EPR-00011 - 70 W Adapter 22-Jan-01
Page 8 of 46
Power Integrations Inc
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an under-voltage detect (100V), over-voltage shutdown (450V) and line feed forward with
DC
MAX
reduction features. Two resistors are used in series to allow low cost ¼ W
resistors which have a lower voltage rating.
The under-voltage detect ensures that the output is glitch free at start-up and shutdown.
With the combined value of R14 and R15 as shown, the power supply does not start
operating until the DC rail voltage reaches 100 VDC. On removal of the AC input the UV
sense prevents the output glitching as C7 discharges turning off the TOPSwitch-GX when
output regulation is lost or when the input voltage falls to 40 V, whichever occurs first.
The over-voltage feature shuts down the supply if the rectified input voltage exceeds
approximately 450V. If exceeded this protects the TOPSwitch-GX from excessive drain
voltages providing an extended AC surge withstand to 700 V
DC
(BV
DSS
rating), ideal for
countries with poor power quality.
Finally the line feed forward feature reduces output line frequency ripple by modulating
the control loop with the line frequency ripple on the DC rail, ideal when using a relatively
small input capacitor.
Resistors R5, R6, and R12 connect to the EXTERNAL CURRENT LIMIT pin (X) and are
used to externally program the current limit level of the device to just above the operating
peak current at full load and low line. This allows use of a smaller transformer core and /
or higher transformer primary inductance for a given output power. Reducing transformer
size and TOPSwitch-GX power dissipation, while at the same time avoiding transformer
core saturation during start-up or output load transient. This resistor network also
reduces the current limit with increasing line voltage. This limits the maximum available
power during overload conditions at high line (see Figure 10) removing the need for any
protection circuitry on the secondary.
The secondary windings are rectified and filtered by D1, D5, C1, and create the 19 V
output voltage. Two windings are used, with separate dual rectifiers (for equal current
sharing), to lower winding losses and maximise efficiency. Inductor L1 provided
additional filtering in conjunction with C4 and C14.
The 19 V output is directly sensed by the series combination of R4 and R13 that form a
divider with R10. These resistors together with the reference node voltage of U3 set the
output voltage. Other output voltages are also possible by adjusting the transformer turns
ratios and value of R4 and R13. Resistor R2 sets the overall DC gain of the control loop
and R7 provides bias for voltage reference U3. Capacitor C11 is the main compensation
capacitor and together with R11, C9, R8 and C10 ensure stability of the control loop.
The TOPSwitch-GX control circuit allows the switching frequency to reduce at light or
zero load conditions, eliminating the need for a pre-load resistor to control the output
