Minimizing destructive HV spikes on square wave push-pull MOSFET drive to transformer

I am not so fond of the wimpy pull-up FET in those drivers but that's another topic.

Can you post a scope shot? And schematics.

TVS'es to muffle spikes is almost like putting oil on squealing brake pads :-)

It would be best to post schematics, layout and maybe snap a picture.

A (very wild) guess would be that the transformer is not optimized for low leakage inductance and low capacitance yet, or maybe the hook-up of that or the FET bridge is a bit loose. But without something to look at, hard to say.

--
Regards, Joerg

http://www.analogconsultants.com/

RCD snubber from drain to ground helps. If you use a dV/dt snub (small capacitor, small R*C time constant), you can slow down the edge, saving switching losses in the transistor and absorbing the overshoot.

If you use a peak snub (large C, enough R to maintain DC level), the overshoot spike is clamped, dumped into the capacitor, and dissipated as heat in a dumb resistor rather than sensitive transistor junctions.

Note that a dV/dt snub carries load current while the voltage swings by. This doesn't allow the stray inductance to discharge, so you still get overshoot. To minimize overshoot, you need a much larger dV/dt snub than otherwise, or you need a peak snub as well.

Switching transient with dV/dt snub, circuit:

Waveform: Top: voltage, bottom: current 1A/V (-4 offset).

- Switch on: current blip due to diode capacitance, represented by C2. Small ringing is due to the inductance between transistor, diode and output load (the 36V supply), represented by L2. This series resonant circuit is damped by Q1's Rds(on), V3's internal resistance (typically capacitor ESR), and any loss components in the circuit (R4 in this case).

- On state: current rises; a lot of nothing else happens.

- Switch off: drain voltage rises and current falls. D2 turns on and C3 charges. Load current is transferred to C3. Technically, this waveform is described not by a triangular slope, but by a fraction of the resonance between L1 and C3.

- Damped "bouncing": as voltage rises past V3, D1 turns on and L2 charges up. Now, the waveform is described by the resonance of C3, L2 and R4. Voltage continues to rise, humps over, and falls. As the voltage falls, L2 discharges and D1 switches off. Now it goes back to the L1-C3 resonance, which makes a short spike before D1, L2 clamp it again. This repeats until the oscillation amplitude is less than Vf, at which point D1 remains on and D2 remains off; the remaining oscillation is damped by R5. This occurs after about two negative-going spikes in this case.

Ringing is well damped by R4, for simulation stability and clarity. Since, in practice, this inductance is due to stray distance between components, it is difficult to dampen directly. An alternative is an R+C damper across the capacitance, in this case D1 and Q1, to dampen the falling and rising edges, respectively.

The equivalent, applied to your circuit:

- L1 + R1 represents the load current, which in this case is delivered by transformer. During the switching transient, it can be assumed reasonably constant.

- Everything past the MOSFET and snubber is behind a transformer, so making certain connections becomes dubious (e.g., placing R4 across only the leakage inductance of a transformer!)

- L2 represents the leakage inductance of your transformer.

- V3 is the voltage which the waveform transitions to after the switching event; since you're driving nearly full duty cycle into a PP transformer, this is the opposite side MOSFET (and its body diode), which in this circuit, is simply twice the supply voltage.

The magnitude of all components can be estimated from circuit geometry and component measurements (you'll likely have to measure your transformer).

- C3 is calculated based on either the load current, delta V and desired rise time (usually 50ns to 5us depending on use; for under-100kHz stuff,

200ns or so is fine), or the desired damping characteristics.

- You want the snubber to discharge appreciably while the transistor is on, so you want 3 * R5 * C3 = t_on(min). This simulation used t_on = 10us, so I picked 5us, close enough. I picked R5 somewhat large to illustrate the type of nonlinear ringing you get from an underdamped diode type snubber.

The waveforms with a peak voltage snubber look similar, except:

- Rising edge is fast (obviously, it's normal)

- A fast ringing overshoot spike, determined by MOSFET-diode-capacitor path inductance as the snubber diode "grabs" on

- "Top" voltage is determined by the capacitor, looking like a 1/4 sine wave (relatively small capacitance) or a flat clamped flyback pulse (high capacitance limit)

- When the leakage is done discharging, voltage relaxes back to its steady value (i.e., the opposing half of the square wave), with ringing determined by stray (diode and transistor junction) capacitance.

Tim

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

"P E Schoen"  wrote in message 
news:jqraib$jm0$1@dont-email.me...
I had posted about success with fast MOSFET gate drivers on my 1500 VA DC-DC
converter. The UCC27321 drivers provide up to 9 amps so the MOSFETs now
switch on and off very quickly compared to the previous drivers, which were
actually gates of an LM324 op-amp. So, although I got better efficiency and
I was able to apply 25 VDC, the 30 volt TVS diodes from each drain to the 25
volt supply overheated, and I removed them.

The output waveform of the transformer was a very clean square wave with a
little bit of ringing during the 1 uSec dead-time transition, but the drain
waveform showed very high spikes at the transition. I was able to reduce the
amplitude somewhat, by adding a capacitor across the drains, as well as a
snubber, but one of the MOSFETs failed shorted. It was probably damaged
before I added the snubber and capacitor. But even after I replaced it, and
I added capacitors from drain to common, there were still unacceptably large
spikes, especially as I increased the voltage from my power supply.

There is also certainly a large current spike due to the load capacitors
being charged with a square wave, but I do have a 100A 100mV shunt from the
sources to common, and an external interrupt that should shut down the PWM
if the current is greater than 100A, but it could easily be much higher
before the PWM removes the gate drive. And then there would be a much larger
dead time. The MOSFETs are HUF75645 which are rated at 100V and 75A
continuous, and at least 450A pulse current for 10uSec. Turn on and turn off
times about 200 nSec. I have two in parallel on each leg of the push-pull.

So, I'm looking at ways to minimize these high voltage transients without
reducing efficiency too much. Some ideas are:

1. Adding capacitance to maintain the current flow through the inductor
during transition. This may be most efficient, but it will cause a lot of
ringing. And the capacitors need to be able to carry the maximum current of
the transformer primary, which could be as high as 50 amps or so. Maybe a
lot more if the current is reflected from the output capacitor charging.

2. Using RC snubbers to limit the peak voltage as well as dissipate the
energy in the resistor. I have tried a single snubber of 0.047uF and 2 ohms
across the primaries, which helped a little. But from my simulation it
seemed like it might be better to add snubbers across the D-S of each
MOSFET. Not really sure of the component values. For 50A peak the 2 ohms
will limit the spike to 100V. That's 5000W but only for less than 1uSec out
of a 250uSec pulse (2 kHz), so average power is about 5000/250 = 20W.
Probably much less.

3.Using TVS diodes to limit the voltage spikes to, say, 80V. The TVS diodes
I used were two 15V in series, so they were trying to suppress the spike at
a lower voltage than necessary, so they got hot. And that was before the
capacitors and snubbers. Probably a good idea to add them, and then design
the snubbers to keep them from absorbing any more than occasional spikes.

4. Slowing down the transitions of the gate drives. These high spikes seemed
to appear after I added the fast gate drivers, and the previous slower
drivers seemed to work well enough, although not as efficiently. So I can
probably add resistance and capacitance to cause slower turn-on and
turn-off, which will essentially quench the inductive spike in the MOSFETs.
Also reduced efficiency, but easier to dissipate the heat in the large heat
sinks than much smaller TVS diodes or snubber resistors.

I'm going to research some of the application notes on MOSFETs and gate
drivers, but some discussion would be appreciated. At least this is actually
about some "cool" electronics and not OT squabbling.

Thanks,

Paul

Have you tried the Parallel RC gate driving network? THe shunt cap should give you a quick response of gate charge change and the R should suppress the ring or greatly reduce it.

Jamie

Tim:

Thanks for the very detailed and helpful reply. I need some time to = absorb=20 all of that and respond, but in the meantime I am attaching a simplified =

LTSpice ASC file that fairly accurately duplicates the waveforms I see = on my=20 scope. I changed the coupling K factor to 0.998. I'm not sure how to = measure=20 the leakage inductance, but using an LCR meter I read 180uH for each = primary=20 winding and 32mH for the secondary, and I used those values for the=20 simulation.

I added 100 ohms gate resistors, but it did not seem to help much. And=20

0.047uF capacitors from drain to ground also didn't help much. I had 80V =

peaks at 20V in, so it will be dangerously close to 100V with 25-28VDC = input=20 (two batteries). Also, the peaks seemed even much higher during = start-up.

In the simulation, I tried various values of gate resistors and = capacitors=20 from gate to ground. The actual gate capacitance is about 4000pF, but I = had=20 to add 0.1uF and 100 ohm to reduce the spikes, especially during = startup.=20 This is an RC TC of about 10uSec, but the linear region is probably = about=20

1/10 that with 10V gate drive, so maybe 1uSec out of 250uSec. The = simulation=20 shows only about 5 watts in each MOSFET, with 330W output, so it does = not=20 seriously affect efficiency. I show 10 watts in each capacitor, and = overall=20 efficiency of 91.6%. So maybe this is a good way to reduce the spikes.

And I can also add the TVS diodes, just to be safe. As long as they = don't=20 get more than warm, it's OK. I'd rather lose a couple watts than the=20 MOSFETs. :) BTW, as I was testing I encountered another overcurrent = fault,=20 and thought I had blown another MOSFET. But I found that the driver had=20 partially shut down so it was effectively driving only one side. It = appeared=20 to be leakage on the perfboard. I cleaned it with alcohol, detergent, = and=20 water, touched up some solder connections, and it works much better.

I did a test with a 300 ohm load and I got an actual efficiency of 75% = with=20

8.8V in at 5.1A (supply current limit). Subtracting the core loss I get = 92%.=20 Once I'm sure I have the spikes under control, I can apply the batteries = and=20 a real load. I'm looking forward to actually taking a ride on my = electric=20 tractor!

Thanks,

Paul

=3D=3D=3D=3D=3D=3D=3D=3D=3D=3D=3D=3D=3D=3D=3D=3D=3D=3D=3D=3D=3D=3D=3D=3D=3D= =3D=3D=3D=3D=3D=3D=3D=3D=3D=3D=3D=3D=3D=3D=3D=3D=3D=3D=3D=3D=3D=3D=3D=3D=3D= =3D=3D=3D=3D=3D=3D=3D=3D=3D=3D=3D=3D=3D=3D=3D Version 4 SHEET 1 880 680 WIRE 256 96 224 96 WIRE 256 128 256 96 WIRE 320 128 256 128 WIRE 432 128 384 128 WIRE 496 128 432 128 WIRE 544 128 496 128 WIRE 560 128 544 128 WIRE 592 128 560 128 WIRE -32 144 -160 144 WIRE 32 144 -32 144 WIRE 96 144 32 144 WIRE 128 144 96 144 WIRE 496 144 496 128 WIRE 224 192 224 96 WIRE 320 208 288 208 WIRE 432 208 432 128 WIRE 432 208 384 208 WIRE 496 208 464 208 WIRE 592 208 592 128 WIRE -640 240 -704 240 WIRE -576 240 -640 240 WIRE -496 240 -576 240 WIRE -448 240 -496 240 WIRE 128 240 128 224 WIRE 128 240 -448 240 WIRE 32 256 32 208 WIRE 128 256 128 240 WIRE 544 256 544 128 WIRE 544 256 496 256 WIRE 288 272 288 208 WIRE 288 272 224 272 WIRE 320 272 288 272 WIRE 416 272 384 272 WIRE -320 288 -384 288 WIRE -80 288 -256 288 WIRE 496 288 496 256 WIRE -640 320 -640 240 WIRE -704 336 -704 240 WIRE -384 336 -384 288 WIRE -256 336 -256 288 WIRE 32 336 0 336 WIRE 80 336 32 336 WIRE 112 336 80 336 WIRE 128 336 112 336 WIRE 80 352 80 336 WIRE 160 352 80 352 WIRE -496 368 -496 240 WIRE -32 368 -32 144 WIRE 80 368 -32 368 WIRE 160 368 160 352 WIRE 256 368 256 128 WIRE 320 368 256 368 WIRE 416 368 416 272 WIRE 416 368 384 368 WIRE 464 368 464 208 WIRE 464 368 416 368 WIRE 496 368 496 352 WIRE 496 368 464 368 WIRE 592 368 592 288 WIRE 592 368 496 368 WIRE -160 384 -160 144 WIRE 0 384 0 336 WIRE 592 384 592 368 WIRE -576 416 -576 240 WIRE -320 464 -320 288 WIRE -208 464 -320 464 WIRE -80 464 -80 288 WIRE -48 464 -80 464 WIRE -704 528 -704 400 WIRE -640 528 -640 384 WIRE -640 528 -704 528 WIRE -576 528 -576 480 WIRE -576 528 -640 528 WIRE -544 528 -576 528 WIRE -496 528 -496 448 WIRE -496 528 -544 528 WIRE -384 528 -384 416 WIRE -384 528 -496 528 WIRE -320 528 -384 528 WIRE -256 528 -256 416 WIRE -256 528 -320 528 WIRE -240 528 -256 528 WIRE -80 528 -240 528 WIRE 80 528 80 512 WIRE 80 528 -80 528 WIRE 160 528 160 512 WIRE 160 528 80 528 WIRE -240 560 -240 528 WIRE -160 560 -160 480 WIRE 0 560 0 480 WIRE 0 560 -160 560 WIRE -544 608 -544 528 FLAG -544 608 0 FLAG 592 384 0 FLAG 560 128 Vout FLAG -448 240 in FLAG 96 144 m1 FLAG 112 336 m2 SYMBOL ind2 112 128 R0 SYMATTR InstName L1 SYMATTR Value 180=B5 SYMATTR Type ind SYMATTR SpiceLine Rser=3D100u SYMBOL ind2 112 240 R0 WINDOW 0 45 35 Left 2 WINDOW 3 41 61 Left 2 SYMATTR InstName L2 SYMATTR Value 180=B5 SYMATTR Type ind SYMATTR SpiceLine Rser=3D100u SYMBOL ind2 240 176 M0 WINDOW 0 21 -5 Left 2 WINDOW 3 -9 113 Left 2 SYMATTR InstName L3 SYMATTR Value 32m SYMATTR Type ind SYMATTR SpiceLine Rser=3D10m SYMBOL nmos -208 384 R0 SYMATTR InstName M1 SYMATTR Value IRFZ44N SYMBOL nmos -48 384 R0 SYMATTR InstName M2 SYMATTR Value IRFZ44N SYMBOL voltage -496 352 R0 WINDOW 123 0 0 Left 2 WINDOW 39 24 132 Left 2 SYMATTR SpiceLine Rser=3D8m SYMATTR InstName V1 SYMATTR Value 24 SYMBOL diode 384 288 M270 WINDOW 0 32 32 VTop 2 WINDOW 3 0 32 VBottom 2 SYMATTR InstName D2 SYMATTR Value MUR460 SYMBOL diode 320 224 R270 WINDOW 0 32 32 VTop 2 WINDOW 3 0 32 VBottom 2 SYMATTR InstName D3 SYMATTR Value MUR460 SYMBOL polcap 480 288 R0 WINDOW 3 24 64 Left 2 SYMATTR Value 1000=B5 SYMATTR InstName C1 SYMATTR Description Capacitor SYMATTR Type cap SYMATTR SpiceLine V=3D400 Irms=3D30 Rser=3D0.02 Lser=3D0 SYMBOL res 576 192 R0 SYMATTR InstName R1 SYMATTR Value 300 SYMBOL voltage -384 320 R0 WINDOW 123 0 0 Left 2 WINDOW 39 -43 57 Left 2 WINDOW 3 228 271 Left 2 SYMATTR SpiceLine Rser=3D100 SYMATTR Value PULSE(0 10 0.5u 10n 10n 499u 1000u 100) SYMATTR InstName V2 SYMBOL voltage -256 320 R0 WINDOW 123 0 0 Left 2 WINDOW 39 -43 57 Left 2 WINDOW 3 100 298 Left 2 SYMATTR SpiceLine Rser=3D100 SYMATTR Value PULSE(0 10 500.5u 10n 10n 499u 1000u 100) SYMATTR InstName V3 SYMBOL diode 320 144 R270 WINDOW 0 32 32 VTop 2 WINDOW 3 0 32 VBottom 2 SYMATTR InstName D1 SYMATTR Value MUR460 SYMBOL diode 384 384 M270 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3 36 40 Left 2 SYMATTR InstName R2 SYMATTR Value 2 SYMBOL cap -336 464 R0 SYMATTR InstName C6 SYMATTR Value 0.1=B5 SYMATTR SpiceLine V=3D250 Rser=3D100u SYMBOL cap -96 464 R0 SYMATTR InstName C7 SYMATTR Value 0.1=B5 SYMATTR SpiceLine V=3D250 Rser=3D100u SYMBOL cap 144 368 R0 SYMATTR InstName C8 SYMATTR Value .047=B5 SYMATTR SpiceLine V=3D250 Rser=3D100u SYMBOL cap 64 368 R0 SYMATTR InstName C9 SYMATTR Value .047=B5 SYMATTR SpiceLine V=3D250 Rser=3D100u SYMBOL res 64 528 M180 WINDOW 0 36 76 Left 2 WINDOW 3 36 40 Left 2 SYMATTR InstName R4 SYMATTR Value 2 SYMBOL res 144 528 M180 WINDOW 0 36 76 Left 2 WINDOW 3 36 40 Left 2 SYMATTR InstName R5 SYMATTR Value 2 TEXT 32 88 Left 2 !K1 L1 L2 L3 0.998 TEXT -536 576 Left 2 !.tran 0 200m 0 1u startup TEXT -352 88 Left 2 ;Primary 2x8 turns 2V/turn at 600 Hz=20

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Is the transformer bifilar wound? This would minimises the leakage inductance between the two halves of the centre-tapped coil - if you were using a centre-tapped coil.

Bifilar winding means winding the centre-tapped winding with twisted pair, which gives you horrible inter-winding capacitance (twisted-pair usually runs at about 150pF per metre) but absolutely minimal leakage inductance (provided that you take the leads off the transformer as twisted pair and only separate them by the bare minimum at the driving transistors (and the current loop represented by the alternative paths through the driving transistors should be minimised too).

Sorry if this is irrelevant, but I once educated Tony Williams on this point (to my immense surprise) and I've acted as if not everybody has absorbed this information ever since.

-- Bill Sloman, Nijmegen

I see your point, but the amount of energy is fairly small, and it is=20 difficult to convert it to a usable form. The zener type TVS, at least, = can=20 handle repetitive surges, unlike the ZnO type surge suppressors that = degrade=20 on each spike and eventually fail.

The schematic is essentially as posted in the simulation. The gate = drives=20 are from a PIC16F684 PWM at 50% duty cycle and 1 uSec dead time.

You can see the transformer and the circuit board and wiring in the=20 following video:

At that time I was running at 500 Hz and the gate drives were from an = LM324.=20 When I tried to run it again, with the battery freshly charged, the TVS=20 diodes burned up and shorted. It seems that the spikes are higher during =

start-up and also the higher battery voltage may have contributed.

The transformer has 100 turns of about #18 AWG wound as the first layer=20 (secondary) on the toroidal core, which was then wrapped with Mylar. = Then=20 the two primary windings were added, using 8 turns each of #10 AWG. = There is=20 not much room left through the core, so it's about maximum fill. It is = not=20 wrapped very tightly, as you can see. I'm trying to determine the pros = and=20 cons of using an iron core transformer at higher frequencies for more = power=20 in the same size. So I expect to get about 1500 VA out of it, which is = about=20

3 times its rating at 60 Hz. I was hoping to be able to get as much as = 3-4=20 kVA, but I think the copper losses will be too large. Possibly a toroid = with=20 a larger hole will allow larger wire, and at the higher frequency not as =

much iron will be needed.

Of course the best design might be using ferrite and 100kHz or higher. = But=20 then there are are other design problems. And you must define "better".

Thanks,

Paul=20

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try changing the name of mosfet to 'irfz44n_withbv' and add the spiceline:

.model irfz44n_withbv ako: irfz44n bv=3D55

-Lasse

Which simulation? If it was a post many moons ago that would have rolled off here.

Why so much? That's a ton of dead time during which things can do plenty of ringading. Can't remember any switcher design of mine that had more than 100nsec.

Muttley looked kind of disappointed that he didn't get a ride :-)

One issue is visible in the video, the rest is hard to see: A few wires from the transformer are flopping about, way too long. They need to be close to the core in an orderly fashion. When they head towards the electronics they must be twisted or at least parallel. Else you'll have lots on unwanted leakage inductance and ... spikes.

Can't see the secondary but try to arrange that primary as mention above. Same for the secondary. You should see a noticeable reduction in spikes. Then it'll be on to the other wiring, that needs some close-ups from the transistor area.

Copper losses, and there'll also be increasing core losses at higher frequencies. Depends on the quality of the laminations in there.

Better in that case would probably mean "smaller". The efficiency won't be that much greater considering that you are propelling a mini-tractor with another 200lbs or so sitting on it, over uneven terrain.

Off-topic but needs to be said: Kudos for taking in Muttley, for giving him a home. That is great. We did the same with a Shepard who is now almost 13. She had major behavioral issues like aggressiveness towards other dogs, it took us years to un-train that because she was more or less a street dog before. But it worked. Tonight there will be three two-legged guests and there will be three Labradors, and not a worry in the sky.

--
Regards, Joerg

http://www.analogconsultants.com/

Short the secondary and measure the primary, A perfect transformer would look like a short circuit. Since there is no such thing as perfect, you'll see some inductance. That is leakage inductance.

Looking at your simulation, the startup current as C1 and C3 charge up is well over the 50 amps specified for your FETs. You need to add some sort of soft precharge arrangement.

I'd advise that you:

Measure the leakage inductance, as above.

Measure mutual inductance, using the usual aiding-opposing technique. You can calculate K from L1 L2, and M.

Make a more comprehensive model of the transformer.

--
"For a successful technology, reality must take precedence 
over public relations, for nature cannot be fooled."
                                       (Richard Feynman)

It was in my post on the "High power 2kHz DC-DC - Success with gate = drivers"=20 and other information was in an older thread (early April) about running = a 3=20 phase motor on an SLA battery. I have now also put the ASC file on my=20 website:

and some screen shots:

I was just "playing it safe". But during startup (and voltage change) = the=20 PWM will be throttled back so I have to quench the spikes anyway. I = can't=20 short the primary as I could with a full H-bridge.

He has to earn his keep. I'm also building a dog cart so he can pull me=20 around. I plan to use PVC pipe and have it so that it can be = disassembled to=20 fit in a car trunk.

He had a long ride in the car over the past weekend. We went camping in = a=20 KOA Kabin in Williamsport, MD.

in

close-ups

I'll try to clean them up and see if there's any improvement.

It seemed like 2kHz was about the best, although 4kHz was not bad. I=20 actually did tests awhile back up to 16kHz, and it still seemed to be OK

higher.

terrain.

Size and weight are not critical for a tractor, especially when it will = take=20 a couple hundred pounds of batteries to make it really practical. For a=20 "real" tractor, with a mowing deck or plow, it will take more power and = more=20 batteries. But there are some working designs "out there". I'm = documenting=20 my progress, and helping and learning from others, on=20

I'm PStechPaul.

I'm glad I was able to take him in. He terrorized my cat, and viciously=20 attacked a young dog in an obedience class, and I almost had him put = down at=20 the recommendation of the instructor. But he's been a great dog and he's =

helped me maybe more than I helped him. His early history and lots of=20 pictures are on his website:

I've been active for a long time on the Cesar Millan website and some of = us=20 have a new website:

We are organizing a big pack meet in Boone, NC in September, and I think = we=20 have about 10 people and 20 dogs!

Thanks,

Paul=20

Just a quick update. I tried adding 0.047uF capacitors to the gates, and = it=20 seemed to make the spikes worse, especially as the voltage increased. = This=20 might have been because the gate drive voltage was still not fully = regulated=20 below 12V, but it did not seem like a very promising method:

10.1V 1.12A 25V peak 12.0V 1.38A 46V peak

Compare to the results without these capacitors:

10.0V 1.10A 12.0V 1.30A 32V peak 20.0V 1.95A 53V peak 25.0V 2.31A 66V peak

I added a 0.22uF capacitor across the drains:

12.0V 1.33A 33V peak 15.0V 1.59A 40V peak 20.0V 2.00A 53V peak

The amplitude of the first spike was about the same, but it just rang=20 longer. So I added a 12 ohm resistor in series for a snubber (in = addition to=20 the 0.047uF and 2 ohm already in place):

12.0V 1.35A 28V peak 15.0V 1.63A 35V peak 16.5V 1.72A 38V peak 20.0V 2.06A 45V peak 25.0V 2.47A 58V peak

That seemed to be some improvement. The 12 ohm 10W resistor was pretty = hot.=20 There is an additional 4 watts at 25V and that seemed about right.

I added a 600 ohm load to the output:

10.0V 3.72A 29V peak 12.0V 4.31A 34V peak

So, that was actually worse than the unloaded test with no snubber. But = the=20 odd thing is that now the spike appears as the MOSFET turns on and the=20 voltage drops. Probably the energy stored in the snubber is being = released=20 as the drive changes state. This is kind of like Whack-a-Mole!

Thanks,

Paul=20

It really looks like your transformer windings are too loose. Having a laminated core at such a high frequency probably won't help either.

With a "loose" transformer an H-bridge can be better.

I think tidying up the transformer and also the wiring in the primary power loops will help a lot. The large those loops are the worse it'll become with the spiek energy. You can snubber much of that but you'll burn lots of energy in the snubber resistors that could have been avoided.

If you could get a hold of a nice big ferrite core that would allow

He is amazingly self-controlled with that steak on the plate. Wish I had that much self-control when it comes to chocolate.

Yesterday we had fun. The dogs did their own thing, picked a toy, licked each other, rolled on their backs, whatever they wanted to do. The highest number we had in the house together was six, all big ones except a young puppy that will (hopefully) be a guide for a blind person some day.

--
Regards, Joerg

http://www.analogconsultants.com/

OK

20kHz.

Yes, that 500Hz whine seemed to startle him and he kept barking for = awhile.=20 It was pretty loud for me, too, and I have significant high frequency=20 hearing loss.

I don't know how long he would have held back if I had gone into the = house=20 for a minute! But I did "test" him once by leaving a box of pizza on the =

chair and I went in for a little while. He was still sitting there,=20 drooling. Of course I handsomely rewarded him with a few bites!

It's wonderful that you can help disadvantaged people in this way. I = have=20 found that Muttley can be dangerous to small, young dogs, for no = apparent=20 reason, and his worst "attacks" were when I was not really paying = attention,=20 so I don't think it was my own "fear" he was reacting to. He seems fine = with=20 most dogs, however:

My dream is to establish an intentional community which is not only=20 dog-friendly, but dog-centric. I want to set it up as a full-featured=20 campground, where people will have their own cabins or small apartments = or=20 camping trailers, but most activities will be shared, such as cooking,=20 eating, recreation, and maintenance. It can operate as a campground as = well,=20 for revenue, and I would also like it to be a dog psychology, training, = and=20 rescue center, as well as perhaps offer veterinary care, boarding, and=20 training for service dogs. Some of my ideas are here:

Back to the project. I just added snubbers of 0.22uF 600V capacitors and =

12=20 ohms, from each drain to GND, and it seems to work beautifully:

With the large filter capacitors out of the circuit:

10.0V 1.01A 22.9V peak 12.0V 1.19A 27.7V peak 15.0V 1.42A 34.6V peak 16.5V 1.53A 38.1V peak 20.0V 1.77A 46.5V peak 25.0V 2.10A 58.8V peak

With the capacitors in parallel (6600 uF at 400V total), and 12.5k = bleeder=20 resistance:

10.0V 1.13A 23.1V peak 12.0V 1.32A 26.9V peak 15.0V 1.60A 33.9V peak 16.5V 1.71A 36.6V peak 20.0V 1.99A 44.7V peak 25.0V 2.38A 55.5V peak

Adding the 600 ohm load:

10.0V 3.62A 24.5V peak 112.7V 21.2W/36.2W=3D58% 12.0V 4.30A 29.5V peak 135.7V 30.7W/51.6W=3D59% 14.0V 5.01A 33.1V peak 159.7V 42.5W/70.1W=3D61% 15.0V 5.38A 34.9V peak 170.9V 48.7W/80.7W=3D60%

The snubber resistors got barely warm, so I figure only 1-2 watts each. = I=20 can live with that, and my MOSFETs are happy and cool!

Paul and Muttley

Actually I have to design something where I need to make sure that service animals won't be bothered by such noise, so I'll have to scope that out later today. To see if 30kHz is high enough.

Dogs are mostly quite peaceful if they are all off leash. Probably like armies, while in uniform they shoot at each other. If they'd all be in T-Shirts and shorts they'd all just have a beer together.

So, that toilet bowl in the background, is that a "canine drinking fountain"? Cats always accuse dogs of drinking out of toilet bowls :-)

Yes, there is a lack of places to down-scale when one gets older. In our area there are some older "micro homes" in some villages. I could imagine living in one some day, but preferably not in some sort of condo.

Those are fairly reasonable values. You can get it better if you clean up that transformer

--
Regards, Joerg

http://www.analogconsultants.com/

On Thursday, June 7, 2012 6:39:03 PM UTC-4, P E Schoen wrote:

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Just winging it with a diode clamp and resistor bleed gets those spikes und= er control with negligible efficiency reduction. The start-up transients ge= t close to the limits, but then again your circuit allows the FETs to peak = at 300A, the maximum obtainable with 10V gate drive, and with considerable = resulting VDS that probably blows them out. If your current sense can turn = the gates off at a peak currrent well below the worst case level correspond= ing to 10V VGS, you should be okay. It wouldn't hurt to clamp each drain wi= th a protective TVS at about 1.5x worst case steady state to catch the off = chance events. Version 4 SHEET 1 880 680 WIRE 0 -32 -352 -32 WIRE -352 0 -352 -32 WIRE -160 0 -272 0 WIRE -352 80 -352 64 WIRE -272 80 -272 64 WIRE -272 80 -352 80 WIRE -352 96 -352 80 WIRE -272 96 -272 80 WIRE 256 96 224 96 WIRE 256 128 256 96 WIRE 320 128 256 128 WIRE 432 128 384 128 WIRE 496 128 432 128 WIRE 544 128 496 128 WIRE 560 128 544 128 WIRE 592 128 560 128 WIRE -160 144 -160 0 WIRE 96 144 -160 144 WIRE 128 144 96 144 WIRE 496 144 496 128 WIRE -352 176 -352 160 WIRE -320 176 -352 176 WIRE -272 176 -320 176 WIRE 224 192 224 96 WIRE 320 208 288 208 WIRE 432 208 432 128 WIRE 432 208 384 208 WIRE 496 208 464 208 WIRE 592 208 592 128 WIRE -640 240 -704 240 WIRE -576 240 -640 240 WIRE -496 240 -576 240 WIRE -448 240 -496 240 WIRE -320 240 -320 176 WIRE -320 240 -448 240 WIRE 128 240 128 224 WIRE 128 240 -320 240 WIRE 128 256 128 240 WIRE 544 256 544 128 WIRE 544 256 496 256 WIRE 288 272 288 208 WIRE 288 272 224 272 WIRE 320 272 288 272 WIRE 416 272 384 272 WIRE -320 288 -384 288 WIRE -80 288 -256 288 WIRE 496 288 496 256 WIRE -640 320 -640 240 WIRE -704 336 -704 240 WIRE -384 336 -384 288 WIRE -256 336 -256 288 WIRE 0 336 0 -32 WIRE 112 336 0 336 WIRE 128 336 112 336 WIRE -496 368 -496 240 WIRE 256 368 256 128 WIRE 320 368 256 368 WIRE 416 368 416 272 WIRE 416 368 384 368 WIRE 464 368 464 208 WIRE 464 368 416 368 WIRE 496 368 496 352 WIRE 496 368 464 368 WIRE 592 368 592 288 WIRE 592 368 496 368 WIRE -160 384 -160 144 WIRE 0 384 0 336 WIRE 592 384 592 368 WIRE -576 416 -576 240 WIRE -320 464 -320 288 WIRE -208 464 -320 464 WIRE -80 464 -80 288 WIRE -48 464 -80 464 WIRE -704 528 -704 400 WIRE -640 528 -640 384 WIRE -640 528 -704 528 WIRE -576 528 -576 480 WIRE -576 528 -640 528 WIRE -544 528 -576 528 WIRE -496 528 -496 448 WIRE -496 528 -544 528 WIRE -384 528 -384 416 WIRE -384 528 -496 528 WIRE -320 528 -384 528 WIRE -256 528 -256 416 WIRE -256 528 -320 528 WIRE -240 528 -256 528 WIRE -80 528 -240 528 WIRE 160 528 160 512 WIRE -240 560 -240 528 WIRE -160 560 -160 480 WIRE 0 560 0 480 WIRE 0 560 -160 560 WIRE -544 608 -544 528 FLAG -544 608 0 FLAG 592 384 0 FLAG 560 128 Vout FLAG -448 240 in FLAG 96 144 m1 FLAG 112 336 m2 SYMBOL ind2 112 128 R0 SYMATTR InstName L1 SYMATTR Value 180=B5 SYMATTR Type ind SYMATTR SpiceLine Rser=3D100u SYMBOL ind2 112 240 R0 WINDOW 0 45 35 Left 2 WINDOW 3 41 61 Left 2 SYMATTR InstName L2 SYMATTR Value 180=B5 SYMATTR Type ind SYMATTR SpiceLine Rser=3D100u SYMBOL ind2 240 176 M0 WINDOW 0 21 -5 Left 2 WINDOW 3 -9 113 Left 2 SYMATTR InstName L3 SYMATTR Value 32m SYMATTR Type ind SYMATTR SpiceLine Rser=3D10m SYMBOL nmos -208 384 R0 SYMATTR InstName M1 SYMATTR Value IRFZ44N SYMBOL nmos -48 384 R0 SYMATTR InstName M2 SYMATTR Value IRFZ44N SYMBOL voltage -496 352 R0 WINDOW 123 0 0 Left 2 WINDOW 39 24 132 Left 2 SYMATTR SpiceLine Rser=3D8m SYMATTR InstName V1 SYMATTR Value 24 SYMBOL diode 384 288 M270 WINDOW 0 32 32 VTop 2 WINDOW 3 0 32 VBottom 2 SYMATTR InstName D2 SYMATTR Value MUR460 SYMBOL diode 320 224 R270 WINDOW 0 32 32 VTop 2 WINDOW 3 0 32 VBottom 2 SYMATTR InstName D3 SYMATTR Value MUR460 SYMBOL polcap 480 288 R0 WINDOW 3 24 64 Left 2 SYMATTR Value 1000=B5 SYMATTR InstName C1 SYMATTR Description Capacitor SYMATTR Type cap SYMATTR SpiceLine V=3D400 Irms=3D30 Rser=3D0.02 Lser=3D0 SYMBOL res 576 192 R0 SYMATTR InstName R1 SYMATTR Value 300 SYMBOL voltage -384 320 R0 WINDOW 123 0 0 Left 2 WINDOW 39 -43 57 Left 2 WINDOW 3 228 271 Left 2 SYMATTR SpiceLine Rser=3D100 SYMATTR Value PULSE(0 10 0.5u 10n 10n 499u 1000u 100) SYMATTR InstName V2 SYMBOL voltage -256 320 R0 WINDOW 123 0 0 Left 2 WINDOW 39 -43 57 Left 2 WINDOW 3 100 298 Left 2 SYMATTR SpiceLine Rser=3D100 SYMATTR Value PULSE(0 10 500.5u 10n 10n 499u 1000u 100) SYMATTR InstName V3 SYMBOL diode 320 144 R270 WINDOW 0 32 32 VTop 2 WINDOW 3 0 32 VBottom 2 SYMATTR InstName D1 SYMATTR Value MUR460 SYMBOL diode 384 384 M270 WINDOW 0 32 32 VTop 2 WINDOW 3 0 32 VBottom 2 SYMATTR InstName D4 SYMATTR Value MUR460 SYMBOL polcap -592 416 R0 WINDOW 3 24 64 Left 2 SYMATTR Value 2200=B5 SYMATTR InstName C2 SYMATTR Description Capacitor SYMATTR Type cap SYMATTR SpiceLine V=3D25 Irms=3D20 Rser=3D100m Lser=3D0 SYMBOL polcap 480 144 R0 WINDOW 3 24 64 Left 2 SYMATTR Value 1000=B5 SYMATTR InstName C3 SYMATTR Description Capacitor SYMATTR Type cap SYMATTR SpiceLine V=3D400 Irms=3D30 Rser=3D0.02 Lser=3D0 SYMBOL cap -656 320 R0 SYMATTR InstName C5 SYMATTR Value .47=B5 SYMATTR SpiceLine V=3D250 Rser=3D100u SYMBOL schottky -688 400 R180 WINDOW 0 24 64 Left 2 WINDOW 3 24 0 Left 2 SYMATTR InstName D5 SYMATTR Value MBRB2545CT SYMATTR Description Diode SYMATTR Type diode SYMBOL res -144 544 R90 WINDOW 0 0 56 VBottom 2 WINDOW 3 32 56 VTop 2 SYMATTR InstName R3 SYMATTR Value .001 SYMBOL cap -336 464 R0 SYMATTR InstName C6 SYMATTR Value 0.1=B5 SYMATTR SpiceLine V=3D250 Rser=3D100u SYMBOL cap -96 464 R0 SYMATTR InstName C7 SYMATTR Value 0.1=B5 SYMATTR SpiceLine V=3D250 Rser=3D100u SYMBOL diode -288 0 R0 SYMATTR InstName D6 SYMATTR Value MUR460 SYMBOL diode -336 0 M0 SYMATTR InstName D7 SYMATTR Value MUR460 SYMBOL cap -368 96 R0 SYMATTR InstName C10 SYMATTR Value 2.7=B5 SYMBOL res -288 80 R0 SYMATTR InstName R6 SYMATTR Value 1K TEXT 32 88 Left 2 !K1 L1 L2 L3 0.998 TEXT -536 576 Left 2 !.tran 0 200m 0 1u startup TEXT 216 -24 Left 2 ;Primary 2x8 turns 2V/turn at 600 Hz

P.S. It's your project so I'm not going to spend a bunch of time spec'ing the components. The schematic is largely symbolic, the details are up to you.

2 volts per turn ain't too hot. err IS too hot, actually.

That was at 600 Hz. I'm now running at 2 kHz so it's more like 6 = volts/turn,=20 which is about right for a 24 V supply. And my readings show that it is = not=20 saturating.

I was able to get the contraption "running" on Monday, but after a = "short"=20 ride there was a "short" in two of the MOSFETs. Here is a possibly=20 entertaining movie of my (mis)adventure:

Enjoy! Now I have to replace those MOSFETs and get some proper TVS = diodes.=20 But I think I may also have to devise a better "soft start". I found = that=20 you can't use the PIC16F684 PWM to drive a push-pull except at 50% duty=20 cycle. So I set up a 2.5 second start-up which used an ISR to provide a =

12%=20 PWM at start-up. Only 256 uSec wide, but still enough to cause a large=20 current surge. I'll have to dig into this a bit deeper. Maybe an = inductor in=20 series with the output capacitors? But it seems like anything big enough = to=20 be effective (like 100mH at 5A) is way too big and expensive. So that's = one=20 reason to go with a much higher frequency, like 50kHz or more, and use=20 ferrite.

But another possibility may be a current limiter on the DC input, even a =

linear device. It would just have to limit at 30 amps and 24 volts for a =

very short time to charge the capacitors. Let's see. 300V and 6600 uF is =

297=20 W-Sec. So an MJ11029 (PNP darlington, 60V, 50A, 300W) would do the job. = I=20 just happen to have a few of them. Maybe set the current limit to 60A or = 80A=20 with two in parallel, so a short circuit will trip the breaker before = they=20 burn up. Probably not the most efficient method, but at 15 amps each = (for=20 normal operation below current limit), they will dissipate 18W each. Not = too=20 bad for a 750W+ converter. But the sense resistor will also dissipate = about=20 5 watts each, and the base drive will take some power as well, although = less=20 than a watt.

At least I think this will protect the MOSFETs from high current=20 destruction, and the TVS diodes (80V) should take care of overvoltage=20 spikes.

Yeah!

Paul=20

it=20

This=20

regulated=20

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hot.=20

the=20

released=20

I fussed with the simulations a lot and the only thing that seems to keep the turn off spikes down is zeners drain to source. =20

?-)