RFrftools.io
EMC

ESD Clamp Diode Selection

Calculate ESD clamp diode peak current, power dissipation, and clamping ratio to verify protection for IEC 61000-4-2 compliance.

Loading calculator...

Formula

I_pk = (V_ESD − V_cl) / Z, P_pk = V_cl × I_pk

V_cl— Clamp voltage (V)
Z— Discharge path impedance (Ω)

How It Works

ESD (electrostatic discharge) clamp diodes protect sensitive ICs from voltage transients exceeding their maximum ratings. In a typical ESD event (IEC 61000-4-2 Level 4: ±8 kV contact), the discharge current flows through the series path impedance Z (typically 330 Ω for the HBM or contact-discharge model). The peak clamp current is I_pk = (V_ESD − V_cl) / Z, where V_cl is the clamping voltage of the TVS diode at peak current. The peak power dissipated in the clamp is P_pk = V_cl × I_pk. For the IC to survive, the clamped voltage (V_cl) must remain below the IC's absolute maximum rating. The clamping ratio V_cl/V_ESD indicates how efficiently the device clamps. A low ratio (< 0.01) means effective clamping.

Worked Example

Problem
An IEC 61000-4-2 Level 2 contact ESD event (V_ESD = 4 kV) strikes a USB line protected by a TVS clamp with V_cl = 15 V and 330 Ω series impedance. Calculate peak current, peak power, and clamping ratio.
Solution
1. Peak current: I_pk = (4000 − 15) / 330 = 3985 / 330 = 12.1 A 2. Peak power: P_pk = 15 × 12.1 = 181 W 3. Clamping ratio: V_cl/V_ESD = 15/4000 = 0.00375 Result: The TVS clamp dissipates 181 W peak (but only for ~1 ns), clamping the line to 15 V. Verify the IC tolerates 15 V on the protected pin. The low clamping ratio (0.37%) confirms effective suppression.

Practical Tips

  • ✓For IEC 61000-4-2 Level 4 (±8 kV), select a TVS with a peak pulse current rating ≥ 15 A (at the relevant pulse width) with adequate margin.
  • ✓Use multi-channel TVS arrays for interfaces with many pins (USB, HDMI, Ethernet) to save board space and reduce routing inductance.
  • ✓Add a small series resistor (10–47 Ω) between the connector and TVS to reduce dV/dt stress on the TVS and help meet ESD limits.

Common Mistakes

  • ✗Selecting a TVS diode based on standoff voltage alone without checking the clamping voltage at peak current — V_cl is always higher than the standoff voltage.
  • ✗Ignoring the capacitance of the TVS diode on high-speed lines — a low-capacitance TVS (< 0.5 pF) is required for USB 3.0 or HDMI.
  • ✗Placing the ESD clamp far from the connector — every mm of trace between the connector and clamp allows the ESD spike to propagate. Place clamps within 2 mm of the connector.

Frequently Asked Questions

ESD TVS clamps are optimised for very fast rise times (< 1 ns) and high peak currents (1–30 A) in very short pulses (< 100 ns). Surge TVS handles longer pulses (milliseconds) with higher energy. Both are bidirectional TVS diodes but sized for different threat profiles.
IEC 61000-4-2 (contact discharge) uses a 330 Ω / 150 pF model with a faster rise time (~1 ns) and higher peak current than the Human Body Model (HBM, 1500 Ω / 100 pF). IEC 61000-4-2 is harsher and is used for system-level testing, while HBM is used for component-level testing.
Not recommended. Zener diodes are not optimised for fast transients — they have slow response times (typically > 10 ns) compared to TVS diodes (< 1 ns). For ESD protection use a TVS rated for the specific ESD model and energy level.

Shop Components

Affiliate links — we may earn a commission at no cost to you.

EMC Shielding Tape

Copper foil tape for EMI shielding and grounding

DigiKey →Mouser →

Ferrite Beads

SMD ferrite beads for suppressing high-frequency noise

DigiKey →Mouser →

Related Calculators

EMC

ESD TVS Diode

Calculate TVS diode clamping voltage, breakdown voltage, peak pulse current, and power rating for ESD protection circuit design.

EMC

RF Shielding

Calculate electromagnetic shielding effectiveness of conductive enclosures

EMC

EMI Margin

Calculate EMI compliance margin accounting for measurement uncertainty and safety margin to predict pre-compliance test pass/fail.

EMC

Ferrite Bead

Calculate ferrite bead filter effectiveness, impedance at frequency, and insertion loss for EMI suppression

EMC

LC EMI Filter

Design an LC low-pass EMI filter for conducted emissions suppression — calculate inductance, capacitance, filter order, and attenuation at the stop band.

EMC

CMC Impedance

Calculate common mode choke impedance, insertion loss, and Q factor at a given frequency for EMC filter design.