Passives for a Solar String Inverter: MOV, High-Voltage MLCC and Divider Resistors

Only 20 of Walsin Technology’s 379 MOV part numbers clear both 510 Vac continuous and 40,000 A peak — the pair of numbers a 400 Vac three-phase string inverter output actually demands. All 20 sit in the Industrial series, in three body sizes: 34 mm Square, 40 mm and 53 mm. Raise the surge requirement to 80,000 A and the set falls to five part numbers, every one of them 34 mm Square.

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Venatech Limited, an authorised Walsin distributor in Hong Kong, holds Walsin Technology (Walsin) full MOV, MLCC and chip resistor parametric exports plus 26 MLCC datasheets. This post is written from those files for the three passive families a solar string inverter design review keeps coming back to: surge protection on the AC output and the DC array input, ceramic capacitors on the DC link, and the sensing divider that measures it. Every part number below is real, and every count is a count of part numbers, not pieces.

Which Walsin MOV clears a 400 Vac three-phase inverter output?

Size the part on the Max Working Volt. column, not on varistor voltage. A 400 Vac line-to-line output at +10 % tolerance sits at 440 Vac, so the continuous rating has to be 510 Vac or above. Walsin’s export carries 56 part numbers at 510 Vac or higher, across all four series — Standard (D), High Energy (E), Super High Energy (V) and Industrial.

Varistor voltage is the DC reference point on the clamping curve. It is always the larger number and it is not the rating to select against.

Part number	Series	Size	Max working voltage	Varistor voltage	Peak current	Match / mismatch
SR821K34R	Industrial	34 mm Square	510 Vac	820 Vdc	40,000 A	Match — 400 Vac three-phase output
SR821K40D	Industrial	40 mm	510 Vac	820 Vdc	40,000 A	Match — same ratings, round body
SR821K34RD	Industrial	34 mm Square	510 Vac	820 Vdc	80,000 A	Match — double the surge, same footprint
SR821K53D	Industrial	53 mm	510 Vac	820 Vdc	70,000 A	Mismatch on surge — 70,000 A, not 80,000 A, in the largest body
SR911K34R	Industrial	34 mm Square	550 Vac	910 Vdc	40,000 A	Headroom option for a 480 Vac line

The 34R and 34RD split is the one to read twice. Both are 34 mm Square and both carry the same voltage ratings; the trailing D is the only thing in the part number that doubles the peak current. Our own published note on this is in MOV varistor series selection — the last character sorts away from its sibling in a spreadsheet, which misfiles 21 parts.

How many MOV part numbers clear both 510 Vac and 40,000 A?

20, out of 379. That is 5.3 % of the varistor catalogue, and it is the number worth carrying into a design review, because a string inverter needs both figures at once and the two filters narrow much harder together than apart. We measured this against the full export; it is not a figure Walsin publishes.

Requirement	Part numbers that clear it	Body sizes available	Series available
510 Vac or above	56	20, 25, 32, 34 mm Square, 40, 53 mm	All four
510 Vac and 10,000 A	32	20, 25, 32, 34 mm Square, 40, 53 mm	Mixed
510 Vac and 20,000 A	24	32, 34 mm Square, 40, 53 mm	Mixed
510 Vac and 40,000 A	20	34 mm Square, 40, 53 mm	Industrial only
510 Vac and 80,000 A	5	34 mm Square	Industrial only

The step from 20,000 A to 40,000 A is where series choice stops being a choice. Below it the Standard, High Energy and Super High Energy lines all contribute; at 40,000 A and above the Industrial series is the whole available set, and the 20 mm and 25 mm bodies drop out entirely. If your surge budget lands at 40 kA, the footprint decision has already been made for you.

Which MLCC cover a 1000 V and 1500 V DC link?

Walsin publishes 2,014 MLCC rows at 1000 Vdc or above, which de-duplicate to 1,874 distinct part numbers. The 1000 Vdc rung is the deepest at 861 rows; 1500 Vdc carries 60 distinct parts across seven case sizes, and the count thins quickly above that.

Part number	Series	Case	Dielectric	Capacitance	Voltage	Match / mismatch
1206B472K152	Middle to High Voltage (200V–4kV)	1206	X7R	4700 pF	1500 Vdc	Match — 1500 V DC-link snubber
1206B101K152	Middle to High Voltage (200V–4kV)	1206	X7R	100 pF	1500 Vdc	Mismatch on body — 1.25 mm thick against 1.60 mm on the 4700 pF part
1825B223K102	Big sizes 1825/2220/2225	1825	X7R	22 nF	1000 Vdc	Match — bulk DC-link decoupling
SH31N470J102	Soft Termination SH (Ag-poly)	1206	NPO	47 pF	1000 Vdc	Match — flex-crack margin on a large board
1206B102K202	Middle to High Voltage (200V–4kV)	1206	X7R	1000 pF	2000 Vdc	Headroom option, same footprint

Two things on that table matter to a layout engineer. First, within one case size and one voltage rating the thickness changes with capacitance: 1206B101K152 is published at 1.25 ± 0.10 mm and 1206B472K152 at 1.60 ± 0.20 mm, so a height-constrained slot does not automatically take both. Second, soft termination is the mechanical answer on the large, flexing boards a string inverter uses — see soft termination MLCC SH, SG, ST and VT for what the polymer layer does and does not buy.

Source for the voltage range and dielectric table: WTC_MLCC_Middle_and_High_Voltage.pdf and WTC_MLCC_Soft_term_SH.pdf.

What is the highest-voltage MLCC in the catalogue, and how many parts is it?

6000 Vdc, on 10 part numbers, all in the 2211 case size. Below that the ladder is uneven in a way that matters if you are planning a second source inside Walsin’s own range:

Rated voltage	Distinct part numbers	Case sizes
6000 Vdc	10	2211
5000 Vdc	155	1808, 1812, 2211, 2220
4000 Vdc	59	1808, 1812, 1825, 2220, 2225
3500 Vdc	1	1812
3000 Vdc	316 rows	1206, 1808, 1812, 2220, 2225
2500 Vdc	23	1206, 1808, 1812, 2220

The 3500 Vdc rung is a single part number — 1812B103K352, X7R, 10 nF, ±10 %. There is no second capacitance, no second case size and no second tolerance at that voltage anywhere in the export. Designing a 3500 V rail against that rung means designing against one part, with 3000 Vdc and 4000 Vdc as the only neighbours to fall back on.

Which resistors suit the DC-link voltage divider?

Walsin’s Thick Film High Voltage Chip Resistor group carries 20,244 part numbers across four series, and the resistance ceiling a divider needs is there: 100 MΩ at 1206, 2010 and 2512.

Part number	Series	Case	Resistance	Tolerance	TCR	Rated power	Match / mismatch
WK12V1004FTL	High Voltage	1206	1 MΩ	±1 %	≦±100	1/4 W	Match — upper divider leg
WK12V1004DTL	High Voltage	1206	1 MΩ	±0.5 %	≦±100	1/4 W	Tighter tolerance, same footprint
WF25N1005FTL	High Voltage, UL Safety Certification	2512	10 MΩ	±1 %	≦±100	1 W	Match — UL-scope divider string
WK12V1004FTLJ	High Voltage (Automotive)	1206	1 MΩ	±1 %	≦±100	1/4 W	Mismatch on scope — automotive marker, not a UL part
WF12N1004FTL	High Voltage, UL Safety Certification	1206	1 MΩ	±1 %	≦±100	1/4 W	UL scope at the smaller footprint

Rated power maps one-to-one onto case size across the whole group, with no overlap at all: 0603 is 1/10 W on 3,924 parts, 0805 is 1/8 W on 3,966, 1206 is 1/4 W on 4,118, 2010 is 1/2 W on 4,118 and 2512 is 1 W on 4,118. There is no 1206 part at anything other than 1/4 W. The resistance ceiling also moves with case size: 0603 stops at 22 MΩ and 0805 at 51 MΩ, while 1206, 2010 and 2512 all reach 100 MΩ.

Why can I not filter the high-voltage resistor export on voltage?

Because the export declares no voltage column. The schema is Part Number, Series Group, Series, Size, Resistance, Tolerance, TCR, Rated Power, Packaging and Termination — ten fields, and the rating the series is named for is not one of them. “High Voltage” is a string inside the Series text, exactly like the “(Automotive)” and “UL Safety Certification” markers that sit alongside it. A numeric filter cannot find these parts; a string match on the Series column can.

Our published note on what the real limit is, and why the square root of power times resistance overstates it, is in high voltage chip resistors and the limit nobody checks. The full range sits on the chip resistor products page.

What should I check before committing any of these part numbers?

Three things, in this order. Confirm the MOV’s energy rating in joules against Walsin’s own datasheet, because the export publishes peak current and not energy, and a string inverter’s surge duty is an energy problem. Confirm the MLCC’s capacitance under DC bias at your actual rail voltage, not at zero volts — a 1000 Vdc X7R part on a 800 V rail does not deliver its nameplate value. And confirm the divider resistor’s rated continuous working voltage per element, because the export is silent on it and a 100 MΩ part in a 1000 V string may need two or three elements in series rather than one.

What Venatech adds past that point is the part of the job a manufacturer’s spec sheet and a public cross tool cannot do: lot and date-code traceability on what you actually receive, a Certificate of Conformance on request, and original sealed manufacturer packaging. A datasheet tells you what the part should be. The paperwork tells you what arrived.

The short version

A string inverter narrows the varistor catalogue harder than any other application we have measured: 20 of 379 part numbers clear 510 Vac and 40,000 A together, all Industrial series, in three body sizes. The MLCC side is deep at 1000 Vdc and thin above 3000 Vdc, with a single part number at 3500 Vdc. Divider resistors exist in volume to 100 MΩ, but the export carries no voltage column, so the series name is the only place the rating appears. Every MOV figure here comes from the export alone, because Venatech holds no MOV datasheet.

Send us the surge, rail and divider numbers for your string inverter and we will come back with the part numbers that clear all three, and the ones that clear only two. One line per requirement is enough to start.

Check it against the catalogue yourself. Cross-reference accepts one part number or a whole BOM column and prints the mismatches as plainly as the matches — qualification stays with you, which is the honest division of labour. Ready stock covers the ceramic capacitor lines held in Hong Kong, as a dated snapshot rather than a live feed.

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Frequently asked questions

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Three ways to get this answered today

1. Cross a part, free. Paste up to 500 part numbers, or drop an Excel or CSV file, into the free cross-reference tool. It is matched against Walsin’s published catalogue and returns candidates field by field, with every difference from your original stated rather than hidden. Filter by commercial or automotive grade, or for soft termination, high temperature, safety certified, high Q / low ESR or low profile. There is no login and no charge.

2. Send the whole BOM. Upload a BOM and request a quote and we will come back line by line — including the lines we cannot help with, which is usually the more useful half of the answer.

3. Check the shelf before you commit. Ready stock shows what was physically in Hong Kong as at 23 September 2026. It is a dated snapshot, never a live feed, and quantities are never permanent.

Why take our word for any of this? Don’t — check it.

Venatech Limited (威納電子有限公司) is an authorised Walsin Technology distributor in Hong Kong. Confirm that with Walsin rather than with us. On the safety capacitor series we hold UL file E182369, which is searchable in UL’s own public database, together with TUV certificates R50381780 and R50195920 and CQC CQC20001280137 and CQC20001280136, against EN 60384-14:2023, IEC 60384-14:2023 and UL 60384-14. We hold 28 Walsin datasheets, 451 pages, and we will send the one covering your part number. Shipments arrive in original sealed Walsin packaging with lot and date-code traceability, and a certificate of conformity is available per shipment on request.

We will also tell you what we do not hold, which no competitor publishes. REACH, SVHC, conflict minerals, PFAS, China RoHS, ISO 9001, IATF 16949, PPAP, IMDS and a Declaration of Conformity appear nowhere in those 451 pages. You would rather hear that from us now than from your own supplier-approval team later. AEC-Q200 is compliant in Walsin’s own words and never certified, we hold no manufacturer’s end-of-life or allocation list for any brand, and qualification in your own circuit stays with you. We will not tell you a part is equivalent. We will show you where it differs.

Venatech Limited · Unit 207, 2/F, Sun Ling Plaza, 30 On Kui Street, Fanling, N.T., Hong Kong · (852) 2628 4777 · Hong Kong, Shenzhen, Singapore and Jiangmen, with Chengdu in preparation.