[ §1 · tank ]

The MMC is sized by current, not by capacitance

The nanofarads are the resonance and take an afternoon. The number of capacitors is the ripple current, and it runs backwards from the instinct: fitting larger parts to be safe gives you a bank that carries less.

CommonGnd24 August 202614 min read

A DRSSTC's tank capacitor is chosen twice. Once for how many nanofarads, which is the resonance and takes an afternoon, and once for how many capacitors, which is the current and is where the money and the failures are.

This page is the second choice. The first is the frequency arithmetic, and it is on the primary page, which is where the ratio of L to C gets decided in the first place.

The reason the second choice is not obvious is that it runs backwards. Fitting larger capacitors to be safe gives you a bank that carries less current, not more, and the datasheets say so plainly once you are reading the right column of the right edition.

What it is and why

An MMC is capacitors in series to hold the voltage and in parallel to carry the current, and the array is sized by whichever of those two runs out first. In a DRSSTC it is almost always the current, because the voltage problem is solved by stacking cheap parts and the current problem is not solved by anything except more of them.

What follows is the ripple rating and how to read it, why smaller parts win, and why the strings do not share the load the way the schematic suggests.

What you decide

Averaged over time, not over the bang

Both the tank and the bus bank get called a bank, both have a ripple rating, and the two ratings are not interchangeable. Nanofarads of film in the primary is this page; thousands of microfarads behind the bridge is the other one.

Same peak, and the bars under them are the heating, which goes as the square of the current across the whole bang. Sizing a bank on the peak reads both of these as the same duty.

I_rms(average) = I_rms(during the bang) · sqrt(duty)
duty = bang length × bangs per second

And the shape of the envelope inside the bang moves the answer by a factor of the square root of five:

a flat top gives I_pk / sqrt(2), so 0.707 of the peak;
a linear rise gives I_pk / sqrt(6), 0.408;
a quadratic rise, which is what a QCW does, gives I_pk / sqrt(10), 0.316.

Dissipation is I_rms^2 · ESR. The catalogue current is quoted for a stated temperature rise, and the figure to look for is ten degrees on most MKP film parts, which is davekni's reading of the catalogues and which he adds is often not printed on the sheet at all. Where it is printed it is not always the ten: of the two TDK sheets cited further down, the B3265* one states 20 °C and the B3264*B one 15, and the 2015 edition of that second sheet stated 10. Whatever the stated rise turns out to be, it gives you the thermal resistance for free and lets you work out the real rise rather than guessing at it. This page used to give twenty degrees as the general figure with nothing behind it; the twenty that survives further down is one sheet's own footnote and holds for that sheet's parts, not as a rule.

And the rise you measure on the first day is not the rise the part settles at. One ceramic tank capacitor has been run to destruction with the heating logged the whole way. davekni took a Samsung CL31C222JHHNNNE, 2.2 nF at 630 Vdc in a 1206 package with no AC rating at all, and characterised it at 450 kHz: 8 to 10 °C of rise at 250 Vrms, 18 to 20 at 340. Carried forward on the square of the voltage, as dielectric loss should go, the 340 V pair predicts 34 to 38 °C at the 466 Vrms he then pushed it to [derived, 18 to 20 times (466/340)²]. He measured 36. The two measured points do not quite follow the square between themselves, an exponent of 2.3 to 2.6 [derived], so the 250 V pair on its own would have said 28 to 35 and the prediction that lands is the nearer one. So far the method on this page works, and the part is characterised rather than guessed: 2.9 Arms into its 160.8 Ω of reactance is 466 V, which is the voltage he states [derived]. Then it kept climbing. Over four weeks of almost continuous running the rise went from that 36 °C to 127 °C, a case at 147 with the room at 20, and the part failed at thirty five days. Nothing in the drive changed. The loss did, by 3.5 times [derived]. So a catalogue current at a stated rise describes a part in its original condition, and this is the only published measurement of what a tank's duty does to that condition. Size for the rise you measure, then go back and measure it again.

Check which edition of the datasheet you are reading

TDK's B32642B0333J is not the part it was on paper. EPCOS's edition of May 2015, mirrored by DigiKey, lists the part at VR = 1000 V DC against VRMS (f ≤1 kHz) = 600 V AC and draws its permissible-voltage curves for "Self-heating TA ≤10 °C". The June 2018 edition lists the same part at 500 V AC and redraws those curves for "ΔT ≤15 °C": a larger rise allowed, and a lower rating anyway. Read the 33 nF curve at 400 kHz off each and it goes from about 68 V to about 34. Half, on the same part number.

Those two voltages are read off graphs, not quoted. Neither edition prints a number at 400 kHz, both give curves, and 68 and 34 are what the 33 nF curve measures on the published plots at that frequency. This page used to print the pair as 70 and 35 with nothing under either. The reading checks against a figure that is printed: TDK's current edition, June 2026, carries the 2018 plot unchanged, plot number KMK1821-E, and adds a column the older sheets did not have, B32642B0333 at 2.9 A RMS at 85 °C and 100 kHz. If that ampere still holds at 400 kHz it is 35 V across 33 nF [derived, 2.9 / (2π · 400 kHz · 33 nF)], against the 34 the curve reads there. So the halving is not an artefact of one edition's draughtsmanship, and it is still in force.

The public capacitor tables everybody links to were built on the older numbers.

Small capacitors carry more current per nanofarad

This is the counterintuitive one, and it is why "I will fit bigger ones to be safe" is backwards. At a fixed bank capacitance, smaller parts give you more current. From TDK's B3265* sheet of June 2026, the B32652 rows at 15 mm lead spacing and VR, DC = 1000 V DC, the amperes being that sheet's IRMS at 85 °C and 100 kHz "for a T ≤ 20 °C":

10 nF: 1.3 A each, 130 mA per nF. A 12.2 nF bank, the one on the machine this site works from, as 9 series by 11 parallel is 99 parts and 14.3 A.
22 nF: 1.9 A, 86 mA/nF. 9 by 5, 45 parts, 9.5 A.
100 nF: 5.4 A, 54 mA/nF. 9 by 1, nine parts, 11.1 nF, 5.4 A.

Nearly a factor of three in current for the same capacitance, and the price is count. Across the two ends of that list it is eleven times the parts for 2.6 times the current [derived, 99 against 9 and 14.3 against 5.4], because every extra parallel string needs its own series stack. That is faster than the square of the current gain, which is what this page used to say it was: 2.6 squared is 7.0 against the 11 the rows actually give.

Nor is a bigger case a guarantee. In the same sheet the B32653 at 22.5 mm pitch and 33 nF, ordering code B32653A0333, gives 1.6 A, less than the B32652 at 15 mm pitch, B32652A0333, and 2.5 A. Both are 1000 V DC rows and both amperes are on the same 85 °C, 100 kHz, 20 °C basis. Read the table, not the outline drawing.

And on a QCW, small in the other sense too

A QCW at the size most people build wants a small bank outright. Gao Guangyan summarises the machine as needing a high impedance primary, "coupling of >=0.3, many turns for primary and with a small tank cap of around 8 - 15nF", and on the forum Hydron told a builder who was planning fifty that "50nF is very large for a QCW coil - the required MMC normally works out to be under 20nF". The bank on the machine this site works from is 12.2 nF and sits inside that.

The reason is the tank's characteristic impedance. At a fixed frequency L and C are tied together, and:

Z = sqrt(L/C),   primary current goes as C,   power goes as C

while the voltage across the bank does not depend on C at all. So the fix for too much current is more primary turns and less capacitance, and the fix for too little is the reverse. Landon Kageler built his first QCW on a 12.35 nF primary capacitor, eight series by three parallel of the same B32642B0333J the edition change above is about, was disappointed by what it made, and then "simply increased the current by increasing the primary capacitance and lowering the inductance" to 30 nF, at which point he was reaching one and a half feet.

And the current does not divide the way you think

Between parallel strings, the current divides in inverse proportion to ESR and path resistance, not to capacitance. At the frequencies a tank runs at, capacitance is not what decides it.

One idea worth disposing of, and the disposal is ours rather than anybody's we can cite: small capacitors do not make a poor bank. A network's ESR does not depend on the size of its unit, because R_bank = R_unit·n/m and tan δ/(2πf·C_bank) are the same number with the unit capacitance cancelling out [derived], and dividing the heat between more parts improves the cooling. This page carried the opposite claim in its first version and it was wrong. What limits it in practice is the number of joints and the symmetry of the paths, not the physics.

One more thing that follows from where the heat goes: the leads are a thermal path and not only a connection. davekni takes his film parts' case rise "at hottest point at center between leads", and notes that in his fixture the "Leads provide heatsinking to copper foil". Cool the leads, and space the parts so air can pass between them.

How the heat divides between the leads and the case is not established on this page. It used to print about two thirds of it leaving along the axis and one third radially. No source for that split was found, and the maker's own model does not work that way: TDK's general technical information sizes the heat a film capacitor can shed as α·A·ΔT with A the surface area of the whole part, not as a fixed share out of the ends. So the split is withdrawn rather than defended. The instruction above does not depend on it.

The rig on the DRSSTC department page draws the bank as a matrix rather than one symbol, because the number of parts is the point. Take the tank capacitor out on the QCW page and you have a ramped SSTC, which is a different machine with a different set of problems.

The two capacitors that were measured here, the Samsung ceramic and davekni's tank, are linked to the threads they were measured in, and so now are Gao Guangyan's range, Hydron's ceiling and Landon Kageler's rebuild. So are the catalogue rows, which used to carry no source at all: the 1000 V family and the B32652 against B32653 comparison are TDK's B3265* sheet of June 2026, and the edition change is TDK's own two editions of the B3264*B sheet, the older of them reachable only through a web archive. The one thing in that comparison that is not quoted is the pair of voltages at 400 kHz, which are measured off the published curves and are marked as such where they appear. The arithmetic on all of them is ours.

What will get you

Reading a current rating without its conditions. Every ampere on this page is quoted with the frequency, the temperature and the permitted self heating it was measured at. An IRMS with those stripped off is not a number, and the two TDK editions above differ on exactly that basis rather than on the part.

Fitting bigger cans to be safe. It is the one instinct this page exists to break. At a fixed capacitance the larger part carries fewer amperes per nanofarad, and a bigger case is not even a reliable guide within one series.

Laying the strings out however they came out. The current divides on ESR and path resistance, so the string nearest the bus takes more of it and gets hotter, and matched capacitors do not help.

What goes wrong

One string runs hot and the others do not. Layout, not parts. Measure the path lengths before you measure the capacitors.

The bank is within its rating and dies anyway. Check which edition of the sheet the rating came from, and check whether the number was VRMS or VR, DC.

A part fails and the rest look fine. A series stack divides the voltage between its members. Losing one hands its share to the survivors, so the stack that killed one part is now over its rating for the rest.

Where next

The bus bank behind the bridge, which is a different capacitor with a different job and is genuinely sized by easy arithmetic, is capacitance is the easy half, the page this one was cut out of.

What the same part does on a ramped machine, where the current is deliberately lower and held fifty times longer, is the tank capacitor on a QCW.

Whether the tank resonates with the primary at all, which is not a given on every topology, is the capacitor that decides what you built.

mmcrippledatasheet