250A electronic load

[Winfield Hill]

Folks who have been paying attention know that
I've nailed the EE-lab techniques of designing
high-amperage current sources. My associate,
Rob Legg, has extended measurements above 1kA.
Now I'm working on high-power electronic loads.

For example, a 2.5kW electronic load working at
5-volts, runs its current at 500 amps.

Electronic loads should handle continuous power
for efficiency and operational measurements,
as well as rapidly-pulsed load testing.

I quickly convinced myself that dissipating the
electronic load power is best done in a bank of
Power MOSFETs, without the use of power resistors,
etc. Die-frames with large areas are best, e.g.,
TO-264, TO-3P and TO-247. These can each easily
dissipate up to 70 to 125 watts.

These packages may all have identical theta_JC,
but the heat-sink-conductance (theta_CS) rules,
even with high-conductance phase-change thermal-
interface materials. Assume we spread the heat
across many MOSFETs, maybe 20, each one with its
own current-sense resistor and feedback loop.

I've been using 5-watt 4320-size wide 5x11mm CS
resistors, but it's a struggle to match parts
to meet the design requirements. Yet it makes
good sense to keep the CS resistors on the PCB.
I'm struggling here, anybody have suggestions?


--
Thanks,
- Win

 

[Winfield Hill]

Winfield Hill wrote...

Folks who have been paying attention know that
I've nailed the EE-lab techniques of designing
high-amperage current sources. [snip]

I quickly convinced myself that dissipating the
electronic load power is best done in a bank of
Power MOSFETs, without the use of power resistors,
etc. Die-frames with large areas are best, e.g.,
TO-264, TO-3P and TO-247. These can each easily
dissipate up to 70 to 125 watts.

These packages may all have identical theta_JC,
but the heat-sink-conductance (theta_CS) rules,
even with high-conductance phase-change thermal-
interface materials. Assume we spread the heat
across many MOSFETs, maybe 20, each one with its
own current-sense resistor and feedback loop. ...

If we need 20 expensive high-performance MOSFETs,
let's try to make a good part choice. Here's my
narrowed-down parts spreadsheet. Besides power
capabilities, price matters. Why spend $200-400,
if $50 to $100 might be enough?

https://www.dropbox.com/s/gz7wuqmq364gyn2/elec-load-MOSFETs.pdf?dl=1


--
Thanks,
- Win

 

[Tim Williams]

"Winfield Hill" <winfieldhill@yahoo.com> wrote in message
news:r2veh701fqv@drn.newsguy.com...

I quickly convinced myself that dissipating the
electronic load power is best done in a bank of
Power MOSFETs, without the use of power resistors,
etc. Die-frames with large areas are best, e.g.,
TO-264, TO-3P and TO-247. These can each easily
dissipate up to 70 to 125 watts.

Resistors are good at higher voltages, where you can afford the lost
low-voltage range. (Say a 400V load capable of full current above 50 or
100V.) That's not going to be much of an option here.

Mind that large packages are much more expensive, and we aren't talking much
silicon here, so you're basically paying for the package.

TO-220 is quite affordable and shouldn't be discounted offhand. It's good
for ~50W in typical application. You would need enough at this scale (40)
that a mechanical solution will be desirable, e.g. a bent plate or machined
bracket that applied spring force to the transistor body.

MAX-247s are nice also; price, YMMV, and they are only spring-mountable.
Clips like MAX07NG are cheap and plentiful; it's just a pain screwing down
oodles of them.

I wouldn't worry about TO-264, SOT-227, etc. 264 maybe worth leaving in the
search criteria but doubtful anything economical will show up.

As for fringe ideas, you could SMT a heat spreader bar very close to
(touching, or overlapping, the tabs, of) an array of D2PAKs, which might be
acceptable on assembly cost. Don't think it would be any more compact than
conventionally assembled TO-220s though, and if you need compactness you're
better off arranging water cooling with some bigger devices (perhaps
including 264s or 227s where the added cost is justified by the size
reduction).

But I digress.

If we need 20 expensive high-performance MOSFETs,
let's try to make a good part choice. Here's my
narrowed-down parts spreadsheet. Besides power
capabilities, price matters. Why spend $200-400,
if $50 to $100 might be enough?

Performance, what's that? I think you will find old school types -- IRFPxxx
in particular -- will show up quite prominently in Pd(max)/$ metrics. Die
area too, relevant to pulsed operation.

Last time I was looking for high voltage types, FQA9N90C was the cheapest,
most powerful (>200W), DC SOA type on DK. And for low voltage TO-220s with
high energy, STP50N06 and IRFZ46N were top. (Should be good for something
like 7ms on-time at 30V, 10A. Modern transistors only do maybe 1-2ms, if
that.)

The FQA9N90Cs I'm using in a DC load:
https://www.seventransistorlabs.com/Images/ActiveLoad2.jpg
The array of them is switching the resistors, probably worth using cheaper
transistors there but whatever; three are on the heatsink for linear
coverage.

Which, I have only partially tested so far, and am procrastinating through a
combination of lack of need (ah, but I want an electronic load to finish the
power supplies I want to test with it--), and shortcomings of the
programmable interface (I need to make an adapter card so I can have the
controller on the front panel, while connecting to the base:
https://www.seventransistorlabs.com/Images/ActiveLoad1.jpg ). It's also
kind of useless without a serial port, or a keypad that I didn't design in.
Should be operable in an on-off-up-down sense but I still need to figure all
that out. Have a nice serial console interface though:
https://www.seventransistorlabs.com/Images/ActiveLoadConsole2.png
and the LCD display runs a nice demo:
https://imgur.com/gallery/oTkxuOY

Tim

--
Seven Transistor Labs, LLC
Electrical Engineering Consultation and Design
Website: https://www.seventransistorlabs.com/

 

[Bill Sloman]

On Monday, February 24, 2020 at 12:45:33 PM UTC+11, Winfield Hill wrote:

Folks who have been paying attention know that
I've nailed the EE-lab techniques of designing
high-amperage current sources. My associate,
Rob Legg, has extended measurements above 1kA.
Now I'm working on high-power electronic loads.

For example, a 2.5kW electronic load working at
5-volts, runs its current at 500 amps.

Electronic loads should handle continuous power
for efficiency and operational measurements,
as well as rapidly-pulsed load testing.

I quickly convinced myself that dissipating the
electronic load power is best done in a bank of
Power MOSFETs, without the use of power resistors,
etc. Die-frames with large areas are best, e.g.,
TO-264, TO-3P and TO-247. These can each easily
dissipate up to 70 to 125 watts.

These packages may all have identical theta_JC,
but the heat-sink-conductance (theta_CS) rules,
even with high-conductance phase-change thermal-
interface materials. Assume we spread the heat
across many MOSFETs, maybe 20, each one with its
own current-sense resistor and feedback loop.

I've been using 5-watt 4320-size wide 5x11mm CS
resistors, but it's a struggle to match parts
to meet the design requirements. Yet it makes
good sense to keep the CS resistors on the PCB.
I'm struggling here, anybody have suggestions?


--
Thanks,
- Win

 

[Bill Sloman]

On Monday, February 24, 2020 at 12:45:33 PM UTC+11, Winfield Hill wrote:

<snip>

I've been using 5-watt 4320-size wide 5x11mm CS
resistors, but it's a struggle to match parts
to meet the design requirements. Yet it makes
good sense to keep the CS resistors on the PCB.
I'm struggling here, anybody have suggestions?

Spreading heat from a small area to big - blown - heat-sink is always tricky.

Heat pipes can soak up a lot of heat, and transfer it - as fast moving water-vapour - along relatively long copper pipes to quite substantial fan-cooled heat-sinks for eventual dissipation to the atmosphere.

You do have to make sure that piping doesn't contain any non-condensible gases.

Sloman A.W., Buggs P., Molloy J., and Stewart D. “A microcontroller-based driver to stabilise the temperature of an optical stage to 1mK in the range 4C to 38C, using a Peltier heat pump and a thermistor sensor” Measurement Science and Technology, 7 1653-64 (1996)

eventually went for this approach, and the guy who got it working had to set up a test rig to make sure that assemblies that got delivered in really had been pumped out hard before the water was put in and the system sealed off.

It wasn't anything complicated - it just put in a little heat and made sure that it was being shifted to the blown end of the assembly without there being too much of a temperature difference.

--

Bill Sloman, Sydney

 

On 23 Feb 2020 17:45:16 -0800, Winfield Hill <winfieldhill@yahoo.com>
wrote:

Folks who have been paying attention know that
I've nailed the EE-lab techniques of designing
high-amperage current sources. My associate,
Rob Legg, has extended measurements above 1kA.
Now I'm working on high-power electronic loads.

For example, a 2.5kW electronic load working at
5-volts, runs its current at 500 amps.

Electronic loads should handle continuous power
for efficiency and operational measurements,
as well as rapidly-pulsed load testing.

I quickly convinced myself that dissipating the
electronic load power is best done in a bank of
Power MOSFETs, without the use of power resistors,
etc. Die-frames with large areas are best, e.g.,
TO-264, TO-3P and TO-247. These can each easily
dissipate up to 70 to 125 watts.

These packages may all have identical theta_JC,
but the heat-sink-conductance (theta_CS) rules,
even with high-conductance phase-change thermal-
interface materials. Assume we spread the heat
across many MOSFETs, maybe 20, each one with its
own current-sense resistor and feedback loop.

I don't like the phase-change stuff. It doesn't squish down thin. The
best cooling is using regular thermal grease between the transistor
and a very flat piece of copper as the first-level heat spreader.

https://www.dropbox.com/s/nmyb9pz0qpma2xa/Amp.jpg?raw=1

Agree about multiple sense resistors and an opamp per mosfet.

I've been using 5-watt 4320-size wide 5x11mm CS
resistors, but it's a struggle to match parts
to meet the design requirements. Yet it makes
good sense to keep the CS resistors on the PCB.
I'm struggling here, anybody have suggestions?

You may as well dump as much voltage/power as possible in the sense
resistors. Heat sink them too. You can get 25 watt DPAK resistors, or
stand-up 20 watt wirewounds. Use multiple of those!

We have a couple of Kikusui load boxes, and they are very handy for
testing power supplies. They can work in CC and programmable ohms
modes, and pulse mode to display supply dynamics. A good load box
should be fast and stable and not very capacitive.



--

John Larkin Highland Technology, Inc

The cork popped merrily, and Lord Peter rose to his feet.
"Bunter", he said, "I give you a toast. The triumph of Instinct over Reason"

 

On Monday, 24 February 2020 01:45:33 UTC, Winfield Hill wrote:

Folks who have been paying attention know that
I've nailed the EE-lab techniques of designing
high-amperage current sources. My associate,
Rob Legg, has extended measurements above 1kA.
Now I'm working on high-power electronic loads.

For example, a 2.5kW electronic load working at
5-volts, runs its current at 500 amps.

Electronic loads should handle continuous power
for efficiency and operational measurements,
as well as rapidly-pulsed load testing.

I quickly convinced myself that dissipating the
electronic load power is best done in a bank of
Power MOSFETs, without the use of power resistors,
etc. Die-frames with large areas are best, e.g.,
TO-264, TO-3P and TO-247. These can each easily
dissipate up to 70 to 125 watts.

These packages may all have identical theta_JC,
but the heat-sink-conductance (theta_CS) rules,
even with high-conductance phase-change thermal-
interface materials. Assume we spread the heat
across many MOSFETs, maybe 20, each one with its
own current-sense resistor and feedback loop.

I've been using 5-watt 4320-size wide 5x11mm CS
resistors, but it's a struggle to match parts
to meet the design requirements. Yet it makes
good sense to keep the CS resistors on the PCB.
I'm struggling here, anybody have suggestions?

Commercial wirewound resistors may be a bit inductive etc, but linear R wire & strip are relatively well behaved. Sure is a cheap way to diss most of that heat.


NT

 

[Winfield Hill]

tabbypurr@gmail.com wrote...

On Monday, 24 February 2020, Winfield Hill wrote:

I quickly convinced myself that dissipating the
electronic load power is best done in a bank of
Power MOSFETs, without the use of power resistors,
etc. Die-frames with large areas are best, e.g.,
TO-264, TO-3P and TO-247. These can each easily
dissipate up to 70 to 125 watts. [snip]

I've been using 5-watt 4320-size wide 5x11mm CS
resistors, but it's a struggle to match parts
to meet the design requirements. Yet it makes
good sense to keep the CS resistors on the PCB.
I'm struggling here, anybody have suggestions?

Commercial wirewound resistors may be a bit inductive
etc, but linear R wire & strip are relatively well
behaved. Sure is a cheap way to diss most of that heat.

Thankfully, most all modern current-sense resistors
are low-inductance. Tim Williams gave us a tour of
his "active" load project, which seems to be based on
PWMing massive banks of huge power resistors, plus a
subset of linear MOSFETs. But I've convinced myself
it makes more sense to do the dissipation mostly with
MOSFETs, rather than with resistors. 50 and 100-watt
resistors are more expensive than high-power MOSFETs,
and much less flexible if you want all-linear w/o PWM.
A MOSFET handles a wide range of voltage and current,
leting us keep our accurate sub-us current-pulse speed.

As my design takes shape, I'm easing the current-sense
resistor dissipation problem with range-switch MOSFETs
(2.5mR FDP8860, etc.), plus a 3157 spdt signal switch.
E.g., each of 20 banks can handle 25A (500A total), by
switching to a 5mR sense resistor. I'll setup the PCB
to handle either 5W SMT resistors or TO-220 heat-sink-
mounted resistors. Bourns PWR221T-30 are under $2 each.



--
Thanks,
- Win

 

[Winfield Hill]

Winfield Hill wrote...

As my design takes shape, I'm easing the current-sense
resistor dissipation problem with range-switch MOSFETs
(2.5mR FDP8860, etc.), plus a 3157 spdt signal switch.
E.g., each of 20 banks can handle 25A (500A total), by
switching to a 5mR sense resistor. I'll setup the PCB
to handle either 5W SMT resistors or TO-220 heat-sink-
mounted resistors. Bourns PWR221T-30 are under $2 each.

Here's a drawing of a single bank.

https://www.dropbox.com/s/75wo1torn7b8gjo/Switched-Range-Resistor.JPG?dl=0


--
Thanks,
- Win

 

[Winfield Hill]

Winfield Hill wrote...

Winfield Hill wrote...

As my design takes shape, I'm easing the current-sense
resistor dissipation problem with range-switch MOSFETs
(2.5mR FDP8860, etc.), plus a 3157 spdt signal switch.
E.g., each of 20 banks can handle 25A (500A total), by
switching to a 5mR sense resistor. I'll setup the PCB
to handle either 5W SMT resistors or TO-220 heat-sink-
mounted resistors. Bourns PWR221T-30 are under $2 each.

Here's a drawing of a single bank.

https://www.dropbox.com/s/75wo1torn7b8gjo/Switched-Range-Resistor.JPG?dl=0

There's a potential shortcoming of my simplified circuit,
in that paralleling 50mR R1 with 5mR R1a includes another
2.5mR from Q2's Rds(on), and there's some uncertainty in
that value. But an 1mR error in the 52.5mR total creates
under 0.2% error in the resulting 4.56mR R1a || R1 value.


--
Thanks,
- Win

 

[Arie de Muynck]

On 2020-02-25 14:50, Winfield Hill wrote:

Winfield Hill wrote...
Winfield Hill wrote...

As my design takes shape, I'm easing the current-sense
resistor dissipation problem with range-switch MOSFETs
(2.5mR FDP8860, etc.), plus a 3157 spdt signal switch.
E.g., each of 20 banks can handle 25A (500A total), by
switching to a 5mR sense resistor. I'll setup the PCB
to handle either 5W SMT resistors or TO-220 heat-sink-
mounted resistors. Bourns PWR221T-30 are under $2 each.

Here's a drawing of a single bank.

https://www.dropbox.com/s/75wo1torn7b8gjo/Switched-Range-Resistor.JPG?dl=0

There's a potential shortcoming of my simplified circuit,
in that paralleling 50mR R1 with 5mR R1a includes another
2.5mR from Q2's Rds(on), and there's some uncertainty in
that value. But an 1mR error in the 52.5mR total creates
under 0.2% error in the resulting 4.56mR R1a || R1 value.

I think even that could have been avoided by placing R1a between GND and
the bottom side of R1, and using Q2 just to short R1. The feedback of
the measured current would then come from (R1+R1a) for lo range and
(R1a) for hi range, without a FET resistance.
Or is there a special reason for the chosen circuit?

Arie de Muijnck

 

[Winfield Hill]

Arie de Muynck wrote...

On 2020-02-25 14:50, Winfield Hill wrote:
Winfield Hill wrote...
Winfield Hill wrote...

As my design takes shape, I'm easing the current-sense
resistor dissipation problem with range-switch MOSFETs
(2.5mR FDP8860, etc.), plus a 3157 spdt signal switch.
E.g., each of 20 banks can handle 25A (500A total), by
switching to a 5mR sense resistor. I'll setup the PCB
to handle either 5W SMT resistors or TO-220 heat-sink-
mounted resistors. Bourns PWR221T-30 are under $2 each.

Here's a drawing of a single bank.

https://www.dropbox.com/s/75wo1torn7b8gjo/Switched-Range-Resistor.JPG?dl=0

There's a potential shortcoming of my simplified circuit,
in that paralleling 50mR R1 with 5mR R1a includes another
2.5mR from Q2's Rds(on), and there's some uncertainty in
that value. But an 1mR error in the 52.5mR total creates
under 0.2% error in the resulting 4.56mR R1a || R1 value.

I think even that could have been avoided by placing R1a between
GND and the bottom side of R1, and using Q2 just to short R1. The
feedback of the measured current would then come from (R1+R1a)
for lo range and (R1a) for hi range, without a FET resistance.
Or is there a special reason for the chosen circuit?

No, that sounds like a good idea.


--
Thanks,
- Win

 

[legg]

On 23 Feb 2020 17:45:16 -0800, Winfield Hill <winfieldhill@yahoo.com>
wrote:

Folks who have been paying attention know that
I've nailed the EE-lab techniques of designing
high-amperage current sources. My associate,
Rob Legg, has extended measurements above 1kA.
Now I'm working on high-power electronic loads.

For example, a 2.5kW electronic load working at
5-volts, runs its current at 500 amps.

Electronic loads should handle continuous power
for efficiency and operational measurements,
as well as rapidly-pulsed load testing.

I quickly convinced myself that dissipating the
electronic load power is best done in a bank of
Power MOSFETs, without the use of power resistors,
etc. Die-frames with large areas are best, e.g.,
TO-264, TO-3P and TO-247. These can each easily
dissipate up to 70 to 125 watts.

These packages may all have identical theta_JC,
but the heat-sink-conductance (theta_CS) rules,
even with high-conductance phase-change thermal-
interface materials. Assume we spread the heat
across many MOSFETs, maybe 20, each one with its
own current-sense resistor and feedback loop.

I've been using 5-watt 4320-size wide 5x11mm CS
resistors, but it's a struggle to match parts
to meet the design requirements. Yet it makes
good sense to keep the CS resistors on the PCB.
I'm struggling here, anybody have suggestions?

Do we really want to burn power in testing?
Perhaps, below 100W, or for transient testing
it's the quick solution; but nowadays, so is a
buck or boost converter feeding a battery, rather
than a resistor bank.

If the DUT's source is DC, couple the load battery
back to the source and get less drain on it.

If the DUT's source in from the AC line, then
Couple the load battery to a grid-tied inverter,
sharing the same wall socket as the DUT, and you
just get reduced wall current draw for the test
or burn-in environment.

The load battery absorbs power and supplies it to
cover transients, turn-on, protection and control
state machine delays/deviations that may be
unpredictable. Doesn't need a lot of capacity.

The net power loss can still be a large fraction of
what may have been burnt. There are static losses
and noise issues, even when nothing is hooked up,
but the hardware costs are probably comparable, and
the battery/inverter can multiply function as a
charger/UPS.

Nailing down the commodity parts for a safe grid tie
is probably the biggest stumbling block, but the
recent growth in DIY PV should make things easier.
If you've already got one, then its battery bus
could be an invaluable two-wire addition to your
test bench, rather than a lot of heatsinks and
fans.

RL

 

On 25 Feb 2020 05:50:20 -0800, Winfield Hill <winfieldhill@yahoo.com>
wrote:

Winfield Hill wrote...
Winfield Hill wrote...

As my design takes shape, I'm easing the current-sense
resistor dissipation problem with range-switch MOSFETs
(2.5mR FDP8860, etc.), plus a 3157 spdt signal switch.
E.g., each of 20 banks can handle 25A (500A total), by
switching to a 5mR sense resistor. I'll setup the PCB
to handle either 5W SMT resistors or TO-220 heat-sink-
mounted resistors. Bourns PWR221T-30 are under $2 each.

Here's a drawing of a single bank.

https://www.dropbox.com/s/75wo1torn7b8gjo/Switched-Range-Resistor.JPG?dl=0

There's a potential shortcoming of my simplified circuit,
in that paralleling 50mR R1 with 5mR R1a includes another
2.5mR from Q2's Rds(on), and there's some uncertainty in
that value. But an 1mR error in the 52.5mR total creates
under 0.2% error in the resulting 4.56mR R1a || R1 value.

Another architecture:

Several, at least 4 but maybe more, little fet-opamp-senseresistor
circuits scattered around a smallish board with a heat sink and a fan.
Possibly a heat sink per fet. Enable the identical current sink
circuits as needed, on one board or many.

Small fets are cheap. Heat sinks are expensive and develop local hot
spots. High currents are hard to handle on a PCB. Just spread out a
bunch of cheap parts electrically and thermally.

We have three class-D amps here, 250 watts per, each with its own fan,
ribbon cable and fastons.

https://www.dropbox.com/s/5nwlbkep2y97baq/3d_4.jpg?raw=1



--

John Larkin Highland Technology, Inc

The cork popped merrily, and Lord Peter rose to his feet.
"Bunter", he said, "I give you a toast. The triumph of Instinct over Reason"

 

[Winfield Hill]

amdx wrote...

Does it do any good to put a heat sink on the topside of
a TO-220 as in, clamp it between heat sinks. I know it's
plastic and has poorer heat transfer, but I still wonder.

It may not help much, but I'm contemplating designing the
PCB for several build possibilities, one of which is small
standing-up heat sinks. Dual heat-sink mounting of Q1 and
Q2 makes sense, because only one will work hard at a time.
There are a number of attractive choices available.



--
Thanks,
- Win

 

On 25 Feb 2020 04:44:15 -0800, Winfield Hill <winfieldhill@yahoo.com>
wrote:

Winfield Hill wrote...

As my design takes shape, I'm easing the current-sense
resistor dissipation problem with range-switch MOSFETs
(2.5mR FDP8860, etc.), plus a 3157 spdt signal switch.
E.g., each of 20 banks can handle 25A (500A total), by
switching to a 5mR sense resistor. I'll setup the PCB
to handle either 5W SMT resistors or TO-220 heat-sink-
mounted resistors. Bourns PWR221T-30 are under $2 each.

Here's a drawing of a single bank.

https://www.dropbox.com/s/75wo1torn7b8gjo/Switched-Range-Resistor.JPG?dl=0

Last time I did something like this, I brought in the control signal
differential. That just added one resistor quad-pack.

You could lay out a smallish PCB with a heat sink and fan all built
in, with fastons for the high-current pins, maybe optional bus bar
ties. Use as many of those as needed.

I like DPAK power resistors. Caddock MP725 and Riedon PFC are cheap
and nanoseconds fast, 25 watts heat sunk. I have TRDs somewhere.




--

John Larkin Highland Technology, Inc

The cork popped merrily, and Lord Peter rose to his feet.
"Bunter", he said, "I give you a toast. The triumph of Instinct over Reason"

 

On Tue, 25 Feb 2020 10:16:24 -0500, legg <legg@nospam.magma.ca> wrote:

On 23 Feb 2020 17:45:16 -0800, Winfield Hill <winfieldhill@yahoo.com
wrote:

Folks who have been paying attention know that
I've nailed the EE-lab techniques of designing
high-amperage current sources. My associate,
Rob Legg, has extended measurements above 1kA.
Now I'm working on high-power electronic loads.

For example, a 2.5kW electronic load working at
5-volts, runs its current at 500 amps.

Electronic loads should handle continuous power
for efficiency and operational measurements,
as well as rapidly-pulsed load testing.

I quickly convinced myself that dissipating the
electronic load power is best done in a bank of
Power MOSFETs, without the use of power resistors,
etc. Die-frames with large areas are best, e.g.,
TO-264, TO-3P and TO-247. These can each easily
dissipate up to 70 to 125 watts.

These packages may all have identical theta_JC,
but the heat-sink-conductance (theta_CS) rules,
even with high-conductance phase-change thermal-
interface materials. Assume we spread the heat
across many MOSFETs, maybe 20, each one with its
own current-sense resistor and feedback loop.

I've been using 5-watt 4320-size wide 5x11mm CS
resistors, but it's a struggle to match parts
to meet the design requirements. Yet it makes
good sense to keep the CS resistors on the PCB.
I'm struggling here, anybody have suggestions?

Do we really want to burn power in testing?
Perhaps, below 100W, or for transient testing
it's the quick solution; but nowadays, so is a
buck or boost converter feeding a battery, rather
than a resistor bank.

If the DUT's source is DC, couple the load battery
back to the source and get less drain on it.

If the DUT's source in from the AC line, then
Couple the load battery to a grid-tied inverter,
sharing the same wall socket as the DUT, and you
just get reduced wall current draw for the test
or burn-in environment.

The load battery absorbs power and supplies it to
cover transients, turn-on, protection and control
state machine delays/deviations that may be
unpredictable. Doesn't need a lot of capacity.

The net power loss can still be a large fraction of
what may have been burnt. There are static losses
and noise issues, even when nothing is hooked up,
but the hardware costs are probably comparable, and
the battery/inverter can multiply function as a
charger/UPS.

Nailing down the commodity parts for a safe grid tie
is probably the biggest stumbling block, but the
recent growth in DIY PV should make things easier.
If you've already got one, then its battery bus
could be an invaluable two-wire addition to your
test bench, rather than a lot of heatsinks and
fans.

RL

We rarely load-test a gadget longer than a few thermal time constants.
Transient response testing can be done in minutes. Why go to all that
trouble and expense to recycle 15 cents worth of electricity?

Who needs a bench full of charged batteries?

This power supply test took about an hour:

https://www.dropbox.com/s/jh2ttpn4vzbepfz/MeanWell_UHP-500_bench.JPG?raw=1

It ran at 500 watts for maybe half an hour.







--

John Larkin Highland Technology, Inc

The cork popped merrily, and Lord Peter rose to his feet.
"Bunter", he said, "I give you a toast. The triumph of Instinct over Reason"

 

[amdx]

On 2/25/2020 9:09 AM, Winfield Hill wrote:

Arie de Muynck wrote...

On 2020-02-25 14:50, Winfield Hill wrote:
Winfield Hill wrote...
Winfield Hill wrote...

As my design takes shape, I'm easing the current-sense
resistor dissipation problem with range-switch MOSFETs
(2.5mR FDP8860, etc.), plus a 3157 spdt signal switch.
E.g., each of 20 banks can handle 25A (500A total), by
switching to a 5mR sense resistor. I'll setup the PCB
to handle either 5W SMT resistors or TO-220 heat-sink-
mounted resistors. Bourns PWR221T-30 are under $2 each.

Here's a drawing of a single bank.

https://www.dropbox.com/s/75wo1torn7b8gjo/Switched-Range-Resistor.JPG?dl=0

There's a potential shortcoming of my simplified circuit,
in that paralleling 50mR R1 with 5mR R1a includes another
2.5mR from Q2's Rds(on), and there's some uncertainty in
that value. But an 1mR error in the 52.5mR total creates
under 0.2% error in the resulting 4.56mR R1a || R1 value.

I think even that could have been avoided by placing R1a between
GND and the bottom side of R1, and using Q2 just to short R1. The
feedback of the measured current would then come from (R1+R1a)
for lo range and (R1a) for hi range, without a FET resistance.
Or is there a special reason for the chosen circuit?

No, that sounds like a good idea.

Does it do any good to put a heat sink on the topside of a TO-220 as
in, clamp it between heat sinks.
I know it's plastic and has poorer heat transfer, but I still wonder.

Mikek

 

[Tim Williams]

"Winfield Hill" <winfieldhill@yahoo.com> wrote in message
news:r3320o0oba@drn.newsguy.com...

Thankfully, most all modern current-sense resistors
are low-inductance. Tim Williams gave us a tour of
his "active" load project, which seems to be based on
PWMing massive banks of huge power resistors, plus a
subset of linear MOSFETs.

Oh-- just to clarify, it's a unary power DAC. Paired with a 10-bit unary
ADC, it's the perfect [approximate] solution; no (repetitive) switching
necessary. Or, it would be if the ADC weren't obsolete now. ;o) (A little
thought should be able to determine which "ADC" I used, and exactly how it's
connected. Nice feature, that!) (But I thought of that, and the resistors
can be controlled via MCU as well.)

Then the linear sink simply fills in the gaps between bits.

As my design takes shape, I'm easing the current-sense
resistor dissipation problem with range-switch MOSFETs
(2.5mR FDP8860, etc.), plus a 3157 spdt signal switch.
E.g., each of 20 banks can handle 25A (500A total), by
switching to a 5mR sense resistor. I'll setup the PCB
to handle either 5W SMT resistors or TO-220 heat-sink-
mounted resistors. Bourns PWR221T-30 are under $2 each.

If you're concerned about dynamics, beware that the ESL of those leaded
parts will set a pretty serious upper limit on performance. Enough I think
that it can dominate over MOSFET capacitance. This can be compensated out
to some extent (put an RC on the feedback sense, thus making the opamp
overshoot just enough), as long as enough voltage is available to do so.

Incidentally, a switching load would be an interesting approach, and needs
only one resistor and a converter; it has the downside of awful dynamics,
because the EMI filter can only be designed for one fixed impedance while
the equivalent DC resistance is supposed to be variable over a huge range.
The further mismatched those impedances are, the worse the filter performs,
in terms of both attenuation and cutoff frequency. Would be good for slow
stuff like battery testing, maybe motors, but definitely not supply
transient testing.

Tim

--
Seven Transistor Labs, LLC
Electrical Engineering Consultation and Design
Website: https://www.seventransistorlabs.com/

 

[Winfield Hill]

jlarkin@highlandsniptechnology.com wrote...

On 25 Feb 2020, legg <legg@nospam.magma.ca> wrote:
On 23 Feb 2020, Winfield Hill wrote:

Now I'm working on high-power electronic loads.
Electronic loads should handle continuous power
for efficiency and operational measurements,
as well as rapidly-pulsed load testing.

Do we really want to burn power in testing?
Perhaps, below 100W, or for transient testing
it's the quick solution ... [snip]

We rarely load-test a gadget longer than a few
thermal time constants. Transient response
testing can be done in minutes. Why go to all
that trouble and expense to recycle 15 cents
worth of electricity? [snip]

This power supply test took about an hour:

https://www.dropbox.com/s/jh2ttpn4vzbepfz/MeanWell_UHP-500_bench.JPG?raw=1

It ran at 500 watts for maybe half an hour.

Last year I bought a birthday present, a Yokogawa
WT310E digital power meter. This instrument takes
0.1% power measurements on waveforms of any type.
It's especially well suited for accurate efficiency
measurements on things like the inefficient Meanwell
power supplies. Taking such measurements requires
a high-power load. Like John, we have collections
of big power resistors well-suited for such a task.
But the lashups are a nuisance, and not well suited
to quick flexible changes in load, so you take what
you can get by re-wiring resistors. Which usually
means one or two max-power tests. I'm looking for
better capability, like my Kikusui electronic loads.

As for operating duration, it'd be nice to run for
an hour at full power, but 30 seconds may be long
enough for transient tests, and five minutes to do
a suite of efficiency measurements. Thermal mass
is our friend. My plan is to design a PCB that
can be built up to cover all the possibilities.

Also, we can make a say 2kW heatsink, but whoa!!
How to get the heat out of the heatsink. Our lab
has big circulating-fluid chillers that can handle
that, if we ever need to implement that capability.


--
Thanks,
- Win