TISP40xxL1AJ/BJ VLV Overvoltage Protectors
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:
di
-
E = – L ----
dt
where:
L
= unsaturated inductance value in H
di
= current change in A
dt
= time period in s for current change di
E
= winding voltage in V
((
Rearranging this equation and working large
∆
changes to saturation gives the useful circuit relationship of:
E x
∆t
= L x
∆i
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 9. 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 though the
protector was in the low voltage condition.
T1
UNSATURATED
Th1
L
SATURATED
Th1 RW
AI4XAO
T1
Figure 9. Transformer Saturation
Figure 10 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.
OVER-
AI4XAN
CURRENT
PROTECT ION
R1
T1
LINE
Th1
SIGNAL
R2
Figure 10. Transformer Winding Protection
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.
TISP
®
Device Voltage Selection
Normally, the working voltage value of the protector, V
DRM
, would be chosen to be just greater than the peak signal amplitude over the
equipment temperature range. This would give the lowest possible protection voltage, V
(BO)
. This would minimize the peak voltage applied to
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.
AUGUST 1999 - REVISED FEBRUARY 2005
Specifications are subject to change without notice.
Customers should verify actual device performance in their specific applications.
BOURNS [ BOURNS ELECTRONIC SOLUTIONS ]
