What Is a Foldback (Snap-Back) TVS? Definition, I-V Curve Reading and Latch-Up Risk Check
In one sentence: a foldback (snap-back) TVS is a TVS whose voltage, once it breaks down, first overshoots and then falls back to a lower holding value before it discharges the surge. The industry calls the version with a shallow fall-back — holding voltage at or above the working voltage — a shallow-foldback TVS, and the version with a deep fall-back, holding voltage below the working voltage, a deep-foldback TVS. Whether one may be used on your rail is not decided by how deep the fall-back looks, but by where the holding voltage V_H sits relative to the node's maximum working voltage U_max — above U_max the device releases itself after the surge; below U_max the node may keep the device held in conduction. This article covers the definitions, how to read the I-V curve, a four-step check, and how to obtain V_H when the datasheet does not give it.
1. Put the names in order first
The same device is called five different things in datasheets, marketing material and design reviews. They are not five products — they are three different viewpoints of one behaviour:
| Name | What it describes | Typical context |
|---|---|---|
| Foldback / Snap-back TVS | the shape of the I-V curve | English datasheets, imported brands |
| Low-clamping TVS | the result (lower Vc at the same grade) | selection and replacement studies |
| Low-residual-voltage TVS | same as low-clamping, another translation | some vendors |
graph LR
A["One device, three viewpoints"] --> B["Foldback / Snap-back<br/>shape of the I-V curve"]
A --> C["Low-clamping<br/>the Vc result at the same grade"]
B --> D["Criterion: does the curve fall back?"]
C --> E["Criterion: the Vc figure at the same grade<br/>and one consistent waveform convention"]A usage worth correcting: those are phenomenon names, not parameter names. The parameters are only four — breakdown voltage V_BR, clamping voltage V_c, holding voltage V_H and holding current I_H. A selection review must land on parameters; adjectives such as "shallow" and "low" cannot be checked.
2. The definition hangs on three quantities on the I-V curve
Below V_RWM a TVS is off with only leakage current; once the voltage exceeds V_BR it turns on quickly and discharges the surge. Everything that separates a standard TVS from a foldback TVS happens after breakdown.
| Quantity | Definition | Where it sits on the curve |
|---|---|---|
Breakdown voltage V_BR | the knee where the device turns on | the first inflection of the high-impedance branch |
Holding voltage V_H | the lowest stable value the voltage falls back to | the bottom of the negative-resistance region |
Holding current I_H | the minimum current that sustains the fallen-back state | the current at the bottom of the fall-back |
Clamping voltage V_c | the maximum voltage read at a specified Ipp | not a knee — a reading that moves with current |
graph TB
A["V below V_RWM: off<br/>leakage only"] --> B["V exceeds V_BR: breakdown<br/>voltage overshoots"]
B --> C{"Where does the voltage go after breakdown?"}
C -->|"keeps rising along the avalanche curve"| D["Standard TVS: Vc rises with Ipp<br/>higher surge, higher residual"]
C -->|"falls back to the valley V_H"| E["Foldback TVS: discharges in the<br/>low-voltage high-current region"]
E --> F{"V_H against U_max?"}
F -->|"V_H > U_max"| G["Releases itself after the surge<br/>shallow foldback: usable on power rails"]
F -->|"V_H < U_max"| H["Node may hold the device on<br/>deep foldback: signal lines only"]The most common error: writing that "the snap-back voltage is the clamping voltage V_c".
V_cis a maximum-voltage reading at a given Ipp and moves with the test current;V_His a knee on the curve, and a given device has only one. Different physics, different measurement, different use. When a document says "the snap-back voltage V_c is below V_RWM", that is really the position ofV_Hrecorded under the nameV_c— always ask whether the number is the valley or the reading at the specified Ipp.
3. Reading the curve: what shallow and deep foldback look like
Plot two foldback parts of the same voltage grade on one I-V chart and only two things differ: how deep the fall-back goes (the position of V_H) and how narrow the valley is (the size of I_H).
| Observation | Shallow foldback | Deep foldback |
|---|---|---|
| Fall-back depth | small, valley near or above V_RWM | large, valley well below V_RWM |
Holding current I_H | large | small |
| Curve after the valley | short, releases easily | long and steep, hard to release |
| Usable on | power rails, active nodes | weak-drive signal lines only |
Three moves when reading the chart: ① find the breakdown knee → V_BR; ② find the fall-back valley → V_H (this is the value datasheets most often omit); ③ read the current at the valley → I_H.
If the datasheet only has a V_c @ Ipp table and no I-V curve, it does not give V_H — do not infer a safe margin from the words "low-clamping"; obtain the data as described in section 5.
4. The real criterion: V_H against the node's maximum working voltage
"Deep" and "shallow" are relative. In engineering they become one comparable quantity: the relation between V_H and the maximum sustained voltage U_max of the protected node in its worst case. U_max rather than the nominal voltage, because a battery bus, a DC link or a node with large capacitance can sit 10–20% above nominal.
V_H versus U_max | Verdict | Consequence | Suitable for |
|---|---|---|---|
V_H > U_max + margin | safe | releases itself, returns to the off state | power rails, active nodes |
V_H ≈ U_max | borderline | temperature and part spread decide | verify by test, use with care |
V_H < U_max | at risk | the node can keep the device held on | weak-drive signal lines only |
This table is the quantitative basis behind the rule of thumb that deep foldback must not be used on a power rail — not because it is "bad", but because its V_H falls below U_max.
Note on conventions: the AMSEMI -N series is designed as shallow foldback (small fall-back, holding voltage at or above
V_RWM) for active power rails such as battery buses. ActualV_H/I_Hvalues must follow the manufacturer I-V curve and datasheet; this article does not substitute for manufacturer data.
5. A four-step check for the "staying on" risk
"Latch-up" here is engineering shorthand for a device that, once triggered into the foldback region, cannot leave it because the node keeps supplying energy. The mechanism differs from a thyristor (TSS) PNPN turn-on — a foldback TVS relies on avalanche negative resistance and is off during normal operation. Whether it can fail to release is checked in four steps:
| Step | Check | Verdict |
|---|---|---|
| ① Node | is it an active node (supplying bus, large capacitor, driver output)? | yes → check ②③ carefully |
| ② Voltage | is V_H clearly above the node's U_max? | no → risk of staying on |
| ③ Current | can the node worst-case deliver sustained current above I_H? | yes → it may be held on |
| ④ Limiting | is there series current limiting (resistor, fuse, wiring, driver impedance)? | yes → risk drops sharply |
If step ② passes (V_H above U_max), the rest is largely moot — which is why the correct choice for a power rail is shallow foldback, not "the deeper the better". On a signal line, by contrast, the drive is weak and series resistors are common, so even a small I_H is hard to sustain and the very low clamping of deep foldback becomes useful.
6. Datasheets usually omit V_H: how to obtain and measure it
Most datasheets at the Littelfuse / Vishay level give V_RWM, V_BR, V_c and Ipp but do not list V_H and I_H separately. Without them the risk check cannot be completed. Two options:
Option 1 — request from the manufacturer (free, preferred)
- the I-V curve for that part number, including the fall-back valley;
- the definition and typical / worst-case values of
V_HandI_H; - the temperature at which the curve was taken (25 °C or 85 °C);
- for a shallow-foldback part, an explicit statement of
V_Hrelative toV_RWM.
Intek Technology, as an authorised distributor of AMSEMI, can request these curves and parameters from the manufacturer on a customer's behalf and state the conventions in writing.
Option 2 — measure it yourself
- Pulsed I-V sweep: apply progressively larger current pulses and read back the device voltage at each point to reconstruct the curve, then read
V_Hat the valley andI_Hat the valley current. Keep pulses short enough that self-heating does not distort the curve. - TDR / curve tracer: good for a quick qualitative "does it fall back", less accurate for
V_H. - On-board reproduction: inject one surge of the same waveform as the field event into the target node and watch whether the device voltage returns to the off state after the surge — the final criterion, and one no datasheet can replace.
V_candIppmay only be compared under one consistent waveform convention (10/1000 µs and 8/20 µs are common and must not be converted into one another). AMSEMI 5.0SMDJ series datasheets (8060014 standard / 8060055 -N foldback) give the 10/1000 µs convention only, with no 8/20 µs data; some imported brands publish 8x20 µs figures as well, which must be attributed explicitly and never subtracted from the AMSEMI numbers.
7. Power rails versus signal lines: the answers differ
| Scene | Node characteristics | Choose | Why |
|---|---|---|---|
| BMS bus / energy-storage DC link | active, sustained supply, low series resistance | shallow foldback | V_H must exceed the bus maximum to release |
| 48 V telecom / server power rail | active, large capacitance | shallow foldback | recovery plus a clamp inside the downstream rating |
| High-speed data / RF / weak-drive IO | weak drive, often series resistance | deep foldback usable | drive is far below I_H, low clamp helps |
| Low-voltage sensing with series resistor | active but large series resistance | shallow foldback safer | the resistor limits the holding current |
| Automotive 12 V / 24 V rail | active, complex transients | shallow foldback + AEC-Q101 | active port, must recover and hold rating |
A one-line memory aid: on a power rail ask "can it release?", on a signal line ask "how low can it clamp?".
Related guides
- The full product line (shallow foldback / snap-back / low-clamping, three power grades) → Foldback TVS Full Series Overview
- Foldback versus standard TVS at the same grade, with imported-brand cross-reference → Which Chinese Brands Make Low-Clamping TVS
- AMSEMI vs Littelfuse foldback TVS comparison → Littelfuse TP5.0SMD-FL vs AMSEMI Foldback TVS
- TVS selection for BMS protection boards → How to Select TVS for BMS Protection Boards
FAQ
Q1: What exactly is a foldback TVS and how does it differ from a standard TVS?
A foldback (snap-back) TVS overshoots at breakdown, then falls back to a lower holding voltage V_H and discharges the surge in the low-voltage high-current region. A standard TVS keeps rising along the avalanche curve after breakdown, so the higher the surge current the higher the residual voltage. The difference is the direction of that post-breakdown segment, which is why at the same voltage grade and the same Ipp a foldback part clamps markedly lower.
Q2: How do shallow and deep foldback differ, and why is deep foldback unsuitable for power rails?
They differ in where the holding voltage V_H sits relative to the working voltage. Shallow foldback falls back little, with V_H at or above the working voltage; deep foldback falls back a lot, with V_H below the working voltage and a very small I_H. Deep foldback is unsuitable on a power bus because an active node keeps supplying energy: once the device is triggered into the foldback region the node can deliver more than I_H and hold it on. A signal line is driven weakly and usually has series resistance, so it cannot supply that holding current, which is why deep foldback works there.
Q3: Is the snap-back voltage the same as the clamping voltage V_c?
No. V_c is the maximum voltage read at a specified Ipp (for example at some current under 10/1000 µs) and moves with the test current; V_H is the valley knee on the I-V curve, and a given device has only one. Release behaviour is judged by V_H; downstream survival is judged by V_c. Two questions, two quantities.
Q4: The datasheet does not give V_H — how do I know whether the part is usable?
Two routes. Ask the manufacturer for the I-V curve including the valley and for the V_H / I_H definition, worst-case value and temperature — free and preferred. Or measure it: a pulsed I-V sweep reconstructs the curve and gives the valley, or inject a same-waveform surge into the target node and watch whether the device returns to the off state. If neither is available, do not leave downstream margin on the strength of the words "low-clamping" — treat it as a standard TVS.
Q5: Can a foldback TVS latch on like a thyristor?
Its mechanism differs from a thyristor (TSS): a foldback TVS relies on avalanche negative resistance, not a PNPN structure, and is off below V_RWM. What must be guarded against is a node holding the device inside the foldback region so that it cannot release — which depends on V_H relative to the node's maximum working voltage, the four-step check in section 5. As long as V_H is clearly above U_max, the device releases itself after the surge.
Contact us
Shenzhen Intek Technology Co., Ltd is an authorised distributor of AMSEMI (Anhui AMSEMI Semiconductor), focusing on low-clamping / shallow-foldback TVS for BMS protection boards, energy storage and telecom power. To request an I-V curve or the V_H / I_H figures for a given grade, or to have your release check reviewed against your node voltage and surge waveform, please contact us.
Shenzhen Intek Technology Co., Ltd — electronic component distributor and system solution provider Tel / WeChat: +86 136-3264-8484 Address: Room 4F, Tower 2, Jingwei Center, 309 Ping'an Avenue, Pinghu, Longgang District, Shenzhen, China Website: www.intek.vip
Disclaimer: This article is technical and engineering communication intended to explain the definition and reading method of foldback (snap-back) TVS only; it is not selection, procurement or business advice. The V_RWM / V_BR / V_c / Ipp conventions referenced come from AMSEMI public datasheets (for example 5.0SMDJ series 8060014 and -N series 8060055); because most datasheets do not list V_H / I_H separately, those must follow the manufacturer I-V curve and the latest datasheet. Waveform conventions differ between vendors — align the convention and attribute it before comparing. Actual designs must be verified against the chosen part's manufacturer datasheet and on-board measurement.