com/product-detail/en/texas-instruments/TLC279IDR/296-1315-1-ND/276583)

So for the initial version the LTC272(4) should be well good enough for the voltage regulation

I think you mean TLC not LTC.

I looked into the input offset voltage, I don't know if it is what you meant by drift. the LT1678 I suggested have superb specs but still 5$ is expensive... kinda worth it too.

I will choose a 2.048v voltage reference with normal 0.1% resistors for the CV\CC and measurements, with something like LT1678 (or the lesser quality TLC277) I guess we'll achieve very nice quality for the price, don't you think so?



I originally picked LT1678 for this.

the shunt of 0.1R will be 10 of 1R resistors in parallel to reduce heating and enhance tolerance. I found this one (all SMD of course): http://www.digikey.com/product-detail/en/stackpole-electronics-inc/RNCP0805FTD1R00/RNCP0805FTD1R00CT-ND/2240534

0.25W per R = 2.5W total. now I2R = 2*2*0.1 = 0.4W which is far less than 2.5W. So one resistor will consume 0.4/10 = 0.04W maximum. pretty solid solution I guess, better than one shunt resistor.

The diodes specified are rather big one, especially with two diodes. The peak current for both diode might reach 20 A, but the average will be only 2 A. So somewhat smaller diodes should be better.

well, I only picked the ones in LTSPICE that has SMD version... I will try and pick another one from digikey. requirements are at least 5A of current and the lowest drop voltage, right? do you have a suggestion?

R22/R34/R36 can be simplified to two resistors (e.g. 10 K and 15 K).

I have done that and made the filtering cap for the -1v now 10n instead of 1n. maybe good for consolidation.

Even with 300 kHz I don't think L3 and L4 need to be so large, 2,2 µH could still be enough.

Hmmm I think they gave me around 6mA ripple or something with 2.2uH, with 4.7uH it is better of course (around 1.2mA or so).

This drives 2 questions:

1- can this design set current be as low as 1mA steps? (1mV for voltage too?)
2- can we do something about the slight error of 1mA or so in the output?

[Kleinstein]

With most of the OP, there will be a significant offset voltage. So one will need a kind of software compensation for this. A 1 mV resolution over 20 V would need an DAC that us good for about 15 Bits. The OP is not really a limit here.

For the current setting the OP also sets a limit. The TLC272 or similar should be good for about 10-100 µV - that would be about 0.1 to 1 mA.

Having the 10 resistors of 1 Ohms relatively close, means they are also thermally coupled and heat sinking might not be that good. 40 mW is than not that low in power. So the temperature rise at full power might reach something like 50 K or even more - the nominal power ratings of SMD resistors assumes a rather large board and good cooling, still leading to a high temperature (thus possibly higher rating with lead free solder). Also the board might add some copper to the resistance. So I am not that convinced that 10 parallel resistors are a good choice.

Not sure why, but the TLC27x seem to be rather expensive at digikey (could be time or just a high price for small quantities). So there might be other alternatives, could be LT1013 if precision is needed.

Using slightly larger MLCC could be an alternative to higher inductance.

[VEGETA]

With most of the OP, there will be a significant offset voltage. So one will need a kind of software compensation for this. A 1 mV resolution over 20 V would need an DAC that us good for about 15 Bits. The OP is not really a limit here.

I planned all along to use 16-bit ADC\DAC since the power of this design is this core circuit. A cheap DAC\ADC can be good enough for me. The other solution was to get a big PIC MCU with internal 12-bit ADC\DAC but this is not a good one. A small MCU is always better, especially whenever there is no need for massive computation power.

2.048v reference voltage is a perfect choice here. 16-bit DAC is 65536 steps which means each bit is equal to 31.25 uV which is awesome. Now this is gonna be x10 for the output and the result is 312.5 uV minimum output voltage step... Amazing.

For the current setting the OP also sets a limit. The TLC272 or similar should be good for about 10-100 µV - that would be about 0.1 to 1 mA.

since it is 200mV maximum, I wanted to get a voltage reference for it with the same voltage, but that is not gonna be good. So will use the 2.048v one. So each bit is 312.5 uA of output, which is good.

Software compensation is a later stage, first this should work.

Having the 10 resistors of 1 Ohms relatively close, means they are also thermally coupled and heat sinking might not be that good. 40 mW is than not that low in power. So the temperature rise at full power might reach something like 50 K or even more - the nominal power ratings of SMD resistors assumes a rather large board and good cooling, still leading to a high temperature (thus possibly higher rating with lead free solder). Also the board might add some copper to the resistance. So I am not that convinced that 10 parallel resistors are a good choice.

well you could say that but it is a good solution so far. 40mW is not that big too, eventually this is the maximum power so 10 resistor will share its heat. Dave mentioned this in the video along with enhancing tolerance.

The other solution is also not that good... getting a 0.1% 0.1R shunt resistance by itself. This alone can cost more than 7$ or even 10$! It will get so hot too xD.

Not sure why, but the TLC27x seem to be rather expensive at digikey (could be time or just a high price for small quantities). So there might be other alternatives, could be LT1013 if precision is needed.

LT1013 is very nice and relatively cheap at 1.98$ at Digikey SMD version, affordable! I can use the 2 op-amps in it for CV and CC while the rest of non-important stuff are left for cheaper op-amps. You mentioned that LT1013 is slow though.

It has 200uV offset voltage which is not good enough to handle our 31.25uV of the DAC/ADC... while LT1678 is offering 20uV which is significantly better that LT1013 and even more than our requirement. It is 5$ which is not cheap but totally worth it, right?

This makes me wonder why EEZ-Supply uses TL072 which has 6mV offset along with 16-bit DAC/ADC?? if the op-amp is not gonna tolerate your DAC resolution, then why bother with such a high resolution DAC anyway? However, they are using OP27GSZ (30uV offset) for current sensing of the shunt. We are doing it along with current limiting with one op-amp.

So I guess with my humble opinion is that LT1678 is the perfect choice here for CV and CC op-amp. The rest are using lesser op-amp.

Using slightly larger MLCC could be an alternative to higher inductance.

like 47uF ceramic caps with the already existing one (1uF I guess) plus 2.2uH?

___________

Anyway, I modified the isolated supply to have 4 series 1N4148 diodes (similar to the ones in the loop) to give similar operation to the previous LED simply because of SMD version (LED didn't have that) plus they are so cheap and used elsewhere. Also, I modified the resistance divider to be 10k with 15k (15k to the negative) which is now outputting a true precise 1v drop voltage. Capacitor is 100nF which will be near Q2 in the practical circuit.

[VEGETA]

using LT1678 made this effect:



I don't know what causes it, perhaps the high speed and high sensitivity of the op-amp? I am going to investigate it.

It happens at 5v and 10v output.

UPDATE: LT1678 showed this in my latest circuit v3.6.5 which has the 4 diodes in series to give negative voltage, while on the previous circuit v3.6.4 which uses your LED negative voltage,it also showed this error while LT1013 didn't in both versions. When this happens it is around 4mS or so. I tried putting capacitors everywhere (FBX, Q2 gate, CV opamp....) but no use. the problem in the CV stage since CC is not active.

So LT1013 works fine now, the questions I have to answer are:

1- what resolution will LT1013 allow for both current and voltage? is it dependent on the offset voltage?

2- why LT1678 didn't work and showed that huge ripple? although being significantly better than LT1013. My guess is it is extremely sensitive and extremely fast so that it produces these stuff.

3- anything to solve this? other than what I tried.

[Kleinstein]

The LT1678 is rather fast and rail to rail. The high GBW might need an extra capacitor to slow it down. The rail to rail input stage means that there is a transition between the two input stages somewhere in the input range in this range the can be extra errors. So rail to rail is not so good for best precision.

The LT1013 should be fast enough for current regulation and the voltage read-back. For the CV loop I am not so sure, but a simulation would show. The TL072 is not such a bad low cost choice - at least better than the lm358, so are MC/RC4558, even if they usually don't have a drift rating, but BJT based OPs are usually not that bad.

The shunt does not need to be high accuracy. So even a 10% tolerance would be OK - the more important parameters are a low TC and high power rating, which means not so much drift due to self heating. The exact value of the resistance and reference voltage is usually considered by an calibration for the final circuit. Similar an offset of the OPs can be compensated in software. So it is not a low offset that is important but low offset drift.

One might need an extra circuit to ensure a clean start up. Usually this is to start with an disabled output. So if the DCDC converter voltage is low, the output should be off be default. So a clean start from output disable is important.

[VEGETA]

Well, I have tried many LT parts and pretty much all worked well except for LT1678 and maybe LT6016. I searched again for good suitable op-amps and found this result:

1- LT6014: around 4.4$ for the 30uV offset and 0.2uV/C drift which is maybe ok.
2- MAX44245ASD or any similar. Around 4$ with 2uV offset and 30nV/NC drift. It is R-R though. This one is 4 OP-AMPs.
3- OP2177, 2 op-amp version. 0.7uV/C drift and 15uV offset.

LT1013 has around 150uV which is too much for 16-bit DAC.

The LT1013 should be fast enough for current regulation and the voltage read-back. For the CV loop I am not so sure, but a simulation would show.

I simulated it for many voltages like 3,5,10,20v and didn't show anything bad to my knowledge. You can verify. However, I want you to try LT6014 too since it's offset and drift are much better. Maybe it can be the true final choice.

The shunt does not need to be high accuracy. So even a 10% tolerance would be OK - the more important parameters are a low TC and high power rating

So this could be a nice first choice: http://www.digikey.com/product-detail/en/ohmite/LVM25FVR100E-TR/LVM25FVR100ECT-ND/6047793

It is 50ppm and 1W (remember our max is 0.4W) with good temperature range.

the full list of resistor search is here.

So it is not a low offset that is important but low offset drift.

Which drives me to think LT6014 is kinda the best here. What is your opinion? remember the 1mV/1mA set capability. We can compensate in software for the extra/missing 1mV/1mA due to several stuff, but set capability should be achieved especially with the power of 16-bit DAC and ADC too.

Imagine my day dream of making a business selling this product when it is finished is complete, how can I calibrate all of them? thus I need a kinda accurate shunt (not absolute accurate). The one I linked is 1% which is the same as getting ten of the 1R 1% resistors or nearly the same... so it is OK right?

One might need an extra circuit to ensure a clean start up. Usually this is to start with an disabled output. So if the DCDC converter voltage is low, the output should be off be default. So a clean start from output disable is important.

Later on I will work on getting output enable circuit ready which can do this job. How to get it done though while having it tracked with the linear (output) stage? I mean shutting linear stage will make pre-pregulator 0v.

[Kleinstein]

The LT6014 is not a good choice: it is compensated for gain >= 5. We need a unity gain stable OP. Otherwise the performance data look good. I would also consider the OPA2170.

Using a 1 W shunt at 0.4 W would mean a temperature rise of about 40% of the nominal value, which is about 50K. So there will be quite some self heating on the shunt. It will work, but not necessarily good.
The offset itself is not a problem unless it gets really large (e.g. > 10 mV). There likely will be a software calibration procedure to adjust at something like 0 V and 0 A and maybe 10 V and at 1 A. So neither offsets nor the absolute values of the reference voltage and the resistors matters. Only stability is important. One kind of needs these points for a test anyway, even if one would decide to use expensive accurate parts (e.g. 0.05% resistors and similar precision reference) to circumvent calibration. To do the calibration one would need something like a good DMM, preferable 5.5 digits or better.

Shuntdown on the linear stage will make the SMPS stage to go towards about 1 V, not 0 V. So the chip is already active.