biasing mosfet for large signal amplification

[jimmyinct]

hello

Im trying to build an ultrasound transducer driver.
The load is a RLC circuit that resonates around 1.5mhtz, this is 20ohms in series wtih 1.8uh and then 9.9nf. The C is the transducer.


The load in either in the common source or drain configuration of the power mosfet in not amplfying the signal.
the mosfet is

http://www.irf.com/product-info/datasheets/data/irfpg50.pdf

In the common drain configuration, the voltage gain is roughly proportinonal to the input signal and VGS is approximatly equal, which should be correct. With a resistor from gate to drain (vcc), a DC offset is given to the load rather than amplify the signal

I need the signal to be amplifed to around 100-500volts peak to peak. So AC voltage gain is needed. In the common source configuation, I am getting a large DC offset on top of the signal rather than having the signal amplified. The DC offset is roughly proportial to VCC wtih the peak to peak roughly equal to the input signal.

What kind of configuation should i be using.
what abuot modeling an audio amplifer but to run at 1.5mhtz?
any other ideas?

 

[jimmyinct]

and there is a vcc, no -vcc
vcc is 300volts

 

[IRstuff]

??

Off/0V
On/500V

TTFN

FAQ731-376

 

[jimmyinct]

yea, thats the idea.
then swictching at 1.5hmtz
Im sure there will be lots of frequency dependent losses.

i think i need to use a push pull configuration.

 

[itsmoked]

Or a full bridge to run off of one source?
Will it take a digital supply square wave instead of a sine wave?

Or develop two 500V supplies from your 300V one and then use a class A or class B push-pull.

I'd be really surprised if the transducer manufacturer didn't have an app note with a recommended drive circuit.

Keith Cress
kcress -

http://www.flaminsystems.com

 

[jimmyinct]

nope, there was no app notes.
its from

http://www.eblproducts.com/transducer.html

These are more specality transducers
For this applicatition, we are driving them at much more power than the average application would use.

It seems very similar to a audio amplifier, just a higher frequency responce and wtih a little added resonation at the center frequency.

Apparently only n type mosfets go to 1000v, the p type only go to -400.. These are the high voltage mosfets orignally made by irf and sold to vishay. Ill figure something out i guess. thanks for the help.

 

[ScottyUK]

Anything wrong with a standard half bridge with one end of the load tied to the mid-point of a series pair of capacitors across Vcc and the other to the switching devices? You could use standard N type switches if you use one of IR's 2xxx series driver ICs. The load sounds akin to some induction heater loads although in this case the C is the part you're interested in instead of the R and L. Without knowing much about ultrasonic transducers, you might consider a parallel tank circuit instead of series resonant. Or maybe not!


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If we learn from our mistakes I'm getting a great education!

 

[jimmyinct]

wow
i didnt know there were driver chips for switching mos in push pull configuration.
that saves a lot of trouble, thanks man!!

the even have high volt ones too.

https://ec.irf.com/v6/en/US/adirect/ir?cmd=catProductDetailFrame&productID=IR2213

Ill give the tank- paraell circuit a try too.
I know series works for now..

 

[ScottyUK]

After a night's sleep I think the tank circuit is a non-starter: the transducer is a voltage-driven device and the series resonant circuit causes voltage magnification near resonance; the tank circuit causes current magnification near resonance, which probably won't be any good for transducer drive.

Pleased you like the IR driver chips - they take away a lot of headaches.


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If we learn from our mistakes I'm getting a great education!

 

[jimmyinct]

i did call up irf today.
the those 2xxx series would be perfect except that they dont really run over 1mhtz.
I need to have a 1.5htz driver.

interstil has 40 mhtz drivers, but they dont really have the output or voltage. pin drivers i believe

 

[ScottyUK]

I'm curious: why do you need such a high frequency? Will the ultrasonic transducers even respond at 1.5MHz? The couple of ultrasonic drivers I've come across in the past operated in the 40 - 60kHz range; those were for big industrial component cleaners. I'm really surprised that 1.5MHz produces any usable mechanical effect in the fluid.


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If we learn from our mistakes I'm getting a great education!

 

[IRstuff]

The OP used "ultrasound":

http://en.wikipedia.org/wiki/Medical_ultrasonography

TTFN

FAQ731-376

 

[ScottyUK]

Thanks IRstuff, I should have remembered those: I spent a few appointments watching our unborn little 'un on one of those. I do remember seeing the high frequency shown on the screen now you've jogged my memory.


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If we learn from our mistakes I'm getting a great education!

 

[Warpspeed]

This certainly stirs up memories from long ago.

First thing to do is plot the transducer impedance with frequency. It will have both a series resonance, and a parallel resonance, fairly close together in frequency. Phase change around these close frequencies will be very dramatic.

It is always easier to feed significant power into the transducer at series resonance, where the output load current is highest.

The trick to driving these transducers to very high power is to make the transducer itself the frequency determining part of a power oscillator. If this is not done, transducer energy and efficiency will fall away very rapidly as the drive frequency moves slightly outside the exact high Q series resonant frequency.

I suggest that you use a current transformer in series with the transducer to provide the feedback voltage for the oscillator. With sufficient power gain, and correct phasing, it will oscillate away quite happily and produce some serious ultrasonic power.

Loading on the transducer will change the oscillation frequency, but it will always run at it's own most efficient self resonant frequency.

Think of this in concept as a very high power crystal (or tuning fork) oscillator with current feedback.

The loaded Q of the transducer at resonance, will still be high enough that driving it with a square wave will work fine.

 

[jimmyinct]

thanks

yup, i think i got the transducer loading down. I think.
Its just a series RLC and i calculated the L to make the trsanducer (C) resonsate.

 

[Warpspeed]

The transducer resonance is actually a physical mechanical resonance of the transducer material.

A crude analogy here might be the speaker cone resonance of a loudspeaker. You dont need to tune the voice coil inductance to 35 Hz with a capacitor to see this low frequency resonant effect.

Likewise, a quartz or ceramic ultrasonic transducer may be cut to mechanically resonate around 1.5 Mhz, but that has very little to do with directly measured electrical capacitance or inductance. The effect of rapidly changing impedance and phase change with frequency, are purely mechanical in nature, reflected back into the electrical circuit.

Do not attempt to tune the transducer by adding external reactance. Just drive it with a plain square wave and pick off the sine wave drive current. At series resonance, current will be maximum and in phase with the drive voltage.

The oscillator will naturally seek out and rapidly increase amplitude at the point of maximum in phase feedback.

 

[Warpspeed]

One further thought. The oscillator needs to be a linear amplifier that will effectively amplify small amplitude signals with high gain.
Any noise "pings" the resonant transducer, and it rings at its self resonant frequency. Amplitude rapidly builds up, and away it goes. The over driven amplifier will quickly saturate into a square wave output.

Using MOSFETS in the power stage will be fine, but they will need to be biased in the linear region somehow, for effective start up.

Do not use schmidt triggers, flip flops, or anything strictly digital. It needs to be able to directly amplify small amplitude noise signals, or it will not easily be able to start up.