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

On a sunny day (Sun, 17 Jun 2012 04:05:13 -0400) it happened "P E Schoen" wrote in :

Since you are a PIC user, a current and voltage regulated supply:

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,

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).

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Haven't looked at the sim yet but I know right off that I wouldn't even con= sider 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 work= ing in conjunction with your PIC, UCC27321(?) drivers, and a dual comparato= r. Then hang a slo-blow fuse off the BATT(+) to kill everything in the even= t of a hard failure.

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

Why not use smaller caps?

snip

looked at something like a IR2086 ?

-Lasse

That may be a good candidate, but it's not recommended for new designs.=20 However, there are similar parts. I found a reference design for a 300W=20 DC-DC converter:

This is mostly a conceptual design. Actually an SG3526 would probably = work=20 well enough. But I'd like to use a PIC so I can combine other features. = I'm=20 using a PIC16F684 for the PWM drive now (with gate drivers), but its PWM = is=20 not suited for push-pull except at 50% duty cycle. Also its internal=20 comparator conflicts with the PWM module. The PIC16F616 has a different=20 comparator module so it might be suited to use as a hardware-based=20 current-limited start-up. But I may also consider using something like = the=20 PIC18F2331 which has the three-phase motor control module.

Another candidate might be

rate, overcurrent protection, and other goodies. I'm not fond of the = tiny=20 package with 0.65mm pitch, but I have worked with them in prototypes. No =

problem for automated production assembly, however.

Thanks,

Paul=20

That looks like a nice unit. But I'm not sure about using a SEPIC to = boost=20 the voltage from 12, 24, 36 or 48VDC to 320 or 640VDC. That's a really = hefty=20 inductor, and I think it would be bigger and heavier and more difficult = to=20 design than a transformer. And I really don't need output adjustment or=20 regulation. All I want is basically a DC transformer about 15/1 ratio.

Thanks,

Paul=20

any=20

thing,

the

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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=20 would probably simulate more quickly by removing those parasitic values.

M2,

It was taking like 1/2 hour to run 100 mSec, so I usually stopped it = once it=20 seemed to settle.

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 =

voltage. I could use an SPDT relay which would first just charge the=20 capacitors and then remove the resistance and apply the voltage to the = VF=20 drive. But if I don't use the doubler I can just use the VFD's internal=20 capacitors, which are probably about 200 uF. I could still monitor its = bus=20 voltage and keep the resistor in the circuit until it reaches about =

That's probably the easiest thing to do now, just to get something = working.=20 Since I already have the PIC, I can monitor the output voltage and just = keep=20 the current limit resistor on until it comes up. Maybe I can use a = MOSFET or=20 my original choice of a power darlington on the input side, and use like = a 5=20 ohm resistor to charge the 2200 uF capacitor to 24V. Then I could use a=20 fairly small relay for the output capacitor charging. I'll have to work = out=20 the details, and see how it responds to a short circuit failure. = Hopefully I=20 can switch it to current limit mode before it reaches extreme levels, = but=20 high enough to trip the breaker quickly enough. And of course a fuse for =

apocalyptic situations :)

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=20 for my tractor under various conditions.

Netlist

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=20 and then save again. Sorry...

Thanks,

Paul=20

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

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some

and

I have updated the sim file to change the Greek mu to "u" and it's = posted=20 at:

I have receive a lot of good ideas here and I just need to look at what = is=20 easiest to get this working, first, and then for a possibly commercially =

viable, reliable design. Such a device may be very useful for EVs,=20 especially smaller ones like tractors, where it will enable the use of=20 standard three phase motors and controllers with just a few 12V deep = cycle=20 batteries which are easily available and well-suited to tractors, = because=20 weight is not a problem and cost is much less than lithium. For a = tractor,=20 if it is used only once or twice a week, the batteries should last up to =

Thanks,

Paul=20

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

The two 3300 uF capacitors were used for my doubler circuit, where I = used=20

Thanks,

Paul=20

Why not just do something like this?

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:

"P E Schoen" wrote in message news:jrk33q$18u$ snipped-for-privacy@dont-email.me... 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.

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:

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

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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current=20

it.

Yes, the peak snubber was suggested by Fred Bloggs and it seems to work=20 well. It might be possible to capture the energy otherwise lost by = replacing=20 the 50 ohm 5W resistor with a DC-DC converter and use it to charge the=20 batteries.

I'd probably use an SG3526 which can drive MOSFET gates directly. But = even=20 better may be the LT1683. Also I think the TC to the CL input on the = circuit=20 you show is too large to be effective. 1 mSec even on a 2kHz converter = may=20 allow several pulses to happen before it shuts down. I'd rather have=20 cycle-by-cycle monitoring and PWM foldback.

Thanks,

Paul=20

On a sunny day (Sun, 17 Jun 2012 17:42:56 -0400) it happened "P E Schoen" wrote in :

Use a transformer. You know, less turns primary, more turns secundary, to up the voltage? I was not giving you a ready design, just some hints on 'current limiting' and programming. Sorry if that confused you.

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

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.

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.

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

I have, somewhere, ferrite cores and bobbins good for about 2000 watts. = I=20 got them on eBay years ago when I first had this idea. I should find out = the=20 cost for new units, and maybe just plan on supplying them, since they=20 (Lodestone Pacific) usually have a high minimum order. But the real = problem=20 may be what sort of wire (or copper strip) to use. It gets tricky with = such=20 high currents. And multiple windings are also a PITA.

I think I can get bare toroid cores from

Another source for toroids is current transformers. They usually have a =

Paul=20