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LT1185-based lead acid battery charger

Started by Piotr Wyderski September 5, 2013
I am designing a prototype charger of a lead-acid battery
used as a backup power supply source. The input voltage
is synchronously rectified 12VAC and the current is <= 40A.
I would like to use LT1185 due to its excellent properties.
Because of other design requirements, the + line is common
and the regulator is connected between the - of the battery
and the GND of the rectifier -- a typical negative voltage
LDO application. The regulator should supply 13.8V/2A when
enabled via the REF pin. However, several questions arise
and I am unable to find an authoritative answer in the PDF.
Will the regulator survive if:

1. it is enabled (via REF) and the voltage on the Vout pin is higher 
than on Vin
(but of correct polarity)? This would happen when the battery is 
connected and
the voltage from the rectifier is in the decreasing sector of the 
abs(sine) wave:

   17V*abs(sin(50Hz)) _____________
			     |	  |
			     |	 ---  +
                              |    -   12V battery
			     |	  |   -
			     -----X   LDO
				  |
		GND  -------------

2. as above, but the regulator is disabled? (Iref < 400nA)

3. the regulator is disabled and its Vin and Vout pins are
shorted (by a 430A-capable NMOS) in order to provide a low
impedance current path from the battery to the supplied device.

                      _____________
		    |        |	  |
		    |	     |	 ---
                   Rload      |    -    12V battery
		    |        |    |
		    |	     |	  *---
		    |	     -----X   | short circuit
		    |		  *---
		    |             |
		GND  -------------

It works in LTSPICE, but I don't know how accurate the simulator is in 
these not very standard conditions. Here's a more detailed diagram:

http://s10.postimg.org/rfhbc28hl/charger.jpg

R1 and R12 are actually switches.

	Best regards, Piotr
Piotr Wyderski wrote:
> I am designing a prototype charger of a lead-acid battery > used as a backup power supply source. The input voltage > is synchronously rectified 12VAC and the current is <= 40A. > I would like to use LT1185 due to its excellent properties. > Because of other design requirements, the + line is common > and the regulator is connected between the - of the battery > and the GND of the rectifier -- a typical negative voltage > LDO application. The regulator should supply 13.8V/2A when > enabled via the REF pin. However, several questions arise > and I am unable to find an authoritative answer in the PDF. > Will the regulator survive if: > > 1. it is enabled (via REF) and the voltage on the Vout pin is higher > than on Vin > (but of correct polarity)? This would happen when the battery is > connected and > the voltage from the rectifier is in the decreasing sector of the > abs(sine) wave: > > 17V*abs(sin(50Hz)) _____________ > | | > | --- + > | - 12V battery > | | - > -----X LDO > | > GND ------------- > > 2. as above, but the regulator is disabled? (Iref < 400nA) > > 3. the regulator is disabled and its Vin and Vout pins are > shorted (by a 430A-capable NMOS) in order to provide a low > impedance current path from the battery to the supplied device. > > _____________ > | | | > | | --- > Rload | - 12V battery > | | | > | | *--- > | -----X | short circuit > | *--- > | | > GND ------------- > > It works in LTSPICE, but I don't know how accurate the simulator is in > these not very standard conditions. Here's a more detailed diagram: > > http://s10.postimg.org/rfhbc28hl/charger.jpg >
You need some capacitance on the input and output. Such LDOs are like the princess on the pea, for the output they need the ESR of this capacitor to be within a defined range. Else they can go berserk.
> R1 and R12 are actually switches. >
http://www.linear.com/docs/2875 The datasheet says the usual on page 10, quote "The LT1185 is designed to allow output reverse polarity of several volts without damage or latch-up, so that a simple diode clamp can be used". So you have to provide a beefy diode. Best to also protect the FB pin via a diode but that one won't have to be big. As for bypass-shorting you'd have to ask LTC but with many regulators that is ok as long as you make sure no BE junctions become reverse-biased past their pain threshold. -- Regards, Joerg http://www.analogconsultants.com/
Joerg wrote:

> You need some capacitance on the input and output.
I will add a small tantalum on the output, but does it make any sense on input? Given the current involved (40A range) no cap is big enough and the rest of the device is powered with rectified AC instead of DC for exactly this reason: I don't want any electrolytic capacitor in the device. It must work flawlessly for decades. > Such LDOs are like the princess on the pea, for the output > they need the ESR of this capacitor to be within a defined range. OK, makes sense.
> The datasheet says the usual on page 10, quote "The LT1185 is designed > to allow output reverse polarity of several volts without damage or > latch-up, so that a simple diode clamp can be used".
As far as I understand, by reverse polarity they mean Vout with respect to GND. In my case Vout is reverse polarized wrt Vin. From the GND point of view the polarity is always correct, especially when the 800uOhm NMOS (R12) enters the stage. So this is my primary concern: what would happen inside the IC then. Unfortunately, I have no LT1185, so I'll use LM337 as a test dummy. > So you have to provide a beefy diode. Or another NMOS...
> As for bypass-shorting you'd have to ask LTC but with many regulators > that is ok as long as you make sure no BE junctions become > reverse-biased past their pain threshold.
I've sent them a query. Thanks, Joerg. Best regards, Piotr
Piotr Wyderski wrote:
> Joerg wrote: > >> You need some capacitance on the input and output. > > I will add a small tantalum on the output, but does it make > any sense on input? Given the current involved (40A range) > no cap is big enough and the rest of the device is powered > with rectified AC instead of DC for exactly this reason: > I don't want any electrolytic capacitor in the device. It > must work flawlessly for decades. >
I never use tantalums because I've seen them explode, out of the blue. Whenever possible I use a ceramic cap. But then again I never use LDOs either because I don't trust them unless I designed them myself on the transistor level. In this case you could use a ceramic cap with a resistor in series that puts it in the middle of the prescribed ESR range. And then pray that the capacitance that the battery presents in parallel to that will not spoil the whole scheme. On the input I'd place at least a small ceramic cap, close to the regulator. Many reasons. Aside from stability it's also because you don't want strong external RF signals to leak into this regulator and mess with the regulator loop. AM stations nearby, shortwave, CB radios, et cetera.
>> Such LDOs are like the princess on the pea, for the output >> they need the ESR of this capacitor to be within a defined range. > > OK, makes sense. > >> The datasheet says the usual on page 10, quote "The LT1185 is designed >> to allow output reverse polarity of several volts without damage or >> latch-up, so that a simple diode clamp can be used". > > As far as I understand, by reverse polarity they mean Vout with > respect to GND. In my case Vout is reverse polarized wrt Vin. > From the GND point of view the polarity is always correct, > especially when the 800uOhm NMOS (R12) enters the stage. So > this is my primary concern: what would happen inside the IC then. > Unfortunately, I have no LT1185, so I'll use LM337 as a test dummy. >
IIUC it's not reversed but when there is no or low input voltage the output voltage is higher (meaning more negative) that the input voltage. When that goes too far it can kill a linear regulator IC unless you have a diode across it.
>> So you have to provide a beefy diode. > > Or another NMOS... >
FETs can work as well, their body diodes are usually rated in a similar current class as the channel of the FET.
>> As for bypass-shorting you'd have to ask LTC but with many regulators >> that is ok as long as you make sure no BE junctions become >> reverse-biased past their pain threshold. > > I've sent them a query. Thanks, Joerg. >
LTC is one of the best companies when it comes to applications support. Maybe even the best. -- Regards, Joerg http://www.analogconsultants.com/
Joerg wrote:

> But then again I never use LDOs either because I don't trust them
> unless I designed them myself on the transistor level. Agreed. My first attempt was to use a design based on discrete elements exactly because I understand better what is really going on in the circuit. It had an 8A NPN in DPAK soldered directly to an IMS based PCB. It had a nice current limiter set to 3.7A and I thought that IMS will be just the right thing to dissipate the power produced by the NPN under short circuit conditions. Unfortunately, while the board's thermal resistance is superior, its thermal impedance is mediocre and after 2 seconds the NPN exploded in a very nasty way. So what I really need is to have thermal/power limiter with a low inertia. As there are no power BJTs with a built-in thermistor, only two ways remain: use a overcurrent/thermally protected NMOS (for example an OMNIFET) + a baroquesque driving circuit or a ready-made IC. The second option is not smarter -- just less crazy.
> On the input I'd place at least a small ceramic cap, close to the > regulator. Many reasons. Aside from stability it's also because you > don't want strong external RF signals to leak into this regulator and > mess with the regulator loop.
Good point.
> IIUC it's not reversed but when there is no or low input voltage the > output voltage is higher (meaning more negative) that the input voltage. > When that goes too far it can kill a linear regulator IC unless you have > a diode across it.
Yes, I've checked several other specs and you are 100% right -- by "reversed" they DO mean exactly the Vin-Vout voltage alone, with no correspondence to GND. I have never used a regulator in this weird sector, so my questions may sound weird for experts -- I'm just trying to learn something new without too many bangs.
> FETs can work as well, their body diodes are usually rated in a similar > current class as the channel of the FET.
I meant that the FET will go in sync with the enable signal, but now I think that it is not the best idea. So one more try: assume LM317 for less pins and the positive regulation side in order not to waste brainpower on sign transformations. Is it allowed connect a Schottky diode between Vout and the load, add a cap between Vout and GND and connect the feedback divider *after* the diode, i.e. to its cathode in order to compensate Uf(temp) changes ? Plus the usual clamping diodes between Vout/Vin and Adj/GND? The reverse current of a 5A Schottky can be as high as 15mA (at 100 degrees) according to the specs, so the clamping diodes are necessary. But the main question: will the feedback circuit happily adapt to this arrangement or is it a no-go? I'll check it, but the question is whether it is allowed, not whether it works with that particular part. Few more square centimeters available and I would go for a decent SMPS, but unfortunately I cannot afford the real estate due to size limits. BTW, I am just a hobbyist. Best regards, Piotr
Piotr Wyderski wrote:
> Joerg wrote: > >> But then again I never use LDOs either because I don't trust them >> unless I designed them myself on the transistor level. > > Agreed. My first attempt was to use a design based on discrete > elements exactly because I understand better what is really going > on in the circuit. It had an 8A NPN in DPAK soldered directly > to an IMS based PCB. It had a nice current limiter set to 3.7A > and I thought that IMS will be just the right thing to dissipate > the power produced by the NPN under short circuit conditions. > Unfortunately, while the board's thermal resistance is superior, > its thermal impedance is mediocre and after 2 seconds the NPN > exploded in a very nasty way. So what I really need is to have > thermal/power limiter with a low inertia. As there are no power > BJTs with a built-in thermistor, only two ways remain: use a > overcurrent/thermally protected NMOS (for example an OMNIFET) > + a baroquesque driving circuit or a ready-made IC. The second > option is not smarter -- just less crazy. >
Not sure about your application but I assume that a thermal shutdown will not be very desirable for the functionality of the whole gear. Why not use a switcher instead? It can be much more efficient. [...]
>> IIUC it's not reversed but when there is no or low input voltage the >> output voltage is higher (meaning more negative) that the input voltage. >> When that goes too far it can kill a linear regulator IC unless you have >> a diode across it. > > Yes, I've checked several other specs and you are 100% right -- by > "reversed" they DO mean exactly the Vin-Vout voltage alone, with no > correspondence to GND. I have never used a regulator in this weird > sector, so my questions may sound weird for experts -- I'm just trying > to learn something new without too many bangs. >
We all have to learn once in a while. I'd ask similar questions if it was a software topic :-)
>> FETs can work as well, their body diodes are usually rated in a similar >> current class as the channel of the FET. > > I meant that the FET will go in sync with the enable signal, but now > I think that it is not the best idea. > > So one more try: assume LM317 for less pins and the positive regulation > side in order not to waste brainpower on sign transformations. Is it > allowed connect a Schottky diode between Vout and the load, add a cap > between Vout and GND and connect the feedback divider *after* the diode, > i.e. to its cathode in order to compensate Uf(temp) changes ? Plus the > usual clamping diodes between Vout/Vin and Adj/GND? The reverse current > of a 5A Schottky can be as high as 15mA (at 100 degrees) according to > the specs, so the clamping diodes are necessary. But the main question: > will the feedback circuit happily adapt to this arrangement or is it a > no-go? I'll check it, but the question is whether it is allowed, not > whether it works with that particular part. >
I can't see why it would not be ok. The important thing is to use protection diodes so neither the output-input path nor that of the ADJ pin can ever become reverse-biased much. However, in this arrangement you will be dissipating tons of power. The LM317 has major drop-out (I think around 2V at that current but haven't looked) and then the Schottky diode will be likely over 800mV unless you use a really fat one.
> Few more square centimeters available and I would go for a decent SMPS, > but unfortunately I cannot afford the real estate due to size limits. > BTW, I am just a hobbyist. >
Look around, there are lots of very tiny switcher chips in the several-amps class available these days. Many can run at frequencies in excess of 500kHz so the inductor will be very small. If you find a synchronous one you won't even have the diode losses. -- Regards, Joerg http://www.analogconsultants.com/
Joerg wrote:

> Not sure about your application
It will be a multichannel 12V lightbulb controller. The ~100Ah accu will allow them to work for several hours when the 230V line is not energized. It must be rock-stable (meaning decades), so I've removed all the quickly decaying elements like the electrolytic caps. Because the bulbs don't care about the exact shape of their powering current, the device is a mixed-mode design. If there power line works, which is the typical case, the bulbs are powered with abs(sin(50Hz)). Otherwise the accu is connected and they get 13.8V DC + some amount of PWM mean power correction. I also want to dissipate as low power as possible because of cooling constraints. Hence the synchronous rectifier + a bunch of very good MOSFETs. The + line is common instead of GND solely because the latter would require a PMOS and the best PMOS available is ~4x worse than an NMOS. However, it implies crazy driving in the charger part, but the current is so much lower -- I've decided it's a lesser evil. The accu charger is just a supplemental functionality and is not expected to be operating very often -- mostly just to compensate the self-discharge current. It should be enabled when the lamps are (mostly) off, so my thermal budget can afford the additional 5 Watts. The board is on IMS, so all the high power parts share a pretty decent radiator. The trick is to not enable all of them at once. :-)
> but I assume that a thermal shutdown will not be very desirable for the functionality of the whole gear.
Actually, there is a global thermal shutdown for safety purposes. It just didn't have a chance to intervene because of the high thermal impedance of IMS and my charger 1.0 exploded. :-) Lesson learned. A built-in thermal protection is a must-have.
> Why not use a switcher instead? It can be much more efficient.
A switcher would be great, but its size is considerably bigger than 3x TO220 which is the real estate I can assign to the task. A 5 Amp diode in SMC is quite big on its own... The LT1185 barely fits, and the main trick is to reuse the large GND trace, because Vin is connected to the tab. But I will rethink this possibility.
> I can't see why it would not be ok.
Then that's great, I have a viable candidate for the charger 2.0. I'll also try to cook something up on UC3843.
> However, in this arrangement you will be dissipating tons of power.
If self-limited, then that's acceptable. The poor NPN didn't have a limiter... > The LM317 has major drop-out (I think around 2V at that current but haven't
> looked)
I assume 3V, but the LM317 was there just to show the key part of the new arrangement. LT1185 has 2 more pins and is negative. ;-)
> and then the Schottky diode will be likely over 800mV unless you > use a really fat one.
The LT has 0.75V@3A and even if the Schottky adds another 0.75, it's just 1.5V -- 2x better than an unprotected LM317 at 2x higher current. It's pretty high, but I can live with that. Best regards, Piotr
Piotr Wyderski wrote:
> Joerg wrote: > >> Not sure about your application > > It will be a multichannel 12V lightbulb controller. > The ~100Ah accu will allow them to work for several > hours when the 230V line is not energized. It must > be rock-stable (meaning decades), so I've removed > all the quickly decaying elements like the electrolytic > caps. Because the bulbs don't care about the exact shape > of their powering current, the device is a mixed-mode > design. If there power line works, which is the typical > case, the bulbs are powered with abs(sin(50Hz)). Otherwise > the accu is connected and they get 13.8V DC + some amount > of PWM mean power correction. > > I also want to dissipate as low power as possible because > of cooling constraints. Hence the synchronous rectifier > + a bunch of very good MOSFETs. The + line is common > instead of GND solely because the latter would require > a PMOS and the best PMOS available is ~4x worse than > an NMOS. However, it implies crazy driving in the charger > part, but the current is so much lower -- I've decided > it's a lesser evil. >
Ok, let me interject here. Contemporary synchronous buck regulators have a boost function which allows the use of NMOS throughout, for the reasons you have stated. If you have many of them on one board you could even contemplate a separate supply for the boost pin. That allows you to get to almost 100% duty cycle which means almost zero dropout.
> The accu charger is just a supplemental functionality > and is not expected to be operating very often -- mostly > just to compensate the self-discharge current. It should > be enabled when the lamps are (mostly) off, so my thermal > budget can afford the additional 5 Watts. > > The board is on IMS, so all the high power parts share > a pretty decent radiator. The trick is to not enable all > of them at once. :-) >
What is IMS?
>> but I assume that a thermal shutdown will not be very desirable for >> the functionality of the whole gear. > > Actually, there is a global thermal shutdown for safety purposes. > It just didn't have a chance to intervene because of the high > thermal impedance of IMS and my charger 1.0 exploded. :-) > Lesson learned. A built-in thermal protection is a must-have. >
Whoops :-)
>> Why not use a switcher instead? It can be much more efficient. > > A switcher would be great, but its size is considerably bigger > than 3x TO220 which is the real estate I can assign to the task.
Nah, easy. I just designed a module with four switchers on there and I've got a 12V/30W switcher in the mix. It is oversized so could easily do twice that power. I just took a look at the layout and that whole 12V section isn't larger than 3x TO220. With switchers the external FETs do not have to be TO220, you can get LFPAK and even smaller. No diode in there, either.
> A 5 Amp diode in SMC is quite big on its own... The LT1185 barely > fits, and the main trick is to reuse the large GND trace, because > Vin is connected to the tab. But I will rethink this possibility. > >> I can't see why it would not be ok. > > Then that's great, I have a viable candidate for the charger 2.0. > I'll also try to cook something up on UC3843. >
That one is a bit long in the tooth, maybe use something more modern and with tons of gate drive. Check out this stuff, just as an example how small things can be these days: http://www.exar.com/Common/Content/Document.ashx?id=761
>> However, in this arrangement you will be dissipating tons of power. > > If self-limited, then that's acceptable. The poor NPN didn't have > a limiter... > >> The LM317 has major drop-out (I think around 2V at that current but > haven't >> looked) > > I assume 3V, but the LM317 was there just to show the key part of > the new arrangement. LT1185 has 2 more pins and is negative. ;-) >
Yes, but LDO's aren't what I'd use in this case.
>> and then the Schottky diode will be likely over 800mV unless you >> use a really fat one. > > The LT has 0.75V@3A and even if the Schottky adds another 0.75, it's > just 1.5V -- 2x better than an unprotected LM317 at 2x higher current. > It's pretty high, but I can live with that. >
Just keep in mind that since you are designing for very high MTBF, every degree this runs cooler will add to the reliability. Less thermal cycling, less problems down the road. -- Regards, Joerg http://www.analogconsultants.com/
Joerg wrote:

> What is IMS?
1.5mm of aluminum + 100um of FR4 + 35um of copper: https://en.wikipedia.org/wiki/Power_electronic_substrate#Insulated_metal_substrate I have no access to the Al2O3-insulated variant, but even the FR4-based one is so much better than generic FR4 in heat conduction. Soldering is a nightmare for exactly the same reason. :-)
> That one is a bit long in the tooth
Sure it is, but it has two advantages: 1. I already have 10 of them on the shelf. ;-) 2. It has a disconnected totem pole driver, while the remaining more modern switchers I own have their integrated MOSFET connected to the Vin line and in my case the Vin line is not filtered, which can cause a problem. IMHO I should provide a small low-power supply for the switcher and use an external NMOS to handle the power part.
> Check out this stuff, just as an example how small things can be these days: > > http://www.exar.com/Common/Content/Document.ashx?id=761
Extremely nice pinout. I have LM2696 capable of 3 amps, but it is almost unroutable on a single-layer PCB and on IMS a via is definitely not what you want.
> Yes, but LDO's aren't what I'd use in this case.
I am not sure whether I can produce a reliable design of such an SMPS. I routinely use switchers, but mostly in arrangements very close to their application notes. This one is so much different -- the + pin of the accu is directly connected to the + side of the rectifier and the main NMOS is on the GND side. This is what I can't change. It means that the switcher must produce positive voltage which follows closely the shape of the rectified voltage in order to maintain the desired 13.8V difference. Not that hard, I can use a TL431 between the accu's terminal and a current mirror composed of 2xBC857 in order to bring the TL's state to the GND side. But how should the inductor and the diode be connected? Cathode to the common +, anode to the NMOs' drain and the - of the accu via an inductor to the drain and a capacitor parallel to the accu? And it must survive the reverse polarity in the Vin < Vbat sector. I've invented something like that (assumed 5V instead of 13.8V for easier dividers): http://s2.postimg.org/bwuao3b2h/smps_charger.jpg but the simulator produces crazy output.
> Just keep in mind that since you are designing for very high MTBF, every > degree this runs cooler will add to the reliability. Less thermal > cycling, less problems down the road.
True. I would very much like to use a switcher, but I must be sure the design is safe. I don't want another explosion, so the linear regulator is just safer to me. Best regards, Piotr
> http://s2.postimg.org/bwuao3b2h/smps_charger.jpg > > but the simulator produces crazy output. >
Could it be inversion in the feedback path? Try feeding the current mirror into the FB pin instead of COMP pin?