LT1185-based lead acid battery charger

[Piotr Wyderski]

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

 

[Joerg]

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.

 

[Piotr Wyderski]

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

 

[Joerg]

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.

OK, makes sense.


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.

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.

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.

 

[Piotr Wyderski]

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

 

[Joerg]

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.

[...]

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

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.

 

[Piotr Wyderski]

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

 

[Joerg]

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?

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

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.


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

If self-limited, then that's acceptable. The poor NPN didn't have
a limiter...


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.

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.

 

[Piotr Wyderski]

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

 

[Joerg]

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.

EMC will probably be an issue as well, I really don't like single-layer
board. At least not for hi-rel stuff. A linear regulator would be tame
in that respect but often they aren't when it come to susceptibility.
Yet that's a test all of my designs have to pass. So the most I can use
is a board with a fat copper core (expensive) or use 4oz copper.

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.

The small low power supply is a good idea and it is almost the only way
to really reach 100% duty cycle, so you can milk the input side to the
limit. Provided there are no power factor requirements.

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.

Thing is, with efficient switchers you may no longer need all this IMS
stuff. My recent design delivers a total of up to 80W at various voltage
levels and it all lives in 2" by 5" board space. I don't have it here
but my client does and their engineer said that none of the
semiconductors become too hot to touch, after running full bore for half
an hour. We could run it well above 100W and it'll be ok.

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.

Well, at some point one has to leave the trodden paths and head out into
an adventure

As the president of one client said, "the most fun projects are those
where you get a serious knot in your stomach after making the commitment".

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.

Can't quite follow here, I don't see any roadblocks. Why does the
positive terminal of the battery have to be connected to the source? I
thought that was only because you did not want to use P-channel devices.
If you make a sync buck switcher with wo N-channel FETs, why not have
this regulator in the positive path?

Of course there are also methods using a bridge converter and
synchronous rectifiers that could go into the negative path but this
gets more expensive.

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.

Maybe because it has no compensation? It should be possible to make
something like this work but there will be the losses in the diode.

You should go into the FB terminal with the feedback. Also, it is
usually cheaper to use an optocoupler even though you don't need isolation.

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.

In my younger days I had more linear regulators explode on me than
switchers

If you use a peak current controlled architecture there really isn't all
that much that could go wrong in terms of a big bang. Of course, one
must watch that the battery will never become overcharged. But that risk
seems higher in a linear regulator, for example if the transistor shorts
out (that happened to me).

 

[piglet]

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?

 

[Piotr Wyderski]

Provided there are no power factor requirements.

No, I don't care about PFC. One specimen only, never going to be sold.

Thing is, with efficient switchers you may no longer need all this IMS
stuff.

Not here, but there are 20 more heaters.
4 big MOSFETs in the synchronous rectifier + 16 channels on
OMNIFETs (basically NMOSes too). The IMS radiator is there
for them, the ability to cool the charger is just a bonus.

Can't quite follow here, I don't see any roadblocks. Why does the
positive terminal of the battery have to be connected to the source? I
thought that was only because you did not want to use P-channel devices.
If you make a sync buck switcher with wo N-channel FETs, why not have
this regulator in the positive path?

Because a diffenent MOSFET is expected to be of N type. The big fat one
which bypasses the charger completely, providing an 800uOhm path between
the - terminal of the battery and GND. It must stand 40A and since
P=I^2R, a PMOS is a no-go. It's the R12 part on my first diagram.
Moving that NMOS to the high side would require more positive voltage
than the most positive potential available, so at least one more charge
pump or fancy voltage doubler. If this NMOS is on the low side, driving
is easy, but the charger becomes a challenge.

You should go into the FB terminal with the feedback. Also, it is
usually cheaper to use an optocoupler even though you don't need isolation.

LTSPICE gets really slow when optos are used.
In a real device I'll use an optocoupler.

Best regards, Piotr

 

[Lasse Langwadt Christensen]

Joerg wrote:






No, I don't care about PFC. One specimen only, never going to be sold.







Not here, but there are 20 more heaters.

4 big MOSFETs in the synchronous rectifier + 16 channels on

OMNIFETs (basically NMOSes too). The IMS radiator is there

for them, the ability to cool the charger is just a bonus.










Because a diffenent MOSFET is expected to be of N type. The big fat one

which bypasses the charger completely, providing an 800uOhm path between

the - terminal of the battery and GND. It must stand 40A and since

P=I^2R, a PMOS is a no-go. It's the R12 part on my first diagram.

Moving that NMOS to the high side would require more positive voltage

than the most positive potential available, so at least one more charge

pump or fancy voltage doubler.

a single part: http://www.linear.com/product/LT1910

-Lasse

 

[Joerg]

No, I don't care about PFC. One specimen only, never going to be sold.


Not here, but there are 20 more heaters.
4 big MOSFETs in the synchronous rectifier + 16 channels on
OMNIFETs (basically NMOSes too). The IMS radiator is there
for them, the ability to cool the charger is just a bonus.

That's a lot. I'd consider a real heatsink, it's cheaper than this
specialty stuff.

Because a diffenent MOSFET is expected to be of N type. The big fat one
which bypasses the charger completely, providing an 800uOhm path between
the - terminal of the battery and GND. It must stand 40A and since
P=I^2R, a PMOS is a no-go. ...

Why? Paralleling three of these would get you there:

http://www.vishay.com/docs/64814/si7145dp.pdf

It is not good to send 40A through a single device anyhow. If you
absolutely want N-channel a small voltage generator would not be a big deal.

It's the R12 part on my first diagram.
Moving that NMOS to the high side would require more positive voltage
than the most positive potential available, so at least one more charge
pump or fancy voltage doubler. If this NMOS is on the low side, driving
is easy, but the charger becomes a challenge.

If you absolutely want N-channel a small voltage generator would not be
a big deal. Much less of a challenge than a negative side switcher.
Since this positive helper voltage does not even have to be regulated it
would fall under the category "piece of cake"

LTSPICE gets really slow when optos are used.
In a real device I'll use an optocoupler.

Then it wold be good to at least model it with a transfer function. But
you need to go into the FB node and use the comp node for what it was
intendedto do, compensation. Otherwise it is tough to get this stable.