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FP38 Datasheet(PDF) 4 Page - Power-One |
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FP38 Datasheet(HTML) 4 Page - Power-One |
4 / 5 page Accessories Filter & Ring Core Chokes FP, L and LP Series REV. SEP 29, 2003 Page 4 of 5 Input Interference Reduction Using L- or LP-series chokes together with an additional external capacitor a similar attenuation can be achieved as with filter blocks. The capacitor between the choke and the converter input is necessary in order to avoid possible oscil- lations caused by the negative input impedance of the regu- lator. This phenomenon could cause the input voltage to leave the specified regulator input range. The relatively high ripple current flowing through the capacitor must be considered for the design. Refer also to: Technical Informa- tion: Installation & Application. The current compensated choke LP183 has a high perme- ability ring core with two identical separate windings. The normal operating current will only see the small stray in- ductance between the windings. However common mode interference will be blocked by the full inductance of the choke. Fig. 7 Low-loss ring core choke with external capacitor (Cex approx. 1000 µF) used as output filter Reduction of Output Ripple Even though switching regulators have an inherently low output ripple, certain sensitive applications need even fur- ther reduction. In such cases, the low-loss ring core chokes designed to reduce disturbances at the input can also be used for reducing the ripple on the output voltage. The chokes in combination with an external capacitor can achieve even better results than the Filter Blocks with re- gard to the ripple and dynamic regulation. The formula for the remaining output ripple at the load RL is calculated as follows: UR = uo • ZC ext/ZLD uo: Output ripple of the regulator ZCex: The impedance of the capacitor at the regu- lator's switching frequency (150 kHz) corre- sponds to the equivalent series resistance (ESR) of the capacitor (please refer to the corresponding data sheet). ZLD =2 π • fS • LD fS: 150 kHz (regulator switching frequency) Through the use of a common mode choke LP 183, the common mode noise at the output can also be further re- duced. Consider that the filter not only affects the output ripple but can also influence the voltage UR across the load RL in the event of load changes. The static regulation increases with the ohmic resistance of the choke, i.e. 6 mV/A for the choke L/LP20-7 and 20 mV/A for the LP34-3. The dynamic regulation is dependent on the size of the ca- pacitor. Generally, the bigger Cex the smaller is the dy- namic, however, recovery will be slower. Typical Application A voltage drop UrGo = rGo • (Io – Ii) is produced across the ground loop resistance rGo. It is superimposed upon the regulators output voltage Uo and generates the voltage UR = Uo – Ur Go across the load resistance RL. Without an input inductance Le the current Ii in the input circuit has a rela- tively high AC component with a basic frequency fs (regula- tor's switching frequency of approx. 150 kHz). This alternat- ing current produces an AC voltage component across rGo which is superimposed upon URL. To prevent this phenomenon, an inductance Le can be in- serted into the input circuit. This causes the AC component of the input current to be supplied entirely from the input capacitor Ce; thus, Ii is a pure direct current. Ce should be wired as close as possible to the regulator's input terminals Vi+ and Gi–. Le and Ce additionally provide protection against input tran- sients and reduce radio interference voltages. External connection of Gi– and Go– or connection via a common ground is not recommended. The internal voltage drop UrG in the regulator would be superimposed on the output voltage. Vi+ Gi– Vo+ Go– PSR Ce Uo U 12014 UR RL RD LD ZLD ZC ext Fig. 5 L/LP type chokes and capacitors used as input filter Vi+ Gi– Vo+ Go– PSR U Cext 1 Cext 2 Cext 3 LP34-3 or L/LP20-7 LP183 12013 Uo RL Vi+ Gi– Vo+ Go– PSR RL Uo U Ce Le UrG rGo rG UrGo Io URL 12015 Fig. 6 Reduction of superimposed interference voltages in grounded power supply systems, caused by ground loops |
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