24V-300V, 1000W DC-DC Push-pull simulation with precharge 90% efficient

[P E Schoen]

You're probably getting tired of this but I've come up with a simulation
that seems like it should reasonably match real-world performance. I added a
0.5 ohm precharge resistor which is bypassed with an NMOS FET when the
output exceeds about 200VDC, and it greatly reduces the initial inrush
current and high power levels seen previously. I found that it worked better
by eliminating the inductor at the output and relying only on the leakage
inductance, which I simulated with two 20nH inductors in series with the
primary legs. I tried various ways of dealing with the high voltage
transients but two RCD snubbers seemed to do the job. At close to 50% duty
cycle the rectified square wave is almost pure DC and the 40uF output
capacitor smoothes the output well enough during the approximate 2 uSec dead
time at 16 kHz.

I had another design which used a MOSFET and some zeners as a 66V high
current shunt regulator and it seemed to work well also as a snubber. I
could also possibly use some TVS diodes.

For a practical design I might use a relay in place of the precharge MOSFET
switch, but it would need to handle over 100A and a couple of MOSFETs (or
even one) should work. There are lots of inexpensive devices with 2 mOhm or
less and voltage of 30-40V which should be enough. So here are the
simulation images and ASC file:

http://www.enginuitysystems.com/pix/24V-300V_CT_Final-1000W.png
http://www.enginuitysystems.com/pix/24V-300V_CT_Final-1000W-20mSec.png
http://www.enginuitysystems.com/pix/24V-300V_CT-16kHz_Final-1000W.asc

Now I suppose I'll need to build the thing and see how it works. I'll try to
take better inductance readings on the transformer first, and then re-run
the simulation if needed. I'll also probably add some current feedback and
shutdown in case of overload.

Paul

 

[Joerg]

You're probably getting tired of this but I've come up with a simulation
that seems like it should reasonably match real-world performance. I
added a 0.5 ohm precharge resistor which is bypassed with an NMOS FET
when the output exceeds about 200VDC, and it greatly reduces the initial
inrush current and high power levels seen previously. I found that it
worked better by eliminating the inductor at the output and relying only
on the leakage inductance, which I simulated with two 20nH inductors in
series with the primary legs. I tried various ways of dealing with the
high voltage transients but two RCD snubbers seemed to do the job. At
close to 50% duty cycle the rectified square wave is almost pure DC and
the 40uF output capacitor smoothes the output well enough during the
approximate 2 uSec dead time at 16 kHz.

I had another design which used a MOSFET and some zeners as a 66V high
current shunt regulator and it seemed to work well also as a snubber. I
could also possibly use some TVS diodes.

For a practical design I might use a relay in place of the precharge
MOSFET switch, but it would need to handle over 100A and a couple of
MOSFETs (or even one) should work. There are lots of inexpensive devices
with 2 mOhm or less and voltage of 30-40V which should be enough. ...

That's what the marketing types say. In reality one has to contend with
sending all the current across a tiny source pin. In your case I see an
average current of almost 50A. So if you absolutly have to use this
brute force method use many FETs.

Also, take a look at the dissipation in M3 peak around 5.1msec. This
peak is half a millisecond wide and goes to almost a kilowatt. That can
greatly exceed the SOA of many devices.

... So here are the simulation images and ASC file:

http://www.enginuitysystems.com/pix/24V-300V_CT_Final-1000W.png
http://www.enginuitysystems.com/pix/24V-300V_CT_Final-1000W-20mSec.png
http://www.enginuitysystems.com/pix/24V-300V_CT-16kHz_Final-1000W.asc

Now I suppose I'll need to build the thing and see how it works. I'll
try to take better inductance readings on the transformer first, and
then re-run the simulation if needed. I'll also probably add some
current feedback and shutdown in case of overload.

Why can't you do the soft-start with the controller? Just make short
pulses in the beginning. There should also be an inductor between D10
and C3, right now you are cramming large current spikes into C3 which
causes increases losses in that cap and the diodes.

 

[legg]

You're probably getting tired of this but I've come up with a simulation
that seems like it should reasonably match real-world performance. I added a
0.5 ohm precharge resistor which is bypassed with an NMOS FET when the
output exceeds about 200VDC, and it greatly reduces the initial inrush
current and high power levels seen previously. I found that it worked better
by eliminating the inductor at the output and relying only on the leakage
inductance, which I simulated with two 20nH inductors in series with the
primary legs. I tried various ways of dealing with the high voltage
transients but two RCD snubbers seemed to do the job. At close to 50% duty
cycle the rectified square wave is almost pure DC and the 40uF output
capacitor smoothes the output well enough during the approximate 2 uSec dead
time at 16 kHz.

http://www.enginuitysystems.com/pix/24V-300V_CT-16kHz_Final-1000W.asc

Hooo boy.

Take a look at the output current waveform, before it hits the
zener/cap/load.

Take a look at the primary current waveform.

All energy transfer occurs in the first 15uSec of the switching phase
period. No energy transfer occurs in the later part of the switching
phase, as primary current ramps linearly.

You've made a push-pull flyback. Yeesh.

This is where you hope that the simulation reflects real hardware,
after you've gone over all the nodes to check for realistic stress
levels.

Those 120V drain spikes might be possible, if the fets's withstand
them without clipping - if they do clip, thats an unanticipated fet
loss and a reduction in the di/dt of the associated circuitry.

The ringing during actual energy transfer may be more lossy than
expected, and include unanticipated harmonics, if any clipping or
snubber/rectifier current reversal actually results.

re: volts per turn. This is a very loose expression but usually
applies as rms/N. You are unlikely to come across a practical example
of 60V/turn in your work. RMS and average values of a displayed period
are easily extracted by your current simulator software GUI by
cntrl/clicking any plotted item.

Higher volts per turn will generate core losses in an exponential
relationship. Peak flux, and proximity to saturation will exhibit an
inverse relationship. Below 40KHz, most ferrite cores are saturation
limited, in commercially available sizes.

RL

 

[P E Schoen]

"legg" wrote in message

Hooo boy.

Take a look at the output current waveform, before it hits the
zener/cap/load.

Take a look at the primary current waveform.

All energy transfer occurs in the first 15uSec of the switching
phase period. No energy transfer occurs in the later part of
the switching phase, as primary current ramps linearly.

You've made a push-pull flyback. Yeesh.

This is where you hope that the simulation reflects real hardware,
after you've gone over all the nodes to check for realistic stress
levels.

Those 120V drain spikes might be possible, if the fets's
withstand them without clipping - if they do clip, that’s
an unanticipated fet loss and a reduction in the di/dt of
the associated circuitry.

The ringing during actual energy transfer may be more
lossy than expected, and include unanticipated harmonics,
if any clipping or snubber/rectifier current reversal actually
results.

re: volts per turn. This is a very loose expression but usually
applies as rms/N. You are unlikely to come across a practical
example of 60V/turn in your work. RMS and average values
of a displayed period are easily extracted by your current
simulator software GUI by cntrl/clicking any plotted item.

Higher volts per turn will generate core losses in an exponential
relationship. Peak flux, and proximity to saturation will exhibit an
inverse relationship. Below 40KHz, most ferrite cores are saturation
limited, in commercially available sizes.

Actually I had meant to do this simulation using the tape wound silicon
steel toroid that I used in my original design. The 60 V/turn was
extrapolated from that and I was trying to see if a transformer with one or
two primary turns of bus bar might be feasible.

So I changed the transformer to my actual measured toroid which I have
tested at 16 kHz, and I ran the simulation again, with 10 nH as the leakage
inductance. The results were actually much better. The power into the load
went up to almost 1200 watts and the efficiency was about 99%. Here are the
plots and ASC files:

http://www.enginuitysystems.com/pix/24V-300V_CT-16kHz_Toroid_16x100-1000W..asc
http://www.enginuitysystems.com/pix/24V-300V_CT_16kHz_Toroid_16x100.png
http://www.enginuitysystems.com/pix/24V-300V_CT_16kHz_Toroid_16x100_Transition.png
http://www.enginuitysystems.com/pix/24V-300V_CT_16kHz_Toroid_16x100_20mSec.png

I do see some problems at the transition, but otherwise things seem to be
OK. But I thought 10nH might be too low for the transformer, so I used
100nH. It seemed even better, perhaps. And I slowed down the transition by
increasing the gate capacitor on M3, and it reduced the peak power spikes to
more reasonable levels. I'll probably use two or more MOSFETs for that
switch, as well as the push-pull drivers. But I was very surprised to find
that they apparently dissipate only about 3 watts each. Here's an updated
plot over 40 mSec:

http://www.enginuitysystems.com/pix/24V-300V_CT_16kHz_Toroid_16x100_100nH_40mSec.png

I still need to look more closely at the waveforms at critical points and
see what I may need to do to reduce the high power transients which will
exceed the SOA.

Thanks.

Paul

 

[Jamie]

You're probably getting tired of this but I've come up with a simulation
that seems like it should reasonably match real-world performance. I
added a 0.5 ohm precharge resistor which is bypassed with an NMOS FET
when the output exceeds about 200VDC, and it greatly reduces the initial
inrush current and high power levels seen previously. I found that it
worked better by eliminating the inductor at the output and relying only
on the leakage inductance, which I simulated with two 20nH inductors in
series with the primary legs. I tried various ways of dealing with the
high voltage transients but two RCD snubbers seemed to do the job. At
close to 50% duty cycle the rectified square wave is almost pure DC and
the 40uF output capacitor smoothes the output well enough during the
approximate 2 uSec dead time at 16 kHz.

I had another design which used a MOSFET and some zeners as a 66V high
current shunt regulator and it seemed to work well also as a snubber. I
could also possibly use some TVS diodes.

For a practical design I might use a relay in place of the precharge
MOSFET switch, but it would need to handle over 100A and a couple of
MOSFETs (or even one) should work. There are lots of inexpensive devices
with 2 mOhm or less and voltage of 30-40V which should be enough. So
here are the simulation images and ASC file:

http://www.enginuitysystems.com/pix/24V-300V_CT_Final-1000W.png
http://www.enginuitysystems.com/pix/24V-300V_CT_Final-1000W-20mSec.png
http://www.enginuitysystems.com/pix/24V-300V_CT-16kHz_Final-1000W.asc

Now I suppose I'll need to build the thing and see how it works. I'll
try to take better inductance readings on the transformer first, and
then re-run the simulation if needed. I'll also probably add some
current feedback and shutdown in case of overload.

Paul

I admire your consistency of getting the inverter to work. If more
people would put that much effort in their work, the world would be a
better place!

But may I suggest something? I have made such an inverter in the past
and suffered as you are with the high wheeling voltage using power fets.

That type of Push Pull inverter works well as long as you keep some
current in the primary field between alternations. This saturation
current or where ever the current maybe at the time helps to reduce the
effects (dump), have you played with over lapping gate signals?

I made one before that simply had delayed trigger on the reference
where each ending cycle would trigger the start of the next for the
other phase however, I used a PIC chip to control that because I also
was doing current monitoring via a ADC to set output demands for soft
ramps and the frequency naturally varied. As current demand drops off,
the overlap got narrow and as current demands increased, I would
increase the overlap time. This made it so that when the secondary side
was up where it belong (low PI error), the current damned on the primary
was low and there was no need for a over lap to speak of, which makes
for low quiescence current.

Have you considered using a "Buck Boost" type of supply? We made one
that had to operate from a 36 volt system and generate ~480 volts DC for
a AC drive on a battery operating equipment. The AC motor was small,only
1/4 so we didn't need a lot there however, we used high voltage fets,
500V (Yes, very close but they worked) types that handles 18 Amps.

The system at 36 volts needed no more than 10 amps from the battery.
We used 4 of those MOSFETS on a widely spread heat sink to keep things
cools. It was most likely over kill but it worked find.
Btw, we used 4 separate coils, one for each MosFet and a switching
diode on each one all tied to gather to the HV cap output side filter.

We also used current mode sensing on that, too. The only trick there
is, to make sure your full magnetizing current has a maximum time limit
before it switches, incase the feed back loop error is greater than the
supply can deliver. Cases like low battery and out of parameter range,
otherwise, you'll get a latch up. Ect..

Have a good day..

Jamie

 

[legg]

So I changed the transformer to my actual measured toroid which I have
tested at 16 kHz, and I ran the simulation again, with 10 nH as the leakage
inductance. The results were actually much better. The power into the load
went up to almost 1200 watts and the efficiency was about 99%. Here are the
plots and ASC files:

http://www.enginuitysystems.com/pix/24V-300V_CT-16kHz_Toroid_16x100-1000W.asc
http://www.enginuitysystems.com/pix/24V-300V_CT_16kHz_Toroid_16x100.png
http://www.enginuitysystems.com/pix/24V-300V_CT_16kHz_Toroid_16x100_Transition.png
http://www.enginuitysystems.com/pix/24V-300V_CT_16kHz_Toroid_16x100_20mSec.png

So you're back into straight DC-DC transformer territory.

If all of your inrush is due to the output filter cap, as you propose,
then why not stick the switched inrush limiter in series with 'it' ?
That way it only has to deal with this specific inrush path, and
doesn't have to deal with continuous power train current, just the
ripple.

Such a control can be on the low side, tailored by whatever small
signal bells and whistles you desire. This isn't an isolated
application, after all.

RL

 

[legg]

<snip>

Like this, as a starting point, anyways:
.............................
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FLAG -656 608 0
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FLAG 592 240 in
FLAG -128 144 m1
FLAG 112 368 m2
FLAG -288 464 g1
FLAG -96 288 g2
FLAG -96 560 s
FLAG 400 80 a
FLAG 400 224 b
FLAG -640 240 batt
FLAG 544 352 0
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SYMBOL ind2 272 128 R0
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SYMATTR Type ind
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TEXT 32 88 Left 0 !K1 L1 L2 L3 1
TEXT -648 576 Left 0 !.tran 0 10m 1u 1u startup
TEXT 48 656 Left 0 ;Leakage 20nH, 47nF+10R
TEXT 48 688 Left 0 ;Load Rl=80, Vout=308, 1191W/1214W = 98.1%
........................

Some wrapping text strings are removed.

 

[P E Schoen]

"legg" wrote in message

So you're back into straight DC-DC transformer territory.

If all of your inrush is due to the output filter cap, as you propose,
then why not stick the switched inrush limiter in series with 'it' ?
That way it only has to deal with this specific inrush path, and
doesn't have to deal with continuous power train current, just
the ripple.

Such a control can be on the low side, tailored by whatever
small signal bells and whistles you desire. This isn't an
isolated application, after all.

That is a good idea, and I tried the simulation you provided in the other
post. The only problem is that the output needs to connect to a VFD, which
normally contains some hefty capacitors on the DC link. But that just means
using a high-side MOSFET. Actually it may be easier to use a simple relay
which can fairly easily handle 320VDC at 3 to 5 amps. There may be some
inrush on the battery side but I really don't need large capacitors. The 40
uF capacitor was chosen for low ESR and high surge currents.
http://www.digikey.com/product-detail/en/C4ATFBW5400A3NJ/399-5949-ND/2704603

It is 40 uF 400V with 1.4 mOhm for about $14 and handles 29A RMS:
http://capacitoredge.kemet.com/capedge2/DataSheet?pn=C4ATFBW5400A3NJ

I really think the DC-DC transformer topology is the way to go. The major
problem was the high current, high voltage, and high power transients. I
still don't know if the steel core toroid will really work well with a 16
kHz square wave, but my earlier testing showed reasonably low core losses of
just about Vin^2/16, so 9W at 12V and estimated 36W at 24V.

But the transformer worked OK at 2kHz with 24V, so I would think it would
work about as well at 16kHz with 1/8 the number of turns, which is where I
got the idea for the single turn primaries. Wouldn't that reduce the core
losses? It would reduce the magnetizing inductance but the current should
stay about the same at the higher frequency. Let's see, it has about
3.2uH/sqrt(T) so 1 turn is 3.2uH which is 0.32 ohms at 24 volts is 75A.
Oops! But 8 turns is 180uH and at 2kHz that's 2.26 ohms and about 12A at
24V. Looks like I'd need 2 turns for 12.8uH and 1.28 ohms or 24A, or go to 3
turns and 28.8uH and 2.89 ohms and about 8.3A.

I see that I've not been figuring this correctly. I had always thought V/t
was constant for a given core (yes) and was linear with frequency (no).
Actually it must go by the square root of frequency, so the transformer that
had 3 volts/turn at 2 kHz would have 8.5 volts/turn at 16 kHz. My initial
estimate was based on about 0.25 V/t at 60Hz and thus 2.5 V/t at 600Hz. I
should have used 0.8 V/t, and 1.44 V/t at 2kHz. In that case, it would be 4
V/t at 16 kHz and my 8 turns is just about right for 24V. AHA!!! That makes
more sense, so that's my theory and I'm sticking to it!

Thanks,

Paul

 

[legg]

post. The only problem is that the output needs to connect to a VFD, which
normally contains some hefty capacitors on the DC link. But that just means
using a high-side MOSFET. Actually it may be easier to use a simple relay

If You want versatility in choice of load, you'll need to use a
smarter converter than unregulated DC transformer.

which can fairly easily handle 320VDC at 3 to 5 amps. There may be some
inrush on the battery side but I really don't need large capacitors. The 40
uF capacitor was chosen for low ESR and high surge currents.
http://www.digikey.com/product-detail/en/C4ATFBW5400A3NJ/399-5949-ND/2704603

It is 40 uF 400V with 1.4 mOhm for about $14 and handles 29A RMS:
http://capacitoredge.kemet.com/capedge2/DataSheet?pn=C4ATFBW5400A3NJ

As previously mentioned, filtering requirements of the unregulated DC
transformer re modest - that includes ripple current rating of the
capacitors. Ideally, no capacitors are needed.

I really think the DC-DC transformer topology is the way to go. The major
problem was the high current, high voltage, and high power transients. I
still don't know if the steel core toroid will really work well with a 16
kHz square wave, but my earlier testing showed reasonably low core losses of
just about Vin^2/16, so 9W at 12V and estimated 36W at 24V.

You've been exposed to other opinions re DC-DC transformer and have
spent some time dealing with the issues involved. If this isn't enough
to convince you otherwise........

At lower voltages, the siren call of the single turn winding is hard
to ignore. You've already run into the mechanical and coupling issues
it raises. These are separate from issues related to the use of the
topology and are a side-tracking obfuscation.

As turns are reduced incrementally, approaching one (why stop there -
why not half or other fractional turns?), it's like moving to another
planet, due to the specific problems involved and differing solutions
available in each iteration. You can end up having to build the core
around the winding, rather than vice-versa - and this is notoriously
less flexible. Check out 'Matrix transformer'.

RL

 

[Fred Abse]

The only problem is that the output needs to connect to a VFD, which
normally contains some hefty capacitors on the DC link.

Doesn't the VFD already incorporate inrush limiting?

Many do, though sometimes on a timer, rather than actual precharge sensing.

 

[Fred Abse]

If all of your inrush is due to the output filter cap, as you propose,
then why not stick the switched inrush limiter in series with 'it' ? That
way it only has to deal with this specific inrush path, and doesn't have
to deal with continuous power train current, just the ripple.

I suggested that, a while back.

 

[Fred Abse]

- but didn't do the 'work' for him. 'fait accomplis' tends to sell.

Not for me to make the horse drink. I might have done it, but couldn't
find a triac model that reflects real life. Using an FET.driven as shown,
produces a nasty dissipation burst in M3, as it goes through its active
region, at 2.5ms. Not helped by Cgs.

The whole simulation business gives me the squitters, when no hardware
is being built. It takes effort to make a simulation represent real
hardware, even after it's built.

Depends on how well you model things. I always check a modeled curve trace
of a new device model against a real life one. I sometimes check device
capacitances, too. I've known models where they were way out.

Only really useful to demonstrate basic priciples or to predict
iterations of subcircuits - and even then you need to add salt.

Useful for proof-of-concept. No substitute for hardware.

 

[legg]

I suggested that, a while back.

- but didn't do the 'work' for him. 'fait accomplis' tends to sell.

The whole simulation business gives me the squitters, when no hardware
is being built. It takes effort to make a simulation represent real
hardware, even after it's built.

Only really useful to demonstrate basic priciples or to predict
iterations of subcircuits - and even then you need to add salt.

RL

 

[P E Schoen]

"legg" wrote in message

- but didn't do the 'work' for him. 'fait accomplis' tends to sell.

The whole simulation business gives me the squitters, when no
hardware is being built. It takes effort to make a simulation
represent real hardware, even after it's built.

Only really useful to demonstrate basic priciples or to predict
iterations of subcircuits - and even then you need to add salt.

Well, in this case I actually have built the hardware, and it worked for a
while but suffered from certain problems such as high voltage spikes and
extreme current surges. So with the help of some more experienced people
here, I was able to see where my design had issues, and now after another
round of simulation, I think I am ready to rebuild the hardware and try
again.

There are some things that do not lend themselves well to simulation. I plan
to use a PIC, probably the PIC16F684 or PIC16F616, but their PWM modules are
not suited to a push-pull topology except at 50% duty cycle. I may be able
to use a low duty cycle PWM to reduce the high current surges from the
output capacitance, and I don't think the small 2HP VFD has a precharge
built in. Some, I think, use SCRs as part of the input rectifier bridge, but
that obviously will work only on AC and not where the DC link is being
powered directly.

There were certainly clues to the problems of the original circuits. When I
powered them using a 5A current limited supply, the current limit always
kicked in until the output came up to near the target voltage. But I was
expecting a surge of up to 50 amps when using the battery. The first
circuit, using a 12V battery, seemed to work OK and the 30A circuit breaker
did not usually trip when turned on. But it used the output of an LM324 to
drive the MOSFETs, so the gate drive was "soft" and that probably reduced
some of the high voltage spikes. Also, for 12V I was using a doubler output
configuration, which may or may not have reduced the stress on the
components. And I was also using 500 Hz.

I think it was when I added proper gate drivers and went up to a 24V supply
and 2 kHz that one of the MOSFETs failed. But that could have been because
of a software bug where I had forgotten about the change to inverting gate
drivers and the start-up code turned both ON for maybe 500 mSec. They might
have survived the current limited supply, but maybe not. I had a 5,000 uF
capacitor on the output of the supply so the surge currents were essentially
unlimited.

There is still much work that can be done using the simulator. I can try
running a low duty cycle for start-up and see if there are any high current
or high power transients. I could use several gate drive sources and set the
delays so that they use increasing duty cycle as the output voltage ramps
up. Or I could make a variable duty cycle driver using a ramp function and a
comparator and perhaps use the current through the MOSFETs to cut the PWM
short, cycle by cycle. Even with the PIC, this may be the best approach, and
I might be able to use the PIC's comparator and an interrupt to end the ON
cycles. This will probably work OK at 16 kHz.

The main objective at this point is to ascertain that the steel core toroid
design will work at the power level I want. Once that has been accomplished
I want to be able to "button up" this unit and just use it for my electric
tractor project as I take other measurements to see what power levels are to
be expected under various conditions. For that, I have made a PIC project
that displays various parameters on an LCD display and streams serial data
to a laptop computer for later analysis.

Eventually, I want to come up with a complete solution, with battery charger
and monitor, DC-DC converter, VFD, and an overall vehicle controller and
dashboard display. I want to use 2 or 3 lead-acid deep-cycle batteries, 105
A-H, 62 lb and $85 each, which should give me a safely usable 1800 W-Hr, and
able to operate at an average of 1-2 HP for up to two hours.

But another option is to use LiPo batteries, and get enough of them for a
300V nominal link voltage without the DC-DC. They are expensive, but I found
some where 30 pieces at $240 would give me 732 W-Hr. Since they are more
efficient and can be discharged and charged up to 10C or even 30C, they
might give me 45 minutes at 1 HP and 5-10 HP peak if needed.
http://www.diyelectriccar.com/forum...g-18650-cells-make-25kwh-battery-75941p3.html

OK, enough discussion for now. Gotta get stuff done.

Thanks,

Paul

 

[P E Schoen]

"legg" wrote in message

- but didn't do the 'work' for him. 'fait accomplis' tends to sell.

The whole simulation business gives me the squitters, when no
hardware is being built. It takes effort to make a simulation
represent real hardware, even after it's built.

Only really useful to demonstrate basic priciples or to predict
iterations of subcircuits - and even then you need to add salt.

Well, in this case I actually have built the hardware, and it worked for a
while but suffered from certain problems such as high voltage spikes and
extreme current surges. So with the help of some more experienced people
here, I was able to see where my design had issues, and now after another
round of simulation, I think I am ready to rebuild the hardware and try
again.

There are some things that do not lend themselves well to simulation. I plan
to use a PIC, probably the PIC16F684 or PIC16F616, but their PWM modules are
not suited to a push-pull topology except at 50% duty cycle. I may be able
to use a low duty cycle PWM to reduce the high current surges from the
output capacitance, and I don't think the small 2HP VFD has a precharge
built in. Some, I think, use SCRs as part of the input rectifier bridge, but
that obviously will work only on AC and not where the DC link is being
powered directly.

There were certainly clues to the problems of the original circuits. When I
powered them using a 5A current limited supply, the current limit always
kicked in until the output came up to near the target voltage. But I was
expecting a surge of up to 50 amps when using the battery. The first
circuit, using a 12V battery, seemed to work OK and the 30A circuit breaker
did not usually trip when turned on. But it used the output of an LM324 to
drive the MOSFETs, so the gate drive was "soft" and that probably reduced
some of the high voltage spikes. Also, for 12V I was using a doubler output
configuration, which may or may not have reduced the stress on the
components. And I was also using 500 Hz.

I think it was when I added proper gate drivers and went up to a 24V supply
and 2 kHz that one of the MOSFETs failed. But that could have been because
of a software bug where I had forgotten about the change to inverting gate
drivers and the start-up code turned both ON for maybe 500 mSec. They might
have survived the current limited supply, but maybe not. I had a 5,000 uF
capacitor on the output of the supply so the surge currents were essentially
unlimited.

There is still much work that can be done using the simulator. I can try
running a low duty cycle for start-up and see if there are any high current
or high power transients. I could use several gate drive sources and set the
delays so that they use increasing duty cycle as the output voltage ramps
up. Or I could make a variable duty cycle driver using a ramp function and a
comparator and perhaps use the current through the MOSFETs to cut the PWM
short, cycle by cycle. Even with the PIC, this may be the best approach, and
I might be able to use the PIC's comparator and an interrupt to end the ON
cycles. This will probably work OK at 16 kHz.

The main objective at this point is to ascertain that the steel core toroid
design will work at the power level I want. Once that has been accomplished
I want to be able to "button up" this unit and just use it for my electric
tractor project as I take other measurements to see what power levels are to
be expected under various conditions. For that, I have made a PIC project
that displays various parameters on an LCD display and streams serial data
to a laptop computer for later analysis.

Eventually, I want to come up with a complete solution, with battery charger
and monitor, DC-DC converter, VFD, and an overall vehicle controller and
dashboard display. I want to use 2 or 3 lead-acid deep-cycle batteries, 105
A-H, 62 lb and $85 each, which should give me a safely usable 1800 W-Hr, and
able to operate at an average of 1-2 HP for up to two hours.

But another option is to use LiPo batteries, and get enough of them for a
300V nominal link voltage without the DC-DC. They are expensive, but I found
some where 30 pieces at $240 would give me 732 W-Hr. Since they are more
efficient and can be discharged and charged up to 10C or even 30C, they
might give me 45 minutes at 1 HP and 5-10 HP peak if needed.
http://www.diyelectriccar.com/forum...g-18650-cells-make-25kwh-battery-75941p3.html

OK, enough discussion for now. Gotta get stuff done.

Thanks,

Paul

 

[Joerg]

"legg" wrote in message



Well, in this case I actually have built the hardware, and it worked for
a while but suffered from certain problems such as high voltage spikes
and extreme current surges. So with the help of some more experienced
people here, I was able to see where my design had issues, and now after
another round of simulation, I think I am ready to rebuild the hardware
and try again.

There are some things that do not lend themselves well to simulation. I
plan to use a PIC, probably the PIC16F684 or PIC16F616, but their PWM
modules are not suited to a push-pull topology except at 50% duty cycle.

And that's exactly where the main problems is. Every mundane switcher IC
worth its salt has a soft-start feature. Other than a little ceramic cap
it requires zero in additional parts.

I don't know what it is with these PICs. They are ok as a uC but why are
people using them as switcher chips? It makes no sense. I've seen
designs where they were used (against my advice) and very painful
compromises resulted. These things cannot do proper current mode
control, there is no good dead time control, there is no easy soft-start
solution, any loop that was being attempted was bog-slow, and to top it
all off the uC cost a lot more than a switcher chip that could have done
a much better job.

[...]

 

[[email protected]]

And that's exactly where the main problems is. Every mundane switcher IC
worth its salt has a soft-start feature. Other than a little ceramic cap
it requires zero in additional parts.

Yep, they're lifesavers.

I don't know what it is with these PICs. They are ok as a uC but why are
people using them as switcher chips? It makes no sense. I've seen
designs where they were used (against my advice) and very painful
compromises resulted. These things cannot do proper current mode
control, there is no good dead time control, there is no easy soft-start
solution, any loop that was being attempted was bog-slow, and to top it
all off the uC cost a lot more than a switcher chip that could have done
a much better job.

To show that they can? Playtime? First do no harm.

 

[legg]

And that's exactly where the main problems is. Every mundane switcher IC
worth its salt has a soft-start feature. Other than a little ceramic cap
it requires zero in additional parts.

I don't know what it is with these PICs. They are ok as a uC but why are
people using them as switcher chips? It makes no sense. I've seen
designs where they were used (against my advice) and very painful
compromises resulted. These things cannot do proper current mode
control, there is no good dead time control, there is no easy soft-start
solution, any loop that was being attempted was bog-slow, and to top it
all off the uC cost a lot more than a switcher chip that could have done
a much better job.

Looks better on a resume.

RL

 

[John Devereux]

"legg" wrote in message



Well, in this case I actually have built the hardware, and it worked for
a while but suffered from certain problems such as high voltage spikes
and extreme current surges. So with the help of some more experienced
people here, I was able to see where my design had issues, and now after
another round of simulation, I think I am ready to rebuild the hardware
and try again.

There are some things that do not lend themselves well to simulation. I
plan to use a PIC, probably the PIC16F684 or PIC16F616, but their PWM
modules are not suited to a push-pull topology except at 50% duty cycle.

And that's exactly where the main problems is. Every mundane switcher IC
worth its salt has a soft-start feature. Other than a little ceramic cap
it requires zero in additional parts.

I don't know what it is with these PICs. They are ok as a uC but why are
people using them as switcher chips? It makes no sense. I've seen
designs where they were used (against my advice) and very painful
compromises resulted. These things cannot do proper current mode
control, there is no good dead time control, there is no easy soft-start
solution, any loop that was being attempted was bog-slow, and to top it
all off the uC cost a lot more than a switcher chip that could have done
a much better job.

[...]

The one time I considered it (with an AVR) the AVR was for something
else. And it was hey, look, I can take this bit away, and that bit, and
do this in software, and even the power supply chip could go if i wanted.

 

[Joerg]

[...]

I don't know what it is with these PICs. They are ok as a uC but why are
people using them as switcher chips? It makes no sense. I've seen
designs where they were used (against my advice) and very painful
compromises resulted. These things cannot do proper current mode
control, there is no good dead time control, there is no easy soft-start
solution, any loop that was being attempted was bog-slow, and to top it
all off the uC cost a lot more than a switcher chip that could have done
a much better job.

To show that they can? Playtime? First do no harm.

But micro controllers can't. That's the problem. Unless you have a
nearly infinite horsepower in the thing which will results in high cost,
a fat chip and wasteful power consumption.