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SR08X Datasheet(PDF) 6 Page - Supertex, Inc |
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SR08X Datasheet(HTML) 6 Page - Supertex, Inc |
6 / 7 page 6 NR040306 SR086/SR087 Line Transformer During initial testing it is tempting to use an isolation transformer or a variable transformer on the AC line. However, the high inductance of the transformer (frequently in the mH’s) interferes with the normal operation of the SR08x and should not be used. This is not a concern with the normal inductance of the AC line or for AC line filters. The SR08x draws current from the AC line in short, high current pulses. The transformer’s high inductance tends to limit the current pulse. Furthermore, inductive kickback on the falling edge of the current pulse can create high voltage spikes which must be absorbed by the transient protector. EMI Capacitor The use of a small-value capacitor from circuit common to earth ground prevents the SR08x from operating and should not be employed. EMI The SR08x circuit as depicted on Page 1, (Typical Application Circuit), meets FCC Class B and CISPR 14-1 (household appliances) requirements for conducted emissions, for combined loads of less than 20mA (I OUT + IREG). SR08x & Circuitry AC Line AC Line SR08x & Circuitry earth ground circuit common Fuse Although the average current drawn from the AC line is low, the RMS current is fairly high due to the current being drawn in short, high-current pulses. Since a fuse is basically a resistor with a power dissipation given by I RMS 2 R, the fuse must be sized for the RMS current, not the average current. For a 1W load and 120VAC the RMS current is 700mA RMS, and for a 0.5W load at 230VAC it is 360mA RMS. Transient Protection The transient protector must be located before the bridge rectifier. The reason for this is to minimize capacitance to allow the rectified AC to fall below V OUT. Since there is no capacitor to absorb AC line transients, complete transient protection must be provided by the TVS or MOV device. Since the recommended IGBT is rated at 1.2kV and the SRO8x never sees the full input voltage, the bridge rectifier becomes the limiting element when selection an MOV. If using a 1.0kV bridge, an MOV having a clamping voltage of < 1.0kV is recommended. Load Total load on the SR08x is the total load current drawn from V OUT (IOUT) and, since the linear regulator is supplied from VOUT, it also includes the current drawn from V REG (IREG). Total load is calculated as follows: I LOAD = IOUT + IREG P LOAD = VOUT (IOUT + IREG ) UPS The SR08x will not operate from uninterruptable power supplies having a square wave output. This type of output is usually referred to as ‘modified sine wave’ |
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