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LNK520GN-TL Datasheet(PDF) 4 Page - Power Integrations, Inc. |
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LNK520GN-TL Datasheet(HTML) 4 Page - Power Integrations, Inc. |
4 / 20 page 4 LNK520 E 2/05 LNK520 LinkSwitch 85-265 VAC VOUT RTN C3 C4 C2 C1 C S D R3 R2 R5 R4 R1 D2 D1 VR1 D3 PI-3703-030404 U1 T1 Figure 6. Power Supply Schematic Outline with Optocoupler Feedback, Providing Tight CV Regulation. The characteristics described above provide an approximate CV/CCpowersupplyoutputwithouttheneedforsecondaryside voltage or current feedback. The output voltage regulation is influencedbyhowwellthevoltageacrossC2tracksthereflected output voltage. This tracking is influenced by the coupling between transformer output and bias windings. Tight coupling improvesCVregulationandrequiresonlyalowvalueforresistor R2.PoorcouplingdegradesCVregulationandrequiresahigher value for R2 to filter leakage inductance spikes on the bias winding voltage waveform. This circuitry, used with standard transformer construction techniques, provides much better output load regulation than a linear transformer, making this an ideal power supply solution in many low power applications. If even tighter load regulation is required, an optocoupler configuration can be used while still employing the constant output current characteristics provided by LinkSwitch. Optional Secondary Feedback Figure 6 shows a typical power supply schematic outline using LinkSwitch withoptocouplerfeedbacktoimproveoutputvoltage regulation. On the primary side, the schematic only differs from Figure 5 by the addition of optocoupler U1 transistor in parallel to R1. On the secondary side, the addition of voltage sense circuit componentsR4,VR1andU1LEDprovidethevoltagefeedback signal. In the example shown, a simple Zener (VR1) reference is used though more accurate references may be employed for improved output voltage tolerancing and to provide cable drop compensation,ifrequired.ResistorR4providesbiasingforVR1. The regulated output voltage is equal to the sum of the VR1 Zener voltage plus the forward voltage drop of the U1 LED. Resistor R5 is an optional low value resistor to limit U1 LED peakcurrentduetooutputripple.Manufacturerʼsspecifications for U1 current and VR1 slope resistance should be consulted to determine whether R5 is required. When the power supply operates in the constant current (CC) region, for example at start up and when charging a battery, the output voltage is below the voltage feedback threshold defined by U1 and VR1 and the optocoupler is fully off. In this region, the circuit behaves exactly as previously described with reference to Figure 5 where the voltage across C2 and therefore thecurrentflowingthroughR1increaseswithincreasingoutput voltage and the LinkSwitch internal current limit is adjusted to provide an approximate CC output characteristic. When the output reaches the voltage feedback threshold set by U1 and VR1, the optocoupler turns on. Any further increase in the power supply output voltage results in the U1 transistor current increasing. The resulting increase in the LinkSwitch CONTROL current reduces the duty cycle according to Figure 4 and therefore, maintains the output voltage regulation. Figure 7 shows the influence of optocoupler feedback on the output characteristic. The envelope defined by the dashed lines represent the worst-case power supply DC output voltage and currenttolerances(unit-to-unitandovertheinputvoltagerange) if an optocoupler is not used. A typical example of an inherent (without optocoupler) output characteristic is shown dotted. This is the characteristic that would result if U1, R4, R5 and VR1 were removed. The optocoupler feedback results in the characteristicshownbythesolidline.Theloadvariationarrowin Figure7representsthelocusoftheoutputcharacteristicnormally seen during a battery charging cycle. The two characteristics are identical as the output voltage rises but then separate as shown when the voltage feedback threshold is reached. This |
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