Can a TVS Diode Go Directly on the AC Input? How to Split Surge Protection on the AC Side
INTEK (www.intek.vip) is an authorized distributor of AMSEMI TVS devices, supplying the full range with selection support. Datasheets, samples and FAE review are available for every model and parameter discussed below.
One-sentence conclusion: No, a TVS cannot be connected directly across a 220 V AC input - and the reason is arithmetic, not taste. 220 Vrms peaks at ±311 V and reverses polarity every 10 ms. Any TVS whose clamping voltage means anything must have a VRWM below 311 V, which means it conducts in reverse breakdown every half cycle; the average dissipation then far exceeds the 10/1000 μs single-pulse power rating in the datasheet, which is a one-shot capability rather than a continuous dissipation rating, and the device thermally fails. Choose a VRWM above 311 V instead and the residual voltage rises to a level that protects nothing downstream, while surge current capability stays far below a MOV. The correct division of labor: the AC port goes to MOVs, GDTs and X/Y capacitors; the TVS sits on the rectified DC bus and low-voltage DC ports, catching fast edges and providing precise clamping.
1. Why the question exists: a good DC-side part invites misuse everywhere
On the DC side the TVS behaves perfectly: fast (avalanche response, sub-nanosecond order), low clamping, and VBR/Vc written explicitly in the datasheet. So the natural thought follows: "if it works on the bus, why not across the AC input for surge protection in one move?"
Where this fails is not a feeling - it is a calculation.
2. The half-cycle account: why a TVS on the AC port must fail
Break down what a 220 V / 50 Hz line does to a TVS:
- Peak voltage ±311 V (220 × √2), polarity reversing every half cycle (10 ms);
- A TVS only clamps in reverse breakdown. To survive the 311 V peak without conducting, the VRWM must exceed 311 V - such grades clamp at around 600 V or higher, which provides essentially no protection margin for the bridge rectifier and bulk capacitors (typically rated 400 V / 450 V);
- Conversely, for the clamping to mean anything (Vc within a few hundred volts), the VRWM must fall below 311 V - so it conducts near every peak. Even if conduction lasts only a few milliseconds per cycle at ampere-level current, the average dissipation is in the tens of watts, while the 3000 W / 5000 W rating is the peak power of a single 10/1000 μs pulse; continuous dissipation capability is only watt-level. The result is thermal failure - predictable, not incidental.
In one line: the TVS capability model (fast, precise, small surge current) and the AC-port requirement model (slow, robust, large surge current) are exactly mismatched.
3. Who actually guards the AC input: split by differential and common mode
Surges enter equipment through differential and common-mode paths (IEC 61000-4-5 type testing). The division of labor:
| Position | Main components | What it does | Why not a TVS |
|---|---|---|---|
| Differential (L to N) | X capacitor + MOV | Filters differential noise; MOV diverts high-current surge | MOV takes large energy; a TVS would conduct every half cycle |
| High-current differential | GDT in series with MOV | GDT fires first; MOV limits follow current and residual voltage | The series pair avoids GDT power-frequency follow current |
| Common mode (L/N to earth) | CM choke + Y capacitors | Filters common mode; routes surge away | Y capacitors are capacity-limited; needs GDT assistance |
| Rectified DC bus | TVS (here) | Catches fast edges the bulk capacitor cannot absorb | —— the correct position for a TVS |
Two engineering points:
First, the GDT-MOV series pair is deliberate. A GDT has the largest surge current capability, but alone across an AC line it has a fatal problem: power-frequency follow current - once the arc strikes, the mains keeps feeding it and the GDT cannot extinguish. In series with a MOV, the MOV interrupts the follow current while the GDT handles the bulk discharge. This is the standard AC-port combination.
Second, the Y capacitor is the weak link of the common-mode path; protect it with a GDT. Safety regulations limit Y-capacitance (leakage current constraints), so the energy it can take is small. A GDT in series with the Y-capacitor path discharges the common-mode surge through the GDT while the Y capacitor keeps its filtering role, avoiding puncture and excessive leakage.
4. TVS vs MOV: division of labor, not competition
Datasheets sometimes list TVS and MOV as alternatives. They are near-perfect complements:
| Dimension | TVS (e.g. AMSEMI 5.0SMDJ series) | MOV (varistor) |
|---|---|---|
| Response speed | Fast (avalanche, sub-nanosecond order) | Slower (tens of nanoseconds, typical) |
| Clamping | Low and precisely predictable (VBR / Vc per grade) | High residual voltage, rises sharply with current |
| Surge current | Small (tens to ~100 A, 10/1000 μs) | Large (kiloampere order, 8/20 μs, typical) |
| Aging | Negligible within ratings | Degrades gradually with each event |
| Placement | DC bus, aux supplies, LV DC and signal ports | AC input, large-energy entry points |
| Waveform standard | AMSEMI Vc / Ipp rated at 10/1000 μs | Surge current rated at 8/20 μs |
Waveform note: MOV surge currents are rated at 8/20 μs. The AMSEMI 5.0SMDJ series (conventional Doc. 8060014 / snap-back -N Doc. 8060055) specifies Vc and Ipp at 10/1000 μs only; the datasheet contains no 8/20 μs data. The two datasets cannot be converted, subtracted or compared directly; see What does the 5000 W on a TVS mean?.
One line of logic: the MOV guards the large energy at the AC port; the TVS guards fast edges and precise clamping on the DC side. The bulk of the surge energy is dissipated at the AC port by the MOV, GDT and capacitor network; only the fast edges and near-end transients reach the TVS.
5. Where the TVS belongs: the rectified DC bus and low-voltage ports
Draw the full AC-DC chain and the TVS position becomes obvious:
flowchart LR
AC["AC input L / N<br/>220 Vac 50 Hz<br/>peak ±311 V"] --> F["Fuse / surge protector"]
F --> CM["CM choke<br/>(common-mode filter)"]
CM --> DM["X cap + MOV<br/>(differential discharge)"]
CM --> GDY["GDT + Y cap<br/>(common-mode discharge)"]
DM --> BR["Bridge rectifier"]
GDY --> BR
BR --> BUS["DC bus ~310 V<br/>bulk electrolytics"]
BUS --> TVS["TVS<br/>(fast-edge clamping)"]
BUS --> PFC["PFC / DC-DC stage"]Why a TVS at all when the bus has bulk electrolytics? The bulk capacitor does absorb most surge energy, but its absorption speed is limited by ESL and layout inductance - the fast edge of a surge arrives before the capacitor can pull the bus down, and the voltage spikes momentarily. The TVS avalanche response is far faster than the capacitor, so it covers exactly the gap the capacitor cannot respond to. The overall three-stage architecture (GDT → MOV → TVS energy grading) is covered in Surge protection: test levels and three-stage design for ESS / BMS products.
Typical placement checklist:
- DC bus after the bridge (~310 V): the standard position at the entry of adapters, chargers and variable-frequency drives;
- PFC output bus (~400 V): behind the PFC stage of ESS inverters and server power supplies;
- Auxiliary supply input: the flyback start-up rail, sensitive to fast edges;
- Low-voltage DC ports (battery side, 48 V rails, signal lines): the main battlefield for low-clamping TVS - in the 48-100 V range the AMSEMI -N snap-back series lowers Vc by roughly 20 % versus conventional parts (54 V grade: 87.1 V → 69.6 V, 10/1000 μs), see AMSEMI snap-back TVS: full model list and cross-reference.
Three-step grade selection on a bus (works for high-voltage buses and LV rails alike):
- VRWM above the highest bus operating voltage - including no-load, PFC overvoltage and line-tolerance scenarios, with derating;
- Vc below the lowest withstand voltage of downstream devices - check the bridge, bulk capacitors and the PFC MOS / IGBT rating floor;
- Check Ipp against the allocated surge level - the TVS only takes the fast-edge portion, but that allocation must be calculated.
The AMSEMI -N snap-back series comes in 3000 W / 5000 W / 8000 W grades, reaching 100 V (5000 W; the 8000 W grade 8.0SMDJ**CA-N covers 48-90 V in 11 steps, AEC-Q101 qualified, automotive suffix -H). For high-voltage bus grade selection and exact Vc / Ipp figures, follow the AMSEMI datasheets Doc. 8060014 / Doc. 8060055 rather than memory.
6. Common pitfalls
| Pitfall | Consequence | Correct approach |
|---|---|---|
| TVS directly across the AC input | Conducts every half cycle, tens of watts average, thermal failure | AC port uses MOV + GDT + X/Y capacitors |
| VRWM above 311 V TVS on the AC line | Survives, but residual voltage near 600 V protects nothing | Accept that TVS does not belong on the AC port |
| GDT alone across L-N | Follow current keeps the arc burning; GDT fails | Use the GDT + MOV series pair |
| MOV only, no TVS on the bus | Fast edges punch through downstream parts in the field | Add a TVS beside the bulk electrolytics |
| Comparing MOV 8/20 μs current with TVS 10/1000 μs Ipp | Selection table computed on mismatched standards | Keep the standards separate and verify each per its own waveform |
Related solutions
- Three-stage architecture: Surge protection: test levels and three-stage design for ESS / BMS products
- TVS fundamentals: What is a TVS diode? Principles and selection basics
- Bus-level selection: How to select TVS diodes for a BMS protection board?
- Full model list: AMSEMI snap-back TVS: full model list and cross-reference
- High-voltage platforms: How to select TVS for 32S / 48S high-voltage BMS?
- Server 48 V rail: How to select TVS for AIDC server 48 V bus?
- Power rating and derating: What does the 5000 W on a TVS mean?
- Inductive loads: Do relays, motors and solenoids need a TVS?
- PFC topology: How to design a PFC circuit?
- Failure prevention: How to prevent TVS failures?
FAQ
Q1: Can a TVS diode be connected directly across a 220 V AC input for surge protection?
No. A 220 Vrms line peaks at ±311 V and reverses every 10 ms. Any TVS whose clamping voltage means anything must have a VRWM below 311 V, so it enters reverse breakdown every half cycle; the resulting average dissipation far exceeds the 10/1000 μs single-pulse power rating in the datasheet, which is a one-shot capability rather than a continuous dissipation rating, and the device thermally fails quickly. If you pick a VRWM above 311 V instead, the residual voltage is far too high to protect anything downstream and the surge current capability is far below a MOV. The AC port belongs to MOVs, GDTs and X/Y capacitors; the TVS belongs on the rectified DC bus and low-voltage DC ports.
Q2: What combination of components should protect the AC input?
Design the differential-mode and common-mode paths separately. Differential mode (L to N): an X capacitor filters low-frequency differential interference and a MOV connected across L-N diverts high-current surges; in high-current cases, a GDT in series with the MOV avoids the power-frequency follow current of the gas discharge tube. Common mode (L/N to earth): a common-mode choke works with Y capacitors, and where creepage is a concern a GDT in series with the Y-capacitor path discharges the common-mode surge without overstressing the Y capacitor. This combination handles the bulk surge energy at the AC port; the rectified DC bus then gets fast-edge clamping from a TVS.
Q3: What is the difference between a TVS and a MOV, and can they replace each other?
They cannot replace each other; it is a division of labor, not a competition. A TVS responds very fast (avalanche breakdown, sub-nanosecond order) with a low, precisely predictable clamping voltage selected by VBR and Vc, but its single-pulse current capability is in the tens to roughly a hundred amperes range (10/1000 μs, per the datasheet). A MOV is slower (tens of nanoseconds, typical), has a higher residual voltage that rises sharply with current, but its 8/20 μs current capability reaches the kiloampere order with large energy throughput, at the cost of gradual aging after each event. So the MOV guards the large energy at the AC port while the TVS guards fast edges and precise clamping on the DC side. Note the different waveform standards: MOV surge currents are rated at 8/20 μs while AMSEMI TVS Vc and Ipp are rated at 10/1000 μs; the two datasets must not be mixed.
Q4: On the rectified DC bus, where should the TVS sit and how do you select it?
Three typical positions: the DC bus after the bridge rectifier (about 310 V), the PFC output bus (about 400 V), and auxiliary or low-voltage DC ports (battery side, 48 V rails, signal lines). The bulk electrolytic capacitor absorbs most surge energy, but its ESL and layout inductance let fast edges spike the bus momentarily; the TVS covers exactly this gap. Three selection steps: VRWM above the highest bus operating voltage (no-load and PFC overvoltage included) with derating; Vc below the lowest withstand voltage of downstream devices; and check Ipp against the surge level allocated to the TVS. On low-voltage DC rails (48-100 V) the AMSEMI -N snap-back series lowers clamping by roughly 20 percent versus conventional parts; for high-voltage buses the grades and parameters follow the AMSEMI datasheets Doc. 8060014 / Doc. 8060055.
Contact us
AC-input protection fails at both extremes: either a clamping device is installed where it must burn, or "a MOV is enough" leaves the bus fast edge punching through downstream parts. The correct approach is energy grading - the AC port to the MOV / GDT / capacitor network, the DC bus and low-voltage ports to the TVS, with the two waveform standards computed separately.
Send us your application (adapter / charger / ESS inverter / server power), line voltage range, downstream withstand window and surge test level, and we will align the AC/DC protection split, TVS grades and Ipp allocation in one pass. AMSEMI 5.0SMDJ conventional and -N snap-back (foldback) series datasheets, samples and FAE review are available - see Contact.
Shenzhen Intek Technology Co., Ltd (深圳市英太科技有限公司) —— authorized distributor of AMSEMI TVS and HOPERF CMT-series digital isolators Phone / WeChat: 136-3264-8484 Website: www.intek.vip
Disclaimer: This article is provided for technical education and exchange only and does not constitute purchasing or design advice. Device parameters follow the latest manufacturer datasheets: the AMSEMI 5.0SMDJ series (conventional Doc. 8060014 / snap-back -N Doc. 8060055) specifies Vc and Ipp at 10/1000 μs only, with no 8/20 μs data; MOV surge currents follow the industry-standard 8/20 μs rating with values per each MOV manufacturer datasheet; response speeds and current orders are typical industry magnitudes, not model-specific commitments; "±311 V peak" and "10 ms half cycle" are physical conversions of a 220 Vac 50 Hz line. Surge testing and protection design must follow IEC 61000-4-5 and applicable end-product certification requirements.