250A electronic load

[Tim Williams]

"Arie de Muynck" <no.spam@no.spam.org> wrote in message
news:5e55367f$0$10279$e4fe514c@news.xs4all.nl...

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?

Nice thing about switching resistors at low voltages, you don't much care
what the gate voltage is, as long as it's enough extra to account for the
resistor's drop.

That, or use differential sense.

Actually, differential sense is probably a requirement if you want any
accuracy (better than 1%, 0.1%?) out of this -- PCB resistances will screw
things up, down in that range.

Tim

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

 

[Winfield Hill]

jlarkin@highlandsniptechnology.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

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

Yes. We got away w/o differential on the single PCB, with
its massive ground, but that can't work with multiple banks.

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'll likely not do more than four banks/section, but might
place at least two. We'll see.

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

How do you heat sink a DPak to 25 watts?


--
Thanks,
- Win

 

[Winfield Hill]

Tim Williams wrote...

Oh-- just to clarify, it's a unary power DAC. Paired with
a 10-bit unary ADC ... the resistors can be controlled via
MCU as well.)

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

Can you tell us more about your power resistors, what values
and specs, etc. Were these in 1-2-4 steps, etc.?


--
Thanks,
- Win

 

[John Larkin]

On 25 Feb 2020 10:24:58 -0800, Winfield Hill <winfieldhill@yahoo.com>
wrote:

jlarkin@highlandsniptechnology.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

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

Yes. We got away w/o differential on the single PCB, with
its massive ground, but that can't work with multiple banks.

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'll likely not do more than four banks/section, but might
place at least two. We'll see.

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

How do you heat sink a DPak to 25 watts?

Hang it off the edge of the board onto the heat sink (or route a hole
in the board) like the fets.

I figure maybe 5 watts or so max, using regular mounting with some
thermal vias.


--

John Larkin Highland Technology, Inc
picosecond timing precision measurement

jlarkin att highlandtechnology dott com
http://www.highlandtechnology.com

 

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.

For high power (kW/MW) dummy loads feeding back the power to the AC
network with an inverter makes sense.

However, in this case, we are talking about only 1-2 kW, i.e. electric
stove power levels. However already at these power level, you can
evaporate a few kettles of water in a long time testing. So getting
rid of the heat during long time testing is an issue.

In this thread, there has ben some discussion, should the power be
dissipated in a resistor or in the junction of a transistor.

Power resistor can handle up to 400 C temperatures, while silicon
transistors can handle only 150 or 200 C at best. From the heatsink
design point of view, dissipating the same heat power from a resistor
to the environment is much easier.

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

 

[Winfield Hill]

upsidedown@downunder.com wrote...

For high power (kW/MW) dummy loads feeding back the
power to the AC network with an inverter makes sense.

Now there's a good idea. But it'd take a special kind
of inverter control system. Maybe one meant for solar
panel roofs. I'm not knowledgeable about how they work.

> Evaporate kettles of water ...

Another good idea, improve low winter-time humidity.


--
Thanks,
- Win

 

[Winfield Hill]

jlarkin@highlandsniptechnology.com wrote...

Last time I did something like this, I brought in
the control signal differential.

Updated, to include yours and Arie's suggestions.

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


--
Thanks,
- Win

 

[Jasen Betts]

On 2020-02-25, Tim Williams <tiwill@seventransistorlabs.com> wrote:

"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.)

ti says LM3914 is still active.

--
Jasen.

 

[Tim Williams]

"Winfield Hill" <winfieldhill@yahoo.com> wrote in message
news:r33tcr0203q@drn.newsguy.com...

Can you tell us more about your power resistors, what values
and specs, etc. Were these in 1-2-4 steps, etc.?

1k 225W x 10. Unary == equal values. Operating range 100-500V 0-4A (and
less current at lower voltages).

The linear sinks are ballasted by smaller resistors (100R 100W x 3), which
should improve capacity and fault tolerance. At the expense of operating
range of course, but I feel the low end is an acceptable sacrifice for a
high voltage load.

Output is also slightly filtered (RC), polarity protected, MOV'd and fused.
You could literally plug it into the wall, forever (i.e. including rare
lightning-induced transients) and not have a problem (fingers crossed).

Tim

P.S. I just noticed, you poor bastard, slumming it on a Yahoo e-mail?
Harvard's really fallen these days... :^)

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

 

[Winfield Hill]

Jasen Betts wrote...

On 2020-02-25, Tim Williams <tiwill@seventransistorlabs.com> wrote:
"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.)

TI says LM3914 is still active.

Cute. Not sure how it works tho.


--
Thanks,
- Win

 

[Tim Williams]

<upsidedown@downunder.com> wrote in message
news:5m7b5fledis9cnulptvbal873u8rm6ldu5@4ax.com...

For high power (kW/MW) dummy loads feeding back the power to the AC
network with an inverter makes sense.

I'd love to make one of those some time, but it's obviously a bit of a pain
to put together.

However, in this case, we are talking about only 1-2 kW, i.e. electric
stove power levels. However already at these power level, you can
evaporate a few kettles of water in a long time testing. So getting
rid of the heat during long time testing is an issue.

Dumping it into car batteries is another option; I did a project a bunch of
years ago, the client was conditioning NiMH cells so I put a lead-acid
"accumulator" on the back end and a bunch of synchronous buck converters in
front. Discharge one cell while charging the other and so on, and the
converters power each other for the most part; the main bus really only
needs a trickle charge to account for charging losses.

In this thread, there has ben some discussion, should the power be
dissipated in a resistor or in the junction of a transistor.

Power resistor can handle up to 400 C temperatures, while silicon
transistors can handle only 150 or 200 C at best. From the heatsink
design point of view, dissipating the same heat power from a resistor
to the environment is much easier.

Resistors are a clear win on size, as you get a heck of a lot more heat into
the same volume of air, or heatsink, at 250C+ than you do at 100-150C. If
you don't mind that the exhaust is hot enough to ignite some materials...

The budgetary win is less clear. Heatsinks aren't the cheapest to buy or
machine or assemble, but resistors need mounting, too. Resistors on custom
combo brackets would be best, but that costs NRE; mounting with the standard
individual clips, isn't much better than heatsinks in terms of hardware
parts count and assembly time. It may turn out a wash.

For Win's case, the low voltage is probably easier to dissipate with
transistors, and the desired dynamic range won't work so well with
resistors.

Tim

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

 

[whit3rd]

On Tuesday, February 25, 2020 at 3:56:17 AM UTC-8, Winfield Hill wrote:

Thankfully, most all modern current-sense resistors
are low-inductance.

The inductance isn't much of a problem, because it can be exactly nulled.
Series inductance adds a voltage that is proportional to what a Rogowski coil picks up,
and you can print the compensation element next to the sense resistor on the PCB.

 

[Winfield Hill]

Tim Williams wrote...

P.S. I just noticed, you poor bastard, slumming it on
a Yahoo e-mail? Harvard's really fallen these days... :^)

My Harvard email account is clogged with over 100k emails.
Mostly all legitimate, sadly. I use Yahoo to get work done.


--
Thanks,
- Win

 

[Winfield Hill]

Tim Williams wrote...

For Win's case, the low voltage is probably easier to
dissipate with transistors, and the desired dynamic
range won't work so well with resistors.

But heat sinks are a struggle. The cute WA-T247-101E
heatsink on my RIS-796 250A pulser is only good for 7.5
watts (100C) or maybe 10 watts (too hot). The pulser can
do 60 volts and 400A = 24kW. I set it to its 30A minimum
for a test, set the supply to 15V, and turned it on. The
TO-247 MOSFET didn't mind the full 450W power level, but
smoke immediately poured from the 20mR ballast resistor.
Rated at 3W, it couldn't handle 18 watts, even for a few
seconds. I had to use under 2% duty-cycle for the tests.
In operation with 10V across the MOSFET, 250A pulses are
limited by the cute heatsink to under 0.4% duty cycle.


--
Thanks,
- Win

 

On Wednesday, 26 February 2020 07:52:16 UTC, Tim Williams wrote:

upsidedown@downunder.com> wrote in message
news:5m7b5fledis9cnulptvbal873u8rm6ldu5@4ax.com...

In this thread, there has ben some discussion, should the power be
dissipated in a resistor or in the junction of a transistor.

Power resistor can handle up to 400 C temperatures, while silicon
transistors can handle only 150 or 200 C at best. From the heatsink
design point of view, dissipating the same heat power from a resistor
to the environment is much easier.

Resistors are a clear win on size, as you get a heck of a lot more heat into
the same volume of air, or heatsink, at 250C+ than you do at 100-150C. If
you don't mind that the exhaust is hot enough to ignite some materials...

temp just depends on the fan, it's not hard to keep 2kW at a safe temp.

The budgetary win is less clear. Heatsinks aren't the cheapest to buy or
machine or assemble, but resistors need mounting, too. Resistors on custom
combo brackets would be best, but that costs NRE; mounting with the standard
individual clips, isn't much better than heatsinks in terms of hardware
parts count and assembly time. It may turn out a wash.

For Win's case, the low voltage is probably easier to dissipate with
transistors, and the desired dynamic range won't work so well with
resistors.

Tim

A combination of both has advantages.

Someone mentioned power R cost somewhere upthread: it can be cheaper to use an array of lower P devices, if you have the space. Not that that's news, but it can wipe out a lot of the R cost.


NT