Push-Pull driver <<1ohm Rdson?

[Joerg]

Hello Folks,

Looking for a staunch chip to drive transformers in the 30-150kHz range.
The most powerful (and available) one I found is the MIC4451 but it's
1ohm Rdson. Way too much.

There are bigger ones but they usually have a charge pump and the
efficiency is pretty poor when operated at less than 3A:
http://rocky.digikey.com/weblib/Infineon/Web%20Data/TDA21201prelimDS.pdf

What I need is n/p channel push-pull output (not two n-channels), no
charge pump because the clock sometimes stops, and ideally 100-200mohm
Rdson. External FETs aren't so hot, too much cross conduction. I was
hoping there'd be n/p synchronous buck converters. But nope, all with
charge pumps.

Any ideas?

--
Regards, Joerg

http://www.analogconsultants.com/

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[John Larkin]

On Sun, 21 Sep 2008 16:44:40 -0700, Joerg
&lt;notthisjoergsch@removethispacbell.net&gt; wrote:

Hello Folks,

Looking for a staunch chip to drive transformers in the 30-150kHz range.
The most powerful (and available) one I found is the MIC4451 but it's
1ohm Rdson. Way too much.

There are bigger ones but they usually have a charge pump and the
efficiency is pretty poor when operated at less than 3A:
http://rocky.digikey.com/weblib/Infineon/Web%20Data/TDA21201prelimDS.pdf

What I need is n/p channel push-pull output (not two n-channels), no
charge pump because the clock sometimes stops, and ideally 100-200mohm
Rdson. External FETs aren't so hot, too much cross conduction. I was
hoping there'd be n/p synchronous buck converters. But nope, all with
charge pumps.

Any ideas?

This is a nice part:

http://www.national.com/pf/LM/LMD18201.html

It has a charge pump, but it also has an oscillator to keep it pumped
even when the input parks at one level for a long time. I've used it
for driving microsteppers, works fine.

John

 

On Sep 21, 6:44 pm, Joerg &lt;notthisjoerg...@removethispacbell.net&gt;
wrote:

Hello Folks,

Looking for a staunch chip to drive transformers in the 30-150kHz range.
The most powerful (and available) one I found is the MIC4451 but it's
1ohm Rdson. Way too much.

There are bigger ones but they usually have a charge pump and the
efficiency is pretty poor when operated at less than 3A:http://rocky.digikey.com/weblib/Infineon/Web%20Data/TDA21201prelimDS.pdf

What I need is n/p channel push-pull output (not two n-channels), no
charge pump because the clock sometimes stops, and ideally 100-200mohm
Rdson. External FETs aren't so hot, too much cross conduction. I was
hoping there'd be n/p synchronous buck converters. But nope, all with
charge pumps.

Any ideas?

--
Regards, Joerg

http://www.analogconsultants.com/

"gmail" domain blocked because of excessive spam.
Use another domain or send PM.

Hi Joerg

Why is a charge pump not acceptable? Are you concerned about the time
it takes to build up the gate charge.

What do you mean when you say "External FETs have too much cross
conduction?

Dick

 

[Joerg]

Jon Slaughter wrote:

"Joerg" &lt;notthisjoergsch@removethispacbell.net&gt; wrote in message
news:u2BBk.721$pr6.427@flpi149.ffdc.sbc.com...
Hello Folks,

Looking for a staunch chip to drive transformers in the 30-150kHz range.
The most powerful (and available) one I found is the MIC4451 but it's 1ohm
Rdson. Way too much.

There are bigger ones but they usually have a charge pump and the
efficiency is pretty poor when operated at less than 3A:
http://rocky.digikey.com/weblib/Infineon/Web%20Data/TDA21201prelimDS.pdf

What I need is n/p channel push-pull output (not two n-channels), no
charge pump because the clock sometimes stops, and ideally 100-200mohm
Rdson. External FETs aren't so hot, too much cross conduction. I was
hoping there'd be n/p synchronous buck converters. But nope, all with
charge pumps.

Any ideas?

--

FS---FDD8424H---Dual N &amp; P Channel half-bridge 40V@20A 54mOhm.pdf

Thanks, Jon. Although they conduct already quite well between 2-3V Vgs
so there'll still be considerable cross conduction. I am operating at
12V. Maybe I'll place zeners in the gate drive to burn off some drive level.

--
Regards, Joerg

http://www.analogconsultants.com/

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[Joerg]

Mike Monett wrote:

Joerg &lt;notthisjoergsch@removethispacbell.net&gt; wrote:

Hello Folks,

Looking for a staunch chip to drive transformers in the 30-150kHz range.
The most powerful (and available) one I found is the MIC4451 but it's
1ohm Rdson. Way too much.

There are bigger ones but they usually have a charge pump and the
efficiency is pretty poor when operated at less than 3A:
http://rocky.digikey.com/weblib/Infineon/Web%20Data/TDA21201prelimDS.pdf

What I need is n/p channel push-pull output (not two n-channels), no
charge pump because the clock sometimes stops, and ideally 100-200mohm
Rdson. External FETs aren't so hot, too much cross conduction. I was
hoping there'd be n/p synchronous buck converters. But nope, all with
charge pumps.

Any ideas?

If you had no luck on a driver and wanted to use a charge pump for the
widest choice of output transistors, could you turn the bottom FET on when
the clock stops?

Yes, that would be no problem.

This would keep the charge pump capacitor primed and ready to go on the
first positive clock. The same thing would work for a full h-bridge. It
might take a bit of glue logic to get things started again in the correct
phase but it shouldn't be too difficult.

I was thinking about glue logic with some "poor man's" one-shots in
there. Certainly an option but I wanted to avoid it, keeping complexity
down.

--
Regards, Joerg

http://www.analogconsultants.com/

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[Joerg]

John Larkin wrote:

On Sun, 21 Sep 2008 16:44:40 -0700, Joerg
notthisjoergsch@removethispacbell.net&gt; wrote:

Hello Folks,

Looking for a staunch chip to drive transformers in the 30-150kHz range.
The most powerful (and available) one I found is the MIC4451 but it's
1ohm Rdson. Way too much.

There are bigger ones but they usually have a charge pump and the
efficiency is pretty poor when operated at less than 3A:
http://rocky.digikey.com/weblib/Infineon/Web%20Data/TDA21201prelimDS.pdf

What I need is n/p channel push-pull output (not two n-channels), no
charge pump because the clock sometimes stops, and ideally 100-200mohm
Rdson. External FETs aren't so hot, too much cross conduction. I was
hoping there'd be n/p synchronous buck converters. But nope, all with
charge pumps.

Any ideas?


This is a nice part:

http://www.national.com/pf/LM/LMD18201.html

It has a charge pump, but it also has an oscillator to keep it pumped
even when the input parks at one level for a long time. I've used it
for driving microsteppers, works fine.

Thanks, John. I had looked at that one and the Rdson is a bit highish,
600mohm max. I've also had some issues with National motor drivers with
obsolescence. Thing is, a lot of my stuff remains in production well
over 10 years.

I wish there was a big brother of the LMD18201 somewhere, 1/3rd of its
Rdson or so. I am running this at 12V supply and cost is not a real
concern for this part of the design. IOW if it was $10 a pop it would be ok.

--
Regards, Joerg

http://www.analogconsultants.com/

"gmail" domain blocked because of excessive spam.
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[John Larkin]

On Mon, 22 Sep 2008 09:03:59 -0700, Joerg
&lt;notthisjoergsch@removethispacbell.net&gt; wrote:

John Larkin wrote:
On Sun, 21 Sep 2008 16:44:40 -0700, Joerg
notthisjoergsch@removethispacbell.net&gt; wrote:

Hello Folks,

Looking for a staunch chip to drive transformers in the 30-150kHz range.
The most powerful (and available) one I found is the MIC4451 but it's
1ohm Rdson. Way too much.

There are bigger ones but they usually have a charge pump and the
efficiency is pretty poor when operated at less than 3A:
http://rocky.digikey.com/weblib/Infineon/Web%20Data/TDA21201prelimDS.pdf

What I need is n/p channel push-pull output (not two n-channels), no
charge pump because the clock sometimes stops, and ideally 100-200mohm
Rdson. External FETs aren't so hot, too much cross conduction. I was
hoping there'd be n/p synchronous buck converters. But nope, all with
charge pumps.

Any ideas?


This is a nice part:

http://www.national.com/pf/LM/LMD18201.html

It has a charge pump, but it also has an oscillator to keep it pumped
even when the input parks at one level for a long time. I've used it
for driving microsteppers, works fine.


Thanks, John. I had looked at that one and the Rdson is a bit highish,
600mohm max. I've also had some issues with National motor drivers with
obsolescence. Thing is, a lot of my stuff remains in production well
over 10 years.

I wish there was a big brother of the LMD18201 somewhere, 1/3rd of its
Rdson or so. I am running this at 12V supply and cost is not a real
concern for this part of the design. IOW if it was $10 a pop it would be ok.

You could certainly make it out of parts... a couple of LM5112's, or a
dual gate driver chip, a couple of non-overlap parts, and two fets.

But I bet one of the fiercer fet gate driver chips would just work...
maybe parallel 2 or 4 sections.

Take a look at the LM5112... it's an interesting part. They cost us
$1.30 in small quantities. Maybe one or three of them could drive your
load.

John

 

[Joerg]

John Larkin wrote:

On Mon, 22 Sep 2008 09:03:59 -0700, Joerg
notthisjoergsch@removethispacbell.net&gt; wrote:

John Larkin wrote:
On Sun, 21 Sep 2008 16:44:40 -0700, Joerg
notthisjoergsch@removethispacbell.net&gt; wrote:

Hello Folks,

Looking for a staunch chip to drive transformers in the 30-150kHz range.
The most powerful (and available) one I found is the MIC4451 but it's
1ohm Rdson. Way too much.

There are bigger ones but they usually have a charge pump and the
efficiency is pretty poor when operated at less than 3A:
http://rocky.digikey.com/weblib/Infineon/Web%20Data/TDA21201prelimDS.pdf

What I need is n/p channel push-pull output (not two n-channels), no
charge pump because the clock sometimes stops, and ideally 100-200mohm
Rdson. External FETs aren't so hot, too much cross conduction. I was
hoping there'd be n/p synchronous buck converters. But nope, all with
charge pumps.

Any ideas?

This is a nice part:

http://www.national.com/pf/LM/LMD18201.html

It has a charge pump, but it also has an oscillator to keep it pumped
even when the input parks at one level for a long time. I've used it
for driving microsteppers, works fine.

Thanks, John. I had looked at that one and the Rdson is a bit highish,
600mohm max. I've also had some issues with National motor drivers with
obsolescence. Thing is, a lot of my stuff remains in production well
over 10 years.

I wish there was a big brother of the LMD18201 somewhere, 1/3rd of its
Rdson or so. I am running this at 12V supply and cost is not a real
concern for this part of the design. IOW if it was $10 a pop it would be ok.

You could certainly make it out of parts... a couple of LM5112's, or a
dual gate driver chip, a couple of non-overlap parts, and two fets.

Yeah, if I don't find anything I'll do that this afternoon. Some logic
and external FETs. I wanted to avoid it but I guess it's the usual, I
might be alone in the marketplace for this stuff so the mfgs don't
bother. Kind of surprising because motor-PWM sometimes needs this stuff.

But I bet one of the fiercer fet gate driver chips would just work...
maybe parallel 2 or 4 sections.

Well, that's just the point, I haven't found any fiercer ones. And none
where I could parallel on the same chip and get to &lt;250mohm. When I
talked to Micrel about paralleling some MIC4451 the engineer said the
delay differences may not work out well unless I make sure they are all
from the same batch. The latter is totally frowned upon by clients these
days.

A back of the envelope calc showed if I paralleled four and one would
really exhibit the 30nsec worst case delay veer versus the other three
that one would quickly mutate into a puff of smoke. Phssst ... *BANG*

Take a look at the LM5112... it's an interesting part. They cost us
$1.30 in small quantities. Maybe one or three of them could drive your
load.

Nah, I did look at that in detail after you mentioned it last year. The
high side is too wimpy, only the BJT in there has enough oomph and that
leaves too much headroom. The MIC4451 is much better. But not good
enough here. Also comes in a cheaper 9A peak edition.

--
Regards, Joerg

http://www.analogconsultants.com/

"gmail" domain blocked because of excessive spam.
Use another domain or send PM.

 

[Joerg]

Joerg wrote:

Hello Folks,

Looking for a staunch chip to drive transformers in the 30-150kHz range.
The most powerful (and available) one I found is the MIC4451 but it's
1ohm Rdson. Way too much.

There are bigger ones but they usually have a charge pump and the
efficiency is pretty poor when operated at less than 3A:
http://rocky.digikey.com/weblib/Infineon/Web%20Data/TDA21201prelimDS.pdf

What I need is n/p channel push-pull output (not two n-channels), no
charge pump because the clock sometimes stops, and ideally 100-200mohm
Rdson. External FETs aren't so hot, too much cross conduction. I was
hoping there'd be n/p synchronous buck converters. But nope, all with
charge pumps.

Any ideas?

Ok, guys, don't bother with this one anymore. I just wrapped it up with
discrete parts. As usual

--
Regards, Joerg

http://www.analogconsultants.com/

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Use another domain or send PM.

 

[Joerg]

Mike Monett wrote:

Joerg &lt;notthisjoergsch@removethispacbell.net&gt; wrote:

[...]

A back of the envelope calc showed if I paralleled four and one
would really exhibit the 30nsec worst case delay veer versus the
other three that one would quickly mutate into a puff of smoke.

Phssst. *BANG*

That's exactly what a small inductor in the top FET will prevent.
Check it out. You might be surprised how effective it can be.

You really have to get off this Phssst. *BANG* habit. It makes too
much noise, and everyone gets nervous. Plus it stinks up the place

I tried the inductor and wasn't too enthused. With a small damper
resistor inparallel it was kind of ok but still cost efficiency. But
it's done, I just lashed up the usual discrete concoction that provides
proper dead time and all that. Now it's moving on to the dreaded
packaging design. As much fun as eating pea soup and I don't like pea
soup. Ok, with some Johnsonville Brats in there I'll eat it.

--
Regards, Joerg

http://www.analogconsultants.com/

"gmail" domain blocked because of excessive spam.
Use another domain or send PM.

 

[Joerg]

Mike Monett wrote:

Mike Monett &lt;None@here.adr&gt; wrote:

Joerg &lt;notthisjoergsch@removethispacbell.net&gt; wrote:

I tried the inductor and wasn't too enthused. With a small damper
resistor inparallel it was kind of ok but still cost efficiency.

But it's done, I just lashed up the usual discrete concoction
that provides proper dead time and all that. Now it's moving on
to the dreaded packaging design. As much fun as eating pea soup
and I don't like pea soup. Ok, with some Johnsonville Brats in
there I'll eat it.

Joerg, I'm surprised it affected the efficiency enough to make a
difference. Did you simulate it or try it on the bench? And did
you use the smallest inductor needed to limit the current? Did it
somehow drastically increase the turnoff times?

I did a small simulation in LTspice. The SPICE model for the
FDD8424H is available from Fairchild, but it is for PSPice. I didn't
want to take the time to make a model for LTspice, so I used IRF530
and IRF7204 for complimentary MOSFETs. I adjusted the rise and fall
time of the gate drive to give about 50ns cross-conduction.

I increased the series inductor between the MOSFETs to 3uH and used
a 47 ohm damping resistor.

The switching losses end up heating the damping resistor, so the
integral of the power dissipated gives the total loss. For this
simulation, the result is 52.114mW.

If the power delivered to the load is 3 watts, using a series
inductor to minimize shoot-through adds 52.114e-3 / 3 = 1.73% to the
total power dissipation. This is a rather small amount for the
simplicity and reliability gained.

Any circuit changes to reduce the power loss in switching will
probably cost additional power, so the overall gain might be small
or negative.

That's just the thing. Whatever you do, in the end you just move the
dissipation from one part to another.

The conclusion is a small series inductor can be a viable option to
minimize shoot-through and reduce circuit complexity.

Yep, so I just did the usual, a concoction of Schmitts, resistors and
diodes. No more cross conduction

The LTSPICE ASC file is below, followed by the PLT file. The .tran
analysis string is set to 1uS to show the switching waveforms.
Increase it to 1ms to calculate the power in R2.

Thanks, Mike, but I am done with that part of the design now. I was just
hoping there was a push-pull driver with much less than 1ohm so I could
drive the transformer directly. Such drivers are usually
process-controlled so their internal cross conduction is minimized.
Something you can't do with tolerance-prone external parts where things
such as Vth stray a lot.

--
Regards, Joerg

http://www.analogconsultants.com/

"gmail" domain blocked because of excessive spam.
Use another domain or send PM.