pnp transistor for level shift circuit.

D

David Grant

Guest
I need a pnp transistor for a basic level shift circuit that brings a 0-5V
swing 400kHz PWM signal down 50V. (supply voltages are +5V, -50V). Shifted
signal is going into a IR2011 gate driver.

The problem I have is that most transistors that have decent enough
switching times (we're aiming for < 40ns rise time and fall time and
something with similar turn on and turn off delays so as to not distort the
signal too much) aren't rated for the Vce or Vbe that we need (most are only
15V). Lots of spec sheets don't even quote the timing specs, or the input
capacitance (which I understand is what determines the switching times).
I've spent several hours looking through spec sheets, and haven't even come
up with something close.

I'm at a loss, does anyone know where to look, or another way to approach
this? I'm pretty inexperienced with searching for parts based on specs
needed. Let me know if more info will help.

Thanks,

Dave
 
"David Grant" <NO_SPAM_PLEASE_jmd_2003@msn.com> wrote in
message news:LBNSd.7336$RM2.2009@read1.cgocable.net...
I need a pnp transistor for a basic level shift circuit that brings a 0-5V
swing 400kHz PWM signal down 50V. (supply voltages are +5V, -50V). Shifted
signal is going into a IR2011 gate driver.

The problem I have is that most transistors that have decent enough
switching times (we're aiming for < 40ns rise time and fall time and
something with similar turn on and turn off delays so as to not distort the
signal too much) aren't rated for the Vce or Vbe that we need (most are only
15V). Lots of spec sheets don't even quote the timing specs, or the input
capacitance (which I understand is what determines the switching times).
I've spent several hours looking through spec sheets, and haven't even come
up with something close.
Click to expand...
A single PNP level shifter is unlikely to yield the
switching speed you need, IMHO. The problem
is that either the load resistance will be too large
to switch fast, or the dissipation will be enough
that a big and slow transistor will be necessary.

I'm at a loss, does anyone know where to look, or another way to approach
this? I'm pretty inexperienced with searching for parts based on specs
needed. Let me know if more info will help.
Click to expand...
I would look into AC coupling the pulse with
some kind of DC restore circuit appropriate
for the expected range of PWM values. You
don't need to rapidly change the -50V level,
do you?

--
--Larry Brasfield
email: donotspam_larry_brasfield@hotmail.com
Above views may belong only to me.
 
Larry Brasfield wrote:

"David Grant" <NO_SPAM_PLEASE_jmd_2003@msn.com> wrote in
message news:LBNSd.7336$RM2.2009@read1.cgocable.net...

I need a pnp transistor for a basic level shift circuit that brings a 0-5V
swing 400kHz PWM signal down 50V. (supply voltages are +5V, -50V). Shifted
signal is going into a IR2011 gate driver.

The problem I have is that most transistors that have decent enough
switching times (we're aiming for < 40ns rise time and fall time and
something with similar turn on and turn off delays so as to not distort the
signal too much) aren't rated for the Vce or Vbe that we need (most are only
15V). Lots of spec sheets don't even quote the timing specs, or the input
capacitance (which I understand is what determines the switching times).
I've spent several hours looking through spec sheets, and haven't even come
up with something close.


A single PNP level shifter is unlikely to yield the
switching speed you need, IMHO. The problem
is that either the load resistance will be too large
to switch fast, or the dissipation will be enough
that a big and slow transistor will be necessary.


I'm at a loss, does anyone know where to look, or another way to approach
this? I'm pretty inexperienced with searching for parts based on specs
needed. Let me know if more info will help.


I would look into AC coupling the pulse with
some kind of DC restore circuit appropriate
for the expected range of PWM values. You
don't need to rapidly change the -50V level,
do you?

Or couple it with a transformer if the 50V level is expected to flop around.
Click to expand...

--

Tim Wescott
Wescott Design Services
http://www.wescottdesign.com
 
Have a look at the 2N2905A or the 2N2907A

http://www.microsemi.com/datasheets/2N2904.PDF

http://www.fairchildsemi.com/ds/PN%2FPN2907A.pdf

They seem to be in the ball-park. The turn-off times from saturation
are a bit slow, so make sure that they don't saturate.

------------
Bill Sloman, Nijmegen
 
I just had an idea... The PWM is generated with a comparator at the input to
the level shifter at present.. Does it make more sense to generate the PWM
using a comparator that's biased at -50V, and instead level shift the audio
signal input to the comparator, hence generating (essentially) the same PWM
signal, but at -50V?

Thanks,

Dave
 
"David Grant" <NO_SPAM_PLEASE_jmd_2003@msn.com> wrote in
message news:tOSSd.7461$RM2.1999@read1.cgocable.net...
"Larry Brasfield" <donotspam_larry_brasfield@hotmail.com> wrote in message
news:Z9OSd.41$z01.1198@news.uswest.net...
"David Grant" <NO_SPAM_PLEASE_jmd_2003@msn.com> wrote in
message news:LBNSd.7336$RM2.2009@read1.cgocable.net...
I need a pnp transistor for a basic level shift circuit that brings a 0-5V
swing 400kHz PWM signal down 50V. (supply voltages are +5V, -50V).
Shifted signal is going into a IR2011 gate driver.

The problem I have is that most transistors that have decent enough
switching times (we're aiming for < 40ns rise time and fall time and
something with similar turn on and turn off delays so as to not distort
the signal too much) aren't rated for the Vce or Vbe that we need
(most are only 15V). Lots of spec sheets don't even quote the timing
specs, or the input capacitance (which I understand is what determines
the switching times). I've spent several hours looking through spec
sheets, and haven't even come up with something close.

A single PNP level shifter is unlikely to yield the
switching speed you need, IMHO. The problem
is that either the load resistance will be too large
to switch fast, or the dissipation will be enough
that a big and slow transistor will be necessary.

The reference design for the Class D power audio amp at
http://www.irf.com/technical-info/refdesigns/iraudamp1.pdf (page 14) uses an
MMBT5401 (http://www.fairchildsemi.com/ds/MM/MMBT5401.pdf) to accomplish
this very thing. Two problems: the datasheet doesn't specify the switching
characteristics (it gives an imput capacitance vs. reverse bias voltage
graph - but i'm not sure how/if timing can be derived from this), plus I
need a device with leads, not surface mount.

Any clarifactions/suggestions appreciated as usual....
Click to expand...
That transistor has 6 pF Cob at 10V. This may go down to
3 pF at your 50V bias. Add that to, say, 5 pF of input C
on the gate driver, and your collector circuit already has a
54 nS time constant. Not very promising to getting to your
40 nS, semi-symmetric rise and fall time goal. The drive
circuit from that schematic will suffer from Miller effect for
the falling output transition, add more dissymmetry.

Note that I did not claim a simple PNP level shifter could
not be built. I claimed it would not deliver the switching
performance you expressed a desire for.

I still believe that an AC coupled PWM logic signal with
a simple DC restore diode would be simpler and give you
the performance you hoped for.

Another approach would be to level shift your low
frequency error signal down to the -50V rail. You can
arrange that its offset error is inside the feedback loop
so that it contributes no error in a class 1 or higher servo.
Then convert it to PWM at the rail where you need that.

--
--Larry Brasfield
email: donotspam_larry_brasfield@hotmail.com
Above views may belong only to me.
 
I just had an idea... The PWM is generated with a comparator at the input to
the level shifter at present.. Does it make more sense to generate the PWM
using a comparator that's biased at -50V, and instead level shift the audio
signal input to the comparator, hence generating (essentially) the same PWM
signal, but at -50V?
Click to expand...
It's not a PWM like you think- it is a self-oscillating sigma-delta
modulator- the level shifter is driven by the integrator, the comparator
is already referenced to the -50V supply. Use the circuit as shown or it
will not work. And when you pose a misleading question to NG, all you
end up with are irrelevant replies.
 
"David Grant" <NO_SPAM_PLEASE_jmd_2003@msn.com> wrote in
message news:bNUSd.7469$RM2.1663@read1.cgocable.net...
"Larry Brasfield" <donotspam_larry_brasfield@hotmail.com> wrote in message
news:3MTSd.74$z01.2782@news.uswest.net...
....
I still believe that an AC coupled PWM logic signal with
a simple DC restore diode would be simpler and give you
the performance you hoped for.

Not sure what you mean by this (sorry - fresh out of university still)

Are you referring to something like:


in --C--+-- out
|
R
|
-50Vwith a diode in somewhere?Dave
Click to expand...
Yes. You would have to ensure that the transient effect
of bringing up the -50V supply did not turn your FETs
on for too long. The diode basically provides a current
path such that the voltage across the output resistor can
have a DC average value. In this case, the anode would
be connected to the -50V rail.

--
--Larry Brasfield
email: donotspam_larry_brasfield@hotmail.com
Above views may belong only to me.
 
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