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Current source design (tricky?)

J

Jamie

Jan 1, 1970
0
Ken said:
There used to be a "constant current" tube that was really a specially
tricked out light bulb.
Yeah, well thats a new one to me.
i remember seeing some of the first
Mac solid state Hi-fi audio gear..
germanium transitor power amps with a
incandescent bulb in the bias circuit
to help get a better linear response on the
on the final stages of the amp!
:)
 
K

Ken Smith

Jan 1, 1970
0
I read in sci.electronics.design that Ken Smith

Well, yes, for sufficiently basic differences. A 'barretter' has an iron
wire filament in a hydrogen-filled envelope.

Yes, thats the device I remember. I had to hunt high and low to find one
for a radio I was fixing.
 
T

Terry Given

Jan 1, 1970
0
Ken said:
John Woodgate said:
I read in sci.electronics.design that Ken Smith
[...]

i remember the days of using an incandescent lamp! :)

There used to be a "constant current" tube that was really a specially
tricked out light bulb.

Well, yes, for sufficiently basic differences. A 'barretter' has an iron
wire filament in a hydrogen-filled envelope.


Yes, thats the device I remember. I had to hunt high and low to find one
for a radio I was fixing.

It kinda makes one wonder just where thermionic technology would be
today, if it had the same $$$ spent on it as Si did.

I read a neat paper a few years back in an IEEE professional comic,
wherein it was shown that Si cant beat TE at high power levels, due to
the operating temperature ranges. If only we could *actually* buy static
induction transistors, thyristors etc. *sigh*

Cheers
 
T

Tony Williams

Jan 1, 1970
0
Winfield Hill said:
Tony Williams wrote...
Assuming the MOSFET is operating in the triode region,
what Vg-s swing is required to hold Id within <5% when
Vd-s changes by 2V (as set by the 12-14V Vs range)?
[snip]
With a big FET like the IRF4905, we're into the constant-current
"saturated" region if Vds is greater than about 500mV at Id = 2A
(this big FET is happy conducting well over 100A, if given a
little Vds to work with, so a low 2A is getting down into its
high-gain subthreshold region). We can estimate the FET's
transconductance to be about 10S at 2A and Vds, above 0.5 to 1V
(datasheet fig 1), so the FET's source-follower gain, G = gm RL
/ (1 + gm RL), would be above 0.5, which isn't too bad...

I see it. You evaded the problems of working in the triode
region by deliberately choosing the appropriate FET, even
though it looks like outrageous overkill.

Yes, going for current overkill on these low-voltage drop
c-c sources looks the obvious design choice now.
It reduces the work the amplifier has to do, which helps
in things like transient PSRR, etc.

Thanks.
 
F

Fred Bloggs

Jan 1, 1970
0
- supply about 2-3 amps (no need for programmability, it can be preset
to a value when constructed).
- have a low minimum voltage drop, of 1 Volt or less (0.5V would be
better). That is, it should work even if Vcc=12V and the load needs 11V
to keep the preset current. of course, it should also work with a
higher Vcc.
- keep a reasonably constant current for both changes in the load, and
(the bigger problem) for a changing Vcc.
- the load is returned to ground, so the source has to be on the "high"
side.
- NO need for good precision, +/-5% is more than ok. the conditions are
DC so bandwidth isn't an issue as well.

You should be safe doing this:

View in a fixed-width font such as Courier.

..
.. -----------------REGULATOR MODULE-------------------
.. | (RM) |
.. | +----------------------+---+ |
.. | | | | PMOS | 2.5A
.. | | I-> | | +-------+ | IL
..VBATT>--------+----/\/\----+---------------------|s d|---->
.. | | Rs 100m | | | | | |
.. | / / | | | g | |
.. | 2k 1% 2k 1% | / +-------+ |
.. | / / | 2k | |
.. | \ \ | / | |
.. | | | | \ | |
.. | + +---+ | | | 220 | |
.. | | | | +-||---|---+---/\/\----+ |
.. |Vref ---/ | | | 22n | | |
.. | // \ -+ / | | | |
.. | --- 200\<-+ +---|--+ |
.. | - | TL431 / | |\ | | | |
.. | |(NTE999) \ +---|-\ | | |
.. | | | | >---+ | |
.. | +------------|------|+/ === |
.. | | | |/ | 100n |
.. | | | LM741 | | |
.. | / / (NTE941)+------+ |
.. | 2k 1% 2k 1% | |
.. | / / | |
.. | \ \ | |
.. | | | | Vref |
.. | +------------+---------+ IL= ---- |
.. | | Rs |
.. -------------------|--------------------------------
.. N
..
..
.. calibration: disconnect load,
..
.. jumper TL431 K-A to short,
..
.. adjust 200R pot for 0.000V at '741 (+)-(-)
..
.. w/ batt applied.
..
..
..
.. TRANSIENT PROTECTION
..
..
.. FAST BLO +--RM---+
.. 5A | |
.. VBATT>---~-----+--------+------------BATT IL---->>--+
.. | | | | |
.. +---+ |+ +---N---+ |
.. 24V | - | === | |
.. TVS |/^ | |35WVDC | |
.. | - | |1000U | +-----+
.. +---+ | | |LOAD |
.. | | | +-----+
.. | | | |
.. +--------+----------------+ |
.. | ---
.. / ///
.. 22 1W
.. / METAL
.. \ OXIDE
.. 1n4007 |
.. +---|<|-----+
.. ---
.. ///
 
F

Fred Bloggs

Jan 1, 1970
0
Fred Bloggs wrote:
scratch that- slight scale factor problem:
View in a fixed-width font such as Courier.

..
.. ----------------------REGULATOR MODULE-------------------
.. | (RM) |
.. | +---------------------------+---+ |
.. | | | | PMOS | 2.5A
.. | | I-> | | +-------+ | IL
..VBATT>--------+----/\/\---------+---------------------|s d|---->
.. | | Rs 100m | | | | | |
.. | / / | | | g | |
.. | 2k 1% 2k 1% | / +-------+ |
.. | / / | 2k | |
.. | \ \ | / | |
.. | | | | \ | |
.. | + +---+-/\/\-+ | | | 220 | |
.. | | | 1k | | +-||---|---+---/\/\----+ |
.. |Vref ---/ | | | | 22n | | |
.. | // \ -+ / / | | | |
.. | --- 100 200\<-+ +---|--+ |
.. | - | TL431 / / | |\ | | | |
.. | |(NTE999) \ \ +---|-\ | | |
.. | | | | | >---+ | |
.. | +-----/\/\-+------|------|+/ === |
.. | | 910 | |/ | 100n |
.. | | | LM741 | | |
.. | / / (NTE941)+------+ |
.. | 2k 1% 2k 1% | |
.. | / / | |
.. | \ \ | |
.. | | | | Vref |
.. | +-----------------+---------+ IL=----- |
.. | | 10xRs |
.. ------------------------|--------------------------------
.. N
..
..
.. calibration: disconnect load,
..
.. jumper TL431 K-A to short,
..
.. adjust 200R pot for 0.000V at '741 (+)-(-)
..
.. w/ batt applied.
..
..
..
.. TRANSIENT PROTECTION
..
..
.. FAST BLO +--RM---+
.. 5A | |
.. VBATT>---~-----+--------+------------BATT IL---->>--+
.. | | | | |
.. +---+ |+ +---N---+ |
.. 24V | - | === | |
.. TVS |/^ | |35WVDC | |
.. | - | |1000U | +-----+
.. +---+ | | |LOAD |
.. | | | +-----+
.. | | | |
.. +--------+----------------+ |
.. | ---
.. / ///
.. 22 1W
.. / METAL
.. \ OXIDE
.. 1n4007 |
.. +---|<|-----+
.. ---
.. ///
 
W

Winfield Hill

Jan 1, 1970
0
Tony Williams wrote...
Winfield said:
Tony Williams wrote...
Assuming the MOSFET is operating in the triode region,
what Vg-s swing is required to hold Id within <5% when
Vd-s changes by 2V (as set by the 12-14V Vs range)?
[snip]
With a big FET like the IRF4905, we're into the constant-current
"saturated" region if Vds is greater than about 500mV at Id = 2A
(this big FET is happy conducting well over 100A, if given a
little Vds to work with, so a low 2A is getting down into its
high-gain subthreshold region). We can estimate the FET's
transconductance to be about 10S at 2A and Vds, above 0.5 to 1V
(datasheet fig 1), so the FET's source-follower gain, G = gm RL
/ (1 + gm RL), would be above 0.5, which isn't too bad...

I see it. You evaded the problems of working in the triode
region by deliberately choosing the appropriate FET, even
though it looks like outrageous overkill.

Yes, going for current overkill on these low-voltage drop
c-c sources looks the obvious design choice now.
It reduces the work the amplifier has to do, which helps
in things like transient PSRR, etc.

It wasn't a form of "outrageous overkill" that drove my MOSFET
selection, although it helped in the issue you raised, but rather
an effort to find a part that could better handle 3A * 15V = 30W
max, in the event someone used the design with a shorted load.

An apparent overkill is common when one uses modern power MOSFETs,
designed for switching use, in linear applications where one wants
to reduce the burden on the heatsink. The IRF4905 is rated at 200W
(for impossible 25C case temp), but this means it has a nice low
0.77 C/W junction-to-case thermal resistance. After adding in say
0.33 C/W for an insulator, the 1.1 C/W total means we add a modest
33C penalty to the heat-sink's job when dissipating 30W.

And I wanted a part that was easy to get and low cost: DigiKey has
10375 in stock, at $2.36 qty 10. The 55V 74A 0.02-ohm IRF4905S
(D2Pak case) is ideal for the 14V automotive industry, $1.88, 1k.
The IRF4905 and IRF4905S are on my jellybean-part lists.
 
K

Ken Smith

Jan 1, 1970
0
Terry Given said:
I read a neat paper a few years back in an IEEE professional comic,
wherein it was shown that Si cant beat TE at high power levels, due to
the operating temperature ranges. If only we could *actually* buy static
induction transistors, thyristors etc. *sigh*

It is starting to look like SITs are just one of those things that never
make it into general use. The IGBT has eaten up some of its market but
not that much. The one device that really seems missing from the spectrum
is something with a Vgs(off) of about -2V and that can handle 600V and.or
50A.
 
K

Ken Smith

Jan 1, 1970
0
Fred Bloggs said:
You should be safe doing this:
I suggest adding a capacitor between the points I've marked "A" and "B" in
the modified drawing below. I suggest the value as 22nF. Imagine a
positive step arrives on the VBATT of the existing circuit. The output of
the LM741 will only step positive by 1/2 that amount so the other 1/2
appears as a step in the Vgs of the MOSFET (ignoring its capacitance).

Unless I've over looked something, the added 22nF will reduce the step to
near zero and match the time constant of the other 22nF so the whole
effect is brought near zero.
 
T

Tony Williams

Jan 1, 1970
0
Winfield Hill said:
It wasn't a form of "outrageous overkill" that drove my MOSFET
selection, although it helped in the issue you raised, but rather
an effort to find a part that could better handle 3A * 15V = 30W
max, in the event someone used the design with a shorted load.
45W.

An apparent overkill is common when one uses modern power
MOSFETs, designed for switching use, in linear applications
where one wants to reduce the burden on the heatsink. The
IRF4905 is rated at 200W (for impossible 25C case temp), but
this means it has a nice low 0.77 C/W junction-to-case thermal
resistance. After adding in say 0.33 C/W for an insulator, the
1.1 C/W total means we add a modest 33C penalty to the
heat-sink's job when dissipating 30W.

Since this appears to be a vehicle application there
may be limited facility for a large heat sink.

In normal operation the MOSFET must be able to
dissipate (15-11)*2.5 Watts. Outside of that it may
be useful to implement a current shutdown/turndown,
either as a foldback (based on the 11 ohm load line),
or temperature sensed (based on a switching PTC).
 
F

Fred Bloggs

Jan 1, 1970
0
Ken said:
I suggest adding a capacitor between the points I've marked "A" and "B" in
the modified drawing below. I suggest the value as 22nF. Imagine a
positive step arrives on the VBATT of the existing circuit. The output of
the LM741 will only step positive by 1/2 that amount so the other 1/2
appears as a step in the Vgs of the MOSFET (ignoring its capacitance).

Unless I've over looked something, the added 22nF will reduce the step to
near zero and match the time constant of the other 22nF so the whole
effect is brought near zero.

That addition definitely makes for an improved transient response- by a
factor of 50x from what I can see. The big 1000u across the regulator
limits the current perturbations to under 5% when subjected to things
like that +30/-300V 20u/30u transient, and the 22 ohm resistor makes for
a nice low pass attenuating Volts-transient to mV-transient applied to
the circuit and on a time scale it can follow. Since we can't put a
component in series with the main current path, the MOSFET is on its
own, with the body diode saving it during negative transients, but no
protection from positive transients. The part numbers suggested thus far
are shy on Vbr,dss.
 
W

Winfield Hill

Jan 1, 1970
0
Tony Williams wrote...
Since this appears to be a vehicle application there
may be limited facility for a large heat sink.

Ahh, but the unit may well be bolted to a slab of metal.
 
J

John Woodgate

Jan 1, 1970
0
I read in sci.electronics.design that Winfield Hill <hill_a@t_rowland-
dotties-harvard-dot.s-edu> wrote (in said:
Tony Williams wrote...

Ahh, but the unit may well be bolted to a slab of metal.
....at 150 C when the engine is running, on a hot day.
 
K

Ken Smith

Jan 1, 1970
0
[.. saving band width ..]
Go back one level to see where I added the capacitor.
That addition definitely makes for an improved transient response- by a
factor of 50x from what I can see.

I think the 50x factor assumes a tighter tolerance than we can expect in
real life. The slightly mismatched capacitors we are likely to have in
real life will still match near exactly on the initial step but the
ext(-t/T) curves will not match.

It would be nice if we could make the circuit not rely on the match. If
we had a rail-to-rail op-amp, or something like the LM301, we could shift
both inputs of the op-amp up near the positivve rail. As it is, the LM741
will work with its inputs up within about 4V of the positive supply. If
we changed the resistor dividers to make them a 1/3,2/3 ratio, we would
get an improved transient performance.

The big 1000u across the regulator
limits the current perturbations to under 5% when subjected to things
like that +30/-300V 20u/30u transient, and the 22 ohm resistor makes for
a nice low pass attenuating Volts-transient to mV-transient applied to
the circuit and on a time scale it can follow.

Since the LM741, IIRC, only draws about 3mA, the 22 ohm resistor in the
minus supply to that section could be increased to lets say 100.

Since we can't put a
component in series with the main current path, the MOSFET is on its
own, with the body diode saving it during negative transients, but no
protection from positive transients. The part numbers suggested thus far
are shy on Vbr,dss.

We can put a small inductance in series. It won't help much except to
round off very short timed pulses. I don't think such things are common
in automotive power systems though.

The problem with looking for a higher Vbr(dss) is that Rds(on) runs about
as the square of Vbr(ds) for the same sized part.
 
W

Winfield Hill

Jan 1, 1970
0
John Woodgate wrote...
Winfield Hill wrote ...
...at 150 C when the engine is running, on a hot day.

Hey, I said "slab" of metal (firewall) not "block" of metal!
 
J

Jim Thompson

Jan 1, 1970
0
John Woodgate wrote...

Hey, I said "slab" of metal (firewall) not "block" of metal!

Why do you think most automotive Electronic Control Units are located
under the driver's seat or in the kick panel to the left of the
"clutch" foot?

Under-hood is VERY inhospitable... when I was in the automotive
electronics biz I designed for a normal high of +140°C.

...Jim Thompson
 
J

John Woodgate

Jan 1, 1970
0
I read in sci.electronics.design that Winfield Hill <hill_a@t_rowland-
dotties-harvard-dot.s-edu> wrote (in said:
John Woodgate wrote...

Hey, I said "slab" of metal (firewall) not "block" of metal!

Practically anything in the engine compartment gets very hot. But it was
just a hint to consider Murphy's Law.
 
W

Winfield Hill

Jan 1, 1970
0
John Woodgate wrote...
Winfield Hill wrote ...
Practically anything in the engine compartment gets very hot.
But it was just a hint to consider Murphy's Law.

Indeed. The OP didn't say anything about operating inside an
actual automobile, let alone in the engine compartment, but if
he has this in mind, he should speak up and let us deal with it!
 
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