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Using FETs in parallel

P

Pooh Bear

Jan 1, 1970
0
Phil said:
I have seen one *misguided* amp design where resistors had been installed
feeding each output BJT's base pin in a parallel group. It only made device
Ic sharing worse.

Not Crest by any chance ? Those 2r2s IIRC ?

They're actually there to stop VHF oscillation ( a la mosfet ).

Graham
 
P

Phil Allison

Jan 1, 1970
0
"Pooh Bear"
Not Crest by any chance ? Those 2r2s IIRC ?


** No.

The amps were the Jands J1000 and J700 (Australian designed & made).

The Motorola and later "Hi-Rel" output BJTs base pins were fed via 10 ohm
0.25 watt resistors.

At high output currents (or under short cct tests), the drop across each 10
ohm was between 0.4 and 0.9 volts !!!

To add to the insanity - the 0.47 ohm, ballast resistors were on a separate
PCB that attached to the one with the BJTs via multipin plugs and sockets -
with the result that the extra copper track lengths altered the actual value
of the ballast by up to 30 % for the ones at the end.

On test, under load - the Ic variation across the 10 BJTs had a ratio of
typically of 2:1 !!!

In a desperate attempt to solve the high resulting *blow up rate* in the
field - Jands went from Motorola MJ15024/25s to the more expensive Hi-Rel
EB204 / ED 204 devices. The Hi-Rels had a much tighter spec for Hfe and Vbe,
so gross mismatching was reduced.

All Jands really had to do was remove all those damn 10 ohm resistors and
run a solder coating along the long thin tracks that fed the distant ballast
resistors.

Done this many times when carrying out repairs:

Voila - near perfect Ic matching.

There were a whole BUNCH of other mindless stuff ups too - like speaker
DC protection relays that didn't.

Cost Jands' reputation very dearly.


They're actually there to stop VHF oscillation ( a la mosfet ).


** I very much doubt that it is VHF.

HF maybe ?

Fusible types are they ??




......... Phil
 
P

Pooh Bear

Jan 1, 1970
0
Phil said:
"Pooh Bear"

< snip .
** I very much doubt that it is VHF.

HF maybe ?

Hmmmm. Yup, you're right.

In the low MHz region IIRC.

Fusible types are they ??

Don't recall now. Would have made sense. Most of the Rs around my own bipolar
outputs ended up as fusibles.

I experienced the oscillation problem myself once btw when I suspect On Semi
changed the process on the MJ15023/4s.

Thanks for the good info btw.

Graham
 
R

rickman

Jan 1, 1970
0
Ken said:
Have you considered using a do it yourself bucker instead of the clamp?

What is a bucker? I know a Buckler, but he lives in Santa Cruz.

By time your got the parts for doing it as a linear regulator, you've
most likely got the cost of a bucker tied up in heat sinking etc.

No major heat sinking required. The over voltage is only a surge
requirement, between 50 and 200 mSec at 100 volts down to 40 volts on a
linear curve.

Vicor makes 46V nominal ones that are good from 21-56V. You could have 2
DC-DC converters and shut off the power to the low voltage one when the
voltage goes high enough that the high voltage one can take over.

That's an idea, but I don't think we need that to handle surges.
 
J

Joseph2k

Jan 1, 1970
0
Phil said:
"Fred Bloggs"


** Likely some hypothetical circuit on an application sheet or web sits -
lots of them simply do not work.

Also, many commercial products have power devices operating in parallel
quite successfully with little or no ballasting.

What outsiders *do not know* is the particular devices in each unit were
selected for exact match in the factory.



....... Phil
If you knew how expensive that is you might not claim it.
 
W

Winfield Hill

Jan 1, 1970
0
Phil Allison wrote...
What outsiders *do not know* is the particular devices in
each unit were selected for exact match in the factory.

Right, using parts from the same batch (tube?), with
a carefully-determined selection test jig and setup.
 
J

John Larkin

Jan 1, 1970
0
Phil Allison wrote...

Right, using parts from the same batch (tube?), with
a carefully-determined selection test jig and setup.

I did that once, about 10 years ago, on my first NMR gradient amp
(which is just now being retired.) Nightmare, even with moderate
source resistors. Now I close a loop around each fet with an opamp
per.

John
 
S

Spehro Pefhany

Jan 1, 1970
0
I did that once, about 10 years ago, on my first NMR gradient amp
(which is just now being retired.) Nightmare, even with moderate
source resistors. Now I close a loop around each fet with an opamp
per.

John

Any interesting stability issues with that approach?


Best regards,
Spehro Pefhany
 
J

John Larkin

Jan 1, 1970
0
Any interesting stability issues with that approach?

Not really. But it pays to use an opamp that has a low open-loop
output impedance, so the fet gate capacitance doesn't make the local
loop too twitchey. The nice thing is that the effective gate
thresholds all go to 0 +-1 mV, and the fet Gm's become predictable and
perfectly linear. (This with feedback taken from the source, using a
small source resistor.) And quiescent current control is now exact.

You can push the fets a lot harder thermally if you are confident
they're sharing the dissipation equally. You could even - just thought
of this - apportion the dissipation according to the fet location on
the heatsink.

John
 
T

Terry Given

Jan 1, 1970
0
John said:
Not really. But it pays to use an opamp that has a low open-loop
output impedance, so the fet gate capacitance doesn't make the local
loop too twitchey. The nice thing is that the effective gate
thresholds all go to 0 +-1 mV, and the fet Gm's become predictable and
perfectly linear. (This with feedback taken from the source, using a
small source resistor.) And quiescent current control is now exact.

You can push the fets a lot harder thermally if you are confident
they're sharing the dissipation equally. You could even - just thought
of this - apportion the dissipation according to the fet location on
the heatsink.

John

now thats a nice though. central FETs do less work....

do you run into any problems with thermal cycling? I saw an interesting
paper not so long ago on thermal cycling failures in TO-247 packages;
its the leading killer in big modules. dT/dt....


Cheers
Terry
 
P

Pooh Bear

Jan 1, 1970
0
Terry said:
do you run into any problems with thermal cycling? I saw an interesting
paper not so long ago on thermal cycling failures in TO-247 packages;
its the leading killer in big modules. dT/dt....

Hah !

Does anyone recall RCA running an ad in the 70s about thermal cycling ? They
reckoned their die attach ? was better than the competition's IIRC.

ISTR a figure of 10,000 thermal cycles being mentioned as a critical level.

Graham
 
J

John Larkin

Jan 1, 1970
0
now thats a nice though. central FETs do less work....

do you run into any problems with thermal cycling? I saw an interesting
paper not so long ago on thermal cycling failures in TO-247 packages;
its the leading killer in big modules. dT/dt....


Cheers
Terry

My NMR gradient drivers push the fets to close to their rated power
(as much as 300 watts dissipation per TO247) at, say, 10% duty cycle,
one pulse per second. Pulse fatigue doesn't seem to be a problem with
the parts I've used, in that we're not seeing any indication of a
wearout mechanism.


John
 
R

Rich Grise

Jan 1, 1970
0
.
You can push the fets a lot harder thermally if you are confident
they're sharing the dissipation equally. You could even - just thought
of this - apportion the dissipation according to the fet location on
the heatsink.

I've seen this done, but open-loop, with different source resistors.
It was in a "Battery Charger-Analyzer" that would determine the condition
of Fibrous NiCd batteries by charging them, discharging them (which is
what the banks of FETs were for), and charging them again. I didn't
design it, I just did some mods to the 68HC11 firmware; but the guy
who did had arranged it so that the hotter FETs were closer to the fan,
and the unit had a definite air flow path.

It seems to have worked - the guy sold about 200 of them to the
US Army.

Cheers!
Rich
 
D

Derek Potter

Jan 1, 1970
0
Does anyone recall RCA running an ad in the 70s about thermal cycling ? They
reckoned their die attach ? was better than the competition's IIRC.
ISTR a figure of 10,000 thermal cycles being mentioned as a critical level.

So an audio amp pumping music at 140BPM will last 1 minute 11 seconds.
 
S

Spehro Pefhany

Jan 1, 1970
0
So an audio amp pumping music at 140BPM will last 1 minute 11 seconds.

You need a cycle in the double-digit seconds to get enough temperature
delta for optimium thermal fatigue. In the early days, SSRs were dying
in two or three months from this cause (running 24/7).


Best regards,
Spehro Pefhany
 
W

Winfield Hill

Jan 1, 1970
0
John Larkin wrote...
... You could even - just thought of this - apportion the
dissipation according to the fet location on the heatsink.

Don't you hate it when the cool ideas come too late for
production? Or everything is already in the field? Let's
hear it for programmable load sharing, with the hardware's
firmware updatable by Internet push from the factory. :)
 
S

Spehro Pefhany

Jan 1, 1970
0
John Larkin wrote...

Don't you hate it when the cool ideas come too late for
production? Or everything is already in the field? Let's
hear it for programmable load sharing, with the hardware's
firmware updatable by Internet push from the factory. :)

I'm just going through the security for something along those lines
with a security expert... ugh.


Best regards,
Spehro Pefhany
 
J

John Larkin

Jan 1, 1970
0
John Larkin wrote...

Don't you hate it when the cool ideas come too late for
production? Or everything is already in the field? Let's
hear it for programmable load sharing, with the hardware's
firmware updatable by Internet push from the factory. :)

On my bigger amps, I digitize everything - heatsink temp, fet
currents, fet voltages - and simulate junction temps in real time,
with software-programmable Tj shutdowns.

John
 
W

Winfield Hill

Jan 1, 1970
0
Spehro Pefhany wrote...
I'm just going through the security for something along
those lines with a security expert... ugh.

Running Windows in your machines again? Didn't you learn?
 
W

Winfield Hill

Jan 1, 1970
0
John Larkin wrote...
On my bigger amps, I digitize everything - heatsink temp, fet
currents, fet voltages - and simulate junction temps in real
time, with software-programmable Tj shutdowns.

Nice, but no software-programmable load-sharing constants,
because you didn't think of it soon enough, right?
 
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