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'[EE]: Want advice on electronic load ( EL )'
2008\11\04@133249 by Michael Algernon

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I have posted a hand drawn schematic at
http://oh-god.com:5080/dir/Electronic-load/

The application is a battery tester.  I want to load a 1.5 to 5V  
battery with a load current of zero to 40A.  To the right you see I  
plan to drive the op amp with a 16 bit DAC.  R3 shuts off the load if  
the DAC is disconnected.  I put in R1 and R2 as insurance.  I am not  
sure I need them at all.  The EL can be very slow.  A response time of  
1 second is okay.  Rsense is .01 ohms.   Thus I figure that Vsense  
will vary from 0.0 to 0.4V.
The FET will be using a heatsink ( and maybe a fan ) and will be rated  
at about 80A and 300W.  I have calculated the power dissipation of the  
FET to be a max of 40A times 5V which is 200W.  ( Some of that heat  
will appear across Rsense )

DAC = DAC8574
FET = FQP90N10V2
OP AMP =   ???

Will this idea work or have I missed the banana boat.  What would be a  
good op amp to maintain precision ?  The op amp need not be fast.  Low  
power is obviously not a factor.

MA





WFT Electronics
Denver, CO   720 222 1309
" dent the UNIVERSE "

All ideas, text, drawings and audio , that are originated by WFT  
Electronics ( and it's principals ),  that are included with this  
signature text are to be deemed to be released to the public domain as  
of the date of this communication .


WFT Electronics
Denver, CO   720 222 1309
" dent the UNIVERSE "

All ideas, text, drawings and audio , that are originated by WFT  
Electronics ( and it's principals ),  that are included with this  
signature text are to be deemed to be released to the public domain as  
of the date of this communication .


 WFT Electronics
Denver, CO   720 222 1309
" dent the UNIVERSE "

All ideas, text, drawings and audio , that are originated by WFT  
Electronics ( and it's principals ),  that are included with this  
signature text are to be deemed to be released to the public domain as  
of the date of this communication .

2008\11\04@194237 by Sean Breheny

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Hi Michael,

By the way, didn't there used to be someone by the name of "Gus" who
used to post to the list with a signature involving WFT Electronics?
Do you know him?

Basically you are on the right track with this. The biggest problem I
see is that it is unrealistic to think that you will be able to
dissipate 200W from one FET in any typical type of package. FET specs
can be very misleading. Usually their max dissipation number assumes
that you manage to keep the case at 25C, which would pretty much
require cooling with liquid nitrogen or lots of water through a copper
heatsink. No real application is going to manage to carry away 200W
from that package without the case getting very hot.

They give junction to case thermal resistance of 0.6 deg C/Watt and
typical case to sink of 0.5. Then, you need to figure at least 1 deg
C/Watt for thermal spreading resistance through the heatsink material.
If you manage to have a huge fan-cooled heatsink, you might manage to
keep the rest of the resistance to ambient at 1 deg C/Watt. That's a
total from junction to ambient of 0.6+0.5+1+1=3.1 deg C/Watt. If
ambient is 25C, and the max junction temperature is 175 C, then this
leaves you with a deltaT of 150C max. 150C/3.1 degC/Watt=48W. Note
that this leaves no margin for calculation approximation error,
accidental overload, part to part variation, or any kind of safety
factor (i.e. keeping the Tj max below 175C to increase semiconductor
reliability).

i think that you are going to end up with multiple devices in
parallel. If you end up doing that, note that you cannot just parallel
FETs directly when they are operating in the linear (half-on) mode as
you are using them here. That means that you would need ballast
resistors and at that point it would probably be better to go with
several BJTs in parallel.

One way to reduce your device power dissipation is to have some power
resistors in the circuit. I once built one of these battery
constant-current dischargers which used two power resistors (call them
R1,R2) in series with several BJTs in parallel across one of the two
resistors (R2). When the BJTs were fully off, the circuit resistance
was R1+R2. With the BJTs fully on, it was roughly R1. I could then
vary the current between those two extremes by varying the drive to
the BJT. The maximum BJT dissipation was then roughly 1/4 of what it
would be if there were no power resistors (max power for the BJT in
this case was at half of max current and half of max voltage).

Another potential problem is op-amp instability. You have the FET in
the feedback loop of the op-amp. The FET has a high gate capacitance
and non-linear behavior, especially as the current gets low. You may
have to add a compensation network (some Rs and Cs) in the op-amp
circuit to prevent oscillation. You can tell by prototyping it,
putting a scope on the op-amp output, and then driving the DAC with
step functions (i.e., command a 0.5A to 10A step) and watch for
ringing on the scope.

As for an op-amp choice, it will be challenging to get near 16 bit
accuracy. There will be Rsense inaccuracies, op-amp noise, and op-amp
offset error. Analog devices makes some amazing zero-drift op-amps now
(which auto-zero their own offset error). A decent general purpose 1mV
offset op-amp is the LMC6482.

Sean


On Tue, Nov 4, 2008 at 1:32 PM, Michael Algernon <spam_OUTpicTakeThisOuTspamnope9.com> wrote:
{Quote hidden}

> -

2008\11\04@205710 by Michael Algernon

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> On Nov 4, 2008, at 5:42 PM, Sean Breheny wrote:
>
> Hi Michael,
>
> ( snip )
> i think that you are going to end up with multiple devices in
> parallel. If you end up doing that, note that you cannot just parallel
> FETs directly when they are operating in the linear (half-on) mode as
> you are using them here. That means that you would need ballast
> resistors and at that point it would probably be better to go with
> several BJTs in parallel.
>
G'day Sean

Why would not three FETs in parallel work in the linear mode ?  Each  
would carry a certain amount of the current and dissipate part of the  
heat.  As  the FET heats up does it carry more or less current for a  
given Vgs ?  According to the spec, I think the resistance goes up  
with temperature.  Thus I would expect FETs in the linear area to  
current share.  Am I wrong ?

MA

2008\11\04@211039 by Sean Breheny

face picon face
FETs have two principal modes: linear mode and ohmic mode.

In the linear mode (which occurs when Vdrain-gate <Vth), the FET acts
like a constant current source/sink. The current depends mainly on Vgs
and not very much on Vds. The current has a positive temperature
coefficient in this mode and parallel FETs in this mode are likely not
to share current equally.

In the ohmic mode (which is when the gate is more than Vth above BOTH
source and drain), the FET acts like a variable resistor where the
resistance is a function of Vgs. In this mode, the RESISTANCE has a
positive temperature coefficient and parallel FETs will share current
just fine.

Sean


On Tue, Nov 4, 2008 at 8:56 PM, Michael Algernon <.....picKILLspamspam@spam@nope9.com> wrote:
{Quote hidden}

> -

2008\11\05@184000 by Michael Algernon

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My new design ( thanks Sean Breheny ) uses 4 FETs.  Three of them  
provide fixed current loads of 10A each.  Each of the 3 can be  
switched on or off.  The fourth provides a variable load of 0 to  
10A.   By mixing and matching , you can get a programmed load from 0  
to 40A.   I am looking for comments and advice.  I calculate the FETs  
will now have to tolerate 10A max and 50W power dissipation.
http://oh-god.com:5080/dir/Electronic-load/

MA


 WFT Electronics
Denver, CO   720 222 1309
" dent the UNIVERSE "

All ideas, text, drawings and audio , that are originated by WFT  
Electronics ( and it's principals ),  that are included with this  
signature text are to be deemed to be released to the public domain as  
of the date of this communication .

2008\11\06@215745 by Tom

picon face
FWIW we use several electronic loads from 300 Watts to 4 kW and they are all
MOSFET based. Maxim had an app note on an op amp based load and I wouldn't
be surprised if IR had information on paralleling MOSFETS. As an aside how
many in this group used paint can full of oil to keep a dummy load cool?

The common problem I see with a current sensing load it the initial turn on
transient. Since the power supply output is zero the MOSFETS are driven into
saturation. Depending on the response of the load and the protection in the
supply this can create some interesting problems. Crowbar protection on a 4
kW supply or a 20 kW inverter for example. The better loads allow for slew
rate control and constant resistance modes that help some.


Tom

2008\11\06@234915 by Xiaofan Chen

face picon face
On Fri, Nov 7, 2008 at 10:57 AM, Tom <tomlivesherespamKILLspamsbcglobal.net> wrote:
> The common problem I see with a current sensing load it the initial turn on
> transient. Since the power supply output is zero the MOSFETS are driven into
> saturation. Depending on the response of the load and the protection in the
> supply this can create some interesting problems. Crowbar protection on a 4
> kW supply or a 20 kW inverter for example. The better loads allow for slew
> rate control and constant resistance modes that help some.

Interesting. Last time I was in a power electronics lab and the electronic
load we have are only for lower power. So when we needed to test
bigger converter or inverter module (up to 5KW), then we had to use a
big bank of resistor with fans for cooling. I also saw people using
a bank of lamps as load. Very good heater in Winter time. But
in summer time, oops.

I am thinking motors can be useful for higher power load as well.

Xiaofan

2008\11\07@005632 by Sean Breheny

face picon face
Hi Tom,

I'm sure you can find some good FET app notes, but just beware that
most FET app notes which talk about paralleling mosfets are talking
about doing so with the FETs used as switches (fully on or off) not in
the linear region.

I'm not sure I understand what you mean with the turn-on transient. Do
you mean when the load is on but nothing is supplying it and then you
suddenly apply the supply to the load?

Sean


On Thu, Nov 6, 2008 at 9:57 PM, Tom <.....tomliveshereKILLspamspam.....sbcglobal.net> wrote:
{Quote hidden}

> -

2008\11\07@103655 by Martin

face
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Xiaofan Chen wrote:
> On Fri, Nov 7, 2008 at 10:57 AM, Tom <EraseMEtomlivesherespam_OUTspamTakeThisOuTsbcglobal.net> wrote:
>> The common problem I see with a current sensing load it the initial turn on
>> transient. Since the power supply output is zero the MOSFETS are driven into
>> saturation. Depending on the response of the load and the protection in the
>> supply this can create some interesting problems. Crowbar protection on a 4
>> kW supply or a 20 kW inverter for example. The better loads allow for slew
>> rate control and constant resistance modes that help some.
>
> Interesting. Last time I was in a power electronics lab and the electronic
> load we have are only for lower power. So when we needed to test
> bigger converter or inverter module (up to 5KW), then we had to use a
> big bank of resistor with fans for cooling. I also saw people using
> a bank of lamps as load. Very good heater in Winter time. But
> in summer time, oops.
>
> I am thinking motors can be useful for higher power load as well.
>
> Xiaofan

I used an old amplifier heatsink and a pile of panel mount 50W 1 ohm
resistors that I got from ebay as my test load for a 14v power supply.
I've also used a 1 ohm 250 watt resistor in a bucket of water to test
high current low voltage sources.
-
Martin

2008\11\07@164027 by Forrest W. Christian

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I recently needed to test some power supplies in the few dozen watt range...

Somewhere I came across an appnote for a constant current "load"
basically using an opamp and a FET...   One of the opamp inputs was tied
to a multiturn pot wired as a voltage divider... the other one was tied
to the top of a low-side current sense resistor.   The output of the
opamp was tied to the input of the FET.

So.. the opamp would increse or decrease it's output (turning the FET on
or off more or less), until the voltage at the top of the current sense
resistor matched the voltage out of the voltage divider...  want more
current, turn up the voltage from the voltage divider.

This worked pretty good, although I was getting some "current ringing",
and I really didn't want to mess with it...  So I went and bought one of
these:  
http://www.circuitspecialists.com/search.itml?icQuery=electronic+load

Perhaps a similar approach (the opamp/fet combination) would work,
although using the output from the ADC to set the opamp input voltage.

-forrest

2008\11\08@021007 by Michael Algernon

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{Quote hidden}

Do you suppose a FET could be operated in a paint can full of oil to  
cool it ?  The max load voltage would be 5V and the gate voltage might  
be 10-12 volts.
MA

2008\11\08@033057 by Clint Sharp

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In message <C13C4F75-5FFD-4A3A-8E92-DA05BBF2700Bspamspam_OUTnope9.com>, Michael
Algernon <@spam@picKILLspamspamnope9.com> writes
>
>I have posted a hand drawn schematic at
>http://oh-god.com:5080/dir/Electronic-load/
>
>The application is a battery tester.  I want to load a 1.5 to 5V
>battery with a load current of zero to 40A.
What type of battery? Is there a specific reason to need to load test it
or can you test using conductance instead?
--
Clint Sharp

2008\11\08@035635 by Michael Algernon

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>
> On Nov 7, 2008, at 1:26 PM, Clint Sharp wrote:
>
> In message <KILLspamC13C4F75-5FFD-4A3A-8E92-DA05BBF2700BKILLspamspamnope9.com>, Michael
> Algernon <RemoveMEpicTakeThisOuTspamnope9.com> writes
>>
>> I have posted a hand drawn schematic at
>> http://oh-god.com:5080/dir/Electronic-load/
>>
>> The application is a battery tester.  I want to load a 1.5 to 5V
>> battery with a load current of zero to 40A.
> What type of battery? Is there a specific reason to need to load  
> test it
> or can you test using conductance instead?
> --
> Clint Sharp
> --

Thanks Clint
My customer wants to program a variable current load on his  
batteries.  I don't know why ......
MA


 WFT Electronics
Denver, CO   720 222 1309
" dent the UNIVERSE "

All ideas, text, drawings and audio , that are originated by WFT  
Electronics ( and it's principals ),  that are included with this  
signature text are to be deemed to be released to the public domain as  
of the date of this communication .

2008\11\08@225955 by Forrest W Christian

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Michael Algernon wrote:
> My customer wants to program a variable current load on his  
> batteries.  I don't know why ......
>  
Is this a one-off?   If so, he/you might want to seriously consider one
of the electronic loads I mentioned in a previous posting to the list.

I also found a couple of circuits which were similar to the one I was
talking about...  The one which is best described is:

http://electronicdesign.com/Articles/ArticleID/4056/4056.html

The schematic is at http://electronicdesign.com/Files/29/4056/Figure_01.gif

What I was suggesting is to hook your ADC up to the + terminal instead
of using the pots as the voltage divider.  The general idea can be used
for constant voltage as well.....  As long as you can produce a voltage
on the - terminal of the opamp which corresponds to the actual current
or voltage on the output, and put the desired matching voltage on the +
terminal, the load will "seek" to the current or voltage, since the
opamp wants it's inputs to match....  If you increase the voltage on the
+ terminal, it will increase the output voltage until the voltage coming
off of the current sense resistors matches.

-forrest



2008\11\09@003528 by Michael Algernon

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>
> On Nov 8, 2008, at 9:01 PM, Forrest W Christian wrote:
>
> Michael Algernon wrote:
>> My customer wants to program a variable current load on his
>> batteries.  I don't know why ......
>>
> Is this a one-off?   If so, he/you might want to seriously consider  
> one
> of the electronic loads I mentioned in a previous posting to the list.

He wants 20 of them.

{Quote hidden}

 WFT Electronics
Denver, CO   720 222 1309
" dent the UNIVERSE "

All ideas, text, drawings and audio , that are originated by WFT  
Electronics ( and it's principals ),  that are included with this  
signature text are to be deemed to be released to the public domain as  
of the date of this communication .

2008\11\11@085022 by alan smith

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Sounds like you picked up that project from guru


--- On Sat, 11/8/08, Michael Algernon <spamBeGonepicspamBeGonespamnope9.com> wrote:

{Quote hidden}

> --

2008\11\11@161414 by Michael Algernon

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Heh heh   are you the guy bidding against me ?
MA

On Nov 11, 2008, at 6:49 AM, alan smith wrote:

Sounds like you picked up that project from guru


--- On Sat, 11/8/08, Michael Algernon <RemoveMEpicEraseMEspamEraseMEnope9.com> wrote:

{Quote hidden}

> --

2008\11\12@085646 by alan smith

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no....I looked at it, talked to them...it appeared to be a can of worms, that I preferred not to get involved with.  When the spec's are not clear, I avoid those projects.


--- On Tue, 11/11/08, Michael Algernon <picSTOPspamspamspam_OUTnope9.com> wrote:

{Quote hidden}

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