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-   -   Looking for HV fets for direct drive (http://www.diyaudio.com/forums/planars-exotics/221050-looking-hv-fets-direct-drive.html)

madtom1999 6th October 2012 04:24 PM

Looking for HV fets for direct drive
 
I'm looking to make some ELS units with direct drive and wondering if anyone has seen any reasonably priced transistors suitable for the job.
Tom

Bazukaz 6th October 2012 06:40 PM

Quote:

Originally Posted by madtom1999 (Post 3191926)
I'm looking to make some ELS units with direct drive and wondering if anyone has seen any reasonably priced transistors suitable for the job.
Tom

I have used IRFBG30/IRFBG20 in the past. The amp was class A bridged; max. output voltage about 600V rms. Suitable for panels having high efficiency only or headphones.
There are many BJT transistors with higher voltage rating but their SOA is usually worse at the same supply voltage. What topology amp you are planning to build?

madtom1999 6th October 2012 09:26 PM

Whatever works!
 
When I find some more I'll just run a lot of spice sims and see what I can get to work best - I used to be a chip designer and got quite good with it many years ago and it should help get me into a nice stable area if there is one.

madtom1999 6th October 2012 09:40 PM

Had a thought
 
Just had a crazy thought - I thought I would ideally need more volts (than 600rms) to get it working well and then thought perhaps I could bridge two amps and then thought, mmmm, I could bridge the speakers!
Could work pretty well up to 2 or 3khz ....
Have you a circuit I could borrow?

Bazukaz 7th October 2012 12:01 AM

1 Attachment(s)
Quote:

Originally Posted by madtom1999 (Post 3192220)
Just had a crazy thought - I thought I would ideally need more volts (than 600rms) to get it working well and then thought perhaps I could bridge two amps and then thought, mmmm, I could bridge the speakers!
Could work pretty well up to 2 or 3khz ....
Have you a circuit I could borrow?

Hi,
Looks like i have lost the complete schematic. Here is the screenshot of one half of the bridge. 600Vrms is already for a bridged amp. It has some severe shortcomings however:
1) low output voltage for most ESLs(except headphones & very sensitive units)
2) high power dissipation(constantly 40w for one bridged module) - active cooling needed
3) possibly lower safety because output voltage is not isolated from the ground.

So I have abandoned the idea some time after.
BTW why not use mains toroids ? Some of them give really excellent results and are readily available. I use 6x 230/8V units for a total stepup ratio of ~1:115.

jcx 7th October 2012 12:28 AM

ESL loudspeakers can require middling kV

IXYS offers MOSFET up to 4 kV - still would need to cascade for some ESL

Class A is hugely power wasting for driving pure C load to 20 kHz when most music has <5 kHz power bandwidth

and even for ES headphones several of the "audiophile" direct drive amps do draw over 50 W continuous power

stratus46 7th October 2012 12:49 AM

Quote:

Originally Posted by madtom1999 (Post 3191926)
I'm looking to make some ELS units with direct drive and wondering if anyone has seen any reasonably priced transistors suitable for the job.
Tom

You might want a look at this idea from EDN. Each amp can hit 1800 Vp-p and in push pull as needed for ESL, 3600 V p-p !! The question is can it deliver enough current to drive that big capacitor.

High-voltage amplifier uses simplified circuit | EDN

G

kevinkr 7th October 2012 01:03 AM

Was looking at that design and my impression is that there are a lot of errors in the schematic, fix the obvious errors and it appears to me given the typical gate capacitance of power mosfets the power bandwidth is quite limited, perhaps even under 1kHz.

geraldfryjr 7th October 2012 02:11 AM

I have many models that I have done in Circuitmaker using the IRFBG30's and similar FET's.
All class A as well, Bridged of course.

I have found that you need about 5Kv to 10Kv peak voltage to make an ESL really sing depending on the size of the panel.
As the bigger the panel (more surface area) the less voltage will be required to reach a certain amount of SPL's.

At the time I started on such designs the IRFBGxx series was the highest voltage that could be found cheaply.
So, in order to get a higher voltage swing I had experimented with stacking (cascoding) the FET's.
This worked very good only I never finished one for such a high range of voltages yet.

I proved my circuit for 200v using 3 or 4 IRF510's about 4 years ago.
Some of my design are similar to Bazukaz circuit only minus the gate driver transistor and they can be found here,

http://www.diyaudio.com/forums/headp...ml#post2596223

This was also before I learned about how to use a current source in place of the resistor in the top half.
This is something that I now understand and hopefully will be continuing such a project soon and I would be very interesting in seeing your results as well.

All I need is some more FET's.

There are many more higher voltage FET's to choose from now, But you want one that has the lowest gate capacitance you can get and these types will generally cost much more although I have found a few that aren't to bad cost wise.

The lower the gate capacitance the less power that will be required to drive them as by the time you get up to 20Khz and higher to faithfully reproduce a square wave the gate current gets quite high in order to have an acceptable slewrate.

Also to fully drive a 1' x 4' panel at these levels it will require on the order of about 900 of power.
Being that an ESL panel has a 6db per octave rise in response after equalization this won't be much of a problem, Just something to consider wile designing such an amplifier.

I have found that considering the current requirements and Pd of some FET's, against cost, That stacking the lower voltage ones may be a viable alternative.
Last year I found some nice 500v to 650v Fet's with a fairly high Pd for like .25$ to .89$ a piece.
I was going to buy up all of the ones for .25$
They were some discontinued parts and I never jump on it, But they may still be available.

Doing it this way will also be a larger construction technique, But having it being spread out will also make it easier to work on and help keep the HV isolated from possible flash over issues between the stages causing a catastrophic failure of the whole thing as each FET stage will need to be adjusted in order to have an equal voltage across each stage.

Then there is the cooling issue and I have thought about putting the whole thing in an oil bath as well.
I have put a lot of thought into these and I still like the idea of a large solid state direct drive amplifier.

I have a design somewhere in my archives that I was going to try using some IRFBG30's stacked up powered from a MOT that I have been saving for some time.
It should produce about 6KV p-p on a 3100v supply.
Even some thing like this may be ample for a larger panel.
But at least twice this will be required for the little panels that I have been experimenting with to get over 105db.

Then there is the issue of the filtering the supply!
A switching supply may be more suitable and cheaper as they are available and used in today's modern microwave ovens.
I found a very nice one on the web that a HAM had done up as it was quite impressive!
But is was only at 1200 volts output.

I had finished my Variable HV supply about a year ago and it is precision regulated and has enough current to pull a constant arc at 5KV.
I have been bitten by this supply as it does a peak voltage of 13.8kv and it is no joke!!

Just a few things to consider.

Cheers !!!

jer :)

geraldfryjr 7th October 2012 02:14 AM

That schematic from EDN has lots of error's and there was never a follow up or any corrections for it.

jer :)


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