Switching current limiter for safe charging of large capacitors, and short circuit protection

[P E Schoen]

In a previous post about minimizing spikes on a square wave push-pull
transformer drive, my simulations discovered a very large and probably
destructive current surge due to the large capacitor load on the output of
the step-up transformer and FWB. I proposed using a brute force linear
current regulator, but the SOA of the transistors would be exceeded. I need
an output of at least 1500 watts, and the energy storage in the capacitors
(6600uF at 300V, or 990 watt-seconds) was such that I would need to use
multiple power transistors and also a controlled charge current with a long
time constant to be reasonable.

But I realized that a switching regulator would be much more efficient and
better for many reasons, so I endeavored to make one using LTSpice. The
basic premise was to apply the battery voltage to an inductor and read the
current, and then switch off the drive when it became a certain maximum
value, such as 100A, and then turn on again when it dropped to about 80A. I
found that a reasonable value was 4.7 uH and I found a commercially
available model good for 60A for about $13.
http://www.newark.com/vishay-dale/ihth1125mzeb4r7m5a/inductor-4-7uh-20-55a/dp/51R6660?Ntt=51R6660

At first I tried using a PMOS but the standard models did not have one that
was suitable. I also had difficulty finding a good gate driver, and the ones
I tried to concoct from op-amps were too slow and caused peak power
dissipation of many kW in the MOSFET, for a significant period of time. So I
changed the design to use an NMOS and a high-side driver:
http://www.newark.com/linear-technology/ltc4440ems8e-pbf/ic-mosfet-gate-driver-high-side/dp/56M9553

So my simulation shows a peak current of about 90A and the voltage at the
transformer stabilizes within about 1.5 mSec with a 2200 uF capacitor. Then
I turn on the gate drives to the MOSFETs for the transformer, and the
current limit goes into effect again, with a maximum current of 150A, while
the series MOSFET dissipates about 35 watts. The circuit oscillates at about
220 kHz. It stabilizes by 90 mSec at which point the battery is supplying
1.15 kW and the output resistor load is 1.06 kW, for an efficiency of 92%.
This includes the current limiter which is about 10W, the switching
transistors which are 20W each, 6W in the inductor, and 15W in the two
output capacitors which are still charging.

Following is the ASC file. I'll have to give this a try before I do any more
testing with the DC-DC converter. And I can probably do the same thing,
essentially, by modulating the gate drives of the transformer driver
MOSFETs. In that case, I will probably need to leave the inductor in the
center tap of the transformer to the battery. But for now, the current
limiter seems to work well, and it may be a good device to make as a
stand-alone current limiter for working with batteries.

Paul

=========================================================

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[Fred Abse]

Following is the ASC file. I'll have to give this a try before I do any more
testing with the DC-DC converter. And I can probably do the same thing,
essentially, by modulating the gate drives of the transformer driver
MOSFETs. In that case, I will probably need to leave the inductor in the
center tap of the transformer to the battery. But for now, the current
limiter seems to work well, and it may be a good device to make as a
stand-alone current limiter for working with batteries.

I see about 24V of HF hash at M3 drain. Apparently due to L5 resonating
with various capacitances.

BTW, what was the thinking behind specifying both series *and* parallel
parasitic resistances for L5? They're each just a different way of
specifying the same quantity. If the 350 ohms is a physical damping
resistor, it should ideally show on the circuit.

Any reason why you restricted the number of cycles of drive to M1 and M2,
rather than letting it run for the full simulation time?

Why not just use a simple precharge resistor, switched out with a
contactor, or maybe a triac? That's the way the VFD guys do it.

Otherwise, your suggestion of PWM-ing the gate drives would work. There
are driver ICs available that current limit in just that way.

Please, pretty please, get rid of that Greek "mu" (Control Panel - Netlist
Options).

 

[P E Schoen]

wrote in message

looked at something like a IR2086 ?

That may be a good candidate, but it's not recommended for new designs.
However, there are similar parts. I found a reference design for a 300W
DC-DC converter: http://www.irf.com/technical-info/userguide/ug-0601.pdf

This is mostly a conceptual design. Actually an SG3526 would probably work
well enough. But I'd like to use a PIC so I can combine other features. I'm
using a PIC16F684 for the PWM drive now (with gate drivers), but its PWM is
not suited for push-pull except at 50% duty cycle. Also its internal
comparator conflicts with the PWM module. The PIC16F616 has a different
comparator module so it might be suited to use as a hardware-based
current-limited start-up. But I may also consider using something like the
PIC18F2331 which has the three-phase motor control module.

Another candidate might be http://www.linear.com/product/LT1683. It is a low
noise push-pull device with integrated MOSFET drivers, programmable slew
rate, overcurrent protection, and other goodies. I'm not fond of the tiny
package with 0.65mm pitch, but I have worked with them in prototypes. No
problem for automated production assembly, however.

Thanks,

Paul

 

[P E Schoen]

"Jan Panteltje" wrote in message

Since you are a PIC user, a current and voltage regulated supply:
http://panteltje.com/panteltje/pic/pwr_pic/index.html

For more current use bigger inductors (physicaly),
and bigger MOSFETS, and a real MOSFET driver.

I have used this as lab supply now for more than year,
and it is absolutely the greatest,

That looks like a nice unit. But I'm not sure about using a SEPIC to boost
the voltage from 12, 24, 36 or 48VDC to 320 or 640VDC. That's a really hefty
inductor, and I think it would be bigger and heavier and more difficult to
design than a transformer. And I really don't need output adjustment or
regulation. All I want is basically a DC transformer about 15/1 ratio.

Thanks,

Paul

 

[P E Schoen]

"Fred Abse" wrote in message

I see about 24V of HF hash at M3 drain. Apparently due to L5 resonating
with various capacitances.

BTW, what was the thinking behind specifying both series *and* parallel
parasitic resistances for L5? They're each just a different way of
specifying the same quantity. If the 350 ohms is a physical damping
resistor, it should ideally show on the circuit.

Those were just from a model that I chose from the inductor menu. Then I
changed the resistance and inductance to match a larger one that I found. It
would probably simulate more quickly by removing those parasitic values.

Any reason why you restricted the number of cycles of drive to M1 and M2,
rather than letting it run for the full simulation time?

It was taking like 1/2 hour to run 100 mSec, so I usually stopped it once it
seemed to settle.

Why not just use a simple precharge resistor, switched out with a
contactor, or maybe a triac? That's the way the VFD guys do it.

That may be a viable option. I figure I would need about 0.5 ohms for 50
amps at 24V. About 1200 watts to start. Or I could put it on the output,
which is about 300V and 4A, so a 75 ohm will work. With the original 6600 uF
the TC would be 0.5 seconds, so about 2.5 seconds until it reaches full
voltage. I could use an SPDT relay which would first just charge the
capacitors and then remove the resistance and apply the voltage to the VF
drive. But if I don't use the doubler I can just use the VFD's internal
capacitors, which are probably about 200 uF. I could still monitor its bus
voltage and keep the resistor in the circuit until it reaches about 250V,
and then kick it out. The VFD draws only about 200 mA or less without a
motor load.

That's probably the easiest thing to do now, just to get something working.
Since I already have the PIC, I can monitor the output voltage and just keep
the current limit resistor on until it comes up. Maybe I can use a MOSFET or
my original choice of a power darlington on the input side, and use like a 5
ohm resistor to charge the 2200 uF capacitor to 24V. Then I could use a
fairly small relay for the output capacitor charging. I'll have to work out
the details, and see how it responds to a short circuit failure. Hopefully I
can switch it to current limit mode before it reaches extreme levels, but
high enough to trip the breaker quickly enough. And of course a fuse for
apocalyptic situations

Otherwise, your suggestion of PWM-ing the gate drives would work.
There are driver ICs available that current limit in just that way.

I will look into those before I make a final design. My purposes now are
more to get something working so I can determine just how much power I need
for my tractor under various conditions.

Please, pretty please, get rid of that Greek "mu" (Control Panel - Netlist
Options).

I thought I had done that. But maybe I updated LTSpice since then. And I
also found that, to take effect, I had to change something in the schematic
and then save again. Sorry...

Thanks,

Paul

 

[legg]

In a previous post about minimizing spikes on a square wave push-pull
transformer drive, my simulations discovered a very large and probably
destructive current surge due to the large capacitor load on the output of
the step-up transformer and FWB. I proposed using a brute force linear
current regulator, but the SOA of the transistors would be exceeded. I need
an output of at least 1500 watts, and the energy storage in the capacitors
(6600uF at 300V, or 990 watt-seconds) was such that I would need to use
multiple power transistors and also a controlled charge current with a long
time constant to be reasonable.

But I realized that a switching regulator would be much more efficient and
better for many reasons, so I endeavored to make one using LTSpice. The
basic premise was to apply the battery voltage to an inductor and read the
current, and then switch off the drive when it became a certain maximum
value, such as 100A, and then turn on again when it dropped to about 80A. I
found that a reasonable value was 4.7 uH and I found a commercially
available model good for 60A for about $13.
http://www.newark.com/vishay-dale/ihth1125mzeb4r7m5a/inductor-4-7uh-20-55a/dp/51R6660?Ntt=51R6660

At first I tried using a PMOS but the standard models did not have one that
was suitable. I also had difficulty finding a good gate driver, and the ones
I tried to concoct from op-amps were too slow and caused peak power
dissipation of many kW in the MOSFET, for a significant period of time. So I
changed the design to use an NMOS and a high-side driver:
http://www.newark.com/linear-technology/ltc4440ems8e-pbf/ic-mosfet-gate-driver-high-side/dp/56M9553

So my simulation shows a peak current of about 90A and the voltage at the
transformer stabilizes within about 1.5 mSec with a 2200 uF capacitor. Then
I turn on the gate drives to the MOSFETs for the transformer, and the
current limit goes into effect again, with a maximum current of 150A, while
the series MOSFET dissipates about 35 watts. The circuit oscillates at about
220 kHz. It stabilizes by 90 mSec at which point the battery is supplying
1.15 kW and the output resistor load is 1.06 kW, for an efficiency of 92%.
This includes the current limiter which is about 10W, the switching
transistors which are 20W each, 6W in the inductor, and 15W in the two
output capacitors which are still charging.

Following is the ASC file. I'll have to give this a try before I do any more
testing with the DC-DC converter. And I can probably do the same thing,

essentially, by modulating the gate drives of the transformer driver
MOSFETs. In that case, I will probably need to leave the inductor in the
center tap of the transformer to the battery. But for now, the current
limiter seems to work well, and it may be a good device to make as a
stand-alone current limiter for working with batteries.

Paul

As the asc file doesn't include the modifications mentioned,
specifically in the inrush limiting section, you shouldn't expect this
proposed schematic to be much use to others. As the load doesn't look
like a battery and is fixed resistive - the simulation does not
illustrate the the battery charging condition or loading effects.
Supposedly you intend to controll these things at some point,
reflecting the mfr's charge and float recommendations for the
batteries used.

It's probably not a good idea to let the RC values of the current
sensor set the switching frequency. This is actually the source of the
first single cycle surge at turn-on, in the posted asc file, as the C
in this sensor's filter charges to an initial value that is not
related to actual switch current. How you handle this as the driver
changes (removing a known fixed hysterisis and altering the threshold
to larger logic levels) is not indicated.

The integrated driver mentioned will not respond nicely to load values
less than the peak limit, as it's gate drive bootstrap source requires
periodic refreshing.

The choice of unsynchronized frequencies can be an issue, as may be
the audible frequency intended for the DC-DC transformer section. The
former introduces a periodic flux imbalance in the downstream
transformer. The latter will just be annoying.

To get a better idea of snubbing energy losses in switching the DC-DC
section, you might apply more realistic coupling coefficients, as the
high power coupler is unlikely to aproach 0.998 .

It's not a wise idea to have two voltage sources (ie capacitive
storage or batteries) on both sides of a DC-DC transformer. At some
time this will look like a short circuit, resulting in incontrollable
current peaks in the switches at both switching and control
frequencies.

RL

 

[P E Schoen]

wrote in message

Haven't looked at the sim yet but I know right off that I wouldn't even
consider this approach. Since the actual current limit is not that
critical,
I would monitor the VDS drop across the RDS of the on MOSFET and
ground the gate drive at IDS of approximately 60A. All you need is some
HCT logic working in conjunction with your PIC, UCC27321(?) drivers, and
a dual comparator. Then hang a slo-blow fuse off the BATT(+) to kill
everything in the event of a hard failure.

I have updated the sim file to change the Greek mu to "u" and it's posted
at:
http://www.enginuitysystems.com/pix/DCDC_CurLim_NMOS.asc

I have receive a lot of good ideas here and I just need to look at what is
easiest to get this working, first, and then for a possibly commercially
viable, reliable design. Such a device may be very useful for EVs,
especially smaller ones like tractors, where it will enable the use of
standard three phase motors and controllers with just a few 12V deep cycle
batteries which are easily available and well-suited to tractors, because
weight is not a problem and cost is much less than lithium. For a tractor,
if it is used only once or twice a week, the batteries should last up to 5
years. For cars, however, the lithium cells may be more cost-effective over
the long run.

Thanks,

Paul

 

[P E Schoen]

"John Larkin" wrote in message

Why not just soft-start the inverter that you already have?

I may do that. But for now I'm trying to avoid a major redesign.

Why not use smaller caps?

The two 3300 uF capacitors were used for my doubler circuit, where I used
12VDC to get 300VDC. But for 24VDC I used a simple FWB, and I have a
position on my switch to leave these capacitors out. Then I will still have
the capacitors inside the VFD, but I think they are probably about 200 uF
and much less problem than 6600 uF.

Thanks,

Paul

 

[Tim Williams]

Why not just do something like this?
http://webpages.charter.net/dawill/tmoranwms/Circuits_2010/12-24_Converter.png

Obviously you'll need something a lot slower, 494 should be okay at KHz
though. The loop components need to be slower of course.

Also, notice the output choke. Forgetting to include it (and a current
limit or soft start) destroys power supplies in exactly the way your
simulations have shown.

The 100uF 50V Vpeak snub cap probably needs film caps in parallel with it.

Tim

--
Deep Friar: a very philosophical monk.
Website: http://webpages.charter.net/dawill/tmoranwms

In a previous post about minimizing spikes on a square wave push-pull
transformer drive, my simulations discovered a very large and probably
destructive current surge due to the large capacitor load on the output of
the step-up transformer and FWB. I proposed using a brute force linear
current regulator, but the SOA of the transistors would be exceeded. I need
an output of at least 1500 watts, and the energy storage in the capacitors
(6600uF at 300V, or 990 watt-seconds) was such that I would need to use
multiple power transistors and also a controlled charge current with a long
time constant to be reasonable.

But I realized that a switching regulator would be much more efficient and
better for many reasons, so I endeavored to make one using LTSpice. The
basic premise was to apply the battery voltage to an inductor and read the
current, and then switch off the drive when it became a certain maximum
value, such as 100A, and then turn on again when it dropped to about 80A. I
found that a reasonable value was 4.7 uH and I found a commercially
available model good for 60A for about $13.
http://www.newark.com/vishay-dale/ihth1125mzeb4r7m5a/inductor-4-7uh-20-55a/dp/51R6660?Ntt=51R6660

At first I tried using a PMOS but the standard models did not have one that
was suitable. I also had difficulty finding a good gate driver, and the ones
I tried to concoct from op-amps were too slow and caused peak power
dissipation of many kW in the MOSFET, for a significant period of time. So I
changed the design to use an NMOS and a high-side driver:
http://www.newark.com/linear-technology/ltc4440ems8e-pbf/ic-mosfet-gate-driver-high-side/dp/56M9553

So my simulation shows a peak current of about 90A and the voltage at the
transformer stabilizes within about 1.5 mSec with a 2200 uF capacitor. Then
I turn on the gate drives to the MOSFETs for the transformer, and the
current limit goes into effect again, with a maximum current of 150A, while
the series MOSFET dissipates about 35 watts. The circuit oscillates at about
220 kHz. It stabilizes by 90 mSec at which point the battery is supplying
1.15 kW and the output resistor load is 1.06 kW, for an efficiency of 92%.
This includes the current limiter which is about 10W, the switching
transistors which are 20W each, 6W in the inductor, and 15W in the two
output capacitors which are still charging.

Following is the ASC file. I'll have to give this a try before I do any more
testing with the DC-DC converter. And I can probably do the same thing,
essentially, by modulating the gate drives of the transformer driver
MOSFETs. In that case, I will probably need to leave the inductor in the
center tap of the transformer to the battery. But for now, the current
limiter seems to work well, and it may be a good device to make as a
stand-alone current limiter for working with batteries.

Paul

=========================================================

Version 4
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SYMATTR SpiceLine Rser=100u
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SYMATTR Value 20k
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SYMBOL res -784 336 R0
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SYMATTR InstName R12
SYMATTR Value 220
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SYMATTR Value 0.47µ
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SYMBOL voltage -1216 336 R0
WINDOW 123 0 0 Left 2
WINDOW 39 -32 107 Left 2
WINDOW 3 -14 53 Left 2
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SYMATTR InstName V4
SYMATTR Value 10
TEXT 32 88 Left 2 !K1 L1 L2 L3 0.998
TEXT -536 576 Left 2 !.tran 0 400m 0 2u startup
TEXT 216 -24 Left 2 ;Primary 2x8 turns 2V/turn at 600 Hz

 

[P E Schoen]

"Tim Williams" wrote in message

Why not just do something like this?
http://webpages.charter.net/dawill/tmoranwms/Circuits_2010/12-24_Converter.png

Obviously you'll need something a lot slower, 494 should be okay at
KHz though. The loop components need to be slower of course.

Also, notice the output choke. Forgetting to include it (and a current
limit or soft start) destroys power supplies in exactly the way your
simulations have shown.

The 100uF 50V Vpeak snub cap probably needs film caps in parallel with it.

Yes, the peak snubber was suggested by Fred Bloggs and it seems to work
well. It might be possible to capture the energy otherwise lost by replacing
the 50 ohm 5W resistor with a DC-DC converter and use it to charge the
batteries.

I'd probably use an SG3526 which can drive MOSFET gates directly. But even
better may be the LT1683. Also I think the TC to the CL input on the circuit
you show is too large to be effective. 1 mSec even on a 2kHz converter may
allow several pulses to happen before it shuts down. I'd rather have
cycle-by-cycle monitoring and PWM foldback.

Thanks,

Paul

 

[Tim Williams]

Also I think the TC to the CL input on the circuit
you show is too large to be effective. 1 mSec even on a 2kHz converter
may

allow several pulses to happen before it shuts down. I'd rather have
cycle-by-cycle monitoring and PWM foldback.

You don't, actually. Cycle by cycle control almost always leads to
instability. In fact, the classic peak-current-mode flyback (e.g., UC3842)
is a classical instance of the logistic function: it exhibits chaos with all
the trimmings.

Continuous mode current limiting is subject to all the normal stability
criteria but that's just a choice of R and C. There's no way to combat
chaos.

Tim

 

[Joerg]

In a previous post about minimizing spikes on a square wave push-pull
transformer drive, my simulations discovered a very large and probably
destructive current surge due to the large capacitor load on the output
of the step-up transformer and FWB. I proposed using a brute force
linear current regulator, but the SOA of the transistors would be
exceeded. I need an output of at least 1500 watts, and the energy
storage in the capacitors (6600uF at 300V, or 990 watt-seconds) was such
that I would need to use multiple power transistors and also a
controlled charge current with a long time constant to be reasonable.

But I realized that a switching regulator would be much more efficient
and better for many reasons, so I endeavored to make one using LTSpice.
The basic premise was to apply the battery voltage to an inductor and
read the current, and then switch off the drive when it became a certain
maximum value, such as 100A, and then turn on again when it dropped to
about 80A. I found that a reasonable value was 4.7 uH and I found a
commercially available model good for 60A for about $13.
http://www.newark.com/vishay-dale/ihth1125mzeb4r7m5a/inductor-4-7uh-20-55a/dp/51R6660?Ntt=51R6660


At first I tried using a PMOS but the standard models did not have one
that was suitable. I also had difficulty finding a good gate driver, and
the ones I tried to concoct from op-amps were too slow and caused peak
power dissipation of many kW in the MOSFET, for a significant period of
time. So I changed the design to use an NMOS and a high-side driver:
http://www.newark.com/linear-technology/ltc4440ems8e-pbf/ic-mosfet-gate-driver-high-side/dp/56M9553


So my simulation shows a peak current of about 90A and the voltage at
the transformer stabilizes within about 1.5 mSec with a 2200 uF
capacitor. Then I turn on the gate drives to the MOSFETs for the
transformer, and the current limit goes into effect again, with a
maximum current of 150A, while the series MOSFET dissipates about 35
watts. The circuit oscillates at about 220 kHz. It stabilizes by 90 mSec
at which point the battery is supplying 1.15 kW and the output resistor
load is 1.06 kW, for an efficiency of 92%. This includes the current
limiter which is about 10W, the switching transistors which are 20W
each, 6W in the inductor, and 15W in the two output capacitors which are
still charging.

Following is the ASC file. I'll have to give this a try before I do any
more testing with the DC-DC converter. And I can probably do the same
thing, essentially, by modulating the gate drives of the transformer
driver MOSFETs. In that case, I will probably need to leave the inductor
in the center tap of the transformer to the battery. But for now, the
current limiter seems to work well, and it may be a good device to make
as a stand-alone current limiter for working with batteries.

Pretty soon you'll have a pre-pre-pre-regulator and things still go
bzzzt

Why do you want to do all this padding around a circuit that could
easily brought to a level where those problems simply go away? If it was
my project I'd do this:

a. An inductor between rectifier and caps. Makes things a _ton_
smoother. It's like the Zoloft or Xanax for bridge and half-bridge
converters. Play around with it:

http://schmidt-walter.eit.h-da.de/smps_e/hgw_smps_e.html

b. Cycle-by-cycle current limiting. If the PIC is too slow then ditch it
and use a real converter chip. UCC-something, TL-something or
LT-something. If you absolutely want to stay with this old laminated
core monster transformer (I really don't understand why) then you'd need
a chip that can get low enough in frequency without requiring lots of
megohms for the timing resistors. Or one that can take an external clock.

c. Consider a ferrite core. Much less turns, smaller, your required
capacitance cuts down to a fraction and Muttley will be much happier
because there's no buzzing in his ears if you keep it well above 40kHz.

Look at the bright side: Now the whole thing becomes so much smaller
that a li'l beer cooler fits into the saved space and the saved energy
can be used to operate the Peltier in that cooler

 

[Fred Abse]

"Fred Abse" wrote in message




That may be a viable option. I figure I would need about 0.5 ohms for 50
amps at 24V. About 1200 watts to start. Or I could put it on the output,
which is about 300V and 4A, so a 75 ohm will work. With the original 6600
uF the TC would be 0.5 seconds, so about 2.5 seconds until it reaches
full voltage. I could use an SPDT relay which would first just charge the
capacitors and then remove the resistance and apply the voltage to the VF
drive. But if I don't use the doubler I can just use the VFD's internal
capacitors, which are probably about 200 uF. I could still monitor its bus
voltage and keep the resistor in the circuit until it reaches about 250V,
and then kick it out. The VFD draws only about 200 mA or less without a
motor load.

That's probably the easiest thing to do now, just to get something
working. Since I already have the PIC, I can monitor the output voltage
and just keep the current limit resistor on until it comes up. Maybe I can
use a MOSFET or my original choice of a power darlington on the input
side, and use like a 5 ohm resistor to charge the 2200 uF capacitor to
24V. Then I could use a fairly small relay for the output capacitor
charging. I'll have to work out the details, and see how it responds to a
short circuit failure. Hopefully I can switch it to current limit mode
before it reaches extreme levels, but high enough to trip the breaker
quickly enough. And of course a fuse for apocalyptic situations

That isn't *quite* what I meant. I meant a precharge resistor in series
with C1 and C3, since that's where the excessive load on startup is. That
will reduce the current capability requirement for your switching device,
contactor or triac, and probably make for more reliable starting.

 

[P E Schoen]

"Joerg" wrote in message

Pretty soon you'll have a pre-pre-pre-regulator and things
still go bzzzt

Why do you want to do all this padding around a circuit that could
easily brought to a level where those problems simply go away? If
it was my project I'd do this:

a. An inductor between rectifier and caps. Makes things a _ton_
smoother. It's like the Zoloft or Xanax for bridge and half-bridge
converters. Play around with it:

http://schmidt-walter.eit.h-da.de/smps_e/hgw_smps_e.html

The problem I see with that is the square wave drive through the transformer
and FWB does not leave any OFF time, and the inductor quickly saturates. Of
course if I use a duty cycle PWM then it might work. But it will be huge if
I use 2kHz or so.

b. Cycle-by-cycle current limiting. If the PIC is too slow then
ditch it and use a real converter chip. UCC-something,
TL-something or LT-something. If you absolutely want to stay
with this old laminated core monster transformer (I really don't
understand why) then you'd need a chip that can get low enough
in frequency without requiring lots of megohms for the timing
resistors. Or one that can take an external clock.

That may be the best. But probably not for 2kHz. The start-up duty cycle
would be very small, unless I added a large inductor.

c. Consider a ferrite core. Much less turns, smaller, your required
capacitance cuts down to a fraction and Muttley will be much happier
because there's no buzzing in his ears if you keep it well above 40kHz.

There are several reasons for using the laminated core toroid.

(1) Just to see what its limitations and possibilities were.
(2) It is more rugged, and better suited to off-road tractor use
(3) Since I may want to offer this as an open-source kit, DIYers can
probably wind their own toroids more easily than a ferrite core, and you can
get toroid cores from junk and surplus, even burned up Powerstats. So cost
may be much less and it's easy to scale up or down as needed.

Look at the bright side: Now the whole thing becomes so much
smaller that a li'l beer cooler fits into the saved space and the
saved energy can be used to operate the Peltier in that cooler

I'm not sure how much smaller it would be using a ferrite core. I have some
1000 watt switching power supplies and they are at least as big and heavy as
what I have now. I can remove the big capacitors and replace them with some
much smaller ones, especially if I can rely on the VF converter's
capacitors. And I think I can greatly reduce the size of the heat sinks from
the monsters I have now. It seems like the losses in the MOSFETs will be no
more than 20-50 watts. I think the easiest thing for now may be the switched
power resistors on the input and/or the output.

For now I just want to avoid the big current surges that are probably
blowing the MOSFETs. I think I have the inductive spikes under control. Then
I want to be able to actually run the tractor under various conditions to
see what is really needed in terms of batteries and motor size. I'm also
going to investigate the effect of removing the gearbox and adding ball
bearing supports for the rear axle shafts. I plan to use a datalogger to
obtain readings of current, voltage, motor RPM, ground speed, and various
temperatures, to determine the overall efficiencies.

I may also consider building a VFD for such applications, and include the
DC-DC conversion and all monitoring and control, so you basically just hook
up a battery pack and a three-phase motor in place of the ICE, and you're
good to go. Also I'll look at using a dedicated three-phase motor for
various implements, particularly mower decks, to see just how much power
they consume, and maybe find an optimum speed related to ground speed for
the highest efficiency.

There are many DIY conversions, but very few use induction motors. Most
still use series wound brushed DC motors, although there are a lot of BLDC
projects, but the motors and controllers are expensive and often
problematic. Also the use of 96V to 700V series battery packs poses a safety
problem as well as a nightmare to maintain. Having multiple power packs with
no more than 48VDC battery inputs greatly improves safety, since all the
high voltage is produced by an active electronic device which can quickly
shut down in case of emergency. There is a real problem with EVs when they
are involved in a crash, and first responders must deal with the possibility
of encountering dangerous high voltages as they try to extricate survivors.
But these concepts are a subject for another discussion, although there are
some very interesting threads in:
http://www.mytractorforum.com/index.php
http://www.diyelectriccar.com/forums/
http://forums.aeva.asn.au/default.asp?C=10&title=technical-discussion

Thanks,

Paul

 

[Joerg]

"Joerg" wrote in message




The problem I see with that is the square wave drive through the
transformer and FWB does not leave any OFF time, and the inductor
quickly saturates. Of course if I use a duty cycle PWM then it might
work. But it will be huge if I use 2kHz or so.

The average current through that inductor can never be more than your
average load current. The larger the inductance the lower the ripple on
it and the smoother things are.

You need variable off-time in order to muffle the huge inrush current.
If the PIC can't do that I'd ditch it.

That may be the best. But probably not for 2kHz. The start-up duty cycle
would be very small, unless I added a large inductor.

It's easiest at low frequencies. I've tried your sim with 30usec on
times and a 2mH inductor. Makes for a nice smooth ramp-up, and the
current rise is so slow that even a PIC should be ble to register it.

There are several reasons for using the laminated core toroid.

(1) Just to see what its limitations and possibilities were.
(2) It is more rugged, and better suited to off-road tractor use

Why that? The heavier something is the higher the chance that it shears
off and goes sailing if you hit a tree stump or the bottom of a ravine

(3) Since I may want to offer this as an open-source kit, DIYers can
probably wind their own toroids more easily than a ferrite core, and you
can get toroid cores from junk and surplus, even burned up Powerstats.
So cost may be much less and it's easy to scale up or down as needed.

On the contary. Winding a toroid is the absolute bear for many. Because
nobody has a winding shuttle in their garage. A pot core with a bobbin
is so much easier plus a lot less turns at high frequency. But mind the
skin effect.

I haven't opened a PC supply in a while, not sure if they still have
bobbins.

I'm not sure how much smaller it would be using a ferrite core. I have
some 1000 watt switching power supplies and they are at least as big and
heavy as what I have now.

I have a little Statpower 300W that I use to operate the jig saw and
stuff from the car battery. 5"*6"*2", and very light.

... I can remove the big capacitors and replace
them with some much smaller ones, especially if I can rely on the VF
converter's capacitors.

Well, you could even use this converter for VF as well. That would be
really cool. But at 1-2kHz this won't work.

... And I think I can greatly reduce the size of the
heat sinks from the monsters I have now. It seems like the losses in the
MOSFETs will be no more than 20-50 watts. I think the easiest thing for
now may be the switched power resistors on the input and/or the output.

Either that or, bog-simple, a 24V headlight bulb for a European truck.

For now I just want to avoid the big current surges that are probably
blowing the MOSFETs. I think I have the inductive spikes under control.
Then I want to be able to actually run the tractor under various
conditions to see what is really needed in terms of batteries and motor
size. I'm also going to investigate the effect of removing the gearbox
and adding ball bearing supports for the rear axle shafts. I plan to use
a datalogger to obtain readings of current, voltage, motor RPM, ground
speed, and various temperatures, to determine the overall efficiencies.

I may also consider building a VFD for such applications, and include
the DC-DC conversion and all monitoring and control, so you basically
just hook up a battery pack and a three-phase motor in place of the ICE,
and you're good to go. Also I'll look at using a dedicated three-phase
motor for various implements, particularly mower decks, to see just how
much power they consume, and maybe find an optimum speed related to
ground speed for the highest efficiency.

What's the advantage versus using a DC motor?

There are many DIY conversions, but very few use induction motors. Most
still use series wound brushed DC motors, although there are a lot of
BLDC projects, but the motors and controllers are expensive and often
problematic. Also the use of 96V to 700V series battery packs poses a
safety problem as well as a nightmare to maintain. Having multiple power
packs with no more than 48VDC battery inputs greatly improves safety,
since all the high voltage is produced by an active electronic device
which can quickly shut down in case of emergency. There is a real
problem with EVs when they are involved in a crash, and first responders
must deal with the possibility of encountering dangerous high voltages
as they try to extricate survivors. But these concepts are a subject for
another discussion, although there are some very interesting threads in:
http://www.mytractorforum.com/index.php
http://www.diyelectriccar.com/forums/
http://forums.aeva.asn.au/default.asp?C=10&title=technical-discussion

I don't see a problem with DC motor of any voltage by just using 24V to
48V battery voltage or whatever is deemed safe, then using a boost
converter to get to the required motor voltage. A synchronous diode
would be nice though and since manufacturers generally do not understand
this yuo usually have to roll your own.

 

[Jasen Betts]

On the contary. Winding a toroid is the absolute bear for many. Because
nobody has a winding shuttle in their garage. A pot core with a bobbin
is so much easier plus a lot less turns at high frequency. But mind the
skin effect.

I haven't opened a PC supply in a while, not sure if they still have
bobbins.

last time I looked ferrite E-I cores for the transformeres with
layered windings on a bobbin and rod and toroid cores for the
filter inductors.

 

[Joerg]

last time I looked ferrite E-I cores for the transformeres with
layered windings on a bobbin and rod and toroid cores for the
filter inductors.

That would be excellent for a DYI project because scrapped PC power
supplies are ubiquitous. Except in some countries the electronic waste
recyclers won't sell back to the public or are prohibited to. But that
would affect laminated toroids even more because they are less prevalent
and expensive. You can only find those in older high-end audio gear.

 

[P E Schoen]

"Joerg" wrote in message

Jasen Betts wrote:

That would be excellent for a DYI project because scrapped PC power
supplies are ubiquitous. Except in some countries the electronic waste
recyclers won't sell back to the public or are prohibited to. But that
would affect laminated toroids even more because they are less
prevalent and expensive. You can only find those in older
high-end audio gear.

You can also use the cores from burned-up Powerstats.

I have, somewhere, ferrite cores and bobbins good for about 2000 watts. I
got them on eBay years ago when I first had this idea. I should find out the
cost for new units, and maybe just plan on supplying them, since they
(Lodestone Pacific) usually have a high minimum order. But the real problem
may be what sort of wire (or copper strip) to use. It gets tricky with such
high currents. And multiple windings are also a PITA.

I think I can get bare toroid cores from www.toroid.com for about $30-$50
each.

Another source for toroids is current transformers. They usually have a 5
amp secondary. So a 250:5 would have 250 turns and two windings of 8 turns
each should produce about 300V.

Paul

 

[Joerg]

"Joerg" wrote in message


You can also use the cores from burned-up Powerstats.

The question is, how many of those burn up per year? I've never had that
happen. There's usually a breaker up front.

I have, somewhere, ferrite cores and bobbins good for about 2000 watts.
I got them on eBay years ago when I first had this idea. I should find
out the cost for new units, and maybe just plan on supplying them, since
they (Lodestone Pacific) usually have a high minimum order. But the real
problem may be what sort of wire (or copper strip) to use. It gets
tricky with such high currents. And multiple windings are also a PITA.

Another method might be to try to find some from 24V -> mains converters.

I think I can get bare toroid cores from www.toroid.com for about
$30-$50 each.

50 bucks? Ouch! This looks much better but I haven't checked if any are
suitable:

http://www.surplussales.com/Inductors/FerPotC/FerPotC-2.html

Another source for toroids is current transformers. They usually have a
5 amp secondary. So a 250:5 would have 250 turns and two windings of 8
turns each should produce about 300V.

Their cores are usually way too small to carry this kind of power.

There is another trick of the trade but that will make the motor
non-isolated from the battery: Place the output side ground on top of
your battery, which adds in the battery voltage "for free". But be careful.

 

[P E Schoen]

"Joerg" wrote in message

P E Schoen wrote:

The question is, how many of those burn up per year? I've never had that
happen. There's usually a breaker up front.

I happen to have about 12 or so powerstat cores that were damaged in an
early design of ETI's breaker test set. They are Staco 2510 and were
supposed to be rated at 9.5 amps and 280V, or almost 3kVA at 60 Hz. But the
unique brush design (a thin flat carbon piece rather than the usual wedge)
overheated, cracked, and then the brush assembly landed on the windings and
caused major damage. I was going to use them in series/parallel to make a
breaker test set rated at 4000 amps and 12V.

But those cores are about 25-30 lb each. However, by running them at 2 kHz I
think it would be possible to get at least 10kVA.

Another method might be to try to find some from 24V -> mains converters.

I tried using some small ones first. They were only 175 and 300W, so they
would power the VFD and spin the motor under no load, but died under power.
My second movie about my project shows this. I was going to get a 2000 or
3000 watt unit, but they are mostly 12 VDC. And they are not isolated. There
is a 150 VDC internal bus, which is probably 300 VDC for the 220V models. So
I could probably pick up the voltage there. But they are about $1/watt. And
I already had the toroid, and the heat sinks, and the big capacitors, and
the heat sinks.

50 bucks? Ouch! This looks much better but I haven't checked if any
are suitable:

http://www.surplussales.com/Inductors/FerPotC/FerPotC-2.html

I've dealt with them before. I'll see what those look like. But I found some
on eBay that are supposed to be good for 3000 to 7000 watts:
http://www.ebay.com/itm/E80-Large-T...330?pt=LH_DefaultDomain_0&hash=item27c830594a

Their cores are usually way too small to carry this kind of power.

They are generally rated at 2VA to 50VA, but those are very conservative
ratings for 1% or better metering. They will probably handle about 5x that
at 60Hz, and I think they will also work on 2kHz to get a much higher
V/turn. I have a box of them so maybe I'll try it. If the secondary winding
can put out 300V at 5A, that's 1500 VA. And the hole in the donut is big
enough for a total of 16 turns of #10 enamel wire, good enough for 60 amps
at 24V at 50% duty cycle.

There is another trick of the trade but that will make the motor
non-isolated from the battery: Place the output side ground on top
of your battery, which adds in the battery voltage "for free". But
be careful.

That is the same principle as an autotransformer. I'm working on a design
for someone who wants to boost 48V to 96V, at 200A or more, and for a 2/1
boost it makes the booster half the size. But for a 24V to 300V, the savings
are less than 10%.

Thanks,

Paul