CTVS Multilayer Varistors for Automotive – Functionality

• The chip of a multilayer CTVS consists of a stack of alternating ceramic/electrode layers. The thickness of the ceramic layers affects the protection level.
• In the active volume of the ceramic chip there are a number of internal electrode layers with an overlapping area affecting the pulse absorption capability. The internal electrodes are connected to the terminals or external electrodes.
• The nickel barrier termination is a suitable for lead-free reflow soldering.

CTVS are operated in one of two modes. These modes concern two different segments of the CTVS V/I curve (1) which lies within a well defined tolerance band (2):

• “High-resistance” mode (< 1 mA)
  If the circuit is operated at normal operating voltage the CTVS has to be highly resistive. Here, the circuit designers may generally want to know about the largest possible leakage current at the given operating voltage. Therefore, the lower limit of the tolerance band is shown.
• “Low-resistance” mode (> 1 mA)
  In an overvoltage event the CTVS has to be highly conductive. Here, the circuit designer’s primary concern is about the worst-case voltage drop across the CTVS. The upper limit of the tolerance band is shown. The clamping voltage V_clamp is the voltage across the CTVS in an overvoltage event.

Example: CT0805K14G
(max. AC operating voltage of V_RMS, max = 14 V and a varistor tolerance K of ±10 %)

The CTVS V/I characteristic curve (1) shows the mean value of the tolerance band between the limits indicated by dashed lines (2). The mean at 1 mA represents the varistor voltage (V_V = V_BR), in this case 22 V. The tolerance K ±10% refers to this value, so at this point the tolerance band ranges from 19.8 to 24.2 V (region (2) ). For the CT0805K14G MLV, a maximum permissible operating voltage of 18 V_DC is specified (3). Depending on the actual value of the MLV in the tolerance band, you can derive a leakage current between 6 x 10^–6 A and 2 x 10^–4 A at room temperature (region (3) ) as well as an increase of the voltage across the MLV to between 65 V and 80 V (region (4) ) assuming a surge current of 100 A.

Pulse energy / peak voltage

Protection principle

Differences