Electronics > Beginners
MC34063 high voltage dc-dc boost converter
dazz:
Damn, thanks for that, MagicSmoker. I'm hopelessly clueless, :-DD
And http://schmidt-walter-schaltnetzteile.de/ seems to work just fine for me. I tried a few pages and got no errors
dazz:
--- Quote from: MagicSmoker on January 02, 2020, 03:13:39 pm ---The pot isn't a standard LTSpice component and simulating 3 seconds is about 2.99 seconds too much (quite literally: 10ms should be enough here). I like to set the simulation time to capture 100 to 1000 switching cycles, or 4-10 AC mains cycles, whichever applies.
--- End quote ---
And sorry about that, forgot about the pot model. I got it somewhere on the web and tweaked it to my liking, but I think it may slow down simulation runs a bit too much, not sure.
I can post the model if needed
ETA: The reason for the long simulation time is because with large filter cap values it takes a while to reach 300V in the output, not sure if there's anything I can do to speed it up
dazz:
Now we're talking. Fixed the tranny's polarity and increased the gate pull down resistor and got a 96% efficiency already. I still need to find a fast switching diode that will take 500V plus peak reverse voltage and perhaps a better mosfet.
ETA: OK, I believe a UF4007 should do for the diode
dazz:
--- Quote from: T3sl4co1l on December 30, 2019, 07:36:34 pm ---By using a transformer, you get all the ratio with none of the wasted capacity. You get an additional wrinkle of leakage inductance (and also stray capacitance, at high voltages), but this isn't new as such, it was always present (stray inductance between the transistor, diode and cap); it's only a matter of correct transformer design, and snubbing if applicable.
--- End quote ---
--- Quote from: MagicSmoker on January 02, 2020, 03:13:39 pm ---Next big issue is there is no RCD or Zener clamp across the primary; switch destruction is guaranteed in the real world, but LTSpice apparently doesn't care... :P
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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
MagicSmoker:
--- 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
--- End quote ---
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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