Switched Mode Power Supply

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Nnnnnnn
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Re: Switched Mode Power Supply

Post by Nnnnnnn »

Another update: I bought some cheap 1kV diodes and capacitors and built a tripler for my test setup. Using this I have reached a new personal record voltage of 3kV (and I can still double the voltage on my variac). After making some arcs I got back to measuring the output voltage. I noticed that when using my MOSFET (IRFP250N) the system saturated somewhere at around 2kV (by saturated I mean the output voltage no longer increased linearly when I increased the Input voltage). Then I put in my IGBT (G4PC50FD, which isn't made for my frequency btw) and easily made it to 3kV. I could have gone further, like I said my variac was only half open. In both cases I was driving at around 200 kHz.

I am wondering why I was getting higher output voltages with the IGBT. Perhaps once you get close to the limits of your transistor it no longer performs as it should (MOSFET max voltage is 200V. I went pretty much right up there and the MOSFET was getting very hot), as a result the MOSFET may not have actually been switching at 200kHz.
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Re: Switched Mode Power Supply

Post by Richard Hull »

Yes, tone that frequency down. 25-50khz is a good bet with the lower frequencies working better in the kilowatt handling range.
The magnetics are also less of an issue.

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Bob Reite
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Re: Switched Mode Power Supply

Post by Bob Reite »

My supply seems to work best at 30 KHz, due to the transformer characteristics.
The more reactive the materials, the more spectacular the failures.
The testing isn't over until the prototype is destroyed.
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Rich Feldman
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Re: Switched Mode Power Supply

Post by Rich Feldman »

Niels, you can get better answers here if you show us a schematic. Whole circuit, including voltage tripler, and your attenuator for HV measurement. I don't know how the voltage triplers are configured in CRT anode power circuits.

>> why I was getting higher output voltages with the IGBT?

The actual breakdown voltage of your IGBT specimen might be higher than that of your MOSFET.

In normal operation of a flyback converter, the secondary current needs to have a DC component (time average of instantaneous current). Its magnitude is the same as the DC current in primary winding divided by the turns ratio, N. In every cycle: When the primary switching device turns off, the secondary voltage is that at which the load conducts. Or N times the voltage at which the primary switch breaks down, if that happens first. Somebody else made that comment recently -- perhaps on another forum I read.

As others have said here: 200 kHz or even 100 kHz is a stupidly high switching frequency, especially for IGBTs, and especially for beginners, even beginners with oscilloscopes. I was just working on a power issue in a board with about a dozen buck converters running at 500 kHz, and could post some salient details if you want.
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Re: Switched Mode Power Supply

Post by Nnnnnnn »

Hey all, I followed your advice: first I lowered the frequency to 100 kHz and noticed I was getting a higher V_out:V_in, however the V_out maxed out for lower V_in. I then lowered the frequency further to 50 kHz, which had the same effect, higher V_out:V_in and lower max V_in. I then increased the number of turns on the primary side which did very little for my max V_in. Then I added a 15nF cap and 100 ohm resistor across the primary to act as a filter against Deltas (primary inductance estimated at 1-5mH). This decreased my V_out:V_in (not in resonance), however it did increase V_in max. So I am thinking about adding an inductor with a known inductance and using that as my filter inductor instead of my unknown primary inductor.

Another sidenote I might upgrade my circuit to a push pull topology, however I see no need for that yet.

As for my circuit diagram:

Image

and

Image

Note I haven't drawn in the filter cap and resistor yet, but it is simple enough to imagine I think. I use a multimeter to measure the current through the 2M\Omega resistor. I have also used a 2M ohm followed by a 100k ohm resistor and used a multimeter to measure the voltage across the 100k ohm resistor. I measured a max voltage of around 3kV.
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Rich Feldman
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Re: Switched Mode Power Supply

Post by Rich Feldman »

What happens if you reconfigure the voltage multipler
so there's a path for DC current in the secondary winding?
For example, from http://www.oldtellys.co.uk/otltbeht.html
ltbeht8.jpg
ltbeht8.jpg (18.74 KiB) Viewed 7273 times
Don't be led astray by the many DIY circuits that use "flybacks" as regular transformers, with drivers that involve more than one switch.

Your HV measurement methods sound fine, if you respect the voltage rating of the high-ohm ( 2 MΩ ) resistor.
If you are using a computer keyboard with a calculator-style number key array, what happens if you type 234 (over there) while the Alt key is depressed? Ω
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Re: Switched Mode Power Supply

Post by Nnnnnnn »

Took apart my transformer and rewound the primary and secondary. I now have 10 primary turns and 37 secondary turns. I checked for the DC current component you mentioned Rich. I did this by putting a large value resistor across the secondary and measuring the DC current through it. I was unable to measure anything. I think there may be some confusion. I am using a driver that one would use with a flyback transformer, however my transformer is not a flyback. It is a toroidal ferrite transformer. Maybe "forward converter" is a more adequate name (though to be clear I do not have an extra inductor. The circuit I posted here is the circuit I am using).

I am getting an issue that my output voltage suddenly drops once I get above a certain input voltage. I believe this is due to breakdown occuring somewhere which causes a short circuit causing the voltage to drop (this happens somewhere in the CW Multiplier, probably soldered it too compact on the perfboard).

Also for 234 on the numpad I get Û
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Rich Feldman
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Re: Switched Mode Power Supply

Post by Rich Feldman »

Well Niels, with the "flyback" drive circuit, you know there's DC current in the primary winding. How many ampere-turns can your toroid take without saturating? We can help you figure that out, if you post the core's dimensions and material properties or inductance index (usually given as some inductance value for some number of turns). Or we can review the calculations you made before you chose a toroid and bothered to wind wire on it.

In a regular transformer, the ideal winding inductance is infinite, and the ideal energy storage is zero. Instant by instant, the power going in at the primary terminals matches the power going out at the secondary terminals. Conversely, finite inductance and energy storage are essential in the magnetic component of a flyback converter. When the primary switch is "on", power goes in and accumulates as stored energy ( L * i^2 / 2 ). When the primary switch turns off, a secondary winding diode turns on. The stored "magnetic" energy goes out as power (volts * amps) through the secondary terminals.

CRT "flybacks" (for horizontal deflection & anode power) generally have an air gap in the ferrite core, at the joint hidden by the winding subassembly. Air gap greatly reduces the winding inductances and greatly increases the ampere-turns limit. Net result is a useful energy storage capacity (with most of the magnetic energy concentrated within the air gap). Became pretty universal before 1960.

If you want to use flyback converter topology with a ferrite toroid core, I think you should learn how to understand and calculate details that you've avoided so far. That knowledge will serve you better than having an oscilloscope. The real world is analog! :-)
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Re: Switched Mode Power Supply

Post by Nnnnnnn »

Darn it. Happened again got logged out when I had typed my response.

So here are my datasheets: http://www.farnell.com/datasheets/15958 ... 1499594956 and http://www.ferroxcube.com/FerroxcubeCor ... t/3c90.pdf (can somebody tell me what Sigma I/A is?)

Yesterday I actually measured the DC current in the Primary to calculate the inductance. At 50kHz and 35V I measured an average current of 66.7mA. From this I concluded that the primary inductance equals 1.3mH. From this I calculated that the DC current at 350V and 50kHz would be something like 337mA. At 10 Primary turns I have 3.37 Ampere Turns. Using the datasheet we find the toroid core is 0.26m Long. Therefore the field equals about 13 A/m, this is far from saturation if I read the datasheet correctly. Also some of this may not be optimal because I did a poor job on my primary windings (some air gaps) because the wire is not ideal for winding (secondary looks a lot better)

As a side Project today I made a sound card silly scope which despite its sampling frequency of 96kHz can only measure signals up to 20 kHz so I think it samples at around 40 kHz. So I lowered the frequency of my driver and hooked up the scope to my seconday windings and played around with the Input voltage a bit. I did not get any saturation this time. I don't recommend anyone try this unless they want a headache from all the high frequency ringing.

I don't think my setup could ever deliver the power needed for fusion in flyback mode. This is why I eventually want to look into push pull and or the half bridge topology. The problem with push pull is the high voltage stress on the switches.
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Rich Feldman
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Re: Switched Mode Power Supply

Post by Rich Feldman »

Love it, Niels. I'm pretty addicted to the stuff, and learned some things from working on answers for you.

On the toroid datasheet, that "Sigma I/A" value is in the magnetical dimensions section. The I is actually a l (lowercase letter L), like the one in the symbol for effective length (l sub e: 255 mm). Unless the Polish word for length begins with the letter between h and j. We see that those sigma term values, and their unit dimensions, match the quotients l/A and l/A^2 of the magnetic length and area parameters. Perhaps they are convenient criteria for some core selection process.

I did an exercise to get the toroid's inductance index, Al, from that Sigma l/A and the initial permeability. In your 3C90 material datasheet, the typical ui value is given as 2300. In absolute terms, that's 0.0029 henries per meter. Dividing by your toroid's le, and multiplying by its Ae, we get 5041 nanohenries [per turn squared], just like the toroid datasheet says!

The primary winding inductance you got, 1.3 mH, sounds perfectly reasonable. That would make the Al value 13000 nH, and relative permeability u = 5930, at the operating point of your measurement. 3C90 datasheet gives u = 5500 at Bmax = 200 mT and t = 100 °C.

I have issues with your extrapolation from 35 to 350 volts, and with your saturation margin numbers. No time to write it now.
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Rich Feldman
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Re: Switched Mode Power Supply

Post by Rich Feldman »

OK, let's make this fast.
The details you gave about the 1.3-mH measurement, operating at 35 V, suggest a primary current waveform that looks like:
niels_flyback.PNG
niels_flyback.PNG (5.57 KiB) Viewed 7170 times
To get DC current of 66.7 mA, the duty cycle needs to be about 50%, with peak current about 4 times greater than average. Have I got that straight? If so, then your Bmax is already up to around 80 mT.

What do you think the secondary current waveform looks like?

How come your expected current at 350 V isn't ten times greater than that at 35 V?

I like your "silly scope" side project.
Suppose you can build a fast-acting voltage sample-and-hold circuit.
Drive it with your Arduino at 49 kHz while the circuit under test is running at 50 kHz.
The output waveform will look like the input, slowed down by a factor of 50. At 1 kHz, your sound card ought to be able to do a pretty high fidelity waveform capture.

Or drive this fast-acting voltage sample-and-hold with your Arduino at 50 kHz, and a programmable phase with respect to your SMPS under test. Like using a stroboscope to "freeze" the view of a rotating part. Take readings manually with a voltmeter, at whatever phases you choose. Or use the Arduino to read an analog to digital converter, which can be as slow as you want.
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Re: Switched Mode Power Supply

Post by Nnnnnnn »

Glad you like it too Rich, you have been very helpful. Thanks for clarifying my question about the datasheet and the example calculation!

Yes that is the waveform I was using in my calculations (no reverse current). You are correct the peak current should be 4 times higher than the average current. Using the 0.0029 H/m you found that means the B_max=66.7mA*4*10Turns*0.0029H/m=150mT! (this explains why saturation is happening so soon). This could be reduced by increasing the number or primary turns, because more primary turns increases the inductance (goes with turns squared) which decreases the current, the amp turns goes linearly with the number of turns, so B_max will drop linearly with the number of turns.

I am a bit confused though: imagine that we sent an AC into the transformer (+35V to -35V square wave). If we look at the current like we just did we fill find that B_max=80mT, however if we use the universal EMF equation for transformers B_max=V_rms/(4.44*f*a*N)=35/(4.44*5*10^4*4.45*10^-4*10)=35mT. And if we drive the transformer with a square wave of 35V to 0V we get a lower RMS voltage which would give a lower B_max.

As for the secondary waveform: in the first phase as the magnetic field builds up the (shorted) secondary should counteract the magnetic field. So first we will see a current in the opposite direction: I_s=-I_p/T, where T is the turns ratio. Then in the second Phase the magnetic field collapses so the current in the secondary reverses its direction instantly. We will see I_s=I_max-k*t, where k is V/L like in the first Phase.

Also you are right the expected current at 350V is 10 times higher than at 35V. I divided by two somewhere in my calculation, my mistake.

I though of a similar way to bypas the frequency limit. I can use a pair of MOSFETS the arduino and the silly scope to do this. I just use the probe I have been using but add some MOSFETS to it which are turned on and off with the arduino at say 1 kHz with a low duty cycle and then using the arduino slowly shift the turn on time. This only works for periodic signals, but that isn't a problem since my Signal is periodic. I will add a drawing in paint to illustrate what I am doing. The red lines are samples which go to the sound Card and can be sampled at 48 kHz.

Image

Also today I took my rectifier from the bread board and soldered it making my Setup a bit cleaner. In addition I added an amp meter in the primary circuit, when I turned it on my IRFP360 MOSFET broke. I replaced it with a IRFP250N, which also broke. I checked the circuit for any shorts, checked wether or not the arduino was creating the correct Signal etc. couldn't find anything. So I put in a new IRFP250N with an extra 100 ohm 50 W resistor on the drain. This time everything worked. However once I removed the resistor the MOSFET broke again. Which is strange because my IRFP360 was working all week without a resistor in place (just the coil). I am also using the 15nF cap to filter the HV feedback. Do you know what is going on here?

I should add I use two power supplies: one is a variac connected to the rectifier and my large 3mF cap, which powers the circuit. The other is a 12V 1A supply to power the switcher and the arduino. I make sure all ground are connected to a common terminal (the ground terminal on my cap) so that nothing floats away. So Arduino, 12V PS and large value cap are all connected.
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Rich Feldman
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Re: Switched Mode Power Supply

Post by Rich Feldman »

I'm giving this a rest for a week. In the meantime, you could find out whether that formula with the 4.44 term is for sine waves (like I warned you about) or square waves. The square and sine coefficients differ by some factor like 2/pi; flyback formula has yet another value in that place. It should be clear if you work it from the laws named for Ampere and Faraday.

In fact for volts-per-turn at some frequency and Bmax, Faraday's law is sufficient. The core needs enough flux swing to match the voltage. The current to get it there (a.k.a. magnetizing current or no-load primary current) is ordinarily very small compared to the operating current, and isn't critical for design choices. Except in flybacks.

For both regular transformers and flybacks, increasing the turns count has the bad effect of increasing the winding resistance and "copper loss" heating. Compensated by using thicker wire, if there's room. If the hole in the core isn't mostly filled with primary and secondary windings, you are wasting core material, and leaving some power efficiency on the table.
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Richard Hull
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Re: Switched Mode Power Supply

Post by Richard Hull »

Power efficiency?

I am a heathen. The very least part of a power supply design I would consider here is power efficiency. The rule is git 'er done!

Why not worry about power efficiency, you might ask....

1. Do you get a significant reward for power efficiency?
2. Are you building this for yourself or for your employer who "wants it all"... small, lightweight, efficient and pleasing to the eye?
3. For some reason is your power company unwilling to supply you with any amount of power for a fixed amount of money?

point number one....

The average working fusion at 2 million fusions per second can easily demand 40kv at 12ma. 480 watts
I pay about 12.4 cents per KWH here and this figure includes all taxes, fuel price adjustment additions, transmission costs, etc.

So....

a 100% efficient supply would cost me about 6 cents per hour to run my fusor at full bore, max power.
a 40% efficient supply would leap the per hour run rate up to a wallet busting 15 cents! Ouch! That stings
A rotten 25% efficient supply would break the bank at almost 25 cents per hour! Oh mercy!

Based on about 15 hours operation at full power, per year of fusor IV, my yearly fusor power bill with my 50% efficient supply is under $2.00
I don't know about you, but I pay almost double my yearly fusor power bill buying a whopper with cheese, and this does not include a drink or fries to go with it.
I have the fusor in my hosehold budget as the wife gets mad spending two bucks a year on fusor operation. She' pushing me to make a more efficient supply to save money.

Point number two

No one is paying you the big bucks to make your supply efficient, light small or nice looking. In addition efficiency is normally a real issue for portable or battery powered gear where every watt is dear and costly.

It seems stupid to try and get 90% efficiency in a home built supply to run a fusion device that is 0.00000001% efficient at doing fusion. The fusor, in all sincerity, is a great space heater when operated in a chilly winter lab. It heats the lab not by fusion energy but by wasted electricity from the wall outlet.

Point number three

Power is cheap when wall power is used at the level of energy and net run power that the average fusioneer will require. The power companies across the land will back you play for pocket change.

I would be far more sympathetic with a 100 fold increase in fusor efficiency to a 0.000001% efficient fusion device. Of course, with a D-D fusor, at amateur level voltages, this goal seems impossible. Thus far, no one has even advanced the fusor to a ten fold increase over the big, "normalized", goal of 2 million fusions per second.


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Re: Switched Mode Power Supply

Post by Nnnnnnn »

I found answers to most of the questions I asked this weekend. First of all I think the MOSFETs were breaking due to the high voltage spike when the switch turned off, so I actually changed my switching topology to a "half bridge" schematic below:

Image

As you can tell this circuit only turns on above a certain Input voltage, when V_gs on the P-MOSFET gets large enough.

I also derived the not so universal EMF equation for a square wave signal: V=N*A*B_max*f/D (V voltage, N turns, A core area, f frequency, D duty cycle).

I don't care about effeciency that much as long as it is above 50% (my variac can deliver 1kVA). I also do not want to lose too much energy in my MOSFETs or core simply because I do not want them to overheat.

The turn count should stay as low as possible because I only have 10 meters of HV wire, however if I use all of that wire it will cover the entire core, so I could wind a second layer using standard wire. There is no way the top layer could cause breakdown through the bottom layer into the primary coil
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Finn Hammer
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Re: Switched Mode Power Supply

Post by Finn Hammer »

Niels,

I have been following this thread from the start, and I feel it is time to chip in.
My background is in the special full bridge converter that you will find in a DualResonantSolidStateTeslaCoil, where I have come up with a couple of engineering "firsts", as well as produced several units for museum, art and television shows. I have also built a handfull of the most common DC-DC converter topologies.
1. About driving mosfets with an Arduino:
Take a look at the datasheet of a IRFP250
https://www.google.dk/url?sa=t&rct=j&q= ... uw&cad=rja
on page 4 you will see this diagram:
Udklip.JPG
This diagram shows how the Drain-Source voltage relates to the charge you transfer to it (the gate). As you will see, the voltage stops rising momentarily when it reaches 6.5 volts, and this horisontal line on the graph is called the miller plateau, this is where the mosfet starts to turn on. You have to drive the gate higher to turn the mosfet properly on, and you can not do this with the output of an arduino, which only puts out 5 volt.
The arduino can also not deliver enough current to turn the mosfet on properly, and that is why there are gate driver chips. You need a totem pole driver with a bootstrap capacitor to also turn the top mosfet on.
2. Topology
There is no need to invent a new topology, as the one you show in your last post. Read a good application note on topologies
https://www.google.dk/url?sa=t&rct=j&q= ... SA&cad=rja

and choose one that is suitable for a high power supply. You will see that most of the topologies stop at around 500W, and from there, there are only the resonant converters that come through. My own pet is the series resonant converter, because it is easy to build and understand, it supports zero current switching, and with that, a lot is gained. If you have ever tried to design snubber circuits for a hardswitching converter you will follow me here. Zero voltage switching is even better, but then you have to build a LLC converter, which is harder to construct.
As a note to topology: My advice, ditch the arduino and get a proper dedicated converter chip that is designed to do just that job. A good old fasioned chip for an entry level converter is the TL494, but there are newer ones also.
3.Layout
When it comes to building power electronics, proper layout practices are key, and along that note, I will encourage you to post actual pictures of your work, because in that case we will probably be able to point at obvious mistakes.
These are the most obvious points I will point at, you have a lot of gumption and that is commendable, and needed, if you are going to see this project to the end with success. The most important advice I can give you is: read the application notes from TI, Linear, etc. They have covered this subject in great detail, and often the notes are written by authors who have exceptionally great command of written language, so that it is well suited for even a lay person studying on his own.
I wish you good luck on this journey, there are a lot of frustrating moments ahead, but there is also a lot of satisfaction to be gained.

Cheers, Finn Hammer
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Re: Switched Mode Power Supply

Post by Nnnnnnn »

Finn,

Thanks for your thoughts and input!

1. I actually do not drive the power MOSFETs directly with the arduino. I use the Arduino to switch a smaller MOSFET (e.g. BS170), which acts as an amplifier for the power FETs (IRFP360, 250 and IRF9640). The drains of these smaller FETs have a 12V Input and are hooked to the gate of the power MOSFETs and a 47 Ohm resistor which limits the current to around 250 mA (this current seems high enough to limit switching heating). You certainly do have a Point, first I was trying to drive the power FET directly with the arduino and this did not work very well. Also before I used the BS170 as amplifier FET I was using IRF540s which should turn on somewhere at around 5V, however this was causing shorts (5 A current with no on the secondary) even at low duty cycles. The BS170s seems to turn on at 5V though.

2. Thank you for the report it is very helpful! I am not sure I know what you mean when you say don't invent your own topology? I think the one I showed here is the half bridge topology, I just added a simple and cheap method to operate the top switch. I have headr of the resonant converters but have not looked into it very much. I think I would need better instrumentation to build one since I would need to know my primary and secondary inductance very precisely, so that I can select the right capacitors for my frequency (or select the right frequency for my capacitor). But it does seem like an interesting project.

3. Thanks I will try and post pictures some time next week. Once I get everything from the tabletop into a box.
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Rich Feldman
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Re: Switched Mode Power Supply

Post by Rich Feldman »

>> I just added a simple and cheap method to operate the top switch.

Its output impedance is almost 1000 ohms! How many mA are there to charge, and to discharge, the P-FET gate?
The charging current, and ultimate Vgs level, are proportional to the bus voltage. For what voltage did you design it?

When are you going to get an analog circuit simulator program and learn how to drive it? There are good ones that can be downloaded and used, honestly, for free. One popular one is SwitcherCAD, from Linear Technology, often informally (and later officially) called LTspice. We'll have to see what happens now that the LT company is part of ADI.

SPICE is but one limb of an evolutionary tree; another is MSINC. https://www.allaboutcircuits.com/textbo ... -of-spice/ gives some history and credit. SwitcherCAD was developed to be good with magnetic components, power switches, and feedback regulation in SMPS circuits.
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Re: Switched Mode Power Supply

Post by Nnnnnnn »

Rich the P Fet will turn on at 100V input this corresponds to 100mA roughly. It is designed to work up to 350V so roughly 350mA at the gate. Perhaps you recall that the n channel fet is getting roughly 250mA to charge the gate. Until now I have had no issues.

The reason I haven't done any simulations yet is because I assumed simulators would cost a lot of money so I didn't bother to look so this is good to know. I will try it out.

I setup my new design and wound my transformer for "full scale operations" 10 primary turns and a bit over 230 secondary turns. I also soldered my quadrupler and made some arcs yesterday. it pulled about 2 amps in the primary while outputing roughly -20kV (this is based on the arc size still need to build a suitable voltage divider for this voltage).
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Re: Switched Mode Power Supply

Post by Nnnnnnn »

As promised some videos and pictures of the supply. First the complete circuit diagram (probably more valuable than pictures of the actual supply):

Image

Now a picture of the actual supply. The switcher part:

Image

It is hard to see what is what if you didn't build this due to the cable jungle (before switching on I always make sure conducting surfaces that should not be touching are not touching). Upper left corner low side switch on a massive heat sink, next component to the right is the 3.3mF cap, to the right of that you will find the high side switch on a not so massive heat sink. In the lower right corner you find the FET I use to switch the high side switch. The reason for different heat sink sizes is simply because I am trying to use what I have laying around (not that much). Just to the left of that FET you will see the diode bridge rectifier. To the left of that on the perfboard you will find the two caps which create the common terminal, two 120 ohm resistors in paralell forming effectivly 1 60 ohm resistor, followed by two 50W 560 ohm resistors in series. On the perfboard you will also find a few leads going to various drains, sources and gates of different MOSFETs. I also have a few ground leads and 1 "common" lead to which one terminal of the transformer is attached. On top of the perfboard you will also find an Arduino UNO and a breadboard. The breadboard has two 47 ohm resistors and 2 bs170 FETs plus the leads of the 12V power supply go here. This setup is used to generate a 12V 250mA PWM signal for the two N-FET gates. You will notice I use soldered wire, crocodile wires and thin wire for the breadboard. The soldered wire is used where there will be 1A or more, the other wires are used where I have lower currents. You will notice that some places that will see more than 1A still have crocodile wire. Don't worry this will be replaced.

The transformer and multiplier (not in oil yet):

Image

You will notice that the secondary of the transformer has mutliple layers. The upper layers are thin insulation while the lower layers are thick insulation. The area with a lot of tape is the solder point. Below the tape are 2 more layers of tape and 2 layers of shrink insulation (not sure if this is the correct term, it is insulation that shrinks when heated, pretty standard). You will also notice that there the blue secondary wire is never on top of the primary. This is to avoid breakdown and a short. You may also notice that the winding pattern of the blue secondary wire is less regular. This is a result of me trying to avoid gaps in the lower layers. If the blue secondary wire were to touch the transformer core anywhere there would probably be breakdown followed by a short.
The multiplier is soldered to a perfboard and connected to the transformer via some aligator clips. I will solder some wire to connect these. I will also solder some 50kV wire to the output and common of the multiplier (at the point where the wire leaves the oil).

Finally a video of an arc I made with this supply:

https://youtu.be/u6_XeROXMhY

EDIT: The arduino code I wrote (you do not need pin 7 or the serial begin. I had that in for testing purposes):

Image
John Futter
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Real name: John Futter
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Re: Switched Mode Power Supply

Post by John Futter »

Niels
The multiplier looks good
The rats nest in the plastic box is sure to please the purveyors of silicon devices
Even at these low frequencies stray inductance and capacitance are destroyers as your circuit goes into parasitc oscillation.
What you do should look like your circuit diagram, print off a copy and lay each bit over its diagram counterpart. That now is the finished size and look of what should be built
Ie no wire longer than it has to be especially the gate drain and source wires of your active devices
Nnnnnnn
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Real name: Niels Geerits

Re: Switched Mode Power Supply

Post by Nnnnnnn »

John,
Thanks. I did not know that stray inductance and capcitance could be that destructive. I thought it would only matter for applications where the form of the signal is absolutely critical. I will work on making it cleaner.

The past few days I distilled and then froze some water in an attempt to make a high value high power resistor for the HV supply and it worked! I also used a small bit of hair wax (parrafin) which almost instantly melted. Tomorrow my 100 10 M ohm resistors shall arrive so I can do some accurate voltage measuring.

I also ran into some trouble. I took an old light bulb and put the negative HV terminal on one of the light bulbs terminals (hoping to ignite a plasma as they have a vacuum inside AFAIK) and put the reference terminal on the glass of the bulb. No plasma but I did see a thin purple beam (basicaly an arc) in the bulb. Eventually it arced through the air instead of the light bulb (huge arc btw). At that point the primary drew more than 5 A (at 200V variac). After that my transformer made a strange noise which can best be compared with putting a small screw in a glass cup and shaking it around. I assumed that this meant I had a short somewhere. The primary current draw made it look like I had a shorted secondary. So I removed most layers of the secondary and found that there were some holes in the tape around the primary. I replaced the primary rewound most of the secondary and now it seems like it is working again.

Now I wonder did the primary break due to high voltage arcing from the secondary or did that current draw >5A break the primary insulation?
Nnnnnnn
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Joined: Tue Jun 13, 2017 6:25 pm
Real name: Niels Geerits

Re: Switched Mode Power Supply

Post by Nnnnnnn »

I succesfully replaced the broken components and rewound the transformer with some extra plastic sheets in between layers for extra protection. I kept the wires short as recommended and added a few extra components in the arduino subsection to limit current (plus some pull down resistors on the bs170s). I also placed 100 10 M ohm resistors and 1 100 k ohm resistor in series behind the multiplier. I measured 2.5V over the 100k resistor with the variac at 30% of max power. With a frozen distilled water load the supply pulls 4mA and stays stable at -25kV.

Here some pics of the supply:

1st the supply without the arduino inside:

Image

The remaining cable jumble goes to the rectifier and variac.

2nd the supply with the arduino. The Bs170s, current limiting resistors and 12V 1A supply are hooked to the breadboard:

Image

And a final note I am getting an oscilloscope to look at the waveforms in my circuit. I want to know if there is anything that might lead to instability.
Nnnnnnn
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Joined: Tue Jun 13, 2017 6:25 pm
Real name: Niels Geerits

Re: Switched Mode Power Supply

Post by Nnnnnnn »

I used my oscilloscope to probe the circuit. I used both channels and used the difference math function, because my circuit is earth referenced. All gate source signals look clean with no ringing, same goes for the drain source voltages except for the P-FET I found some ringing there during turn off. Here is a picture of the waveform (note its inverted):
scope.jpg
I am thinking about using some zener diodes and a resistor to clip the ringing at 400V (FET is rated for 500V) it seems like the simplest adjustment to protect the FET.

The pulse train you see after turn off is from the transformer as far as I know (this was also present in simulations)
RickyTerzis
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Joined: Mon Aug 28, 2017 1:42 pm
Real name: Ricky Terzis

Re: Switched Mode Power Supply

Post by RickyTerzis »

Hi...i am a new user here. As per my knowledge one never goes anywhere near Bmax for a core as if you do the core effectively dissappears at this time all your silicon devices turn into silicon oxide and smoke.So now using around 0.3 * Bmax redo your sums
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