04-Oct-2005
EP-85 6 V, 330 mA Low Cost Charger
3 Schematic
Figure 3 – LNK564 Low Cost Charger Schematic.
4 Circuit Description
4.1 Input and EMI Filtering
AC input differential filtering is accomplished with the very low cost input filter stage
formed by C1 and L1. The proprietary frequency jitter feature of the LNK564 eliminates
the need for an input pi filter, so only a single bulk capacitor is required. This allows the
input inductor L1 to be used as a fuse as well as a filter component. This very simple
Filterfuse input stage further reduces system cost. The L1 is sleeved to allow it to function
as a fuse. An optional fusible resistor, RF1, may be used to provide the fusing function.
Input diode D2 may be removed from the neutral phase in applications where decreased
EMI margins and/or decreased input surge withstand is allowed.
4.2 LinkSwitch-LP Feedback
The power supply utilizes simplified bias winding voltage feedback enabled by LNK564
ON/OFF control. The resistor divider formed by R1 and R2 determine the output voltage
across the transformer bias winding during the switch off time. In the V/I constant voltage
region, the LNK564 device enables/disables switching cycles to maintain 1.69 V on the
FB pin. Diode D3 and low cost ceramic capacitor C3 provide rectification and filtering of
the primary feedback winding waveform. At increased loads, beyond the constant power
threshold, the FB pin voltage begins to reduce as the power supply output voltage falls.
The internal oscillator frequency is linearly reduced in this region until it reaches typically
50% of the starting frequency when the FB pin voltage reaches the auto-restart threshold
voltage (typically 0.8 V on the FB pin, which is equivalent to 1 V to 1.5 V at the output of
the power supply). This function limits the output current in this region without fold back
until the output voltage is low.
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