Well, I've done a quick search on the site, and haven'tquite been able to find what I am looking for..
Basically, I am about half way finished with my first SMPS, I've mounted everything but the rectification and reservior capacitors. While testing the waveform though, I get a good square out of the controller chip, but I get a fairly large spike on the rising edges of the primary winding wave. Since this is my first shot, I'm not really sure what I am looking for, or how to correct the problem. I've tried using 100nF caps to 'snub'. I've also considered that this may diminish once I add the reservoir caps and diodes. Any suggestions?
Basically, I am about half way finished with my first SMPS, I've mounted everything but the rectification and reservior capacitors. While testing the waveform though, I get a good square out of the controller chip, but I get a fairly large spike on the rising edges of the primary winding wave. Since this is my first shot, I'm not really sure what I am looking for, or how to correct the problem. I've tried using 100nF caps to 'snub'. I've also considered that this may diminish once I add the reservoir caps and diodes. Any suggestions?
I placed one from the center tap on the primary to each primary winding, with a resistor. After a little more experimentation, I've found that hooking one of these networks from the center tap to one of the primaries does nothing, while without hooking one up to the other primary, my spike becomes a ringing, so instead of a plateau, it lust looks like I have a couple of spikes in descending amplitude. I since I have a fairly clean square wave from the controller, do you think it's one of the mosfets?
In a push pull SMPSs, you can expect to see such overshoot because the winding is not directly but inductively coupled to the free wheeling (body) diode of the mosfet on the other winding. Continue construction of the secondary part of the circuit and place a small load on it and see what happens to help determine what kind of snubbers you may then want on the primary side.
The big problem was my transformer core. I was told that Iron Powder would work, but I don't think the permeability is great enough at the frequency I am running, and it lets too much current through. I should actually do the math BEFORE I start, what a novel idea. Anyway, I found that I had blown one of the mosfets (probably due to this). Replacing it put the ringing in its place! There is still a tiny spike on the rising edge, but no huge ocillation as before.
Ok, now I've put it all together and everything works fine....except that I have +/-60V rails now instead of the intended 30V. Looking at the primary, I have 23V pek square waves on each primary. Not sure what went wrong there. On the primary I did 4 turns, center tap, 4 turns, the secondary was 10 turns, center tap 10 turns, and I only have a 12V supply. I'm wondering if I made some minor mistake and it's acting as an auto-transformer. Back to winding again...
Ok, now I've put it all together and everything works fine....except that I have +/-60V rails now instead of the intended 30V. Looking at the primary, I have 23V pek square waves on each primary. Not sure what went wrong there. On the primary I did 4 turns, center tap, 4 turns, the secondary was 10 turns, center tap 10 turns, and I only have a 12V supply. I'm wondering if I made some minor mistake and it's acting as an auto-transformer. Back to winding again...
In push-pull circuits it is normal for the voltage output to be the same as a half-bridge operating on twice the voltage. Instead of the two power supplies being stacked on top of each other, they are side-by-side. The mosfets still have to handle as much voltage as with a totem pole because when the opposing mosfet goes low, it forces the drain of the "off" one to go high.
smps issues
I would definitely perform the magnetics computations first, and then wind the transformer. I would use ferrite core material. As far as the 23 V on each primary winding goes, a push-pull converter topology places a voltage stress on each MOSFET drain equal to twice the input supply voltage. About 24 V is what I would expect when the input is 12 V. As far as winding a transformer goes, minimum leakage inductance is mandatory. The biggest problem with custom transformers for SMPS is leakage inductance. Also, use a low leakage inductance core geometry, having a wide window and shallow height. I would keep each winding at one layer Do not put the primary and secondary on the same layer. Each winding should span the full breadth of the window, leaving a small margin on each end, and this way magnetic coupling is maximized. I would use bifilar windings for both primary and secondary, making the center tap connections externally. Each half of the C.T primary should be vertically aligned (directly above or below) its corresponding C.T. secondary half. At 100 kHz switching frequency, I would use wire no thicker than AWG 26. At 50 kHz, up to AWG 23 is ok. Parallelling conductors (if bifilar AWG 26 isn't quite enough, use quadfilar, or hexfilar, etc.) increases current capability, while keeping skin effect under control. Also, multifilar conductors helps fill the window width when the number of turns is small.
Last, you are using an output filter inductor, aren't you? Also, you are using feedback, correct? Otherwise, you will get an unregulated output. For anything more than a few watts, I avoid inductorless circuits. Without an inductor, ripple current on the output caps is very large, making large sized caps necessary. Also, noise is much worse without an inductor. As far as the overvoltage output, my guess would be that under no load a substantial secondary leakage inductance could over charge the output caps. Hard to tell from here. Best regards.
I would definitely perform the magnetics computations first, and then wind the transformer. I would use ferrite core material. As far as the 23 V on each primary winding goes, a push-pull converter topology places a voltage stress on each MOSFET drain equal to twice the input supply voltage. About 24 V is what I would expect when the input is 12 V. As far as winding a transformer goes, minimum leakage inductance is mandatory. The biggest problem with custom transformers for SMPS is leakage inductance. Also, use a low leakage inductance core geometry, having a wide window and shallow height. I would keep each winding at one layer Do not put the primary and secondary on the same layer. Each winding should span the full breadth of the window, leaving a small margin on each end, and this way magnetic coupling is maximized. I would use bifilar windings for both primary and secondary, making the center tap connections externally. Each half of the C.T primary should be vertically aligned (directly above or below) its corresponding C.T. secondary half. At 100 kHz switching frequency, I would use wire no thicker than AWG 26. At 50 kHz, up to AWG 23 is ok. Parallelling conductors (if bifilar AWG 26 isn't quite enough, use quadfilar, or hexfilar, etc.) increases current capability, while keeping skin effect under control. Also, multifilar conductors helps fill the window width when the number of turns is small.
Last, you are using an output filter inductor, aren't you? Also, you are using feedback, correct? Otherwise, you will get an unregulated output. For anything more than a few watts, I avoid inductorless circuits. Without an inductor, ripple current on the output caps is very large, making large sized caps necessary. Also, noise is much worse without an inductor. As far as the overvoltage output, my guess would be that under no load a substantial secondary leakage inductance could over charge the output caps. Hard to tell from here. Best regards.
This is beginning to drive me crazy!. Don't get me wrong, I won't give up, but I am so close to having it done but for one problem. Everything works, I have a beautifully straight DC line, one positive, one negative, but no matter what I have done, one rail is always significantly higher than the other. Not exactly double, but about that. I've gone through, rewound the transformer a couple of times, checked the orientations of all the capacitors/diodes, and can't find anything out of the ordinary. I mean, it's such a simple principle to rectify a wave. The nearest I can figure is that I have some little error somewhere that is causing a voltage doubler with my caps and diodes. I guess the next step is to remove the caps and see what happens...
Actually, for half-bridge or push-pull or full bridge for those into that topology, the rectification part is important because it may be helpful to draw power off the transformer during each waveform cycle. So I recommend avoiding a half-wave rectifier in these cases. For flyback topology, a half wave rectifier may be useful. Also, be sure to include a choke between the rectifiers and the filter capacitor. It took me a long time to discover its importance.
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