LNK520
Output Voltage
Inherent
CC to CV
transition
point
Tolerance envelope
without optocoupler
Typical inherent
characteristic without
optocoupler
Voltage
feedback
threshold
Characteristic with
optocoupler
Load variation
during battery
charging
Output Current
PI-2788-092101
Figure 7. Influence of the Optocoupler on the Power Supply Output Characteristic.
Output Voltage
Inherent
Tolerance envelope
without optocoupler
CC to CV
transition
point
Typical inherent
characteristic without
optocoupler
Voltage
feedback
threshold
VO(MAX)
Characteristic with
optocoupler
Power supply peak
output power curve
Load variation
during battery
Characteristic observed with
load variation often applied during
laboratory bench testing
charging
Output Current
PI-2790-112102
Figure 8. Output Characteristic with Optocoupler Regulation (Reduced Voltage Feedback Threshold).
is the characteristic seen if the voltage feedback threshold is
above the output voltage at the inherent CC to CV transition
point also indicated in Figure 7.
optocoupler feedback regulates the output voltage until the
peakoutputpowercurveisreachedasshowninFigure8. Under
theseconditions,theoutputcurrentwillcontinuetoriseuntilthe
peakpowerpointisreachedandtheoptocouplerturnsoff. Once
the optocoupler is off, the CONTROL pin feedback current is
determined only by R1 and the output current therefore folds
back to the inherent CC characteristic as shown. Since this type
of load transition does not normally occur in a battery charger,
theoutputcurrentneverovershootstheinherentconstantcurrent
value in the actual application.
Figure 8 shows a case where the voltage feedback threshold
is set below the voltage at the inherent CC to CV transition
point. In this case, as the output voltage rises, the secondary
feedback circuit takes control before the inherent CC to CV
transition occurs. In an actual battery charging application, this
simply limits the output voltage to a lower value. However, in
laboratory bench tests, it is often more convenient to test the
power supply output characteristic starting from a low output
current and gradually increasing the load. In this case, the
In some applications it may be necessary to avoid any output
currentovershoot,independentofthedirectionofloadvariation.
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POWERINT [ Power Integrations ]
