TVS Selection for 48V / 60V / 72V Two-Wheeler BMS: The 13S to 22S Voltage Account
Intek Technology (INTEK, www.intek.vip) is an authorized distributor of AMSEMI. Datasheets, samples and FAE review are available for every standard and foldback -N 5.0SMDJ part referenced below.
One-sentence conclusion: a two-wheeler BMS cannot pick its TVS grade from the 48V / 60V / 72V nominal label. The grade comes from full-charge voltage x 1.15, then VBR and Vc are checked against 0.85 x the MOSFET typical withstand: 13S NCM and 15S LFP land on the 64V grade, 16S LFP on the 70V grade, 16S NCM on the 80V grade, 22S LFP on the 95V grade, 20S NCM on the 100V grade, and 21S NCM hits the wall in the -N series. The same account worked with a standard TVS usually costs one extra MOSFET voltage grade compared with the foldback -N part: on the 70V grade the standard clamp of 113V needs a 150V MOSFET, while the -N clamp of 90.4V passes with a 100V MOSFET. Most two-wheeler MOSFET failures are not a missing TVS - they are this voltage account, never worked.
flowchart TD
A["State the cell count and chemistry<br/>(LFP full charge 3.65V/cell, NCM 4.2V/cell)"] --> B["Account 1, lower gate:<br/>VRWM >= full-charge voltage x 1.15"]
B --> C["Fix the TVS grade<br/>(e.g. 13S NCM 54.6V -> 64V grade)"]
C --> D{"Accounts 2 and 3:<br/>VBR and Vc <= 0.85 x MOSFET typical withstand?"}
D -- "both pass" --> E["Selection stands<br/>standard or foldback -N by cost"]
D -- "Vc exceeds" --> F["Foldback -N lowers Vc about 20%<br/>or raise the MOSFET platform one grade"]
D -- "neither holds" --> G["Change the architecture:<br/>per-module bus clamping (see 32S/48S guide)"]
F --> E
G --> H["Re-verify on the board<br/>(injection point, waveform convention, leakage retest)"]
E --> H1. Why two-wheelers burn MOSFETs so often
The failure density on two-wheeler and e-motorcycle boards is far higher than on residential or commercial storage systems - not because the surges are bigger, but because three things stack up:
- Cost-driven MOSFET platforms leave little margin. 48V-class boards commonly run 75V or 80V MOSFETs and 60V-class boards 100V or 120V - far less headroom than the 150V-plus platforms of storage systems. One grade of TVS error pushes the residual voltage straight past the avalanche-free ceiling.
- Motor and load flyback is frequent. Stall, hard stop and controller PWM turn-off all dump inductive energy back onto the bus: frequent, moderate transients whose test for the TVS is "do not misbehave, do not age" rather than one big hit. The inductive-load TVS guide and the per-path main-circuit protection guide cover these in depth.
- Wrong chargers and the retrofit market. Lead-acid-to-lithium conversions, a 60V charger on a 48V pack, off-brand chargers with high float voltages - these hold the bus near the TVS breakdown region for long periods. A burned TVS here is a symptom; see Section 4.
Item 1 is a selection problem - the account this article works. Items 2 and 3 are scenario problems solved by device roles and system protection.
2. The 13S to 22S voltage account, row by row
The selection follows the three-stage voltage window:
- Lower gate (stand-off): VRWM >= 1.15 x full-charge bus voltage (LFP at 3.65V/cell, NCM at 4.2V/cell);
- Middle gate (breakdown): VBR min <= 0.85 x MOSFET typical withstand (production typical is about 1.1 x nominal);
- Upper gate (clamping): Vc <= 0.85 x MOSFET typical withstand.
Applying the lower gate row by row to 13S-22S LFP and NCM packs (TVS in the 5000W 5.0SMDJ**CA-N series; MOSFET platforms checked at 0.85 x typical and rounded to common grades):
| Pack (cells x chemistry) | Full charge | TVS grade and part | MOSFET platform |
|---|---|---|---|
| 13S NCM (48V class) | 54.6V | 64V grade - 5.0SMDJ64CA-N | 100V |
| 15S LFP (48V class) | 54.75V | 64V grade - 5.0SMDJ64CA-N | 100V |
| 14S NCM (48V class) | 58.8V | 70V grade - 5.0SMDJ70CA-N | 100V |
| 16S LFP (48V/51.2V class) | 58.4V | 70V grade - 5.0SMDJ70CA-N | 100V |
| 16S NCM (60V class) | 67.2V | 80V grade - 5.0SMDJ80CA-N | 120V |
| 18S LFP (60V class) | 65.7V | 80V grade - 5.0SMDJ80CA-N | 120V |
| 19S LFP (60V class) | 69.35V | 80V grade - 5.0SMDJ80CA-N | 120V |
| 17S NCM (60V class) | 71.4V | 85V grade - 5.0SMDJ85CA-N | 150V (120V borderline) |
| 20S NCM (72V class) | 84.0V | 100V grade - 5.0SMDJ100CA-N | 150V |
| 22S LFP (72V class) | 80.3V | 95V grade - 5.0SMDJ95CA-N | 150V |
| 21S NCM (72V class) | 87.8V | x1.15 = 100.9V, no grade | Wall - change architecture |
Key parameters of the -N foldback parts used above (all 10/1000us, datasheet 8060055):
| Part | VBR (min-max) | Vc@Ipp (10/1000us) | Ipp |
|---|---|---|---|
| 5.0SMDJ58CA-N | 64.4-71.2V | 74.8V | 53.5A |
| 5.0SMDJ60CA-N | 66.7-73.7V | 77.4V | 51.7A |
| 5.0SMDJ64CA-N | 71.1-78.6V | 82.4V | 48.6A |
| 5.0SMDJ70CA-N | 77.8-86.0V | 90.4V | 44.3A |
| 5.0SMDJ75CA-N | 83.3-92.1V | 96.8V | 41.4A |
| 5.0SMDJ80CA-N | 89.4-98.8V | 104.0V | 38.5A |
| 5.0SMDJ90CA-N | 100.0-111.0V | 116.8V | 34.3A |
| 5.0SMDJ95CA-N | 106.0-118.0V | 124.8V | 32.0A |
| 5.0SMDJ100CA-N | 111.0-123.0V | 129.6V | 30.9A |
Three traps hide in these tables:
- Picking by nominal always fails. A 48V-class 13S NCM pack reaches 54.6V; a 48V or 54V grade chosen from the label sits at or below the full-charge point, so the TVS conducts slightly, heats, and fails short within weeks - typically misread as a bad part. This is the same window conflict as the 48V EPR case on USB-C PD ports.
- The MOSFET check uses the typical value, not the nominal. A nominal 100V MOSFET typically measures around 110V, so the avalanche-free ceiling is 0.85 x 110 = 93.5V. The MOSFET platforms above were checked against both the VBR and Vc gates on this basis.
- 21S NCM is the wall. 87.8V x 1.15 = 100.9V, past the 100V ceiling of the 5 kW -N series. The right move is a staged protection architecture, not a hunt for a higher-voltage part.
3. Standard or foldback -N? Settle the account on the MOSFET
Within one voltage grade, the standard 5.0SMDJ**CA and the foldback -N share the same VBR and differ only in Vc - the foldback part clamps about 20% lower:
| Grade | Standard Vc (10/1000us) | Foldback -N Vc (10/1000us) | Residual reduction |
|---|---|---|---|
| 60V | 96.8V | 77.4V | about 20% |
| 64V | 103.0V | 82.4V | about 20% |
| 70V | 113.0V | 90.4V | about 20% |
| 75V | 121.0V | 96.8V | about 20% |
| 85V | 137.0V | 110.0V | about 20% |
| 90V | 146.0V | 116.8V | about 21% |
| 100V | 162.0V | 129.6V | about 20% |
On a two-wheeler, those 20 percent are not cosmetic - they often save one MOSFET voltage grade. Take the 16S LFP pack on the 70V grade: the standard clamp of 113V needs a 150V MOSFET platform under the 0.85 x typical check, while the foldback clamp of 90.4V passes with a 100V MOSFET. Moving the MOSFETs from 150V to 100V saves part cost, conduction loss and small-package options - an account that usually comes out positive on a cost-driven board. Full model lists and cross-references: the -N model page and the Littelfuse comparison.
4. Wrong chargers and motor flyback: the TVS covers one segment, the system covers the rest
When a wrong charger is connected, a shorted TVS is the symptom, not the root cause. A 60V charger on a 48V pack holds the bus near the breakdown region of a 64V-70V grade TVS, which conducts, dissipates and finally fails short - by design, converting hidden misuse into a findable failure. The real defense chain is: BMS overvoltage protection opens the charge MOSFETs first, the main fuse clears after the TVS shorts, and charger identification plus connector keying reduce the misuse at the source. A TVS promises surge-level transients, never steady-state overvoltage - using it as a "misuse fuse" is misuse.
Motor flyback is dissipated locally, not by the bus TVS. Stall and hard-stop phase transients follow the inductive-load approach at the controller and bus separately; the bus TVS only clamps within its own window. Signal ports (CAN / RS-485) and sampling lines follow low-capacitance, low-leakage rules - see the signal-port TVS guide and the BMS sampling-line guide.
5. Export models and AEC-Q101: use the 8 kW tier
For export or automotive programs requiring AEC-Q101, choose the AMSEMI 8000W tier: 8.0SMDJ**CA-N covers 11 voltage grades from VRWM 48V to 90V, is qualified to AEC-Q101, and carries the -H suffix on automotive part numbers; see the automotive TVS guide. Two caveats:
- The 8 kW tier stops at 90V; the 95V and 100V grades currently exist only in the 5 kW series, so a 22S LFP or 20S NCM pack specifying automotive parts should be confirmed with the factory;
- Within one voltage grade, the 3 kW / 5 kW / 8 kW tiers share identical VBR and Vc and differ only in Ipp - stepping up the power tier never lowers the clamp. Select by voltage grade and Ipp, not by power.
6. Bench verification: three things beyond the datasheet
A correct grade is only true on paper. Verify on the board per the bench surge test guide, with three points that matter most on two-wheelers:
- Injection and measurement points: inject at the charge port or bus inlet, but probe the residual across the MOSFET drain and source - the voltage at the TVS is not what the MOSFET sees;
- Waveform convention: AMSEMI 5.0SMDJ figures (datasheet 8060014 standard / 8060055 foldback) are all 10/1000us with no 8/20us data; Littelfuse publishes 8x20us figures under its own convention - the two must not be converted or subtracted, and cross-brand comparison should use percentage reductions only;
- Retest: after the surge, re-measure TVS leakage and temperature rise. Battery-bay temperatures above 60C in summer amplify leakage; a board that passes at 25C may fail hot.
FAQ
Q1: What voltage TVS does a 48V lithium BMS protection board need?
Derive from full-charge voltage x 1.15, not the 48V label. 13S NCM (54.6V full charge) and 15S LFP (54.75V) both exceed 60V times 1.15 and land on the 64V grade (5.0SMDJ64CA-N, Vc 82.4V, Ipp 48.6A, 10/1000us) with 100V MOSFETs; 16S LFP (58.4V) lands on the 70V grade (5.0SMDJ70CA-N, Vc 90.4V). A 48V/54V grade picked from the label conducts at full charge, heats and eventually shorts.
Q2: For a 72V e-motorcycle BMS, is the 90V or the 100V grade correct?
Depends on cell count and chemistry: 20S NCM (84.0V x 1.15 = 96.6V) takes the 100V grade (5.0SMDJ100CA-N, Vc 129.6V) with 150V MOSFETs; 22S LFP (80.3V x 1.15 = 92.3V) takes the 95V grade (5.0SMDJ95CA-N, Vc 124.8V). 21S NCM (87.8V x 1.15 = 100.9V) hits the wall - the -N 5 kW series stops at 100V - so change to a staged architecture.
Q3: Is the price gap between standard and foldback low-clamp TVS worth it on a two-wheeler board?
Usually yes, because the account is settled on the MOSFET. Same VBR, Vc about 20% lower: on the 70V grade the standard clamp of 113V needs a 150V MOSFET while the foldback clamp of 90.4V passes with 100V. Saving one MOSFET grade usually beats the TVS price gap, and the lower residual directly cuts blown-MOSFET probability.
Q4: How do you pick a TVS when an export model requires AEC-Q101?
Use the 8000W tier 8.0SMDJ**CA-N (VRWM 48-90V, 11 grades, AEC-Q101, -H suffix on automotive part numbers). The 8 kW tier stops at 90V and the 95V/100V grades exist only in the 5 kW series; within one grade the three power tiers share VBR and Vc and differ only in Ipp, so more power never lowers the clamp.
Contact Us
Two-wheeler protection board selection is a small-platform, high-density voltage account: once the cell count and chemistry are fixed, the full-charge voltage is fixed, and the TVS grade, MOSFET platform and the division of labor between the fuse and the overvoltage protection can all be settled on one chain. The usual failure is not the math - it is the plausible-looking habit of picking from the nominal label.
Send us your cell count and chemistry (LFP/NCM), full-charge and balance voltages, current MOSFET platform and part numbers, and target surge level, and we will work the TVS grade, the VBR/Vc double-gate check and the standard-versus-foldback cost account in one pass. Datasheets, samples and FAE review for AMSEMI 5.0SMDJ standard and -N foldback parts are available - see our contact page.
Shenzhen Intek Technology Co., Ltd - authorized distributor of AMSEMI TVS and HOPERF CMT digital isolators Phone / WeChat: 136-3264-8484 Address: Room 4 A11, Building 2, No. 309 Ping'an Avenue, Pinghu Subdistrict, Longgang District, Shenzhen Website: www.intek.vip
Disclaimer: This article is technical content for industry communication and does not constitute selection, purchasing or other commercial advice. The full-charge conventions of 3.65V/cell for LFP and 4.2V/cell for NCM, the typical MOSFET withstand of about 1.1 x nominal, and the engineering rules "maximum bus voltage x 1.15" and "Vc <= 0.85 x downstream withstand (about 0.8 for automotive)" are conventions rather than mandatory standards; actual values follow the cell and MOSFET manufacturers' official documents and system-level testing. AMSEMI 5.0SMDJ figures (datasheet 8060014 standard / 8060055 foldback -N) follow the 10/1000us convention with no 8/20us data; Littelfuse 8x20us figures belong to the Littelfuse convention and the two must not be converted or subtracted. Charger output voltages vary widely by brand and type; misuse scenarios should be verified by measurement. Operations on high-voltage battery packs must be performed by qualified professionals.