AUGUST 1999 - REVISED JANUARY 2007

Specifications are subject to change without notice.

Customers should verify actual device performance in their specific applications.

Figure 4. Transformer Saturation

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Th1

T1

UNSATURATED

L

RW

Th1

T1

SATURATED

TISP40xxH1BJ VLV Overvoltage Protector Series

APPLICATIONS INFORMATION

Transformer Protection

The inductance of a transformer winding reduces considerably when the magnetic core material saturates. Saturation occurs when the

magnetizing current through the winding inductance exceeds a certain value. It should be noted that this is a different current to the

transformed current component from primary to secondary. The standard inductance-current relationship is:

where:

Rearranging this equation and working large ∆ changes to saturation gives the useful circuit relationship of:

A transformer winding volt-second value for saturation gives the designer an idea of circuit operation under overvoltage conditions. The

volt-second value is not normally quoted, but most manufacturers should provide it on request. A 50 Vµs winding will support rectangular

voltage pulses of 50 V for 1 µs, 25 V for 2 µs, 1 V for 50 µs and so on. Once the transformer saturates, primary to secondary coupling will be

lost and the winding resistance, RW, shunts the overvoltage protector, Th1 - see Figure 4. This saturated condition is a concern for long

duration impulses and a.c. fault conditions because the current capability of the winding wire may be exceeded. For example, if the on-state

voltage of the protector is 1 V and the winding resistance is 0.2 Ω, the winding would bypass a current of 1/0.2 = 5 A, even through the

protector was in the low voltage condition.

EL

di

dt

-----

–

=

Ex

x

∆tL

∆i

=

( (

Figure 5 shows a generic protection arrangement. Resistors R1 and R2, together with the overcurrent protection, prevent excessive winding

current flow under a.c. conditions. Normally these resistors would only be needed for special cases, e.g. some T1/E1 designs. Alternatively, a

split winding could be used with a single resistor connecting the windings. This resistor could be by-passed by a small capacitor to reduce

signal attenuation.

Figure 5. Transformer Winding Protection

Th1

SIGNAL

T1

OVER-

CURRENT

PROTECTION

LINE

R1

R2

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Overcurrent protection upstream from the overvoltage protector can be fuse, PTC or thick film resistor based. For very high frequency circuits,

fuse inductance due to spiral wound elements may need to be evaluated.

the transformer winding and increase the time to core saturation.

In high frequency circuits, there are two further considerations. Low voltage protectors have a higher capacitance than high voltage protectors.