Author Topic: MC34063 high voltage dc-dc boost converter  (Read 21431 times)

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Offline dazzTopic starter

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Re: MC34063 high voltage dc-dc boost converter
« Reply #50 on: January 03, 2020, 03:37:11 pm »
Damn, of course. A cap is a short for AC signals. Electronics 101.  |O  :palm:

EDIT: The zener seems to be clamping the signal, but as anticipated by MagicSmoker, it's not doing a great job.
I'll try the RCD snubber next
« Last Edit: January 03, 2020, 04:08:29 pm by dazz »
 

Offline MagicSmoker

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Re: MC34063 high voltage dc-dc boost converter
« Reply #51 on: January 03, 2020, 04:52:27 pm »
R2 in your circuit is a bit of kludge in that it provides some damping whenever the Zener/TVS conducts, but it has to be a realistic value given the expected peak current because the voltage drop across it adds to what the switch must withstand (keep in mind the leakage inductance acts like a current source).

 

Offline dazzTopic starter

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Re: MC34063 high voltage dc-dc boost converter
« Reply #52 on: January 03, 2020, 07:53:46 pm »
R2 in your circuit is a bit of kludge in that it provides some damping whenever the Zener/TVS conducts, but it has to be a realistic value given the expected peak current because the voltage drop across it adds to what the switch must withstand (keep in mind the leakage inductance acts like a current source).

I've tried anything from 200 up to 10k for R2 and a bunch of different zeners from 15V to 100V with no apparent luck, not sure if I'm doing something wrong, but no combination reduced the power dissipation in the mosfet significantly.  :-//

So D4 is a TVS? I was wondering what it's doing there. Picked a uF400x because I read somewhere that it needs to be a fast switching diode. But then again the MRA4007 that they're using in the schematic I posted above is a standard recovery diode according to the datasheet...  :-//
 

Offline David Hess

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Re: MC34063 high voltage dc-dc boost converter
« Reply #53 on: January 03, 2020, 08:20:48 pm »
The diode has to turn on fast which is generally not a problem even with a standard recovery diode unless it is defective.  Diodes can do weird things sometimes.  The same of course applies to the zener or TVS.  Since high voltage zener and TVS diodes work in avalanche mode, I would expect them to turn on very quickly in reverse breakdown.

The alternative snubber uses a diode in series with a parallel RC load so the capacitor absorbs the spike and the resistor slowly bleeds off charge.  No zener or TVS is used.
 

Offline dazzTopic starter

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Re: MC34063 high voltage dc-dc boost converter
« Reply #54 on: January 03, 2020, 09:02:38 pm »
The diode has to turn on fast which is generally not a problem even with a standard recovery diode unless it is defective.  Diodes can do weird things sometimes.  The same of course applies to the zener or TVS.  Since high voltage zener and TVS diodes work in avalanche mode, I would expect them to turn on very quickly in reverse breakdown.

The alternative snubber uses a diode in series with a parallel RC load so the capacitor absorbs the spike and the resistor slowly bleeds off charge.  No zener or TVS is used.

OK, so you mean only the Ton time matters here, because that's when the spikes happen, right?
 

Offline David Hess

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Re: MC34063 high voltage dc-dc boost converter
« Reply #55 on: January 03, 2020, 10:31:30 pm »
OK, so you mean only the Ton time matters here, because that's when the spikes happen, right?

Exactly, and even slow diodes can turn on very quickly.  Some diodes do have problems turning on quickly under some conditions but I think this is a manufacturing problem.  It happens even with fast recovery diodes.
 

Offline MagicSmoker

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Re: MC34063 high voltage dc-dc boost converter
« Reply #56 on: January 03, 2020, 10:49:36 pm »
R2 in your circuit is a bit of kludge in that it provides some damping whenever the Zener/TVS conducts, but it has to be a realistic value given the expected peak current because the voltage drop across it adds to what the switch must withstand (keep in mind the leakage inductance acts like a current source).

I've tried anything from 200 up to 10k for R2 and a bunch of different zeners from 15V to 100V with no apparent luck, not sure if I'm doing something wrong, but no combination reduced the power dissipation in the mosfet significantly.  :-//

Yep, you're doing something wrong... namely, throwing random values at the schematic to see if anything works. Sometimes you can learn a lot that way, but most of the time you just end up jogging in place (ie - going nowhere).  >:D

In this case, first look at the peak primary current then pick a resistor value that will result in around 10-20V of drop at that current. For example, if the peak current is 2A then a 5-10R resistor would be more appropriate, not 100R or higher! Keep in mind the Zener/TVS (D5 in the above) diode needs to be able to handle this peak current, too, and it absolutely must have a breakdown voltage significantly higher than the supply voltage (more specifically, it needs to be higher than the reflected voltage from the secondary, as I already explained).
 

Offline dazzTopic starter

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Re: MC34063 high voltage dc-dc boost converter
« Reply #57 on: January 03, 2020, 11:01:33 pm »
OK, so you mean only the Ton time matters here, because that's when the spikes happen, right?

Exactly, and even slow diodes can turn on very quickly.  Some diodes do have problems turning on quickly under some conditions but I think this is a manufacturing problem.  It happens even with fast recovery diodes.

Cool, thanks David.

BTW, I wasn't getting very good results in the sim with any combination of components in the snubber network, so I went back as I usually do, to re-read the thread. Tim said a few pages back that my tranny inductors were far too large. Trying much lower values now to reduce the leakage inductance, and efficiency/power loss in the mosfet are already much improved. When I find the optimal values of inductance for the transformer I'll go back to testing snubbers in the sim. I know this trial and error strategy is probably not the best way to go about understanding how thing actually work, especially since these are no real world tests, but I hope this will give me a general idea of what works and what doesn't, and then I can better understand why once I google the concepts. I dunno, we'll see.
 

Offline dazzTopic starter

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Re: MC34063 high voltage dc-dc boost converter
« Reply #58 on: January 03, 2020, 11:21:10 pm »
R2 in your circuit is a bit of kludge in that it provides some damping whenever the Zener/TVS conducts, but it has to be a realistic value given the expected peak current because the voltage drop across it adds to what the switch must withstand (keep in mind the leakage inductance acts like a current source).

I've tried anything from 200 up to 10k for R2 and a bunch of different zeners from 15V to 100V with no apparent luck, not sure if I'm doing something wrong, but no combination reduced the power dissipation in the mosfet significantly.  :-//

Yep, you're doing something wrong... namely, throwing random values at the schematic to see if anything works. Sometimes you can learn a lot that way, but most of the time you just end up jogging in place (ie - going nowhere).  >:D

In this case, first look at the peak primary current then pick a resistor value that will result in around 10-20V of drop at that current. For example, if the peak current is 2A then a 5-10R resistor would be more appropriate, not 100R or higher! Keep in mind the Zener/TVS (D5 in the above) diode needs to be able to handle this peak current, too, and it absolutely must have a breakdown voltage significantly higher than the supply voltage (more specifically, it needs to be higher than the reflected voltage from the secondary, as I already explained).

Yeah, you're right, I need to stop trying to brute force things by mindlessly trying values when I don't know what they'll do. And thanks for those pointers. May I ask why the 10-20V drop in the resistor? I would have thought that it would need to sink the same voltage as the zener clamp, or about 2x the reflected voltage in the primary, in my case 2x(300V/12.5)=2x24V=48V
 

Offline MagicSmoker

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Re: MC34063 high voltage dc-dc boost converter
« Reply #59 on: January 03, 2020, 11:41:34 pm »
Yeah, you're right, I need to stop trying to brute force things by mindlessly trying values when I don't know what they'll do. And thanks for those pointers. May I ask why the 10-20V drop in the resistor? I would have thought that it would need to sink the same voltage as the zener clamp, or about 2x the reflected voltage in the primary, in my case 2x(300V/12.5)=2x24V=48V

There's no hard-and-fast rule for sizing the resistor - in fact, it can be eliminated if an RC damper is also present - so don't get too hung up on the voltage drop; I just ballparked something that looked like it would be about right based on experience.

 

Offline dazzTopic starter

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Re: MC34063 high voltage dc-dc boost converter
« Reply #60 on: January 03, 2020, 11:51:53 pm »
There's no hard-and-fast rule for sizing the resistor - in fact, it can be eliminated if an RC damper is also present - so don't get too hung up on the voltage drop; I just ballparked something that looked like it would be about right based on experience.

The RC dumper is in my things-to-google list, thanks again, MS
 

Online magic

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Re: MC34063 high voltage dc-dc boost converter
« Reply #61 on: January 04, 2020, 09:41:10 am »
Actually, I have a dumb question ;)

What if we take the RCD capacitor and return it to the negative supply, in parallel with the switch, rather than to the positive supply?
This would seem to absorb the energy of the whole loop, including capacitor and PCB, not just the transformer. Energy which otherwise rings with the switch and/or avalanches it, I suppose.
The only drawback I see is that now the process of discharging the clamp needs to deal with supply ESL, but that's relatively slow and resistively damped so meh (?):-//
 

Offline T3sl4co1l

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Re: MC34063 high voltage dc-dc boost converter
« Reply #62 on: January 04, 2020, 11:21:17 am »
Yes, it is electrically preferable to put the clamp across the switch.

It is done to the supply, when the supply inductance is low enough not to mind; the advantage is easier layout.

Instead of an RCD clamp snubber, an RCD rate snubber is also often seen, which allows some savings in turn-off losses.  The RC can also be dimensioned to dampen the transformer's free ringdown waveform.  (This tends to be impractical when leakage is high, as the rate snubber will not significantly reduce the peak voltage.)

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Offline dazzTopic starter

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Re: MC34063 high voltage dc-dc boost converter
« Reply #63 on: January 04, 2020, 02:20:50 pm »
I found what looks like a great series of videos about flyback design.

https://www.youtube.com/user/rbola35618/search?query=analysis+and+design+of+a+flyback
 

Offline dazzTopic starter

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Re: MC34063 high voltage dc-dc boost converter
« Reply #64 on: January 04, 2020, 11:34:22 pm »
OK guys, I'm really confused about the results I'm getting with different snubber configurations. I attached screenshots of the drain voltage with no snubber, with a zener clamp, with a RCD snubber, and a combination of zener+RCD. First thing I don't understand is the zener clamp doesn't seem to be clamping at all. The zener in the pic has a 20V reverse voltage, yet most of the signal while the mosfet is on is above 20V. It does seem to do a good job at eliminating the ringing while the during the off time of the mosfet though (pic #2). The RCD filter, OTOH, does the oposite apparently. It filters out the ringing while the mosfet is on (pic 3). So I tried a combination of both, which seems to be working pretty well (pic 4).

One thing I noticed is that the voltage in the drain while the mosfet is off, is not 24V like in the input, but 48V. It must have something to do with the transformer, because if I decrease the inductance in the secondary, off drain voltage increases... but shouldn't it be the other way around?
 

Offline T3sl4co1l

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Re: MC34063 high voltage dc-dc boost converter
« Reply #65 on: January 05, 2020, 06:30:23 am »
24V zener, 24V supply, what's 24+24V? ;D

RCD, try more like 1k and 100nF.

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Online magic

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Re: MC34063 high voltage dc-dc boost converter
« Reply #66 on: January 05, 2020, 07:56:38 am »
The peaks are higher than 24+24V. That's because primary current at turn-off is presumably several amps. This means significant voltage drop across the 10Ω resistor and trouble for the tiny 0.5W zener.

BTW, it seems that off-time primary voltage approaches zener breakdown voltage. That's not a good idea, you may end up dissipating all flyback transformer energy in the zener rather than the secondary ;)
« Last Edit: January 05, 2020, 08:05:56 am by magic »
 

Offline MagicSmoker

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Re: MC34063 high voltage dc-dc boost converter
« Reply #67 on: January 05, 2020, 01:10:31 pm »
OK guys, I'm really confused about the results I'm getting with different snubber configurations.
...
One thing I noticed is that the voltage in the drain while the mosfet is off, is not 24V like in the input, but 48V. It must have something to do with the transformer, because if I decrease the inductance in the secondary, off drain voltage increases... but shouldn't it be the other way around?

Well, someone's not paying attention. Go back and (re-)read posts #44, 46 and 56 as I already covered all of the above questions!

The voltage across the switch when it turns off is the sum of the supply voltage and the "reflected" output voltage (ie - the output voltage transformed by the sec:pri turns ratio [note the order]). If the output is 300V and the pri:sec turns ratio is 1:12, then the ratio going in the other direction is 12:1, so the secondary voltage will be divided by 12 when seen at the primary, or 25V in this example. The reflected 25V from the secondary adds to the 24V supply to give a minimum voltage the switch must withstand at the instant of turn-off of 49V (not including ringing, leakage spikes, etc.). If the Zener clamp across the primary breaks down at a voltage lower than 25V then energy stored in the transformer during the switch on time will preferentially exit through the clamp during the switch off time (ie - the flyback period, hence the name), rather than from the secondary. To use a classic military abbreviation, that's NFG, hence why I suggested in one of the aforementioned posts to set the Zener voltage to between 36V and 51V.

 

Offline dazzTopic starter

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Re: MC34063 high voltage dc-dc boost converter
« Reply #68 on: January 05, 2020, 01:44:45 pm »
Yes, I need to take a break from the sim and read more instead. The reason I used such a low voltage zener is simply because it seemed to work. The higher voltage ones weren't clamping at all, perhaps because of the high frequency of the ringing, not sure. It's also obviously true that the zener I picked was dissipating far too much power, some 15W on average. I probably should also swap the mosfet for one with lower Cds, I think that will reduce the ringing a bit, correct? Is the output capacitance in the datasheet the same as Cds?

ETA: One question, please. Is the snubber supposed to improve overall efficiency? Or it simply lowers the dissipation in the mosfet by the same amount the snubber dissipates?
« Last Edit: January 05, 2020, 02:41:28 pm by dazz »
 

Offline dazzTopic starter

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Re: MC34063 high voltage dc-dc boost converter
« Reply #69 on: January 05, 2020, 03:09:53 pm »
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).

OK, just reread this and I see now you answered many of my questions there. Yep, I'm a bit slow, sorry about that

ETA: This looks much better, right? (Added screenshot)
« Last Edit: January 05, 2020, 03:20:37 pm by dazz »
 

Offline MagicSmoker

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Re: MC34063 high voltage dc-dc boost converter
« Reply #70 on: January 05, 2020, 03:12:28 pm »
Yes, I need to take a break from the sim and read more instead. The reason I used such a low voltage zener is simply because it seemed to work. The higher voltage ones weren't clamping at all, perhaps because of the high frequency of the ringing, not sure. It's also obviously true that the zener I picked was dissipating far too much power, some 15W on average. I probably should also swap the mosfet for one with lower Cds, I think that will reduce the ringing a bit, correct? Is the output capacitance in the datasheet the same as Cds?

ETA: One question, please. Is the snubber supposed to improve overall efficiency? Or it simply lowers the dissipation in the mosfet by the same amount the snubber dissipates?

1. R2 is artificially boosting the clamping voltage at the instant of turn-off; try setting it to 1m to effectively take it out of the circuit without actually removing it.

2. Cds is a bit of a moving target because it varies inversely - and non-linearly - with the applied drain-source voltage (when off, of course; this capacitor is obviously shorted when the MOSFET is on). A MOSFET with lower Cds with all other parameters (except cost) being the same will store less energy which eventually needs to be snubbed so, yes, that is a good direction to go. It's a bit of a 2nd order problem at the moment, though; in other words, concentrate on getting the converter to work properly first.

Note, also, that Cds (plus other stray capacitances) forms two different resonant networks in the flyback: with the leakage inductance of the primary when the secondary diode is conducting, and with the magnetizing inductance if the diode turns off some time before the switch turns on again (ie - operating in Discontinuous Mode). Basically, you can damp the former but not the latter.

3. Standard dissipative snubbers just move losses from the switch to themselves, they don't reduce losses (they actually increase the total loss somewhat because of pesky thermodynamics). There are a class of so-called "lossless" snubbers that utilize resonant networks to recycle energy from switching and/or ringing back to the supply (or output), but don't even think about messing around with them for now.

 

Offline dazzTopic starter

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Re: MC34063 high voltage dc-dc boost converter
« Reply #71 on: January 05, 2020, 03:32:34 pm »
1. R2 is artificially boosting the clamping voltage at the instant of turn-off; try setting it to 1m to effectively take it out of the circuit without actually removing it.

2. Cds is a bit of a moving target because it varies inversely - and non-linearly - with the applied drain-source voltage (when off, of course; this capacitor is obviously shorted when the MOSFET is on). A MOSFET with lower Cds with all other parameters (except cost) being the same will store less energy which eventually needs to be snubbed so, yes, that is a good direction to go. It's a bit of a 2nd order problem at the moment, though; in other words, concentrate on getting the converter to work properly first.

Note, also, that Cds (plus other stray capacitances) forms two different resonant networks in the flyback: with the leakage inductance of the primary when the secondary diode is conducting, and with the magnetizing inductance if the diode turns off some time before the switch turns on again (ie - operating in Discontinuous Mode). Basically, you can damp the former but not the latter.

3. Standard dissipative snubbers just move losses from the switch to themselves, they don't reduce losses (they actually increase the total loss somewhat because of pesky thermodynamics). There are a class of so-called "lossless" snubbers that utilize resonant networks to recycle energy from switching and/or ringing back to the supply (or output), but don't even think about messing around with them for now.

OK, I have calculated my leakage inductance at 60nH (which seems low, right? I guess I cheated a bit by lowering the inductances of the transformer too much) and the mosfet output capacitance is 380pF according to the datasheet, I guess I can do a lot better than that. So with the clamping voltage at 50V I still get spikes in excess of 60V, a 80V mosfet would be the way to go, correct?
 

Offline MagicSmoker

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Re: MC34063 high voltage dc-dc boost converter
« Reply #72 on: January 05, 2020, 09:12:04 pm »
...
OK, I have calculated my leakage inductance at 60nH (which seems low, right? I guess I cheated a bit by lowering the inductances of the transformer too much) and the mosfet output capacitance is 380pF according to the datasheet, I guess I can do a lot better than that. So with the clamping voltage at 50V I still get spikes in excess of 60V, a 80V mosfet would be the way to go, correct?

Gah... It took me less time to just edit your .asc file than type another response. My usual disclaimer applies: I did not optimize everything to leave something for you to do, but I got you into the ballpark.

That ratio extender trick isn't needed if you select a reasonable base switching frequency vs. inductance vs. peak current so that the flyback doesn't really need to go much above 50% duty and operates as close to continuous current mode as possible without actually entering it. Hence why the timing capacitor was reduced and the primary inductance increased.

An RC damper was added across the switch to deal with the high frequency ringing - it only costs about 0.3W of loss and stops your flyback from doing double duty as an AM (or FM!) radio transmitter. As mentioned earlier, this damper can be placed across the primary, instead, I just felt it kept the schematic neater putting it across the switch.

The primary clamp is now an RCD type with the (51V) Zener acting as more of a backstop. Loss in the clamp once the converter reaches equilibrium should be around 0.5W or so with peak voltage well controlled for a 100V MOSFET.

Output capacitor increased to a more realistic value and output diode changed to a 1A/600V SiC Schottky, though as long as you keep the flyback in DCM you can use a conventional fast recovery diode (ie - with <100ns reverse recovery time [given as tt in the .model statement, btw]).

Finally, the simulation parameters were tweaked to give more accurate results and the total time reduced to just a little more than is needed for the output voltage to reach equilibrium (at which point the '34063 goes into burst mode). All in all, waveforms look pretty realistic now and you have a reasonable jumping off point to keep tinkering.

 

Offline dazzTopic starter

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Re: MC34063 high voltage dc-dc boost converter
« Reply #73 on: January 05, 2020, 09:28:17 pm »
...
OK, I have calculated my leakage inductance at 60nH (which seems low, right? I guess I cheated a bit by lowering the inductances of the transformer too much) and the mosfet output capacitance is 380pF according to the datasheet, I guess I can do a lot better than that. So with the clamping voltage at 50V I still get spikes in excess of 60V, a 80V mosfet would be the way to go, correct?

Gah... It took me less time to just edit your .asc file than type another response. My usual disclaimer applies: I did not optimize everything to leave something for you to do, but I got you into the ballpark.

That ratio extender trick isn't needed if you select a reasonable base switching frequency vs. inductance vs. peak current so that the flyback doesn't really need to go much above 50% duty and operates as close to continuous current mode as possible without actually entering it. Hence why the timing capacitor was reduced and the primary inductance increased.

An RC damper was added across the switch to deal with the high frequency ringing - it only costs about 0.3W of loss and stops your flyback from doing double duty as an AM (or FM!) radio transmitter. As mentioned earlier, this damper can be placed across the primary, instead, I just felt it kept the schematic neater putting it across the switch.

The primary clamp is now an RCD type with the (51V) Zener acting as more of a backstop. Loss in the clamp once the converter reaches equilibrium should be around 0.5W or so with peak voltage well controlled for a 100V MOSFET.

Output capacitor increased to a more realistic value and output diode changed to a 1A/600V SiC Schottky, though as long as you keep the flyback in DCM you can use a conventional fast recovery diode (ie - with <100ns reverse recovery time [given as tt in the .model statement, btw]).

Finally, the simulation parameters were tweaked to give more accurate results and the total time reduced to just a little more than is needed for the output voltage to reach equilibrium (at which point the '34063 goes into burst mode). All in all, waveforms look pretty realistic now and you have a reasonable jumping off point to keep tinkering.

Thanks, much appreciated. I will keep on searching for info to learn how all this works and do those calculations myself, if I can.

ETA: WTF? 98% efficiency? I know this is just a simulation, but wow!
« Last Edit: January 05, 2020, 09:34:04 pm by dazz »
 

Offline dazzTopic starter

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Re: MC34063 high voltage dc-dc boost converter
« Reply #74 on: January 23, 2020, 03:39:52 pm »
I've put aside the flyback research for the moment while I build and test the original boost converter.
I have most of the components already, but I've come to the realization that I need to learn more about inductors and how they're built before I do this. The thing is I simply searched for 470uH 3A toroidal inductors in ebay, and got these crappy ones that I'm pretty sure won't even handle 1A. What's worse, the ones I got had 0.3mm wires, not the 0.7mm advertised. So yeah, all but useless.

I've spent an afternoon googling about toroidal inductors and it's a bit daunting TBH. Apparently it's not as simple as picking a core size that can fit the necessary number of turns of the necessary wire gauge to obtain the desired inductance and current rating/resistance. Looks like there's a host of different core materials that affect the calculations.

I'm going by this right now: https://cromwell-intl.com/radio/copper-wire/
There's a table specifying the different inductance indexes for every core type, and the formulas to derive the necessary number of turns for a given target inductance. Seems straightforward enough.
I also did a search in Digikey to get an idea of the resistance and core size of inductors in the ballpark of what I need. Most seem to be 1.280" Dia x 0.650" W (32.51mm x 16.51mm). Unfortunately the datasheets I checked don't say anything about core material or wire gauge.

Is there a standardized method to derive the kind of inductor you need? Would you star by picking the core material, then the size, wire gauge, etc? The problem is, as far as I can tell, all those things are interconnected: a thicker gauge requires a larger core. Materials with larger Al index can be smaller, but then the wire needs to be thinner too, but a thinner wire will produce more resistance with the same number of turns... What a mess!  |O

I'm sure there's an easy way to approach this. Any pointers appreciated
 


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