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TISP40XXH1BJ Datasheet(PDF) 7 Page - Bourns Electronic Solutions |
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TISP40XXH1BJ Datasheet(HTML) 7 Page - Bourns Electronic Solutions |
7 / 9 page AUGUST 1999 - REVISED FEBRUARY 2005 Specifications are subject to change without notice. Customers should verify actual device performance in their specific applications. Figure 4. Transformer Saturation AI4XAO 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: 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: 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 PROTECT ION LINE R1 R2 AI4XAN 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, VDRM, 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. |
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