Switched Mode Power Supply

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Nnnnnnn
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Switched Mode Power Supply

Post by Nnnnnnn »

I would like to share my understanding of how to make a switched mode power supply. I have read the FAQs and read many threads in this section so hopefully I am not asking basic questions.

As I understand it one can make a SMPS as followed: Variac connected to the wall socket rectify AC current coming from the variac -> use arduino and high power MOSFET switcher to generate a high frequency (10kHz-100kHz) square wave. -> Feed square wave to transformer with ferrite core. The ferrite core has a B_max of 2T, so even for a small core one only needs a few primary turns (I calculated N=3 for Vrms=200 f=50kHz B_max=2T and core diameter 1.6cm, this is very low so there might be a mistake somewhere). The transformer has a turn ratio of 1:50 to 1:100, which brings the voltage up to 10kV-20kV. I don't go higher so that the insulation on the secondary coil does not need to be very thick. Finally the signal is fed into a voltage trippler which should give me around 60kV DC.

As for my questions: I know there is a thread on making your own transformer but I am having trouble finding it again. If I recall correctly somebody said that there is more to building a transformer than just a core and the turns. I forgot what else one has to watch out for. Is there anything else I must watch out for that I have not mentioned in this post?
I found a ferrite E-Core that is rated for 500W. Does this mean it gets too hot at 500W and I could go past 500W if I had some forced cooling?
The MOSFET will probably also need cooling. Again does this need forced cooling (peltier, water or fan?) or just a radiator?

Any other major plot holes in my plan?

Again hope this hasn't been asked a million times. I did search the forums and could not find it.
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Re: Switched Mode Power Supply

Post by John Futter »

Niels
its no where as easy as you make out
one never goes anywhere near Bmax for a core as if you do the core effectively dissappears (ie permability drops to 1) at this time all your silicon devices turn into silicon oxide and smoke.
So now using around 0.3 * Bmax redo your sums
Of course you will have to take into account leakage inductance mosfet turn on and turn off times multiplier diode recovery time etc etc
all of these not perfectly designed will add to your bucket full of dead silicon devices.

If it was as easy as you make out everybody would be doing this
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Richard Hull
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Re: Switched Mode Power Supply

Post by Richard Hull »

I really have no excuse for not poking around at making a switch mode supply. I am retired and have a ton of silly-con on hand, about (40) 800 volt FETs and (30) 1200 volt 50 amp IGBTs. I also have a large Hi-Volt corp 25khz 6kW flyback style xfmr in hand. I could grossly underdrive the thing, I guess. It was part of one of their 80kv @ 60ma power supply. I even have 3 or 4 of the Hi-volt negative multiplier "pies". The issue is I haven't any reason to upgrade over my Xray xfrmr of these long years running my fusor. If I do it it will be for the experience and not the need. I might be able to get by with a single driver silly-con device using rectified 240volts. Full bridge is gross overkill and even a half bridge might be over doing it.

Gotta' poke around at maybe 24 volts drive and find out some data on the big xfrmer as frequency and duty cycle is monkeyed with.

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Garrett Young
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Re: Switched Mode Power Supply

Post by Garrett Young »

Richard,

If you end up not using the Hi-Volt corp 25khz 6kW flyback, I would be interested in buying it from you.
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Re: Switched Mode Power Supply

Post by Nnnnnnn »

Thanks for the info John. That is good to know that you should stay way below B_max. I redid the calculations for 0.3Bmax and 100 kHz. I do not think it is sensible to go beyond 100 kHz due to rise and fall times of all the semiconductors involved (as you mentioned). I think rise and fall times can be brought down to 100ns for a reasonable price (which at 100kHz should be good enough). Another good point on the diode recovery time! I also didn't take the forward voltage into account in my initial calculations. For affordable fast diodes (again roughly 100ns) this is rather high which bites into your output voltage. The leakage inductance is my main concern. I will have to look at this some more and at some point just start experimenting, not at high voltage of course.

I did not mean to make this sound easy or cheap. I think it seems like a nice challenging project, a way to test the things I learned in my EM and electronics courses. And perhaps it is a nice alternative to the almost extinct massive x-ray transformer and finished power supplies from ebay (again probably not cheaper).
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Re: Switched Mode Power Supply

Post by Dennis P Brown »

This should be an extremely interesting project and I look forward to it in the future; getting a viable core (for current, response to the high frequency, and inductance performance) might be an issue so maybe that should be something to investigate at the start.

A voltage multiplier can be a project and and requires rather good performance diodes and caps which can be a bit expensive. Look into that carefully before making a decision on the transformer voltage. Also, a three stage multiplier will NOT take 20 kV and yield 60 kV! Performance of follow on stages can drop fast - especially current output! Total voltage/power depends on many factors such as components (high freq voltage and current capabilities), design and other factors (like proper cables/wiring even.) Your reading on the subject of multipliers needs to be increased. Don't make assumptions about such so-called trivial aspects of a power supply design like saying: "I will just add a voltage multiplier". These side issues can kill a project if not given careful consideration.

Also, power supplies in the under 20 kV range with enough power tend to be available on ebay and not very expensive. While one does pay for power, as Richard points out in the FAQ's, higher voltage does offer advantages in the lower power ranges. So building a 15 kV or so power supply might not be worth the effort considering these can often be found and fairly cheaply on ebay if willing to make the multiplier (I built a rather powerful 60 kV multiplier that is feed by a fairly high current NST (just 60 Hz. This was a test level unit but I never went to the higher frequency's because I lucked upon a proper x-former.) The door knob caps (needed for high freq. performance) weren't cheap even though I got a great surplus price on them.) The issue is making the multiplier supply enough power at the voltage - that requires careful design and building the unit to error on more power since reality may cause lower power performance than what the design says ... just some issues to keep in min before building any power supply. There are reasons few do ... .
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Rich Feldman
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Re: Switched Mode Power Supply

Post by Rich Feldman »

Re. ferrite vs iron cores: Yes, the B_max of a transformer design should be well below the saturation flux density.

In case nobody has already said it in this thread:

1) The flux density in saturated ferrites is about 4 times lower than that in saturated steels. Roughly 0.5 T, not 2 T.
Here is a figure snipped from somebody else's snip from Handbook of Small Electric Motors, by Yeadon.
yeadon_table.PNG
That table is widely snipped on the Internet, usually including the page number but no other attribution. :-(

2) Core loss in steel designs is mostly from eddy current V^2/R.
Core loss in ferrite designs is mostly from hysteresis loss. Directly proportional to frequency and, I bet, roughly the square of operational B_max. Generally tabulated in material datasheets in terms of watts per unit volume or mass at common (f,B_max) values.

p.s. what do Arduinos have to do with transistor bridges in power inverters?
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Richard Hull
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Re: Switched Mode Power Supply

Post by Richard Hull »

Arduinos can form well controlled pulse widths at given frequencies and using the A-D ports you can manually control same.

However, numerous switch mode PWM ICs with special internal timing and drivers are probably a better choice. Why fight a battle in software when for a buck or two Signetics, Fairchild, TI, or FUJI have won that battle in a single chip with tightly controlled hardware.

Regardless, it is all a plug and pray situation and the neophyte needs to have a bit of spare silly-con on hand in development mode as the magic is released in the occasional puff of smoke.

Richard Hull
Progress may have been a good thing once, but it just went on too long. - Yogi Berra
Fusion is the energy of the future....and it always will be
The more complex the idea put forward by the poor amateur, the more likely it will never see embodiment
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Re: Switched Mode Power Supply

Post by Nnnnnnn »

Aw man! Typed out my reply and got auto logged out. Now its gone so here I go again!

Dennis I have been looking at cores made by Ferroxcube, they seem to make cores that are good for this application. Eventually I will just have to start experimenting though.

I agree I will only start working on the multiplier once the transformer is complete and will base the design of the multiplier on what I actually can get out of the transformer. I should have noted in the OP that I am aiming for 40-50kV, the 60kV in my initial calculations was to add a small buffer. I did do some initial calculations on the multiplier though at 100kHz 50kV 20mA out and 1nF caps the voltage ripple will be at around 2%. Like you said these capacitors are not cheap! Same goes for the diodes.

I agree the 20kV supplies are actually pretty affordable, however as you pointed out the x section of DD fusion rises a lot between 20 and 100kV. Fusion at 20kV is wasting a lot of power if you can also do it at 50kV. The problem with these 20kV supplies is that you need a switcher that can handle 20kV if you want to step up the voltage to 50kV. If you know about any affordable MOSFETS that can handle this kind of voltage I would like to hear about them. This project would probably be simpler than what I am about to embark on.

Rich: What Richard said! I have a few arduinos here and they are great for switching a MOSFET on and off at the required frequency. In my OP I stated that Bsat of ferrite is 2T, that is my fault I read 20000G somewhere one zero too much. It is more like 0.2T. Though I did find a few that are rated at 0.47T
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Garrett Young
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Re: Switched Mode Power Supply

Post by Garrett Young »

The front end driver is straight forward (half or full bridge) - see viewtopic.php?f=11&t=11146&start=146
I designed this produce 2.4kW of high frequency AC to drive a high voltage transformer/multiplier. The total part cost is about $100.

The output multiplier is slightly harder but still accessible.

Selecting the correct ferrite material and winding configuration takes more knowledge but most could figure this out.

I think the deal breaker is actually winding the transformer with low secondary capacitance and the proper isolation. In my opinion, if you have a HF transformer with sufficient power capacity and insulation then you're golden and success is within reach.

I prefer to hijack supplies with good HF transformers and multipliers but faulty or insufficient front end drivers. With the typical low duty cycle and operating lifetime requirements of fusors many name brand supplies are a prime targets to push past their original power rating.
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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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Re: Switched Mode Power Supply

Post by Rich Feldman »

>> 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 7231 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

Post by Nnnnnnn »

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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