Transformator Flux balancing capacitor selection

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For a full bridge phase shift converter I need to place a series capacitor to the bridge transformer . This will prevent dc current in the trnsformer core and avoid core saturation.

 

The bridge has a Bus voltage of 540VDC. The switching frequency is 50khz-100khz.

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The capacitor triangle current is (+/-) 30A peak to peak. 

 

In steady state the capacitor voltage is low. But if the duty cycle change in a short time is high ,the voltage on the capacitor rises for a short period of time.

 

The capacitor voltage rating is 800V-1000V. (regarding dynamic state and worst case with safety margin.). In steady state the voltage rating of the capacitor is very low.

 

I investigated film capacitors. But I noticed that they have a sharp decline of voltage rating versus frequency. For example a MKV polypropylene capacitor with a voltage rating of 1000VDC have only 25VDC voltage rating when driven by 10khz.

 

I am afraid that the only choice is the ceramic MLCC.

 

2 questions:

 

1- Which capacitor type would you prefer.

 

2- Is it more economic to limit duty cycle change range so that the voltage rating of the capacitor will be less. Is this method reliable enough? My regulation loop does not need fast response. So I can limit duty cycle change for a given time frame. This will introduce some problems for short circuits. 

If I change my control scheme from voltage control mode to peak current control mode, I can get rid of the capacitor but I think that voltage mode control has more advantage and more easy to implement for my application (requires significant less engineering work)

 

Any recommendations?

 

Thanks for your input.

 

 

This topic has a solution.

________________________________ We dream of a world where current does not need the voltage to flow.

Last Edited: Mon. Feb 11, 2019 - 03:56 PM
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I don't think ceramics a particularly good at high voltages either, in that capacitance drops off like a stone at high voltages.

 

I did see an installation where power factor correction capacitors were fitted. They were huge with substantial screw terminals and no doubt frightfully expensive. IIRC they were metallised poly-something film but of course only had to work at 50Hz.

 

I think you're into specialist stuff here and you may as well be building a flux capacitor. Although having said that I wonder what solar panel converters use.

 

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The Bus voltage of the LC network is 540VDC. (3 phase 400VAc rectified). 

 

In the simulation , If I choose an overdimensioned C like 100uF the voltage on the capacitor does not exceed 15Vpeaktppeak. The huge part of the voltage stays on the inductor.

 

But some design paper suggest to use high voltage rated capacitors even if the capacitor voltage does not exceed 15Vpeaktopeak in steady state.

 

I do not trust the simulation software in dynamic state. I can avoid huge changes of the PWM duty cycle in the embedded software code. But even then there may be some situations like ex. where i should handle shorted secondary of the transformer and in this case there may be a huge fluctiuation in the volt-second balance of the transformer. This may increase the voltage on the series capacitor. But I am not sure of that.

 

The critical question is that in an LC network if we choose overdimensioned C (ex 100uF) and a small enough L, can we *always* guarantee that in all working condition (both steady state and dynamic ) the voltage on C stays below a certain limit (ex: 20V) 

 

I assume the obligatory condition is that we are working far away from the resonant frequecy of the LC network. But even then I am not sure if some complication can occur at startup and shutdown of the network.

 

My working frequency ist 50khz.  The resonance frequency of LC circuit ist at 3.5khz.

 

I fear if at startup or shutdown some portion of the morphing frequency tends to get some amplification nearing the resonance frequency and the capacitor is damaged because of high voltage. 

 

  

 

 

  

________________________________ We dream of a world where current does not need the voltage to flow.

Last Edited: Sun. Feb 10, 2019 - 02:31 PM
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Way out of my league!

 

That said, I would love to see a photo of the setup when you get it up and running!

 

I still recall a few exploding caps when I was learning how to build a Xenon Strobe driver for a vehicle lightbar.

350V-ish at mA's of current, not 30 A!!!

 

Please be careful!!

 

JC

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The first implementation will be up to 15A @50khz.

 

If I stay far from resonance frequency of 3.5khz, it seems that the cap voltage will never go beyond 20V.

 

I constructed a capacitor bank consisting of 50V,10uF MLCCs because of cost optimisation.

 

The group1 of capacitor bank is made of  4 series connected 50V 10uF (MLCC ,case 1206), each balanced with 47k paralel resistor network.

Group1 resulting cap is 200V 2.5uF

 

x10 group1 connected parallel so the resulting cap is 200V, 25uF.

 

The resulting PCB with heavy copper (105um) ist poured into resin epoxy.

 

Does anyone have experience with balancing the series connected capacitors through paralel resistor networks when working under high frequency?

________________________________ We dream of a world where current does not need the voltage to flow.

Last Edited: Sun. Feb 10, 2019 - 04:03 PM
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This bit came up at Dangerous Prototypes recently for TDK CeraLink Capacitors - DC link, filter or snubber applications.

 

        App note: CeraLink® Capacitors 

 

The dielectric is a PLZT ceramic (lead lanthanum zirconium titanate), where capacitance increases with bias voltage.

 

There are three basic values of 0.25, 0.5 and 1.0 µF  with rated voltages of 900, 700 and 500 VDC. Available singly or in flex assemblies of 2, 3 or 10 paralleled capacitors.

 

They're quite impressive and pricey.

 

Last Edited: Sun. Feb 10, 2019 - 06:23 PM
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incal wrote:
For example a MKV polypropylene capacitor with a voltage rating of 1000VDC have only 25VDC voltage rating when driven by 10khz.
This limitation is probably (I'm guessing here) due to heating.

Just for fun: Calculate the current through your capacitor @1000Vac and 10kHz.

 

incal wrote:
But some design paper suggest to use high voltage rated capacitors even if the capacitor voltage does not exceed 15Vpeaktopeak in steady state.
Yes, of course. Your capacitors will have to swallow high peaks during switching, and you want them to survive that.

 

It seems like tye MKV you selected earlier is perfect for the job. It can withstand the 500V peaks and during normal operation you apparently have 15Vac over your capacitor, which is also within your specified limits.

Doing magic with a USD 7 Logic Analyser: https://www.avrfreaks.net/comment/2421756#comment-2421756

Bunch of old projects with AVR's: http://www.hoevendesign.com

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Yes, I believe I should design the LC tank so that in steady state the capacitor voltage does not exceed 20V. At start up and shut down or for transformer secondary short circuit handling routines , there may be moments that the morphing frequency accidentally or mathematically gets near to resonance frequency. This can induce for a very short period of time a high voltage on the cap.

 

And for this purpose it seems that the polypropylene MKP is the right choice. It is also the most expensive solution. (everything has its price).

 

The regular MLCC (except the ones with special PLZ ceramics from sbennet's link above) are probably less reliable than MKP caps.

 

Thanks for the recommendation.

 

Edit:  this is the link of Kemet 1uF 1000VDC (275VAC) capacitor: At page 6.

 

At 10Khz Voltage Rating is 80Vrms

    20khz Voltage Rating is 45Vrms

    30khz Voltage Rating is 30Vrms

    50khz  Voltage Rating is 20Vrms.

 

At my working frequency of 50khz , it is suprisingly on the limit.  They cost 0.3Euro/each for a quantity of 1000 (mouser). So my 25uF cap. bank will cost 7.5 Euro.

I think this is the cheapest alternative.

 

 

http://eu.mouser.com/datasheet/2/212/KEM_F3030_R71-1314182.pdf

 

________________________________ We dream of a world where current does not need the voltage to flow.

Last Edited: Sun. Feb 10, 2019 - 08:59 PM
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They seem to be excellent capacitors. But IMHO for 25uF @1000V they will be much more expensive than the alternative film capacitors.

 

For this kind of severe current range jumping from -15A to 15A, I also can not predict if the ceramic body mechanically withstand the osscilations. A series connected pulse capacitor carrying the whole line current with a ripple current of +/-15A is a very different story from a smootening or snubber capacitor. 

 

They are really expensive:

 

700VDC 1uF cap cost around 13.5 Euro/each for a quntity of 1000. My 25uF cap bank should cost around 337.5 Euro. The film capacitor bank costs around 7.5 Euro, but is not the 1:1 alternative. 1000Vdc film cap has only 20V Voltage rating at 50khz. 

 

http://eu.mouser.com/ProductDetail/TDK/B58035U7105M062?qs=sGAEpiMZZMtGtrSL1omHbHPJTSb0%2fcOqdqjlg8RO1I8%3d

________________________________ We dream of a world where current does not need the voltage to flow.

Last Edited: Sun. Feb 10, 2019 - 08:59 PM
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When in the dark remember-the future looks brighter than ever.   I look forward to being able to predict the future!

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Thanks for the links. I have investigated the above links a while ago. The problem is :

 

If you increase C the voltage stress on c decreases, but you will need to increase the resonant inductor so that the minimum current where the soft switching begins remains low. This is not so straighforward. They don't explain in the paper (chineese) how they overcome the problem that the minimum current for the soft switching becomes too high. It would be better that they give the Lr and Cb values so that I could simulate it to see if the claims are justified.

 

If you decrease the value of C, you have more room for soft switching because the inductor has enough energy to balance the enegry stored in Capacitor. But then you face 2 main problems: 1-The voltage rating of the capacitor will be very high and it will be very expensive. 2-This will also limit the power transfer and duty cycle.

 

All these can be overcomed by nearing to resonance frequency and controling the power flow through a kind of frequency modulation. It is the famous LLC converter. But it has also drawbacks. 

 

Short: Nothing is perfect.  

________________________________ We dream of a world where current does not need the voltage to flow.

Last Edited: Sun. Feb 10, 2019 - 10:34 PM
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incal wrote:
25uF @1000V 
Aah, a value for a capacitor.

 

If I remember well, and toying a bit with Octave:

 

octave:1> f= 10000
f =  10000
octave:2> W= 2*pi*f                  # W = Omega, my greek ain't too good.
W =  62831.85307
octave:3> C=25e-6                    # 25uF Capacitor.
C =  0.000025000
octave:4> i
ans =  0 + 1i                        # Gosh, Octave understands complex numbers (Never tried that before).
octave:5> Z= -i/(W*C)
Z = -0.00000 - 0.63662i              # Impedance of capacitor.
octave:6> abs(Z)
ans =  0.63662
octave:7> 1000/ans                   # Voltage / impedance = Current.
ans =  1570.8

I think it's somewhat understandable that a cap that can handle 1500 Amps @ 1000V is going to cost some money, and even at 20V you're looking at serious currents. ( 31.416 A).

I may have made some gross error in this calculation, my head has been a bet cloudy lately, so please check the numbers.

A maximum of 30A sounds reasonable though for a "normal" capacitor.

Doing magic with a USD 7 Logic Analyser: https://www.avrfreaks.net/comment/2421756#comment-2421756

Bunch of old projects with AVR's: http://www.hoevendesign.com

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Yes absolutely true,

 

Q=C.V

 

And unfortunatelly, it is not a resisitive element. It means any fluctuations in V means the existence of current. they are connected.

 

dQ/dt = C. dV/dt

 

And if we are talking abaout frequency, it means that dV/dt must be huge so the term dQ/dt. dQ/dt is nothing others than the  I, current.

 

Passing huge amounts of current through huge dV/dt means extreme power dissipation:

 

W= 1/2 * C* V^2.  This is the stored energy, but the dissipation is also very high because of very high ripple currents.

 

Thanks for your input.

 

 

 

 

  

 

________________________________ We dream of a world where current does not need the voltage to flow.

Last Edited: Mon. Feb 11, 2019 - 06:30 AM
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have you actually tried looking at sites of manufacturers directly?

 

I have found in the past that although you give a number of parameters in a distributors search engine that all the results come up.

you could try avx, kemet, panasonic, tdk, Yageo see if they have capacitors in their inventory. Then with the real part numbers check at the distributors end if they have them or similar ones.

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I investigate the distrubutors inventory (mainly mouser). 

 

My main problem for MLCC ceramics is that the ESR is not well defined.

 

For example for the film cap 1uF, 1000VDC the internal ESR is given as 9mOhm. If I=15A. I can calculate that the power dissipation is 15x15x0.009= 2watts. The case of film caps are huge (27.5mmx10mmx15mm). So I can be sure that 2W is compatible for the dimension of the case.

 

But for the ceramics there is a problem with the ESR. Ok, we know that it is very low, but as far as I noticed in the datasheets it is also heavyly frequency dependant.  I can not calculate it precisely. Is it 10mOhm or 1mOhm?. Not clear from the logaritmic graphs that is available in the datasheets. The case of the ceramics are also very small. So any misscalculation of the effective ESR will destroy the chip . think how small is the 1206 case. It can definitely not dissipate 2W. There is also the risk of a mechanical crack.

 

I investigate further.

________________________________ We dream of a world where current does not need the voltage to flow.

Last Edited: Mon. Feb 11, 2019 - 07:23 AM
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Some folks have a hobby of heating things with an induction coil.

The Chinese accomodate this by making ver cheap kits.

They consist of a tank circuit made from an inductor in the form of a copper pipe and fairly beefy film caps.

(They also have some MOSfets to pup power into the circuit).

 

"ZVS" (Zero Voltage Switching) is a good magic word to find them:

https://www.aliexpress.com/wholesale?SearchText=zvs+induction

 

These boards come in lots of different sizes.

The Capacitors all the different Chinese use for these things look like:

(Often thyey are black instead of blue)