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Old 12th February 2004, 04:01 AM   #21
Symx is offline Symx  United States
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Default Mosfet Overshoot

[QUOTE]Also, be sure to include a choke between the rectifiers and the filter capacitor. It took me a long time to discover its importance.
Yes, the choke should be the same type of core or larger than the transformer with opposing windings for the positive and negative rails, on the same core, ( to cancel common mode noise between rails ).
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Old 12th February 2004, 04:51 PM   #22
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Thanks for the link to the other thread. Though I would like to think that I am more competent then simply being able to build scematics like that individual, this project is making me believe otherwise, but it will be good for the experience. OK, I have taken some pics for your viewing pelasure!

Unloaded secondary waves (note overshoot I mentioned) Pay no attention to the RMS on these, they are square waves out of phase, and I believe the ocilliscope just averages what shows onscreen. Note the amplitude matches.

unloaded output Huge difference! The arrows for channel 1 and 2 show ground, and though they look to be the same distance, arrow 2 is going off of a 50V per major division scale, while Ch. 1 is only 20V.

loaded secondary waves The waves are still very close to the same, and I am beginning to think that the voltage problem is in the spike, though I've been told that a spike is normal.

Loaded output About a six volt difference between the two...still pretty bad in my opinion. they should be 3-6 volts difference UNloaded, and the same loaded.

and finally, just a pic of the board I have built so far, to help you all empathize!(pic)

My circuit is fundementally the same as the circuit used in your link, probably because it's just your average rectification circuit.[link]


I am beginning to realize my misconceptions now that the project is loaded. (again, where actually experience outweighs bookwork) I remember being surprised that I was getting 24 Volts on my primaries, hearing that this was normal, and adjusting my windings to try and get about 35V, basically 1:1.5... well, loaded, I am getting 1.5 times my 12V, so I'll have to "wind" back to where I originally was(1:3).

About those spikes now...are they normal, or are they gargantuan? I have rewound the darn thing five times, used two different cores, but the reaction only changed when I changed mosfets, and I ended up getting something similar to the loaded waves while unloaded, but I was also pulling five amps unloaded(so I switched my switchers back). Again, my IC ocillator gives a clean square. The only other thing I can think of is that my huge caps on the primary might be giving a giant inrush every time it switches? Don't know why, I thought they would only help.

Ok, here is how I am winding. I have taken 8 strands of 26 gauge in parallel, and wound 6 turns. then I liink another set of wires exactly the same, and wind another 6 in the same direction. This makes my primary, and I cover the entire toroid, so each wire covers half, and they meet in the opposite side form the link. Now I start at the link of the primary (ground), and wind 10 strands of 30ga. in parallel in the opposite direction of the primary it is on top of, again covering half of the toroid. Then go back to the starting point and wind the other half, so basically both ground points start at the same position, and all ends end together. I realize that my pic looks just a little sloppy, I just soldered it in quickly to take the pics, but I assure you , the results have always been the same on the O scope, no matter how I try to fix the transformer. I have also tried 6 and 8 strands of 22 ga. for secondary and primary.

So...what's the word?
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Old 12th February 2004, 06:43 PM   #23
Eva is offline Eva  Spain
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This negative spike could be caused by very slow diode turnoff combined with high leakage inductance and poor coupling in the transformer [it's like there were flowing very high current when the positive rail diodes turn off]

Anyway, your transformer is improperly wound so you'll habe to wind it back, but this time please do it right :

Isuggest 4T + 4T primary, 10T + 10T secondary [your core appears to be too big to require 6 turns at 55Khz]

At 55Khz and for a simple prototype you don't need to fight with litz wires, just use single solid wire for each winding [for light loads there isn't any difference]

All windings have to be thightly coupled and must have the lowest leakage inductance. This means you must spread EACH winding over the entire core

Wind both primeries first, in a bifilar fashion

Then do the same with both secondaries at the same time, and wind everything in the same direction

Winding this way you will reduce leakage inductance by an order of magnitude [ringing and overshoot is almost proportional to leakage inductance]

Insert some dead time in the control circuit [25-50%] to be able to see clearly how the system behaves when both mosfets are turned off [the spikes could be caused by parasitistic turn-on of one of the mosfets when the other turns-on, look for gate waveforms, or simple cross conduction and/or core saturation due to too low dead time]

I use a potentiometer in my prototypes to regulate duty cycle and look for spikes, cross conduction, core saturation, etc... as the ducy cycle increases

Also, when doing the captures try to use 2uS/div and 10V/div so wen can see clearly the turn-on and turn-off transients

Is it normal to see that much noise or shadow in the traces? If it's ripple from the 12V supply, don't you have a hold function in she digital scope?

Are the output diodes all equal, not blown and properly wired?
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Old 12th February 2004, 09:33 PM   #24
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What core material are you using?
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"If transistors are blueberries and FETs are strawberries, then tubes must be.. pears"
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Old 13th February 2004, 12:11 AM   #25
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I am using a ferrite material, with a permeability of approx. 3000. Was previously using iron powder, but couldnt get good results below 200kHz, and by good I mean similar to what I have now.

I don't think my diodes are the problem (data sheet), because I get the spikes on the secondaries out of circuit as well, and had them before I added the bridge or capacitors.

I will try the duty cycle adjustment, and rewiring the transformer. Here is how my diodes are wired, I believe it is correct. Now, when you say to cover the entire toroid with the windings, are you saying go all the way around with each primary/secondary, or half way, to meet in the middle, and does it matter if you go beyond that and overlap?
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Old 13th February 2004, 05:19 AM   #26
Eva is offline Eva  Spain
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These Diodes are a good choice. Do they get hot without load?. If the reply is no then the problem is in the primary side, secondary side is too simple and too pasive altough RC damping networks are missing, but this shouldn't make a great difference anyway

If the waveforms are almost the same with secondaries disconnected, I think you may be experiencing heavy cross conduction at MOSFET turn-on

About winding the transformer, you have to go all the way aroud the core with each winding and each pair of windings has to be perfectly matched to avoid asymmetric voltage drop in them and core saturation at high powers, so I suggest bifilar [bi-strand] winding

10-20% dead time is also very important to give time to the core to 'center itself' between switching cycles and prevent saturation

Remember that control ICs are not perfect and that the core is not AC coupled so if positive pulses are 1% longer than negative pulses then you'll have some DC applied to the core, 12 * .01 = .12V, enough to saturate it

Each winding by itself has to cover the entire toroid [really, all it has to cover is all the other windings and vice-versa, the toroid itself has nothing to do with leakage inductance or HF coupling, but people seems to think so]

When you have half the windings in one side and half the windings in the other, there is very poor HF coupling between sides of the symmetric windings and this causes huge ringing and overshoots due to huge leakage inductances between halves

Could you post the schematic or the PCB of the primary side?

PD: Transformer winding is more tricky than it appears to be at first
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Old 13th February 2004, 03:45 PM   #27
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Dang woman, you're beautiful! It works perfectly now, from what I can see! It was the transformer winding after all. So hey, nobody ever taught me how to wind a transformer, seems intuitive, right? Wrong. Well, I am glad it was something that simple, at least now I don't feel completely inept.

By the way, the diodes still don't get very hot, maybe just a little warm with lightbulb loads. Should I be worried? it's working fine...

I was reading all these posts, and I was thinking, what's this bi-something winding that everyone keeps talking about. So I did a little studying, and hey, that Tesla guy's not too shabby, he should be famous(just kidding).

I am going to post some pics just in case someone else has a similar problem later, and to confirm that I got a good result with you guys.


Click the image to open in full size. To those of you who don't know how to wind a transformer, you're not alone! Make sure you use your Ohmmeter so you don't connect the same wire to itself in the middle after you wind the two parallel primaries.

Click the image to open in full size. Look good? This is under load, with very little load, the duty cycle shortens to about 15% and the squares become ramps (I assume this is a short square pulse with a trailing discharge.)

Thanks guys.
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Old 13th February 2004, 04:40 PM   #28
Eva is offline Eva  Spain
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You have reduced duty cycle to about 60%, haven't you?

These ramps you can see are caused by the magnetizing inductance of the core discharging freely during dead time

As you can see, the flux in the core is somewhat asymetric but not enough to enter deep saturation [when flux is symetric, waveforms are symetric during the time you let the core 'free' to 'flyback' in the direction it wishes]

Now, the transformer appears to have so low leakage inductance that overshoot and ringing are too small to be appreciated at first sight. [Try to remove the RC on the primary and look to the waveforms at MOSFET drains]

Now, you can try to increase the duty cycle to 90% and see what happens [high duty cycle is needed to get good efficiency]

Could you post a capture of the voltage waveforms at the *gates* of the MOSFETS with the maximun load you can use? [with 60% and maximum duty cycles]

I suspect you may have a pending problem of parasitistic MOSFET turn on, altough with the good coupling in the new transformer it no longer screws up the circuit

About diodes : They should get equally hot but only with load [for heavy loads they will need a heatsink], getting hot without load means someting is going wrong

The core may also get 5-10șC hotter than ambient temperature [independently of load] but this is normal

MOSFETS should not get hot witn light loads or no load, only with heavy load
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Old 14th February 2004, 10:28 AM   #29
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I was under the impression that the duty cycle should be just under 50% on each leg, making two square waves that are never high at the same time. I haven't changed anything from the previous pictures I posted duty cycle wise, but I did zoom in on the time axis like you had asked. I will post a waveform from the gates when I get home. It looks the same as the last picture I posted above, just a little more square rather than rounded off on the edge. I tend to think that you're still right, because I calculated my efficiency at about 54%.
With my lightbulb load, mosfets are about 90 degrees F. , diodes are just slightly warm to the touch. not so surprising I guess, where I am only pulling 300mA through the diodes, and 2.8 amps on the mosfets. 11.8V in@ 2.8 amps=33 watts, Two supplies that are 30V@300mA=18Watts, 54% efficiency, right?
All in all , I am still very excited to actually have a split supply with even readings, and no huge spike. Now for fine tuning.
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Old 14th February 2004, 01:57 PM   #30
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Unloaded at gate
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Loaded at gate
Click the image to open in full size.
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