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TVS & Surge Clamp

Protection & EMC

Does this TVS survive the surge and keep the pin below its absolute maximum?

Inputs

Surge: open-circuit peak, 500 V to 4 kV. ESD: contact charge voltage, 2 kV to 8 kV (type 8k).
V
IEC 61000-4-5: 2 Ω power line to line, 12 Ω power line to ground, 42 Ω all other lines.
Ω
Highest steady voltage on the line, with tolerance and ripple.
V
Protected part's datasheet, absolute maximum ratings, this pin.
V
TVS datasheet, VRWM (VR).
V
TVS datasheet, VBR max at IT.
V
TVS datasheet, VC max at IPP.
V
TVS datasheet, IPP on the pulse the datasheet states.
A
Series resistor and derating
A resistor or PTC between the source and the TVS. It must be rated for the pulse.
Ω
The part's peak-pulse derating curve read at your ambient (often 100 % up to 25 °C).
%

Results

Clamp voltage at the pin VC'
21.23V
Pass

Checked: VC' ≤ 0.9 × Vabs (design margin), failing above Vabs: 0.708 × your 30 V pin, with Rser 0 Ω
Show working
  1. Dynamic resistanceR_dyn = (V_C − V_BR,max) / I_PP = (24.4 − 18.5) / 24.6 = 239.8 mΩ
  2. Surge currentI = (V_s − V_BR,max) / (R_src + R_ser + R_dyn) = (500 − 18.5) / (42 + 0 + 0.2398) = 11.4 A
  3. Clamp voltage at the pinV_C' = V_BR,max + I × R_dyn = 18.5 + 11.4 × 0.2398 = 21.23 V
  4. Current against the derated ratingI / (k_T × I_PP) = 11.4 / (1 × 24.6) = 11.4 / 24.6 = 0.463
Surge current I
11.4A
Stand-off margin VRWM/Vline
1.087:1
Series R for 0.9·Vabs Rser

VC' is at or below 27 V, 0.9 × Vabs, with no series resistance
0Ω
All results (3)
Dynamic resistance Rdyn
239.8mΩ
Peak power in the TVS P

at the peak of the surge
242W
Derated IPP kT·IPP
24.6A
TVS clamp linesurge source

Accuracy

Source
  • STMicroelectronics AN4275
  • IEC 61000-4-5
  • Texas Instruments SLVAE37
  • IEC 61000-4-2

STMicroelectronics AN4275, IEC 61000-4-5 standard overview: the combination wave and its 2 Ω, 12 Ω and 42 Ω coupling paths the equation

IEC 61000-4-5, surge immunity test: the surge current shapes on the 12 Ω and 42 Ω coupling paths the equation

Texas Instruments SLVAE37, How to select a surge diode: match the rating pulse to the test pulse, or read the pulse-rating curve the equation

IEC 61000-4-2, electrostatic discharge immunity test: the contact-discharge current, 3.75 A/kV at the first peak and 2 A/kV at 30 ns, as summarised by the ESD Association the equation

Checked against: The datasheet of your TVS: maximum breakdown and clamping voltages; peak pulse current and its rating pulse; the peak-pulse derating curve

Precision
Exact arithmetic on a straight clamp line through the two datasheet maxima, fed from the IEC 61000-4-5 generator taken as its open-circuit voltage behind its source resistance.

A real clamp curve is not a straight line, and a real part usually sits below its datasheet maxima: its clamp is lower and its current slightly higher than shown. The maximum breakdown voltage sets most of the clamp at low surge currents, and the derating read off the part's own curve sets the current check. In ESD mode the clamp is an estimate only.

Most of the uncertainty comes from Breakdown voltage, max. Tighten that first.

Verified against

2 independent cases. See the working.

This is a design aid. The engineer remains responsible for the design and for checking the standard itself.

Parameter sweep

Vary one input over a range and see the answer and verdict at each step, as a table and a curve.Pro

Worst-case corners

Put a tolerance on each input and get the worst-case band around the answer.Pro

Parts that track move together.Inputs in the same group move together, like the parts of a matched network. Leave independent parts ungrouped: that is the safe choice.

Circuit

The principle

A TVS diode stays off below its breakdown voltage. Past it, it conducts and holds the line near its clamping voltage while the surge current flows through it to ground. This page models the surge generator as its open-circuit voltage behind its source resistance (IEC 61000-4-5), and the TVS as a straight clamp line through its two datasheet maxima. From them it finds the surge current and the voltage left at the protected pin, and checks the pin against its absolute maximum, the current against the TVS's derated pulse rating, and the stand-off against the line voltage.

Clamp line through the datasheet maxima
V=VBR,max+I RdynRdyn=VC−VBR,maxIPPV = V_{BR,max} + I\, R_{dyn} \qquad R_{dyn} = \frac{V_C - V_{BR,max}}{I_{PP}}
A straight line through VBR,max at about zero current and VC max at IPP, a design choice. A line through VBR,min would understate the clamp below IPP. Above IPP the line is extrapolated.
Surge current and the clamp at the pin
I=max⁡(0,  Vs−VBR,maxRsrc+Rser+Rdyn)VC′=VBR,max+I RdynI = \max\left(0,\; \frac{V_s - V_{BR,max}}{R_{src} + R_{ser} + R_{dyn}}\right) \qquad V_C' = V_{BR,max} + I\, R_{dyn}
The generator is Vs behind Rsrc: 2 Ω power line to line, 12 Ω power line to ground, 42 Ω all other lines (IEC 61000-4-5, as summarised in ST AN4275). With I = 0 the TVS does not conduct, and VC' = Vs.
Pulse rating at temperature
IPP(TA)=kT IPPIkT IPP≤0.8I_{PP}(T_A) = k_T\, I_{PP} \qquad \frac{I}{k_T\, I_{PP}} \leq 0.8
kT is read off the part's peak-pulse derating curve, which derates PPP and IPP together against the initial junction temperature. The page takes TA as that temperature, as datasheets of this kind state (TJ initial = TA). 0.8 is this page's design margin.
ESD contact-discharge current
Ipeak=3.75  A/kV×VESDI30=2  A/kV×VESDI_{peak} = 3.75\; \t{A/kV} \times V_{ESD} \qquad I_{30} = 2\; \t{A/kV} \times V_{ESD}
IEC 61000-4-2: the contact-discharge current at its first peak and at 30 ns. VC' = VBR,max + Ipeak·Rdyn is an estimate only, because the clamp line comes from a microsecond pulse.
Series resistance for a target clamp
Rser=(Vs−VBR,max) RdynVt−VBR,max−Rsrc−RdynVt=0.9 VabsR_{ser} = \frac{(V_s - V_{BR,max})\, R_{dyn}}{V_t - V_{BR,max}} - R_{src} - R_{dyn} \qquad V_t = 0.9\, V_{abs}
The algebra of the lines above, floored at zero. No series resistance reaches Vt when Vt is not above VBR,max.
  • VBR,maxV_{BR,max}breakdown voltage, the datasheet maximum at its test current IT
  • VCV_Cclamping voltage, the datasheet maximum at IPP
  • IPPI_{PP}peak pulse current, on the datasheet's rating pulse
  • RdynR_{dyn}dynamic resistance, the slope of the clamp line
  • IIsurge current through the TVS
  • VC′V_C'voltage at the protected pin
  • VsV_ssurge open-circuit peak voltage
  • RsrcR_{src}generator source resistance, by coupling path
  • RserR_{ser}added series resistance between the source and the TVS
  • kTk_Tpulse rating at TA, as a fraction of IPP
  • VESDV_{ESD}ESD contact charge voltage
  • Ipeak, I30I_{peak},\,I_{30}ESD contact current at its first peak and at 30 ns
  • VtV_ttarget clamp, 0.9 × Vabs
  • VabsV_{abs}absolute maximum voltage of the protected pin
  • VRWM, VlineV_{RWM},\,V_{line}TVS stand-off voltage; highest steady line voltage
More detail

Stand-off first

In normal use the TVS must stay off. Its stand-off voltage VRWM, where the datasheet specifies the leakage IR, sits above the highest steady line voltage. 1.1 × Vline is this page's design margin.

Then the clamp and the current

On a low-voltage line there is little room between VBR,max and the pin's absolute maximum. The source resistance sets the current: 42 Ω on a signal line passes far less than 2 Ω across a power line. Added series resistance, a resistor or a PTC, lowers both the current and the clamp. It must be rated for the pulse.

The rating pulse

A datasheet states IPP on one pulse shape, often 10/1000 µs. The IEC 61000-4-5 surge is far shorter, and the part's pulse-rating curve allows a higher peak on a shorter pulse, so checking it against the 10/1000 µs rating is conservative.

Not modelled

The clamp's real curve between the two datasheet points, the change of VBR with temperature (the datasheet's temperature coefficient, where it gives one), the junction heating during the pulse, repeated pulses, the line's inductance and the protected pin's own clamp structure. For ESD the page gives the IEC 61000-4-2 currents and an estimate only.

Judge the clamp at the pin against its absolute maximum, and the stand-off at the highest line voltage.
References: IEC 61000-4-5, surge immunity: the combination wave, 1.2/50 µs open circuit and 8/20 µs short circuit, the 2 Ω, 12 Ω and 42 Ω coupling paths, and the surge current shapes on the 12 Ω and 42 Ω paths. STMicroelectronics AN4275, IEC 61000-4-5 standard overview, summarises it. Texas Instruments SLVAE37, How to select a surge diode. IEC 61000-4-2, electrostatic discharge immunity: the contact-discharge current, 3.75 A/kV at the first peak and 2 A/kV at 30 ns, as summarised by the ESD Association. The TVS maker's datasheet: VBR max, VC at IPP, the rating pulse and the derating curve. Consult the standards and the datasheet themselves for anything that matters. Related: Fuse Sizing, Inrush & I²t.

Engine version ⁨1.26.4⁩