Transient EM/Circuit Co-Simulation in XFdtd®:
A Closer Look at TVS Diodes for ESD Protection
80.0

15 kV Human Body Model (HBM) Waveform

70.0
60.0
Current (A)Transient voltage suppression (TVS) diodes
help protect electronic circuits from surge
voltages associated with an electrostatic
discharge (ESD) event and are commonlyplaced alongside I/O lines, audio hardware,human interface devices, power supplies,
and an antenna’s RF front end (RFFE).

50.0
40.0

30.0
TVS diode simulation presents certain
challenges. Full-wave time-domain
20.0
electromagnetic (EM) solvers can analyze
10.0
a device’s 3-D CAD geometry, but the0.0
circuit component models are limited.
0102030405060
Alternatively, circuit solvers can simulate
Time (ns)complex nonlinear elements, but 3-D EM
effects and signal delay are approximatedas inductors, capacitors, or distributed elements, such as a transmission line.

708090100

This paper introduces XFdtd’s transient EM/circuit co-simulation capability, which combines
the strength of 3-D full-wave electromagnetic simulation with the flexibility of circuit solvers.
This approach embeds a transient nonlinear circuit solver in the full-wave simulation so that allelectromagnetic phenomena are accounted for in a single time-domain simulation.
RFFE Connected to an External Antenna
As an example, this paper considers a TVS diode protecting thecircuitry of a matching network. Motorola Mobility provided the3-D CAD model, which was then modified for demonstration
purposes to include an external antenna that was matchedusing a simple series capacitor and shunt inductor. The TVS
diode protects these components, as well as a 50 ohm resistor
acting as the integrated chip (IC) terminal.
An ESD gun with a current source using a 15 kV human bodymodel (HBM) waveform is applied to the external antenna. TheTVS diode is placed in front of the matching network, redirecting
the surge current to ground. The manufacturer provided
the current-voltage characteristics of this TVS diode as anequivalent SPICE circuit model.

315 S. Allen St., Suite 416 | State College, PA 16801 USA | +1.814.861.1299 phone | +1.814.861.1308 fax | sales@remcom.com | www.remcom.com

Transient EM/Circuit Co-Simulation in XFdtd: A Closer
Look at TVS Diodes for ESD Protection
Full-Wave EM Simulation with FDTD
The finite-difference time-domain (FDTD) solution is a full-wave technique based on Maxwell’sequations. 3-D CAD geometry is discretized into rectilinear cells and a material property is associatedwith each cell edge.
At the beginning of the FDTD simulation, allfields are initialized to zero and the current
source introduces fields into the simulation
space at the tip of the ESD gun based on the15 kV HBM input waveform. As the simulation
progresses, the electromagnetic fieldspropagate through space and interact with 3-Dmaterial structures along the way.
The TVS diode circuit model is an unknown
concept in terms of traditional FDTD
methodology. Full-wave equations don’t compute the voltage and currents through the complex
nonlinear component, and a traditional FDTD simulation can therefore only simulate the ESD eventwith an unprotected IC.
Each cell edge has an associated electric field that varies with time. Each cell edge is also surroundedby four magnetic fields, which are used to compute the corresponding current. In this manner,currents both at the source and at the IC terminal are calculated as a function of time. Transientcurrents are high in this unprotected configuration.
Circuit Simulation

In this example, the current source employing
a 15 kV HBM waveform is connected to thematching network and IC terminals. The TVS
diode is placed before the matching network.

8.0
7.0
6.0
5.0
Current (A)The simulation space for circuit simulators isdefined in terms of terminals, elements, andedges. This network is formulated into a nodal
matrix equation describing the components,
as well as their connectivity information.
An iterative solver computes the nonlinear
system of equations based primarily on
Kirchoff’s Current Law (KCL), which statesthat the algebraic sum of currents leaving any
node must be zero.

4.0
3.0
2.0
1.0
0.0
-1.0
-2.0

0

12

3

4

56Time (ns)78910

Transient EM/Circuit Co-Simulation in XFdtd: A Closer
Look at TVS Diodes for ESD Protection
The 3-D geometry of the device is unaccounted for in the circuit simulation. If there is priorknowledge of the geometry’s behavior–such as transmission lines and structural inductance andcapacitance–additional circuit elements could be added. However, it would be only an estimate of theelectromagnetic effects of the structure.
The current at the IC terminal is plotted as a function of time. The lack of time-domain variability
indicates an overly simplified representation of the 3-D model.
Transient EM/Circuit Co-Simulation
XFdtd’s transient EM/circuit co-simulation
uses both computational techniques
simultaneously in a single simulation. A celledge in a full-wave FDTD simulation is linked tothe pair of terminals spanning the TVS diode in
the circuit solver. The circuit solver simulation
space is reduced to only the two terminals,
six elements, and numerous edges associatedwith the TVS diode circuit model.
The circuit solver’s timestep is synced with theFDTD timestep. At each timestep, the FDTD solver passes the cell edge’s current to the circuit solver,which computes the TVS diode’s system of equations. It then returns the voltage to the FDTD solver
for use, thereby including the diode effect in the FDTD simulation space.
The protection provided by a TVS diodeis apparent when comparing the resultsof a transient EM/circuit co-simulation
containing a diode and an FDTD simulation
without one. The current at the IC terminal
is significantly reduced when the diode ispresent.

4.50
4.00
3.50

Current (A)

3.00
2.50
2.00
1.50
1.00
0.50
0.00
-0.50
-1.00

01

2

3

4

5

6

78

9Time (ns)Without TVS Diode

With TVS Diode

10In addition to verifying protection, theringing in the time-domain voltage confirmsthat all electromagnetic phenomena areaccounted for. The ringing is caused by theinductive and capacitance properties andcoupling to nearby structures.

Transient EM/Circuit Co-Simulation in XFdtd: A Closer
Look at TVS Diodes for ESD Protection
Summary
Effectively simulating TVS diode protection requires a transient nonlinear circuit solver to beembedded in a full-wave electromagnetic solver. This type of simulation simultaneously captures allelectromagnetic phenomena from the 3-D CAD geometry, as well as the complex nonlinear behaviorof the circuit model. ESD engineers can apply this simulation technique in XFdtd in order to analyze
device protection measures and mitigate ESD events more effectively.

To learn more about XFdtd’s transient EM/circuit co-simulation capability, please visit:www.remcom.com/xf-transient-em-circuit-co-simulation
Contact sales at:
sales@remcom.com

MOTOROLA and the Stylized M Logo are trademarks or registered trademarks of Motorola Trademark Holdings, LLC.

Remcom, Inc.
315 S. Allen St., Suite 416
State College, PA 16801 USA+1.888.7. REMCOM (US/CAN)+1. 814 . 861.1299 phone+1. 814 . 861.1308 fax

315 S. Allen St., Suite 416 | State College, PA 16801 USA | +1.814.861.1299
phone |sales@remcom.com© 2021 Remcom Inc.
All rights reserved.
TVSDiode-0621

Visit www.remcom.com for more information+1.814.861.1308 fax | sales@remcom.com | www.remcom.com