DER-107 10 W Compact Power Supply October 26, 2005
Page 6 of 26
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4 Circuit Description
The schematic in Figure 2 shows an off-line Flyback converter using the TOP245R. The
circuit is designed for 90 VAC to 300 VAC input and 6 V, 1.67 A output, with a transient
load requirement of 3 A for 2 minutes in duration.
4.1 Input EMI Filtering
Capacitor C1, C2 and L1 form in input p-filter for differential-mode conducted EMI.
Common-mode conducted EMI is reduced with the ESHIELD winding technique
employed in the transformer construction. A input X-capacitor and a Y-capacitor to bridge
the isolation barrier are not required, due to the ESHIELD transformer construction and
frequency dithering of the TOPSwitch-GX.
4.2 TOPSwitch Primary
Rectifier bridge BR1 and C1, C2 provide a high voltage DC BUS for the primary circuitry.
The DC rail is applied to the primary winding of T2. The other side of the transformer
primary is driven by the integrated MOSFET in U1. Diode D4, R7, R3 and C6 clamp
leakage spikes generated when the MOSFET in U1 switches off. Resistor R8 sets the
low-line turn-on threshold to approximately 69 VAC, and also sets the over-voltage
shutdown level to approximately 320 VAC. R2 sets the U1 current limit to approximately
75% of its nominal value. This limits the output power delivered during fault conditions.
C5 bypasses the U1 CONTROL pin. C4 has 3 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. R1 adds a zero to stabilize the power
supply control loop. Diode D3 and C12 provide rectified and filtered bias power for U3
and U1. The Frequency pin (F-pin) of U1 is tied to the Control pin (C-pin) to set the
operating frequency of the U1 to 66kHz.
4.3 Output Rectification
The output of T2 is rectified and filtered by D6, C9, and C10. Inductor L2 and C11 provide
additional high frequency filtering.
4.4 Output Feedback
Resistors R9 and R10 divide down the supply output voltage and apply it to the reference
pin of error amplifier U2. Shunt regulator U2 drives optocoupler U3 through resistor R12
to provide feedback information to the U1 CONTROL pin. The optocoupler output also
provides power to U1 during normal operating conditions.
Components C4, C13, R1, R11, and R12 all play a role in compensating the power
supply control loop. Capacitor C4 rolls off the gain of U1 at relatively low frequency.
Resistor R1 provides a zero to cancel the phase shift of C4. Resistor R12 sets the gain of
the direct signal path from the supply output through U2 and U3. Components C13 and
R11 roll off the gain of U2.
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