Switched Mode Power Supply

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Rich Feldman
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Re: Switched Mode Power Supply

Post by Rich Feldman »

Sounds good, Niels and Dennis.

For others who want to roll their own transformers, beware of turns-count formulas based on RMS voltage of sinusoidal AC. The RMS voltage of square wave AC is the same as its "peak" voltage, and is substantially less than the RMS (much less the peak) voltage of a sinusoid with same frequency and Bmax on the same core and coil.

As a checkpoint, here is a data point figured independently from fundamentals. It's supposed to show everyone that there's nothing to be afraid of in there.

Many modern SMPS cores have Bmax = 0.2 T as an ordinary design point on the datasheet, so let's use that.
If we change your core diameter from 1.600 cm to 1.596 cm, its area becomes exactly 2 cm^2, and the other numbers all end up nice and round.

The associated flux swing is exactly 80 microwebers. The product of core area in m^2, flux density Bmax in teslas, and 2 (for operation between negative and positive extremes).

As a linear ramp over 10 microseconds, dF/dt is 8 webers/second and induces 8 volts per turn.
For example, 200 volts on a 25-turn primary, and 10000 volts on a 1250-turn secondary.

That would fit an operating frequency of almost 50 kHz. Cycle period includes 10 us for flux ramp up, 10 us for flux ramp down with reversed voltages, and some switching time and/or dead time.
The core area and adopted Bmax dictate a limit of 80 volt-microseconds per turn, per half-cycle, regardless of frequency or waveshape.

The obligatory picture is snipped from an app note by West Coast Magnetics. http://wcmagnetics.com/wp-content/uploa ... .28.10.pdf
Maybe it's already among your references. It has lots of practical information about winding and insulation issues for real commercial designs, not to mention references to some of those Ferroxcube materials you've become familiar with.
wcm_cores.PNG
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Garrett Young
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Re: Switched Mode Power Supply

Post by Garrett Young »

Rich,

Is your primary turns based on a half or full bridge at 200V DC rail (and 0.2T in a 2cm^2 core)? 25 Turns is correct for a full bridge but not half. Likely the secondary would need more turns to account for leakage inductance (which is usually quite high for high voltage secondaries because of isolation requirements).

The math isn't hard ... it's the doing.
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Rich Feldman
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Re: Switched Mode Power Supply

Post by Rich Feldman »

Yup, that analysis omits lots of real-world complications, to quickly show the physics behind turns-count formulas. Given a frequency and a volt-microseconds limit, it was trivial to get the maximum square wave voltage per turn. Optional exercise: get the sine wave voltage per turn.

And yes, it was sort of assuming a full bridge (H-bridge). It does say "200 V across 25 turn primary for not more than 10 us, then 200 V the other way for the same amount of time".

With a DC bus voltage of 200 volts, rail to rail, the inverter's output can get +200 V or -200 V, measured between symmetrical "hot" terminals. The switches can also be set to deliver 0 volts with low impedance. That state happens for a chosen fraction of each half-cycle in "modified sine wave" 60 Hz inverters. A DC motor, driven reversibly with an H-bridge, stops faster in the 0 volt drive state than the high-impedance drive state.

I think that ordinary (non-resonant) half bridge drivers switch just one end of the load back and forth between the DC rails. The other end of the load is held at an intermediate voltage by at least one relatively large-value capacitor. That voltage will automatically match the average voltage on the switched side.

In the example with 200 volt DC bus, a half bridge would put +100 V or -100 V on the primary winding. For the same frequency and flux density, the primary could get by with only half as many turns.

Have I got that right, Garrett?

The Spellman -70 kV power supplies extensively discussed on these forums in late 2016 have the half-bridge topology. I never got around to powering mine up after replacing a diode and capacitor. Should talk less and do more.

Another isolated converter topology with only two switches uses a center-tapped primary. Flyback converters are isolated and need only one switch, but they aren't really transformers. Good for low power SMPS, CRT anode power, and small engine ignition.
Last edited by Rich Feldman on Sat Jun 17, 2017 2:47 am, edited 1 time in total.
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Garrett Young
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Re: Switched Mode Power Supply

Post by Garrett Young »

You are correct, Rich.

I'm partial to the half bridge topology, since I think it's a good balance between cost, efficiency, and complexity.
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Re: Switched Mode Power Supply

Post by John Futter »

I hate half bridge as you have to allow even more headroom so that core flux walking doesnt suddenly give saturation on a half cycle
Poof more beach sand (SiO2).
It can take a long time to catch the conditions that cause core flux walking ie product out in the real world with unexplained high number of failures under warranty
Only exception to this is current fed current mode which was basically invented to get around flux walking in the core in half bridge designs. CFCM has the switches overlapping in on time and it makes for a very clean output with fewer artifacts so snubbing becomes alot easier. you can take this even further with resonant CFCM so everything looks like sine waves making EMI mitigation much easier.
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Re: Switched Mode Power Supply

Post by Garrett Young »

Between a "stiff" centerpoint and current mode control they usually behave nicely ...
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Re: Switched Mode Power Supply

Post by Nnnnnnn »

Thanks for the info Garret and Rich. I am going to go get some parts (might need to order) this weekend. I agree that winding the transformer properly seems tough with regards to tightness and insulation. I plan to use a single MOSFET and an arduino to create a square wave going from 0 to roughly 300 V (max output of the variac). The AC to DC conversion happens later in the multiplier. I could create a square wave that goes from -300V to 300V, which means I would have a higher peak to peak to work with, but also requires more semiconductor devices. To me the purely positive square wave seems like a simpler project for a beginner. Unless someone knows a reason why that would be a terrible idea?
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Re: Switched Mode Power Supply

Post by Rich Feldman »

It's good to hear that you are getting your hands dirty, Niels.

How will you get a 0 to +300 volt drive with one MOSFET? An open switch is not the same as driving 0 volts.

Don't forget that the voltage averaged over a whole cycle needs to be the same on both ends of a transformer primary winding. If the switched end is alternately driven to 0 V and +300 V (using two MOSFETs), with equal ON times, the other end should be at +150 V. This is the half-bridge topology spoken of previously.

With just one MOSFET, you could experiment with the flyback converter topology. As a lab exercise you don't need a special (gapped) core, but you do need a secondary winding and at least one diode connected to it.

Do you have an oscilloscope?

Do you have a good grasp on the concept of electric current? Suppose an electric circuit is isolated within a box. A hole in the box gives you access to one wire of the circuit. The wire will have a current, possibly zero, which you could measure. But the "voltage" of that single wire is undefined, right? Nothing to measure (not counting any small IR drop between different places on the wire).
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Re: Switched Mode Power Supply

Post by Nnnnnnn »

Rich right you are! That was embarassing on my part of course an open switch is not the same as driving at 0V, I was thinking a bit too quick there.

I looked up the conventional half bridge circuit diagram:
Image
And I am probably wrong but it seems like the loop V_dc - C1 - C2 - V_dc is shorted out. So I simulated it (falstad circuit simulator) and put a small resistor in that loop. I did indeed get a square wave output, but it was nowhere near 100V DC I put in in my test. To be honest I do not think that the falstad circuit simulator is working properly. I should be getting some components soon so I can play around with it (got some extra MOSFETS in case I ruin one).

I don't have an oscilloscope. I could arrange to use the one at my university every once in a while should I need it.

As for the thought experiment. A Voltage is a potential difference the wire itself has not change in potential and therefore no voltage drop (unless you look at its resistance). It does have a current which depends on the impedance of the other components. Basically voltage is potential energy, resulting from an electric field. The electric field creates a force on charged particles which causes them to move. Technically with no resistance anywhere the electons would accelerate until they reach the lowest potential. However in a conductor the electrons scatter against the lattice (comparable with friction) which causes an equilibrium velocity of the electrons to settle in. From this velocity the charge the electron density (material property I guess) and the area of the conductor one can optain the equilibrium current. Btw each scattering event in the conductor can be seen as a voltage drop, because the electron loses its kinetic energy.
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Re: Switched Mode Power Supply

Post by John Myers »

Doesn't look like a conventional H bridge to me, at least what I'm use to seeing.
The low side output ( point O) will be half Vdc. It creates a positive and negative voltage swing with point O as the common.
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Re: Switched Mode Power Supply

Post by Nnnnnnn »

John. I noticed that with the Vdc/2 too. I left out the capacitors while simulating the circuit. Have almost gotten it to work as I thought it would just got to do some more tweaking.
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Re: Switched Mode Power Supply

Post by Nnnnnnn »

So most of my supplies have arrived (except high voltage wiring so I am limited to 1kV tests right now). My arduino is now creating two 300 khz 40% duty cycle out of phase switch signals (I measure the frequency using the arduino too). I have also been doing so reading: I found out that the topology I referred to in my initial post is called the flyback topology this only requires one MOSFET to function. I have also been thinking about the half bridge topology. I am having trouble finding sources on how it actually works. If MOSFETS were actually just switches it would be easy. A problem I can see is that in the drawing I posted here there is no load on the MOSFET drain. So I was wondering if anyone has any sources that I can read to learn more about the half bridge topology.
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Re: Switched Mode Power Supply

Post by John Futter »

300kHz is way too fast for HV supplies
this causes dIdt problems with the multiplier caps and the diodes
most modern HV supplies use 30 to 100kHz max with HV diodes for 100kHz costing many many dollars each
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Rich Feldman
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Re: Switched Mode Power Supply

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>> wondering if anyone has any sources that I can read to learn more about the half bridge topology.

Uh, LMGTFY. Here's the top result from a Google search for half bridge topology. http://www.irf.com/electronics/topology-fundamentals Looks like it would not be a bad place to start. It has some details I could quibble about, but not now.


>> in the drawing I posted here there is no load on the MOSFET drain

Unless you've changed that half-bridge schematic snippet, it shows no MOSFETs and no drains. (hint: IGBT)

Sure, you could build a switch bridge with the load connected to nothing but drains (or collectors), and DC rails connected to nothing but sources (or emitters). Just use complementary devices as the top side switches, and change the gate drive appropriately. In fact, that's the most popular by a factor of a billion or so.
cmos_inverter.gif
cmos_inverter.gif (2.18 KiB) Viewed 7212 times
Or put the complementary devices on just the bottom side. Then all switches have drain (collector) on a rail, and source (emitter) on the output. Yet another set of drive waveforms. That option has been around since the dawn of BJT's.
complementary_class_B.gif
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Re: Switched Mode Power Supply

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Hey Rich I found that link too I just could not get that much out of it. You hinted at the IGBTs (never heard about these in any of my electronics courses which now I find strange), so I looked them up, got a few and played around with them. I think I get how they work now. It seems though that they are slower than MOSFETS.

When I was talking about drains earlier I assumed you could replace the IGBTs in the drawing with MOSFETS. However using a p-Mosfet and an n-Mosfet makes sense.

I booked some success this weekend. I put together a flyback driver (two mosfets, one low voltage attached to the arduino and one high voltage with the gate connected to the drain of the low voltage mosfet). I used a 100 ohm 50W resistor to limit current and measured the voltage on the secondary of my transformer. I measured using a simple multimeter. At first I saw nothing because the multimeter averages the voltage. So I added an LED (yes. I ended up destroying about 7 of them) because I don't have any spare diodes and then there was light. I put a resistor in series with the LED to measure the voltage drop. It seems I am getting a voltage ratio of 1:1 even though my turns ratio is 1:2. In addition if I increase the voltage over the primary the voltage drop over my 100 ohm resistor decreases. It makes sense for the voltage drop over the resistor to decrease as I increase the switching frequency as the current will not have enough time to rise to its maximum value (coil impedance etc.). I still need to make sense of the decrease in voltage drop when the overall voltage is increased (perhaps it has to do with the core being saturated?). I will do some math today and solve the DE for a simple LR-circuit with a pulsed input. And I will get some diodes and capacitors to double and rectify the output voltage.
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Re: Switched Mode Power Supply

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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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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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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 7275 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

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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

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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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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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