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Design of SMPS - Not sure what these spikes are being caused by.

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MagicSmoker:
There's 2 mistakes: change the K to K1 (for the first set of coupled inductors; K2 for the second set; etc.), and change the RC filter on the current sense line to something like 100R and 220p because you generally want the time constant of the RC filter to be in the range of 0.5x to 2x the turn-on rise time of the switch.

EDIT - by not specifying K correctly the inductors weren't coupled, so you had a boost converter feeding into an LC filter and they really don't like that. Hence the ringing. In fact, in the real world the switch would be destroyed after the first switching cycle.

EDIT 2 - I did not check the rest of your circuit.

Zog:
They are 2 separate inductors so the K1 trick did not work but putting K1 L1 L2 0 ie: not coupled produced this ! Like magic  :-+
Still singing a little on L2 though. But wayyy better.

Once again I have to thank you guys for helping me out here. It's actually more enjoyable learning this now than I thought it would be. Just a few LTSpice wrinkles to iron out. Like you can't use M for megohm apparently ?
Onwards and upwards I will be doing tank circuits soon .... yer right.


MagicSmoker:

--- Quote from: Zog on September 18, 2019, 10:33:48 am ---They are 2 separate inductors so the K1 trick did not work but putting K1 L1 L2 0 ie: not coupled produced this ! Like magic  :-+
Still singing a little on L2 though. But wayyy better.
--- End quote ---

Wait, whut?! The circuit in post 104 shows what should be a tapped boost, and in such the windings must be coupled tightly for it to work. You cannot take the output of the first boost inductor and have it feed directly into another (separate) inductor or the switch will be destroyed. So you really should have something like K1 L1 L2 0.995 (or 0.9975, which gives 1% and 0.5% leakage, respectively).


--- Quote from: Zog on September 18, 2019, 10:33:48 am ---Like you can't use M for megohm apparently ?
--- End quote ---

Yep, one of the quirks of LTSpice and in defiance of almost everything else is that you have to use Meg to specify a million of something; M and m are interchangeably treated as milli, or a thousandth of something.

Zog:
This post ?
https://www.eevblog.com/forum/projects/design-of-smps-not-sure-what-these-spikes-are-being-caused-by/msg2696312/#msg2696312
You are right of course. I was forgetting everything I learnt about the tapped toroid I did before.
Too deep in spice and I "looks" like it is working so I throw the brain out.
To self, repeat after me. I need a transformer .... I need a transformer. :palm:

Well K1 did not produce any difference. That would of course explain my sudden output voltage drop.

MagicSmoker:

--- Quote from: Zog on September 18, 2019, 11:28:38 am ---...
To self, repeat after me. I need a transformer .... I need a transformer. :palm:

Well K1 did not produce any difference. That would of course explain my sudden output voltage drop.

--- End quote ---

So, there's a number of things wrong with 5.asc above which I have cleaned up and attached here as 5a.asc. It's not a complete design - this is my job and you ain't paying me, after all - but it should get you much closer to your (wacky) goal of powering a hundred Nixie tubes or whatever daft adventure you've set out yourself.

Note that I changed the following things:

1) Deleted extraneous specs from the input and output capacitors; first get a design working, then go back and add in ripple current and voltage ratings if you like (always include ESR, however, even if it's just a guess).

2) Added a gate resistor - always, always, always use a gate resistor when driving MOSFETs and IGBTs.

3) Changed the values on the RC filter for the current sense signal - note that this IC has a very low current sense threshold of 0.11V which makes filtering noise and spikes out of it even more critical.

4) Added an RC damper across the switch - you always need this with a tapped boost (or any configuration that has leakage inductance) - to eliminate much of the ringing. I did not optimize these values, but my guesses usually aren't too far off the mark. Usually you start with a C value that is twice the switch output capacitance then select R to be somewhere between 0.5x and 2x the characteristic impedance of the resonant network between the total capacitance and total stray inductance. Given that these parameters are sometime difficult to know ahead of time some guessing is inevitably involved. Note, also, that if the ratio of L1 to L2 is much different than 1:1 that it will become neccessary to use separate RC dampers across each inductor, rather than a single one across the switch, because the leakage inductances will be too different for one RC network to effectively damp.

5) Changed the transient simulation parameters - note that if you have to skip initial operating point in LTSpice to get a circuit to simulate, especially with one of their IC models, then something is probably wrong. It usually helps to specify the maximum timestep, however; something in the range of 0.1x to 0.5x the switching period usually ensures LTSpice doesn't inadvertently skip over ringing and other such phenomena.

6) Changed the feedback resistors to something more sensible which still gives Vout = 200V.

7) Changed the K1, L1 and L2 values to something more realistic for 300kHz CCM operation. Note that L2 has 3x as many turns as L1, so its inductance is 9x higher (inductance is proportional to turns squared). It wouldn't be unreasonable to go up to as high as 6x L1 for L2, though damping the ringing will become progressively more difficult as the turns ratio of the tapped boost increases farther away from 1:1.

8) Changed the output rectifier to the 600V SiC Schottky; this isn't strictly necessary, but sometimes a recalcitrant HV boost will behave better with this diode (as long as the average output current is <1A).

I didn't touch compensation though that almost certainly needs to be changed, and there are still some glitches in the MOSFET switching (doubled then skipped pulses - usually the result of noise on the current sense signal, though that looks pretty good at this point).

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