Electronics > Beginners
MC34063 high voltage dc-dc boost converter
dazz:
--- Quote from: MagicSmoker on January 03, 2020, 11:14:23 am ---
--- Quote from: dazz on January 03, 2020, 03:28:40 am ---Learning about this stuff right now. Already understand what you mean by leakage inductance and why I need a snubber network, either RCD or Zener, and I'm in the process of learning how to pick the right clamping voltage to have the best compromise between limiting the voltage overshoot in the mosfet and the power losses in the snubber itself. I can see in LTS the massive spikes in the mosfet's drain voltage (and current). Let's fix that
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In the real world - especially if input voltage range is wide - the RCD clamp is usually the better choice (and is more forgiving of sloppy component selection) but the Zener clamp seems to be easier for beginners to understand and is an excellent choice at lower power levels and/or when the input voltage is more or less fixed.
The basic idea is to pick a Zener voltage that is around 1.5x to 2x the reflected voltage from the secondary, or the secondary voltage divided by the turns ratio going from secondary to primary. For example, the voltage reflected back to a 1 turn primary from a 300V secondary that has 12 turns is 25V (300 / 12); a Zener (or TVS diode) rated for 36V to 51V (using standard values) would be appropriate. You want the clamping voltage to be as high as possible to both reset the leakage inductance quickly and minimize the loss in the clamp - current will not flow out of the secondary until the leakage inductance is reset - but not so high that you have to increase the voltage rating of the switch so much that efficiency suffers from the inevitably higher Rds[on] (keeping price constant). Note that the Zener clamp doesn't help snub any high frequency ringing - it can make it worse, in fact - so you might need a light RC damper across the primary, too (but this is getting into one of motivations for using an RCD clamp... a bit advanced of a topic for the moment).
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Awesome, thanks. I've been trying zeners in that range of voltage, and sure enough, they don't do anything for the large, almost instant spikes in the rising edge of the waveform.
I'm currently trying this:
What I don't understand is why the clamp is connected to the positive line instead of GND, wouldn't that derive the spikes right back into the tranny?
I will also try the RCD clamp, good to know it's a better option here
MagicSmoker:
--- Quote from: dazz on January 03, 2020, 11:40:30 am ---What I don't understand is why the clamp is connected to the positive line instead of GND, wouldn't that derive the spikes right back into the tranny?
I will also try the RCD clamp, good to know it's a better option here
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You can return the clamp to ground - ie, wire it across the switch - but then it has to withstand the sum of the input voltage + the reflected voltage (ie - same as the switch). The transformer (and switch) don't really care one way or the other. The same applies to the RC damper, if used. BTW - a good rule of thumb for the RC damper is to make C about 3x the output capacitance of the switch and R somewhere around 1x to 3x the characteristic impedance of the LC network formed between the total capacitance of switch and damper and the leakage inductance. For example, if there is 2.4uH of leakage and the switch output capacitance is 50pF then a damper comprised of 150pF and 100-330R will likely clean up the highest frequency ringing (the lower frequency ringing in the flyback is between the magnetizing inductance and the lumped capacitance and can't really be suppressed as it is invariably too close to the switching frequency).
One other thing is that 1% leakage is a more realistic minimum for a typical E-core design (and even that requires considerable care in winding geometry), so set the coupling coefficient, K1 (2, 3, etc.) to 0.995 (leakage factor is 1-K2).
T3sl4co1l:
As long as C2 is close to the snubber and transistor, it acts to turn the power rail into a supernode, i.e., in AC terms, equivalent to GND. In an AC-equivalent circuit, all rails are drawn as GND, which looks very strange the first few times you see it, but helps illustrate how things work.
Tim
dazz:
Thanks guys. Plenty of things to google in those two last posts. If you find it takes a while to load google.com, bare with me, this is important :-DD
David Hess:
--- Quote from: dazz on January 03, 2020, 11:40:30 am ---What I don't understand is why the clamp is connected to the positive line instead of GND, wouldn't that derive the spikes right back into the tranny?
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As is usually the case, the schematic does not reflect what is actually happening with the circuit layout. (1) The leakage inductance spike which needs to be snubbed comes from the transformer primary so minimizing the loop area means placing the snubber electrically close to the primary connections.
As T3sl4co1l points out, C2 makes the positive and negative input the same AC ground but besides the issue MagicSmoker identified, this also increases the loop area to extend through C2 lowering the effectiveness of the snubber.
(1) There is a way to indicate proper layout on the schematic with 45 degree connections but hardly anybody bothers.
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