DER-94
LCD Monitor Internal Supply
September 12, 2005
4 Circuit Description
The schematic in Figure 1 shows an off-line flyback converter using the TOP246. The
circuit is designed for 90 VAC to 265 VAC input, with two outputs: 5 V / 2 A and 12 V / 3
A.
4.1 Input EMI Filtering
Capacitor CX1 and the L1 leakage inductance filter differential mode conducted EMI.
Inductor L1 and CY1-CY3 filter common mode conducted EMI. L4 is a ferrite bead
connected between secondary return and safety ground that reduces high frequency
(> 20MHz) conducted EMI.
4.2 TOPSwitch Primary
The AC line voltage is rectified and filtered to generate a high voltage DC bus via D1-4
and C1. Diode D5, C3, and R2-4 clamp leakage spikes generated when the MOSFET in
U1 switches off. D5 is a glass-passivated normal recovery rectifier. The slow, controlled
recovery time of D5 allows energy stored in C3 to be recycled back to the high voltage
bus, significantly increasing efficiency. A normal (non-passivated) 1N4007 should not be
substituted for the glass-passivated device. The U1 “F” pin is connected to the control pin
to program 65kHz operating frequency. Resistor R5 sets the turn-on voltage of the supply
to approximately 76 VAC. C4 bypasses the U1 control pin. C5 has three functions. It
provides the energy required by U1 during startup, sets the auto-restart frequency during
fault conditions, and also acts to roll off the gain of U1 as a function of frequency. R8
adds a zero to the control loop to stabilize the power supply control loop. Resistor R7
programs the U1 current limit to 75% of the nominal value. Resistor R6 acts to depress
the U1 current limit as a function of line voltage, making the maximum overload power
more independent of line voltage.
4.3 Output Rectification
The T1 output is rectified and filtered by D7 and C8-9 for the 12V output, and by D8 and
C11-12 for the 5V output. Components C8 and R12 provide snubbing for D12.
Components L2, L3, C10, and C13 provide additional high frequency output filtering.
4.4 Output Feedback
Resistors R15 and R16 are used to set the +5V main output voltage. Resistor R25
provides a small amount of feedback from the +12V output to improve cross regulation.
Shunt regulator U3 drives optocoupler U2 through resistor R11 to provide feedback
information to the U1 control pin. The optocoupler output also provides power to U1
during normal operating conditions. Capacitor C16 applies drive to the optocoupler during
supply startup to reduce output voltage overshoot. Capacitor C15 and R14 provide
frequency compensation for error amplifier U3. Components C14 and R12 improve the
control loop phase margin by providing gain and phase boost near the control loop 0dB
crossover frequency.
Components C5, C14, C15, R8, R11, R12, and R14 all play a role in compensating the
power supply control loop. Capacitor C5 rolls off the gain of U1 at relatively low
Power Integrations
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