Bob Cordell Interview: BJT vs. MOSFET

[Edmond Stuart]

Re: Re: The Real World

Kanwar,
.......................
It is difficult to speculate on what is wrong with your test setup, as you have not provided an exact schematic. My current suspicion is parasitic oscillations, but who knows? What value of gate series resistance were you using in your test setup (you were using one, weren't you?)?
Bob

Bob, I admire your patience!

Cheers,

 

[kASD]

The Real World Vs Real World

Kanwar,

I repeated your experiment and did not get the results you showed. I got results that conform with the data sheets published by IR, Fairchild, and others. I therefore think that you are wrong, and that there is something dreadfully wrong with your test setup. What it is, I don't know. Maybe parasitic oscillations that you are unaware of.

I took an IRFP240 and biased it at Vgs = 3.5, 3.75 and 4.0 V, and measured drain current with a 1-ohm resistor in the drain as a function of applied Vds. The source was directly grounded. The gate series resistor was 100 ohms, and there was a series R-C gate shunt to ground of 100 ohms and 150 pF. The whole arrangement was bypassed carefully. All probes to gate or drain were isolated with 100-ohm resistors. The HEXFET was mounted to a heat sink. All measurements shown here were done on a continuous basis (not pulsed), as the currents were not so high as to raise thermal concerns. To the extent that there are any thermal effects here, they will tend to slightly increase the drain currents at higher Vds.

As a base line, drain current at Vds = 10V were as follows:
Vgs = 3.50V => 4.4 mA
Vgs = 3.75V => 28.5 mA
Vgs = 4.00V => 127 mA


For Vgs = 3.75 V, Id as a function of Vds was as follows:
10V => 28.5 mA
15V => 29.3 mA
20V => 29.7 mA
25V => 29.9 mA
30V => 30.4 mA
35V => 31.2 mA
40V => 31.6 mA
45V => 32.0 mA
50V => 32.4 mA

For Vgs = 4.00 V:
10V => 127 mA
15V => 131 mA
20V => 136 mA
25V => 140 mA
30 V => 145 mA

You can see that the id dependence on Vds is very mild, as one would expect from a MOSFET. It is also in pretty good agreement with the data sheets. There is no evidence of the effect you describe and measure.

We all have at some time or another made measurements and results that seem out of whack with what we have learned or what is published in device data sheets. When this happens to me, I rigorously question my setup, measurement procedure and assumptions. I will often try to observe the effect with a different approach or setup. I will always suspect something wrong, even if I don't see it explicitly. I think you need to do the same before you insistently send others off on a wild goose chase.

It is difficult to speculate on what is wrong with your test setup, as you have not provided an exact schematic. My current suspicion is parasitic oscillations, but who knows? What value of gate series resistance were you using in your test setup (you were using one, weren't you?)?

Bob

Bob,

The setup I used for the testing and measurement is very much the same like yours....
Source is not directly grounded but it has a series 0.1Ohm resistor, Gate resistor is 100Ohms and a 10uFD cap from gate[before gate resistor] to ground, input voltage to gate is held constant by 6VDC battery and VGS is set by potentiometer....A 10Ohm 1W resistor in series with Drain terminal and VDS is applied from variable voltage source with 10mS to 100mS variable trigger level...The device was also mounted on heatsink...
Thats my setup...I have double checked it for any type of parasitic oscillations...they were not there...

But you havenot checked the results with VDS higher than 50V, you could also test at 150V for a 200V device atleast...

I will try to test some more devices from different batch numbers and also from different manufacturers as well....IRF ,APT, Fairchild, IXYS etc...Lets see what is the next outcome....


Kanwar

 

[kASD]

Are you trying to seek attention by these remarks ?

What about me, with 10 Ohm Rgate and 500Mhz & 1GHz Tek, but i see no parasitic oscillations....

Well any chance of seeing your modern day miracle ??

I guess not. Too much hype/talk......

Kevin

Kevin first of all see this link: There is no gate resistor at the gate of the mosfets in this schematic....and thats not a miracle, its a unique simple approach...

http://www.qscaudio.com/support/library/schems/Current/PowerLight Series/PL6.0II.pdf

Kanwar

 

[Fanuc]

Kevin first of all see this link: There is no gate resistor at the gate of the mosfets in this schematic....and thats not a miracle, its a unique simple approach...

http://www.qscaudio.com/support/library/schems/Current/PowerLight Series/PL6.0II.pdf

Kanwar

Congragulations on the schematics. Looks like a well thought out design and implementation.

It misses out crucially on the ruling edifice of my viewpoint. It does _not show_ the PCB layout with respect to the mosfets. Or any attention to detail in this respect.

I ruefully admit, but you can not be taken seriously without disclosing trade secrets.....

Regards Kevin

 

[Bob Cordell]

Re: The Real World Vs Real World

Bob,

The setup I used for the testing and measurement is very much the same like yours....
Source is not directly grounded but it has a series 0.1Ohm resistor, Gate resistor is 100Ohms and a 10uFD cap from gate[before gate resistor] to ground, input voltage to gate is held constant by 6VDC battery and VGS is set by potentiometer....A 10Ohm 1W resistor in series with Drain terminal and VDS is applied from variable voltage source with 10mS to 100mS variable trigger level...The device was also mounted on heatsink...
Thats my setup...I have double checked it for any type of parasitic oscillations...they were not there...

But you havenot checked the results with VDS higher than 50V, you could also test at 150V for a 200V device atleast...

I will try to test some more devices from different batch numbers and also from different manufacturers as well....IRF ,APT, Fairchild, IXYS etc...Lets see what is the next outcome....


Kanwar

It's hard to speculate on what you are doing wrong. My results agree with the manufacturers' data sheets, so I think the burden of proof is on you.

Your results clearly indicated a large increase in Id well before 50V, whereas mine did not. There is no need to go beyond 50V to make the point.

I hope you will be able to discover what is wrong with your approach and obtain results that are in line with the manufacturers' data sheets.

Try a different topology, perhaps one with source grounded and sense resistor in drain. Try a different gate drive arrangement, maybe using 220 ohms. Try better bypassing and minimization of inductance, especially in the drain and source circuits. Don't connect to any node with a probe that is not isolated by at least 100 ohms at the tip.

Bob

 

[Bob Cordell]

Congragulations on the schematics. Looks like a well thought out design and implementation.

It misses out crucially on the ruling edifice of my viewpoint. It does _not show_ the PCB layout with respect to the mosfets. Or any attention to detail in this respect.

I ruefully admit, but you can not be taken seriously without disclosing trade secrets.....

Regards Kevin

This is a QSC design. Does Kanwar work for QSC?

Bob

 

[peranders]

He works for this company
http://www.workhorsetechnologiesindia.com/

Kanwar, never thought of filling your website with info

 

[kASD]

Re: Re: The Real World Vs Real World

I think the burden of proof is on you.

This is a QSC design. Does Kanwar work for QSC?

Bob

Really but I don't think so, because I have just tested APT parts and they weren't showing anything different from Datasheets....
There is nothing wrong with my setup...

This design was done by Paul F. Ierymenko, He approached Pat Quilter [Ceo of QSC] with his design,a deal was fixed with QSC for filing the patent in 1998 and the patent was granted to QSC for this design in 2000, This patent took 2 years because there was already a patent present on Grounded Bridge amp by Gerald Stanley [The Crown Audio Mastermind engineer]....but it got expired...and QSC was enjoying their pinnacle of technology, while for Crown it was their yesterday's technology.....

Bob, have you checked the patents....

Kanwar

 

[jan.didden]

Re: Re: Re: The Real World Vs Real World

Really but I don't think so, because I have just tested APT parts and they weren't showing anything different from Datasheets....
There is nothing wrong with my setup...

This design was done by Paul F. Ierymenko, He approached Pat Quilter [Ceo of QSC] with his design,a deal was fixed with QSC for filing the patent in 1998 and the patent was granted to QSC for this design in 2000, This patent took 2 years because there was already a patent present on Grounded Bridge amp by Gerald Stanley [The Crown Audio Mastermind engineer]....but it got expired...and QSC was enjoying their pinnacle of technology, while for Crown it was their yesterday's technology.....

Bob, have you checked the patents....

Kanwar

???? How can you patent something that was patented by someone else many years ago, even if it expires? How can anything be 'novel' if another guy patented it ages ago?
Are the PTO losing the plot?

Jan Didden

 

[Nelson Pass]

Kevin first of all see this link: There is no gate resistor at the gate of the mosfets in this schematic....and thats not a miracle, its a unique simple approach...

http://www.qscaudio.com/support/library/schems/Current/PowerLight Series/PL6.0II.pdf

Of course you will note that an earlier version had Gate resistors,
and when they went to EL2257's for drivers, they took the
gate resistors out. But if you look at the EL2257, you see that
they have internal resistance, probably enough to limit the output
current to 90 ma, and the drivers are run locally open loop.

Me, I would not want to deal with an amplifier that requires 22
pages of schematics.

 

[jacco vermeulen]

Me, I would not want to deal with an amplifier that requires 22
pages of schematics.

We can forget about P-A Labs, right ?

Am i right in assuming from this part of the thread that the use of gate resistors is another contraversion between professional and personal power amplifiers ?

(i'd love a WorkeHorse power amp picture for my collection)

 

[kASD]

Re: Re: Re: Re: The Real World Vs Real World

???? How can you patent something that was patented by someone else many years ago, even if it expires? How can anything be 'novel' if another guy patented it ages ago?
Are the PTO losing the plot?

Jan Didden

Jan its sometimes hard to believe but the both patents exist for grounded bridge, one for Crown and another for QSC....I think to grant a patent the USPTO just need their fees, not the clarifications, money is the name of the game....

Kanwar

 

[GK]

G'day all.

Does anyone here have experience with parallel-connecting Exicon lateral MOSFET's?

http://www.profusionplc.com/pro/gex/prodGen.html?prdtyp=lateral mosfet

I've played with these before, but I've never paralleled lots of pairs.

Below approx 200mA Id, these devices have a practically zero Vgs/Id temperature coefficient, which is really nice and something the manufacturer boasts about. The manufacturer also boast about the ease of parallelling these devices, but doesn’t give a lot of information on what kind of degree of Vgs variation can be expected between devices.
An Australian distributor of these parts (Altronics) sells a power amp kit using these MOSFET’s, which curiously uses source ballast resistors to equalise the drain currents of the multiple pairs, so I’m not sure how tight the Vgs/Id tolerance is.

Cheers,
Glen

 

[Fanuc]

G'day all.

Does anyone here have experience with parallel-connecting Exicon lateral MOSFET's?

http://www.profusionplc.com/pro/gex/prodGen.html?prdtyp=lateral mosfet

I've played with these before, but I've never paralleled lots of pairs.

Below approx 200mA Id, these devices have a practically zero Vgs/Id temperature coefficient, which is really nice and something the manufacturer boasts about. The manufacturer also boast about the ease of parallelling these devices, but doesn’t give a lot of information on what kind of degree of Vgs variation can be expected between devices.
An Australian distributor of these parts (Altronics) sells a power amp kit using these MOSFET’s, which curiously uses source ballast resistors to equalise the drain currents of the multiple pairs, so I’m not sure how tight the Vgs/Id tolerance is.

Cheers,
Glen

The Exicon's are made by magnatec in the UK. What I like about lateral's is there very low threshold voltage. Obviously they have lower transconductance compared with verticals. Charles Hansen once said the P Chl Hitachi 2SJ162 was not as good as the complimentary P Chl. Exicons/Magnatecs for some reason. Forgot the reason now.

Charles Hansen and Nelson Pass had a great exchange on the merits of boths of these. I will try dig up the post later.

Best Regards

Kevin

 

[AndrewT]

Hi,
I used the Hitachi version 2sk1058/j162.
The Nchannel spread in Vgs was very tight. I got groups of 4 from 16pair within 3mV @ 50mA and @ 100mA.
The Pchannel spread is about double @ 6mV + one rogue.
It runs slightly warmer than it's partners, from memory it's Vgs is 10mV low. The source resistors 0r1 are matched.
The amp (Sugden P128, but on 4pairs) runs resistor biasing for 300mA (75mA/Q). I wish the sinks were bigger, I would like to try the Borbely recommendation of >=100mA/Q & 500mA total Iq.

 

[SpittinLLama]

Kevin,

Since these are made by Magnatec, do you know if they are the same die with different part numbers than some that Magnatec sells? I have been trying to get a hold of the BUZ900DP or 901DP and the complements to them, the BUZ905DP and 906DP. The cureves look awefully similar to the EC20N16/20 and EC20P16/20 parts. These look like very good parts and with such low Vt they should work great with a LM4702.

Anyone know a source for these in the U.S.A? The link by Glen will ship to the US but at $50 for shipping.

Thanks,
-SL

 

[EUVL]

> The curves look awefully similar to the EC20N16/20 and EC20P16/20 parts

They are the same part. I wrote to Magnatec about a year ago and they confirmed that by writing. And you can still order either of those.

> Anyone know a source for these in the U.S.A? The link by Glen will ship to the US but at $50 for shipping.

I ordered 25 pieces a year ago directly from Magnatec. They charged 55 GBP (about 80 USD ?) for shipment. And cash in advance by bank transfer. Had I known beforehand, I would have just stuck to Hitachi (or now Renesas).


Patrick

 

[SpittinLLama]

thanks Patrick.

 

[fab]

G'day all.

Does anyone here have experience with parallel-connecting Exicon lateral MOSFET's?

http://www.profusionplc.com/pro/gex/prodGen.html?prdtyp=lateral mosfet

I've played with these before, but I've never paralleled lots of pairs.

Below approx 200mA Id, these devices have a practically zero Vgs/Id temperature coefficient, which is really nice and something the manufacturer boasts about. The manufacturer also boast about the ease of parallelling these devices, but doesn’t give a lot of information on what kind of degree of Vgs variation can be expected between devices.
An Australian distributor of these parts (Altronics) sells a power amp kit using these MOSFET’s, which curiously uses source ballast resistors to equalise the drain currents of the multiple pairs, so I’m not sure how tight the Vgs/Id tolerance is.

Cheers,
Glen

If you are willing to pay the price Borbely (http://www.borbelyaudio.com/audiophile_components.asp) can match them for you. Each type (N and P) had respectively the same lot number and they were matched from N to P to within 10% VGS from what I recall. I use 5 pairs in my power amp.
Source resistor is not necessary at all when using matched lateral mosfet devices.

Also, have in mind that Lateral mosfet are a lot more expensive than vertical ones...maybe because of low volume...

 

[GK]

Thanks all for the info on the Exicon’s. I’m not prepared to go through the trouble of matching devices, so I guess that if I choose to use these MOSFET’s, I’ll use low value source resistors just to be on the safe side. I just though that since the manufacturer boasts about the ease with which these devices can be paralleled, they might be good enough to get away unmatched without the resistors.

Cheers,
Glen