Do You Still Need a TVS Next to an Isolator? Surge Protection and Two-Stage Coordination for Isolated Interfaces
Bottom line: Yes, and the division must not be mixed — the isolator solves common-mode voltage and ground-potential difference (insulation capability), the TVS solves the transient energy from lightning, hot-plug, inductive-load kickback and ESD (discharge capability). The protection of an isolated interface walks four steps: ① separate the responsibility boundary (not either/or) → ② find all four protection locations (external bus port / secondary isolated-supply port / sampling and gate-driver output side / inter-board signals) → ③ two-stage grading with decoupling (TVS on the isolator outer side for front coarse-clamp, then after decoupling near the protected device for rear fine-clamp) → ④ align four parameters to the ratings (VRWM covers the working swing, Vc aligns with the bus fault-protection voltage, junction capacitance matches the speed, package energy matches the surge level). Using Shenzhen HOPERF (HOPERF) CMT-series isolators and AMSEMI TVS as references, isolation parameters are from HOPERF official materials, TVS parameters are labeled by the datasheet waveform caliber, and actual design follows the selected model datasheet and whole-system measurement.
flowchart TD
A["Two kinds of threats to an isolated interface"] --> B{"Is it a transient energy event?<br/>lightning / hot-plug / kickback / ESD"}
B -- "Yes (high energy, short time)" --> C["TVS responsible: clamp and discharge<br/>must sit on isolator OUTER side (bus-port side)"]
B -- "No (only ground-potential diff / common-mode drift)" --> D["Isolator responsible: common-mode rejection and insulation<br/>5kVrms is test value, not surge-withstanding"]
C --> E["Two-stage grading<br/>front coarse-clamp -> decoupling -> rear fine-clamp"]
E --> F["Check Vc <= transceiver bus fault-protection voltage<br/>e.g. ±70V class"]
F --> G["Check junction capacitance vs bus speed<br/>CAN FD 5Mbps / RS-485 12Mbps"]
D --> H["Check operating isolation voltage<br/>not 5kVrms test value"]
G --> I["Secondary isolated supply + barrier keep-out + fail-safe"]
H --> I
I --> J["Power-up sequence: supply secondary isolated supply first<br/>then enable isolator and transceiver"]First, two most-common questions
Q: The isolator is installed, so why is the bus port still damaged? Can't the isolator prevent surge?
The isolator's strength is common-mode: it electrically separates the bus side from the local control side ground so that tens of volts or even kilovolts of ground-potential drift will not pour into the control circuit; its isolation-withstand is insulation capability, not surge-withstanding capability. A surge is a transient energy event — lightning coupling, cable hot-plug, inductive-load interruption kickback, human-body ESD — the energy must find a path to dissipate in a very short time. The isolator's own surge rating (general isolators typically ~8kV class) is a backstop, not the main force: if a surge is allowed to pour directly into the isolator, at best data bit-errors, at worst it punches through the barrier or burns the bus-side pins. The isolator "holds the common-mode", the TVS "prevents transient damage" — neither can be missing.
Q: With both an isolator and a TVS on the bus, who should be closer to the connector?
The TVS must be on the isolator outer side, as close to the connector (surge entry) as possible. The reason is protection must be as early as possible: when the surge comes in from the cable it is first dissipated by the TVS and clamped to the bus, then attenuated by the decoupling element before entering the isolator and transceiver. If you put the TVS on the isolator inner side (control side), you let the surge cross the isolator first, and the barrier and bus-side pins bear all the energy first — the order is wrong and the protection is void.
Step 1: first separate the responsibility boundary
| Item | Isolation device (e.g. HOPERF CMT series) | TVS (e.g. AMSEMI 5.0SMDJ series) |
|---|---|---|
| Problem solved | Ground-potential difference, common-mode voltage, cross-barrier insulation | Transient overvoltage energy discharge (lightning / hot-plug / kickback / ESD) |
| Key metric | Isolation-withstand (test value), operating isolation voltage, CMTI, common-mode range | VRWM, VBR, Vc, Ipp, junction capacitance, peak power |
| Failure manifestation | Common-mode over-limit bit-error, barrier punch-through, cross-side short | Clamp failure, protected device punched through, TVS itself open or short |
| Can they replace each other | No: insulation capability != surge-withstanding capability | No: discharge capability != common-mode rejection capability |
Remember the boundary in one line: common-mode finds the isolator, transient finds the TVS. At system design, check each of these two first and you will not get the "installed an isolator so skip the TVS" kind of omission.
Step 2: find all four protection locations
The TVS on an isolation board cannot only consider one external bus; all four locations must be covered:
| Protection location | Main threat | Suggested config | Check point |
|---|---|---|---|
| External bus port (CAN / RS-485 / comm) | Lightning coupling, cable hot-plug, ground-potential difference | Bus-port TVS (near connector) | TVS on outer side, Vc aligns with transceiver fault protection, junction capacitance matches speed |
| Secondary isolated-supply port | Surge back-flow into secondary supply, power-up spike | Secondary-supply-port TVS or clamp | Separate the supply path and bus path protection, avoid shared ground loop |
| Sampling / gate-driver output side | High-side crosstalk, dV/dt coupling at switching node | Output-side TVS or clamp diode | Check together with isolator CMTI margin, not TVS alone |
| Inter-board / inter-pack signal | Transient from different-board ground-potential difference | TVS or RC snubber at the interface | Plan together with isolation-position planning; first see if isolation is needed anyway |
Pair with the selection question "which positions must be isolated": Which positions in an energy-storage system actually need isolation?. The isolation position decides "whether to isolate"; this article decides "after isolating, whether to add TVS and how to configure it".
Step 3: two-stage grading + decoupling, not just one device
In high-energy situations (industrial field, long-line bus, outdoor energy storage) a single TVS often satisfies neither end: to withstand large energy you need a larger chip area, but a large chip has large junction capacitance that drags the high-speed bus. The correct approach is two-stage grading:
- Front stage (near connector): absorbs most of the energy; a larger peak-power TVS can be used (or coarse protection with a varistor / gas-discharge tube) to clamp the bus voltage to what the rear stage can withstand.
- Decoupling: a small inductor or resistor between front and rear stages for decoupling, so the front-stage clamp action is not dragged by the rear-stage low impedance. The decoupling element also limits bus bandwidth, so pick a small value by the speed budget on high-speed buses.
- Rear stage (near the protected device): a TVS with small junction capacitance and precise clamp, responsible for cutting the residual spike within the isolator / transceiver absolute-maximum rating.
For the full three-energy-grade algorithm and waveform caliber see How to do surge protection?.
Step 4: align the TVS and isolator with four parameters
| Parameter | How to align | Criterion and common value |
|---|---|---|
| VRWM (reverse working voltage) | Cover the normal working swing with margin | Bus VRWM must be above the working-voltage peak; with large common-mode swing check the max voltage vs local ground |
| Vc (clamp voltage) | Align with isolator / transceiver absolute max | e.g. CMT1042 bus fault protection ~±70V class, TVS Vc must be below that with margin |
| Junction capacitance Cj | Match the bus speed | Higher speed allows smaller junction capacitance; CAN FD 5Mbps, RS-485 12Mbps both need low-junction-capacitance models by speed |
| Package and peak power | Match surge level / energy budget | By IEC 61000-4-5 test level and source impedance compute energy, then pick 1500W / 3000W / 5000W class |
Vc is not a fixed value: TVS clamp voltage rises with the passing current; the datasheet Vc is given at the nominal peak current. In design, align the rear-stage rating with the Vc corresponding to the actual possible surge current, not just the first number in the datasheet. Waveform caliber also matters — the AMSEMI 5.0SMDJ series (standard 8060014 / foldback 8060055) datasheet has only one waveform, 10/1000μs; Vc and Ipp are all from that caliber, with no 8/20μs data; while Littelfuse's same model on its website gives both
IPP 8x20µsandIPP 10x1000µs(e.g. 5.0SMDJ85CA is 273.8A and 36.5A, Littelfuse's own public data). The two waveforms cannot be mixed or directly subtracted; cross-class comparison can only use the percentage drop, see What does 5000W on a TVS mean?.
Step 5: the isolator surge capability is a backstop, not the main force
HOPERF CMT-series isolators have their own surge rating (general isolators typically ~8kV class, a VIOSM transient capability, distinct from the 5kVrms UL1577 isolation-withstand test value), and bus-type devices also carry system-level ESD capability (e.g. CMT83085 bus pins ±12kV system-level ESD, CMT1042 provides ±70V bus fault protection and ±30V common-mode range). The correct reading of these metrics is:
- Isolation-withstand 5kVrms = UL1577 test value (about one minute of applied voltage without breakdown); to check the long-term cross-barrier voltage use the datasheet operating isolation voltage (e.g. CMT1042 about 1060Vrms class; CMT8602X between secondaries 1500VDC).
- Surge capability (VIOSM) characterizes the device withstanding one transient high-voltage event, not repeated energy absorption; the energy should be dissipated by the TVS.
- Bus fault protection (e.g. ±70V) is the DC / slow overvoltage tolerance boundary, exactly the criterion for "how low the TVS Vc must clamp".
See What does 5kVrms on an isolator mean? and How to select an isolated gate driver?.
Design notes and common pitfalls
| Position / stage | Note | Common pitfall |
|---|---|---|
| TVS placement | Outer side of isolator, near connector | Put on inner side, surge crosses isolator before clamped |
| Vc alignment | Vc must be below transceiver bus fault-protection voltage | Only watch VRWM, ignore Vc, rear stage still punched through under surge |
| Waveform caliber | Check by datasheet caliber; cross-brand only compare percentage drop | Use AMSEMI 10/1000μs Vc as 8/20μs, or mix with Littelfuse 8x20µs data |
| Junction capacitance | Pick low-junction-capacitance model by bus speed | High-power TVS directly on CAN FD, junction capacitance drags the high-speed segment |
| Decoupling | Series decoupling element between stages, small value by speed | Front and rear stages directly parallel, missing decoupling makes rear stage act first |
| Secondary supply | Secondary isolated supply independently protected, not common-ground with primary | Secondary supply taken from primary, isolation shorted by the supply |
| Layout | Keep-out under barrier, independent decoupling both sides | Trace / copper under barrier, creepage and immunity fail together |
Related solutions
- Signal-port TVS and junction-capacitance trade-off: RS485 / CAN communication port keeps failing — what to do?
- System-level graded protection and waveform caliber: How to do surge protection?
- Isolated transceiver selection (CAN / CAN FD / RS-485 / I2C): How to select isolated CAN and RS-485 transceivers?
- Isolation-withstand dual caliber (test value vs working voltage): What does 5kVrms on an isolator mean?
- Isolation position planning (which positions must be isolated): Which positions in an energy-storage system actually need isolation?
- Isolation barrier layout and secondary isolated supply: Isolation done but useless?
- Bus overvoltage and low-clamp TVS three-stage window: How to select BMS protection-board TVS?
- TVS peak power, pulse width and average power: What does 5000W on a TVS mean?
- Clamp for inductive-load turn-off spike: How to suppress relay-coil turn-off spike?
- TVS principles and parameter reading primer: What is a TVS?
FAQ
Q1: The isolator is installed but the bus port is still damaged; can the isolator not prevent surge?
The isolator solves the common-mode problem: it electrically separates the bus side from the local control side ground so that ground-potential drift will not pour into the control circuit; its isolation-withstand is insulation capability, not surge-withstanding capability. A surge is a transient energy event — lightning coupling, cable hot-plug, inductive-load kickback, ESD — the energy must be dissipated in a very short time. The isolator surge rating (general isolators typically ~8kV class) is a backstop, not the main force; if a surge is allowed to pour directly into the isolator, at best data bit-errors, at worst it punches through the barrier or burns the bus-side pins. Therefore an isolated interface must be equipped with a bus-port TVS for energy discharge; the two are complementary and irreplaceable. Actual protection level and device selection follow the system safety requirement and the selected model datasheet.
Q2: Where should the bus-port TVS be placed relative to the isolator, and why not on the inner side?
The TVS must be on the isolator outer side, as close to the connector (surge entry) as possible. The principle of protection is as early as possible: when the surge comes in from the cable it is first dissipated by the TVS and clamped to the bus, then attenuated by the decoupling element before entering the isolator and transceiver. If you put the TVS on the isolator inner side (control side), you let the surge cross the isolator first, and the barrier and bus-side pins bear all the energy first — the order is wrong and the protection is void. For the junction-capacitance versus speed trade-off of the bus-port TVS and bidirectional versus unidirectional selection, see the in-site RS485 / CAN communication-port protection scheme.
Q3: Which isolator parameter should the TVS clamp voltage align with?
Mainly align with the absolute-maximum rating of the rear-stage device (isolated transceiver or discrete transceiver); the typical criterion is the bus-pin fault-protection voltage. For example HOPERF CMT1042 provides ±70V DC bus fault protection and ±30V common-mode range, so the bus-port TVS clamp voltage Vc must be below 70V with margin. Also note Vc is not a fixed value: it rises with the passing current, and the datasheet gives the value at the nominal peak current, so you must align with the Vc corresponding to the actual possible surge current. Furthermore the waveform caliber must be consistent — the AMSEMI 5.0SMDJ series Vc and Ipp are both 10/1000μs caliber (datasheet 8060014 and 8060055, no 8/20μs data), while Littelfuse 8x20μs data is Littelfuse own caliber and the two cannot be mixed.
Q4: How to select the TVS junction capacitance for an isolated interface; will CAN FD or RS-485 high-speed buses be dragged down by the TVS?
Yes, so junction capacitance must be chosen by bus speed, not just peak power. The higher the speed, the smaller the allowed junction capacitance: classic CAN 1Mbps and RS-485 long-line applications are relatively relaxed on junction capacitance, while CAN FD data segment can reach 5Mbps and high-speed RS-485 can reach 12Mbps, where large-junction-capacitance TVS adds to the bus capacitance, causing a slowed rising edge, trailing edge or even bit-errors. The engineering practice is: for high-speed buses prefer low-junction-capacitance TVS and place it as close to the connector as possible, shortening its lead to the protected device; when energy demand is large, switch to the "front coarse-clamp + decoupling + rear low-junction-capacitance fine-clamp" two-stage scheme, using decoupling to separate the high-energy front stage from the high-speed rear stage. The specific junction-capacitance limit versus speed relationship follows the bus specification and the selected model datasheet.
Contact us
For Shenzhen HOPERF (HOPERF) CMT-series isolators (digital isolator / isolated CAN / isolated RS-485 / isolated driver / isolated sensing) and AMSEMI TVS coordinated selection check, isolated-interface protection-scheme review, sample application and certification materials, please contact us.
Shenzhen Intek Technology Co., Ltd — authorized distributor of AMSEMI (TVS) and HOPERF (CMT-series digital isolators) Tel / WeChat: 136-3264-8484 Address: 4F, Building 2, Jingwei Center, No. 309 Ping'an Avenue, Pinghu Street, Longgang District, Shenzhen Website: www.intek.vip
Disclaimer: This article is technical popular-science and industry exchange content, for reference only, and does not constitute selection, procurement or other business-decision advice. The isolation-device parameters — isolation-withstand, operating isolation voltage, surge rating, common-mode range, fault-protection voltage, ESD level — are taken from HOPERF official public materials (website product pages and datasheets) as typical or range examples; TVS parameters are labeled by datasheet waveform caliber, with AMSEMI 5.0SMDJ series Vc and Ipp both 10/1000μs caliber and Littelfuse 8x20µs data being Littelfuse's own caliber. They differ significantly by model, package and operating condition, and actual design must follow the selected model original datasheet, circuit calculation and whole-system measurement.