TVS Diode VRWM, VBR, and VC: Parameter Relationship Guide

Three voltage parameters appear on every TVS diode datasheet — VRWM, VBR, and VC — and confusing their relationship is the single most common cause of TVS selection errors. VRWM defines when the device should remain inactive, VBR defines where conduction begins, and VC defines what voltage actually reaches the protected circuit during a transient. Selecting a TVS based on VRWM alone, without verifying VC against the protected component's absolute maximum rating, is a design error that passes initial inspection but fails in the field or during surge testing.

This guide explains each parameter, their mathematical relationship, and provides a complete selection calculation.

What Do VRWM, VBR, and VC Mean?

VRWM (Reverse Working Voltage): The maximum voltage the TVS can sustain continuously without entering significant conduction. This is the "safe zone ceiling" — normal circuit operation must always stay below this value.

VBR (Breakdown Voltage): The voltage at which the TVS begins avalanche conduction, typically specified as a range (VBR min to VBR max) measured at a small test current (often 1 mA). VBR is always somewhat higher than VRWM by design.

VC (Clamping Voltage): The voltage measured across the TVS terminals when a specified high pulse current (matching the IPP rating) flows through the device. VC is always higher than VBR because the diode's dynamic resistance, multiplied by the high current, adds additional voltage drop.

The relationship:

VRWM < VBR < VC Example datasheet values for a typical 15V TVS: VRWM = 15V VBR(min) = 16.7V, VBR(max) = 18.5V VC(max) @ IPP = 24.4V ## How These Parameters Map to the TVS V-I Curve

Voltage │ │ ________ VC (at rated IPP) │ / │ / │_________________/ │ VBR (breakdown point) │ │ VRWM (normal operating region, high impedance) │_____________________________→ Current 0 Below VRWM: the TVS presents high impedance (normal operation, essentially no current flows). At VBR: the TVS transitions into avalanche conduction. At VC: under the specified pulse current, the TVS voltage has risen to this clamped level — this is the maximum voltage the protected circuit will experience during the transient.

The Critical Selection Rule: VC Must Be Checked Against the Protected IC's Absolute Maximum Rating

The selection criterion that matters most for circuit protection is:

VC(max) ≤ Absolute Maximum Voltage Rating of the protected IC A common mistake is verifying only that VRWM exceeds the normal operating voltage and stopping there. This confirms the TVS won't misfire during normal operation, but says nothing about whether the protected circuit survives an actual transient event.

Step-by-Step VRWM and VC Selection Calculation

Scenario: A 12V system with normal operating voltage up to 14.4V (including ripple), protecting an MCU with an absolute maximum VDD rating of 20V, and an expected maximum surge current of 10A (8/20 µs waveform).

Step 1 — Select VRWM:

VRWM ≥ 14.4V × 1.1 = 15.84V → Select VRWM = 15V (e.g., a part in the SMA04J15V family) Step 2 — Check VC at the expected current:

Datasheet value: VC(max) at the rated test current ≈ 24.4V Step 3 — Compare against the protected IC rating:

VC (24.4V) > Absolute Max Rating (20V) ✗ FAILS Conclusion: This selection does not work. The TVS clamps the transient, but the clamped voltage (24.4V) still exceeds what the MCU can survive (20V absolute maximum). The design needs either: a lower-clamping-voltage TVS family, an additional front-end MOV/GDT stage to reduce the current reaching this TVS (and thus reduce its VC), or confirmation with the MCU vendor of a higher-voltage-tolerant part.

This example illustrates the central point of this guide: satisfying VRWM does not mean the selection is complete — VC is the final test.

Why a Higher VRWM Is Not Automatically Safer

A common but incorrect instinct is to select a VRWM far above the actual operating voltage "to be safe." This backfires because VBR and VC scale upward together with VRWM — a higher VRWM TVS has a correspondingly higher VC, which can push the clamped voltage above the protected circuit's tolerance.

Better practice: Select VRWM close to the actual maximum operating voltage (10–20% margin), keeping VC as low as possible while still avoiding false triggering during normal operation.

How VC Changes with Pulse Current

VC is not a single fixed number — it rises as the current through the TVS increases. Datasheets typically specify VC(max) at the rated IPP test condition, but quality datasheets also provide a VC-vs-current curve.

If your application's expected surge current differs significantly from the datasheet's standard test condition, request the full VC-vs-current curve from the manufacturer rather than relying on the single-point VC(max) value, especially when working close to the protected circuit's voltage tolerance limit.

How Power Package Affects VC at High Current

For TVS parts with the same VRWM but different power packages (e.g., SMA 400W vs. SMB 600W vs. SMC 1,500W), VC at low test currents is nearly identical across packages. However, at higher surge currents, the larger-package part typically shows a smaller VC rise (larger junction area, lower dynamic resistance).

Practical implication: If your application involves large surge currents, selecting a higher power-rated package — even with the same VRWM — can yield a lower actual VC and better protection margin.

Frequently Asked Questions

Q: The protected IC's datasheet only lists a "recommended operating voltage range" without an explicit absolute maximum rating. How should VC be evaluated? A: Contact the IC manufacturer to confirm the absolute maximum rating. As a rough industry rule of thumb, digital ICs' absolute maximum rating is often 20–50% above the recommended operating voltage (e.g., a 3.3V part might have a 4.0–5.0V absolute max), but this is only an estimate — verify with the manufacturer before finalizing a design, especially before high-volume production.

Q: How much does VC vary across different current levels, and how do I obtain a complete curve? A: Reputable TVS suppliers provide a VC-vs-IPP characteristic curve, not just a single-point value. If the datasheet only shows one VC value, request the manufacturer's full curve, or ask for VC data at 2–3 different current points to properly evaluate protection at different surge levels.

Q: Does ASIM provide complete VC-vs-current curve data for its TVS products? A: ASIM's TVS datasheets provide VC(max) under standard test conditions and can provide supplementary data for specific customer application requirements. Contact ASIM engineering at +86-400-014-4913 for detailed curve data needed for precision selection calculations.

About ASIM Electronics: ASIM (阿赛姆电子) is a Shenzhen-based manufacturer of TVS protection components, founded in 2013. TVS product line covers 1,200+ models across VRWM from 3.3V to 500V, with full VRWM/VBR/VC(max) parameters documented for precision engineering selection. Contact: +86-400-014-4913 | asim@asim.com.cn | Published: June 2026