Power supply topology for 150kV

[Marco]

Lets say 10 times their rated continuous current during reverse recovery. Driven just right/wrong some even supposedly soft recovery diode can get snappy and switch that current off in well under a ns, the smoke is more likely to come out of other parts when they do.

[BootstrapBill]

I'm really surprised no one has seriously considered a series resonant topology such as LLC for this application. You can use IGBT's at a much higher frequency than if they are hard switched. The leakage inductance (which is huge due to the isolation requirement) can be lumped into the resonant inductor and you need no output inductor. Also EMI is almost non existent compared to a hard switched converter.
The dynamic range issue can be a problem but there are ways around that also. The transformer doesn't see any high frequency ringing currents which often plague high power switched converters causing them to run hotter than expected. It's nirvana from all angles!

LLC is what contemporary x-ray generators use. Its pretty insane how small they are. I would love to be able to do this I just feel the control and R&D would take time and be even more dangerous. Is there a preferred off the shelf control IC you would recommend?

Regarding the buck current-fed full bridge.. is it feasible to go any higher in frequency from the voltage-fed? Are switching losses any lower?

PS I have an oem equipment solution for this particular application but am still interested in making a low voltage/ low power version for educational purposes and possibly for the future.

[Richard Head]

With such a high turns ratio in the transformer the inter-winding capacitance of the secondary will be huge. Once this is reflected through to the primary (by the square of the turns ratio!) it will be obvious that this capacitance will drive the topology of choice. I initially suggested an LLC but in order to absorb the reflected secondary capacitance an LCC topology would probably be a better option. This topology allows you to absorb the parallel capacitance into the lower resonant capacitor. It unfortunately only has lossless switching on  the turn-on edge. Turn-off edge is lossy.
At high power levels I reckon that a regenerative snubber should be considered for the turn-off transition. Don't go for a half bridge and split resonant caps as it'll require a transformer with twice the turns ratio (with 4x the capacitance also!) as a full bridge.
The LLC and LCC arrangements are strictly speaking multi-resonant topologies as their natural resonant network frequency moves with load as does the excitation frequency.
The control strategy is variable frequency working above resonance. Always work above resonance (inductive mode) or lossless switching will be lost.
Light load operation at high line can be a problem as the transfer function is unfavourable in this regime. Sometimes pulse skipping is required at no-load to keep the voltage from running away.
These things are a bitch to design but once it's working as intended and the niggley problems addressed it should beat any hard switched approach hands down.

[MagicSmoker]

...
These things are a bitch to design but once it's working as intended and the niggley problems addressed it should beat any hard switched approach hands down.

This is precisely why I did not recommend any kind of resonant or quasi-resonant topology to the OP. This will be his first smps design, after all...

In fact, anything more complicated than a low voltage buck converter is likely to be more of a learning experience than the OP expects, but rather than flat out discourage him (or insult him into hiring a pro to design it for him - a curious marketing strategy, that), I instead chose a topology which is rather forgiving to design and highly tolerant of abuse.

For example, any kind of modulated bridge converter is almost a non-starter because the layout and timing of the bridge switches is very critical which means extra attention needs to be paid to the gate driver design and layout, and this is experience that one tends to acquire from, well, experience. Ie - not only blowing stuff up, but learning from the bits of shrapnel and escaped magic smoke. Thus no type of modulated bridge is suitable for a beginner.

Resonant (frequency modulated) converters are downright nightmarish to get working correctly, and operating on the wrong side of the resonant hump, or with the wrong Q, or about a dozen other arcane to mundane restrictions, can get you into trouble fast. It's hard enough for people like me who've been designing smps for 20+ years; I certainly wouldn't recommend it to a beginner. Frankly, I only recommend resonant converters for specific and highly reactive loads like, for example, transverse RF-excited CO2 lasers or high power ultrasonic transducers.

Quasi-resonant (aka resonant transition, lossless transition, etc.) can combine the wide load range performance of hard-switched with the (sometimes, not always) lower losses of resonant, but also seem to combine the worst traits of both converter families when it comes to designing them (especially those which require an auxiliary switch network to achieve lossless switching).

Even if you possess the requisite knowledge and experience to design any kind of converter, real world constraints often steer you away from the more exotic even if theoretically ideal topologies. Unless you design LLC converters every day I can guarantee you it will take much longer to get one working than a buck fed bridge, and at some point just the sheer difference in design time outweighs any potential operational efficiencies, especially for an intermittent load as I imagine an x-ray tube to be.

[Richard Head]

Quite frankly you'd be insane to try and build a power supply like this as a first ever SMPS. There are potential pitfalls everywhere when it gets to such high powers. Issues that are a small irritant with a 250W supply can be a show stopper at 100kW. For example it's not a problem to snubb the ringing across the output diodes of a LV 1kW supply but when the ringing is at 200kV on the secondary side and the snubbing power lost perhaps 500W - 1kW the viability of a hard switched topology will be questioned unless operated at 10khz maybe. The main problem with this type of supply as I see it is that there is a very high leakage inductance due to the high isolation requirement AND there is a huge reflected capacitance from the secondary. The high leakage will resonate with the parallel capacitance causing terrible ringing across the output rectifiers which are almost impossible to snub at those voltages. My suggestion is not to fight physics but use it to your advantage whenever possible.

[MagicSmoker]

Quite frankly you'd be insane to try and build a power supply like this as a first ever SMPS. There are potential pitfalls everywhere when it gets to such high powers. Issues that are a small irritant with a 250W supply can be a show stopper at 100kW....

Again, I completely agree, but let's just say that often times experience is the best teacher, hmm? In this case, a buck fed bridge is likely to survive long enough for the OP to make some measurements and, perhaps, figure out how to address the issues such as you (me, T3sl4co1l, Marco, etc...) have raised along the way.

[Richard Head]

Even if you possess the requisite knowledge and experience to design any kind of converter, real world constraints often steer you away from the more exotic even if theoretically ideal topologies.

I agree. Very often this is the case. And as you mentioned, design time can be a huge factor if you don't have decent volumes to amortize the development cost over.

[BootstrapBill]

The MONSTERBUCK 5000..

Actually the only thing 5000 about it is what the load is rated for.. in water.

I have a ~700uH inductor I wound. Core is a 58337 HighFlux series from Mag-inc. The Load is a 12ohm water heater element.

Operated at about 100-150vdc input. Was able to go from 0 to ~80 volts out. Very smooth output.

The waveforms are across the diode and igbt. The diode is the one with the severe ringing and the igbt waveforms are of a before and after snubber.

[jbb]

Operated at about 100-150vdc input. Was able to go from 0 to ~80 volts out. Very smooth output.

The waveforms are across the diode and igbt. The diode is the one with the severe ringing and the igbt waveforms are of a before and after snubber.

OK, now we're talking :-)  It's nice to have some goodies on the bench.

First, practical safety improvements:

• Add a discharge resistor across the main DC cap.  That way it will discharge without the aid of a screwdriver.  Also, electrolytics have an interesting trick whereby they actually recharge themselves after you remove the shorting link.
• Adding a big red LED (+ resistor) onto the main DC cap will let you see if it's charged.
• Ideally, get some kind of box with a clear lid so you can put the higher voltage stuff under cover before testing. Much safer.
• I see you're using a breadboard to hold a cable down.  You need to anchor this stuff so it doesn't slide around. (A 200V capacitor sliding onto your hand will hurt quite a bit or worse...)
• If that's a shared table, a suitable sheet of plywood or similar can be helpful. You can mount your stuff to it and reduce the risk of damage when moving.  And protect the table from soot marks.
• To go further you'll need a current sensor. Something like a LEM HAS 50-S (available ex Digikey) might fit the bill.  Without knowledge of the current control schemes are generally limited to guess-and-bang methods.

I see you've got some ringing. We can help with that.  Ringing is caused by LC circuits.  In this case the C is the stray capacitance of the IGBT and diode.  You can't do much about those. The L is due to leakage inductance.  Have a look at the power circuit consisting of main DC cap, IGBT and diode.  The wires are quite far apart.  That means that the area of the loop they form is large. Loop area makes leakage inductance.  So try:

• getting some cable ties just squashing the wires together.
• shortening the wires
• getting a combo IGBT + diode module (you can also use a half bridge IGBT module with the gate of the unusued IGBT shorted to its emitter)

Then take a look at the gate-drive circuit;

• Get the gate-emitter loop nice and tight to reduce leakage inductance
• It looks like you're using an opto-isolated driver. That's good.  I suggest you add a little isolated power supply to protect your bench supply from accidents. It might not like having the mains connected to it.
• If the IGBT has a separate emitter terminal next to the gate terminal, use that.
• Mount the gate driver board close to the IGBT. Inductance is your enemy here.
• Consider using a gate driver with desaturation protection.

Next, you can improve the fundamentals of the circuit.  I see that the power supply return on your output filter cap goes back to the diode and then daisy-chains to the main input cap.  I suggest that it should go straight from the output cap to the common point on the main cap (like a star ground).

Good luck

[BootstrapBill]

Quasi-resonant (aka resonant transition, lossless transition, etc.) can combine the wide load range performance of hard-switched with the (sometimes, not always) lower losses of resonant, but also seem to combine the worst traits of both converter families when it comes to designing them (especially those which require an auxiliary switch network to achieve lossless switching).

Looks like these guys are using this..

http://www.glassmanhv.com/glassman_tech.shtml

Any passive way of doing this with the buck current-fed bridge? I know you mention it's just as big of a pain as the resonant but it's the frequency modulation and more importantly the narrow output voltage range that turns me off. The articles I've read on this all seem to be quite elaborate, sometimes more than doubling the original part count. Also haven't found too many for the full bridge.

[Siwastaja]

Re photos: is this a joke? For real? Please just stop now, you clearly are not even close to be able to even start this kind of project. I'm sorry!

"Arduino" talk indeed was a good sign of total cluelessness, once again.

[BootstrapBill]

Arduino is a due. Planning on using atmel studio 7. Yes I am aware of the delays/interupts with the arduino language.