Bipolar transistors for SMPS?

[Joerg]

Hello All,

What is your favorite (cheap) bipolar transistor for switcher
applications? I mean something that can do 100V or so, tolerate an amp
or more and turn off in much less than 100nsec.

Reason I ask is that lots of low voltage gear cannot drive the gate of a
FET high enough to guarantee a resonable conduction. There are some
very-low Vgs versions but usually they can't do 100V or so. Same for
bipolar, lots of gold-doped RF gems but they cannot stomach higher
voltages either.

Regards, Joerg

http://www.analogconsultants.com

 

[Joerg]

Hi John,

Thanks. Seems they make some good stuff in the UK. But these transistors
have pretty long storage times. That's ok in slow converters but when
you want to keep the magnetics small it can become tight.

Regards, Joerg

http://www.analogconsultants.com

 

[John Popelish]

Joerg wrote:

Hi John,

Thanks. Seems they make some good stuff in the UK. But these transistors
have pretty long storage times. That's ok in slow converters but when
you want to keep the magnetics small it can become tight.

Regards, Joerg

http://www.analogconsultants.com

The specified storage times apply to the specified conditions. Pull
charge out of the base faster and it goes quicker.

--
John Popelish

 

[Tim Wescott]

Joerg wrote:

Hello All,

What is your favorite (cheap) bipolar transistor for switcher
applications? I mean something that can do 100V or so, tolerate an amp
or more and turn off in much less than 100nsec.

Reason I ask is that lots of low voltage gear cannot drive the gate of a
FET high enough to guarantee a resonable conduction. There are some
very-low Vgs versions but usually they can't do 100V or so. Same for
bipolar, lots of gold-doped RF gems but they cannot stomach higher
voltages either.

Regards, Joerg

http://www.analogconsultants.com

I don't, but Zetex has some nice app notes on properly driving bipolars
for fast turn-off, mostly involving actively pulling the base below the
emitter to suck those darn carriers _out_ of the base rather than just
letting them die natural deaths due to recombination.

--

Tim Wescott
Wescott Design Services
http://www.wescottdesign.com

 

[Ken Smith]

In article <jKAHd.12569$wZ2.6374@newssvr13.news.prodigy.com>,
Joerg <notthisjoergsch@removethispacbell.net> wrote:
[...]

The only option would really be the good old baker clamp while giving up
some efficiency for not reaching the famously low Vcesat.

A few suggestions:

Disable the Baker-clamp for the first 75% of the on time. If you are
making a simple squarewave converter you can do that.

Add an anti-cross-conduction circuit to live with the storage time.

Make a small DC-DC converter to make enough voltage to drive MOSFET gates.
--
--
kensmith@rahul.net forging knowledge

 

[legg]

On Wed, 19 Jan 2005 21:58:39 GMT, Joerg
<notthisjoergsch@removethispacbell.net> wrote:

Hello John,

The specified storage times apply to the specified conditions. Pull
charge out of the base faster and it goes quicker.



Sometimes that ain't so easy. Often all you have is a 3.3V supply, or
down to 2.5V in two-cell operation. No negative swings that could help
in wiping the charge, except for some capacitor tricks. Then you have to
drive as hard as the min hfe spec says, plus some margin.

The only option would really be the good old baker clamp while giving up
some efficiency for not reaching the famously low Vcesat.

Or a cascode circuit.

RL

 

[John Larkin]

On Wed, 19 Jan 2005 20:21:08 GMT, Joerg
<notthisjoergsch@removethispacbell.net> wrote:

Hello All,

What is your favorite (cheap) bipolar transistor for switcher
applications? I mean something that can do 100V or so, tolerate an amp
or more and turn off in much less than 100nsec.

Reason I ask is that lots of low voltage gear cannot drive the gate of a
FET high enough to guarantee a resonable conduction. There are some
very-low Vgs versions but usually they can't do 100V or so. Same for
bipolar, lots of gold-doped RF gems but they cannot stomach higher
voltages either.

Regards, Joerg

http://www.analogconsultants.com

Considered cascodes? Cheap hv fet on top of a sot-23 low threshold
fet, or something like that.

John

 

[Joerg]

Hi Ken,

A few suggestions:

Disable the Baker-clamp for the first 75% of the on time. If you are
making a simple squarewave converter you can do that.

That is a good suggestion, it could eke out a few more percent in
efficiency especially when running on fairly low voltages. It would add
another little transistor though but that one can be a 'penny device'.

75% would be a bit much at todays frequencies. Most of my switchers run
at several hundred kHz or even above a MHz. Then when the transistor
storage time is >500nsec I've got a wee problem.

Add an anti-cross-conduction circuit to live with the storage time.

It's usually single transistor circuitry, no cross conduction. The
concern is that the situation becomes odd when the storage time
represents a good part of the desired duty cycle, or worst case could
exceed it. Light load is another matter. That would require to enter
into a pulse skipping mode and then the whole switcher begins to sound
like a wood splitter hitting a knot.

Make a small DC-DC converter to make enough voltage to drive MOSFET gates.

That's what Winfield Hill had suggested in another thread about really
low voltage single cell applications. It does make things complex though
because most of the time the driving device itself isn't rated for use
above 3.3V. That would require a level translator in addition to the
helper-SMPS.

Regards, Joerg

http://www.analogconsultants.com

 

[Ken Smith]

In article <rWSHd.3834$8Z1.3670@newssvr14.news.prodigy.com>,
Joerg <notthisjoergsch@removethispacbell.net> wrote:
[...]

Unless, you run the main switcher chip on the helper supply.



Well, yes if you use an off-the-shelf PWM chip. But that is often too
costly and then you are stuck with either logic chips or a micro
controller which won't be happy above 3.5V or so.

There's a job for JT. Do a micro-controller in Supertex's high voltage
CMOS process. Just think a PIC that does 60V swings!

--
--
kensmith@rahul.net forging knowledge

 

[Jim Thompson]

On Thu, 20 Jan 2005 20:27:23 +0000 (UTC), kensmith@green.rahul.net
(Ken Smith) wrote:

In article <rWSHd.3834$8Z1.3670@newssvr14.news.prodigy.com>,
Joerg <notthisjoergsch@removethispacbell.net> wrote:
[...]
Unless, you run the main switcher chip on the helper supply.



Well, yes if you use an off-the-shelf PWM chip. But that is often too
costly and then you are stuck with either logic chips or a micro
controller which won't be happy above 3.5V or so.

There's a job for JT. Do a micro-controller in Supertex's high voltage
CMOS process. Just think a PIC that does 60V swings!

--

Dreamer! Don't you realize what happens to device dimensions when
voltage goes up?

I haven't been following this thread... really busy doing real work
;-)

What was the original problem?

...Jim Thompson
--
| James E.Thompson, P.E. | mens |
| Analog Innovations, Inc. | et |
| Analog/Mixed-Signal ASIC's and Discrete Systems | manus |
| Phoenix, Arizona Voice480)460-2350 | |
| E-mail Address at Website Fax480)460-2142 | Brass Rat |
| http://www.analog-innovations.com | 1962 |

I love to cook with wine. Sometimes I even put it in the food.

 

[Joerg]

Hi Nicholas,

Lets get this straight:

Vce max = 100 volts
base/gate drive of 2V
Ic > 1A
Toff ~ 35nS
~$0.02

And I imagine (why not):

hfe > 200min
Vce sat < .2V @ 1A

That would be nice to have and 2 cents sounds even better.

This sounds like two devices. Or is this a design for
a 2-260V input power supply?

It's not an actual design, just a rough patch I keep running into at
times. Others do, too, for example folks who want to control big stuff
with new micro controllers that were migrated to a new process and can't
be operated at 5V anymore.

In my case it is sometimes the generation of variable voltages, say,
3-50V or 3-80V kind of like a function generator. All from low battery
voltages, ideally just a couple of cells. Well, ideally one cell but
that's mostly not in the cards because logic chips become expensive in
that range.

At what Vce do you need the 1 amp Ic?

Near or at saturation. In continuous current mode it could be a few
volts but that can be avoided.

At what Ic do you need the 100 volt Vce?

Close to zilch, when turned off.

Regards, Joerg

http://www.analogconsultants.com

 

[John Larkin]

On Thu, 20 Jan 2005 02:03:24 GMT, Joerg
<notthisjoergsch@removethispacbell.net> wrote:

Hi John,

What is your favorite (cheap) bipolar transistor for switcher
applications? I mean something that can do 100V or so, tolerate an amp
or more and turn off in much less than 100nsec.

Reason I ask is that lots of low voltage gear cannot drive the gate of a
FET high enough to guarantee a resonable conduction. There are some
very-low Vgs versions but usually they can't do 100V or so. Same for
bipolar, lots of gold-doped RF gems but they cannot stomach higher
voltages either.


Considered cascodes? Cheap hv fet on top of a sot-23 low threshold
fet, or something like that.



Sorry, should have mentioned that it's not just the available devices
that are limited in voltage but usually there isn't much there in supply
voltages besides 3V or so.

Otherwise a cascode or something with active level translation would
work. Without a decent VCC all there is left would be a step-up
transformer or a dedicated lil' helper switcher to create a higher rail
but that usually blows the cost budget out of the water.

Regards, Joerg

http://www.analogconsultants.com

In that case, you need a fast, high-voltage, hi-beta bipolar
transistor, in my professional opinion.

Hey, how about a darlington? The big guy doesn't saturate so turns off
fast. First switcher I ever did was a 24-to-5 buck, with a 2N2905
driving a 2N3055 at 20 KHz, pseudo-PNP connection. The 3055 didn't
saturate so turned off quick. Error amp was an uncompensated 709, I
think.

John

 

[Joerg]

Hi Ken,

Dreamer! Don't you realize what happens to device dimensions when
voltage goes up?



I assume volts per foot remains constant in all directions unless you
raise the resistivity of the silicon too, but why grumble. A micro that
would (at least) run on a 9V battery would be a nifty thing to have. I'd
bet if someone brought one out they'd make litterally dozens of dollars on
it.

It doesn't have to be big. Once we designed a chip that was running low
voltage for RF amps, muxes and so on but it had a high voltage
extension. Actually the process claimed 60V just as you had mentioned
but in the end we had to keep things below 50V. It was about 2mm by
10mm, not bad considering that it could handle 64 input and 16 output
channels. Electro-migration is another issue but this was for a
disposable product.

What was the original problem?



The poor fellow has a switcher based on some micro (PIC like I think) that
is now running on 3V instead of 5V. He needs to redesign the output
section to run with a 3V input.

More likely it would be an MSP430, a family that migrates from 5V to
3.6V and less. But the same is true for a lot of logic. Also, there is a
desire to run equipment with the least number of cells, typically two.

Regards, Joerg

http://www.analogconsultants.com