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Design of SMPS - Not sure what these spikes are being caused by.
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Zog:
Gotcha,
Well worth the 5 bucks I paid !  ;) cough  ;D
Cheers Mate.
Zog:

--- Quote from: treez on May 19, 2018, 09:57:53 pm ---your mosfet you say is switching unevenly.
You can  often correct this by judicious adjustment of the current sense filter and the fet gate series resistor.
But there is no gate series resistor in your schem. I think you need one.

Sometimes of course, uneven fet switching can be caused by insufficient slope compensation in current mode designs.
Sometimes it can be caused by insufiicient delta current in the current signal, ie you have used too big inductance and the on state current ramp is so shallow that it easily hits the sense threshold through noise instead of in the normal way.

Also, here is my pcb layout doc attached

Oh by the way, a  lot of the spikes i saw are just "pickup"...induced into the scope probes impedance by all the noise that an smps chucks out.
There is a test to see if it is pickup....connect your  scope ground clip to the probe tip  and see if you see the spikes still, with the probe tip touching whichever node on the circuit.

--- End quote ---


Thanks very much for your input treez very much appreceated mate. Thank you for taking the time to share that write up.
I "know" some of that but your tip on how on connecting the scope ground clip to the probe tip and probing around is one I would never of thought of.
Thanks again  :-+

P.S. You didn't do all that writing just for this topic did you ? or was it something you had already done ?
Zog:
Question for anyone who knows about 1:10 ratio coupled inductors.
If the primary turn has an inductance of 25uH what would be the inductance of the secondary ?

The usual way is to do this right ?
M2 = L1+L2
M2=Mutual Inductance

But the datasheet does not supply L2 Only L1

Sooo .. I am guessing that the datasheet is just showing the total mutual inductance and there is no way to work it out.  Even using the ratio ? (btw I am not great at maths  :P)

T3sl4co1l:
Datasheets never give M, it's a rather useless figure, it arises from physics but you never measure it in circuit.

Hmm, well I suppose you can, since the definition is:
Vp = Lp * dIp/dt (single inductor, Vp = primary voltage, Ip = primary current)
Vs = M * dIp/dt (coupled inductor, Vs = secondary EMF, Ip = primary current)
You'd apply a constant AC current to the primary, and measure the secondary current.  M drops out from the secondary, while Lp drops out from the primary (and if you apply the current to the secondary, you measure Ls instead).

Anyway, you're more likely to measure Lp, Ls and k (coupling factor).  https://en.wikipedia.org/wiki/Leakage_inductance
LL is easily measured by shorting one winding and measuring the other winding inductance.  LL, k, and M (as a product of Lp, Ls) are all easily related.

Since they don't give any of k, M or LL, it's impossible to know them, or Ls exactly.  Presumably, Ls is around (10/1)^2 * Lp, or 2500uH.

Incidentally, you use the inductance ratios in SPICE, not the turns ratio.  So to model this, you need to enter the inductances accordingly, and put in a coupling statement ("K12 L1 L2 0.98", say).

Tim
Zog:
Thanks for the explaination Tim,

Ok back to the chase.

Edit: Here is a pretty picture of how I tapped the torroid.



This is a better drawing of the circuit with the test points marked.
Edit: yes I know that it does not show the original input caps as per the original circuit ... but they are there. I have just been working on a different input circuit so have not shown them on this sheet.



First point to note. Is I can only get a maximum of 85mA's out now. Not the 140mA I was getting before.
Only thing changed is I have center tapped the torroid as shown above.

Second point. As the shot below shows I am now not missing any gate pulses. So the tap is working ? I guess.
Still a lot of ringing but no snubber yet so that is expected.

Taken at TP2 at only 35mA


Following Shots Taken at TP3 at 85mA



Zoomed in a little



Close up



Hoping that the rest of the snubbers will fix this.

BUT .. why the lower max mA's out ?












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