pnp transistor for level shift circuit.

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

--

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

Have a look at the 2N2905A or the 2N2907A

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 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

A 10-15Mbps optoisolator might be just the ticket. Guaranteed

30-40nsec response times (if you can get ahold of any). eg NEC PS9121

Best regards, Spehro Pefhany

--
"it's the network..."                          "The Journey is the reward"
speff@interlog.com             Info for manufacturers: http://www.trexon.com
Embedded software/hardware/analog  Info for designers:  http://www.speff.com

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" wrote in message news:tOSSd.7461$ snipped-for-privacy@read1.cgocable.net...

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.

0-5V

Shifted

the

only

input

come

The reference design for the Class D power audio amp at

(page 14) uses an MMBT5401 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....

Dave

message

distort

are

times).

even

an

switching

That'd do it, thanks!

Dave

message

0-5V

distort

determines

uses an

switching

Thanks for pointing this out.

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

"David Grant" wrote in message news:bNUSd.7469$ snipped-for-privacy@read1.cgocable.net...

....

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.

So, the obvious answer is to use a 2n5401 TO92 device

How about using a P channel MOSFET? They have fast and near equal switching times. Take a look at Supertex.

You could also move the entire PWM system down to the -50V rail by making a -50+5V to run the circuit on and level shifting the input.

--
--
kensmith@rahul.net   forging knowledge

input to

PWM

audio

PWM

Thanks for the tip. I realize the IR reference circuit is self-oscillating... I'm modifying their design to use a comparator instead. When I said "at present" I meant currently in *my* design. Sorry for the confusion.

Simple.

Don't use the IR2011. That's for one gnd referenced output plus a high side drive. As in SMPSs.

HP ( or whatever they call themselves these days ) make a lovely opto-isolated super fast gate driver. 2A peak gate current IIRC.

Sorry don't have the device number to hand but sounds the ticket to me.

Graham

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.

You're not going to "modify" it without mucking it up- you have already run into the requirement for a fast level shift which the IRF design does not require. The IRF design looks streamlined to the minimum- you will not improve it.

It should do... after all, many smt devices are based on their forebear leaded devices and have the same generic number identifier. ie. MMBT5401 = 2N5401. There will usually be only slight differences in characteristics and in many cases the leaded device will have better specs.