DER-98
41W (53W pk) DVR
September 12, 2005
5 Circuit Description
The schematic in Figure 2 shows an off-line flyback converter using the TOP246Y. The
circuit is designed for 85 VAC to 265 VAC.
5.1 Input EMI Filtering
X-capacitor C32, and common-mode choke L1 act as an input filter to reduce common
mode and differential mode EMI. The AC line voltage is rectified and filtered to generate a
high voltage DC bus via D1-4 and C1.
5.2 TOPSwitch Primary
Diode D5, C6, and R4, R45 and VR5 clamp leakage spikes generated when the MOSFET
when U1 switches off. D5 is a glass-passivated normal recovery rectifier. The slow,
controlled recovery time of D5 allows energy stored in C6 to be recycled back to the high
voltage bus, significantly increasing efficiency. A normal (non-glass-passivated) 1N4007
should not be substituted for the glass-passivated device. C5 bypasses the U1 control pin.
C4 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 help stabilize the power supply
control loop. Diode D6 and capacitor C3 provide rectified and filtered bias power for U1 and
U2. Components R39, R48 and R9 provide a signal to the U1 X pin to reduce current limit
at high line to keep the maximum output power consistent with low line. R47 and R38
provide OV/UV protection.
5.3 Outputs
The T1 output is rectified and filtered by Diodes D8-D12 and filtered by inductor/capacitor
networks on most outputs. NOTE: Large capacitors were used for the 12 V output in order
to prevent possible voltage overshoots from the reverse current coming from the Hard Disk
Drive motor. Testing with the actual unit may show that these capacitors can be reduced
for cost reduction.
5.4 Output Feedback
Output feedback is used from a combination of the 5 V and the 3.3 V rails. Resistors R31,
R32 and R30 develop a feedback voltage, which is fed to the reference regulator U3. U3
drives optocoupler U2 through resistor R27 to provide feedback information to the U1
control pin. The optocoupler output also provides power to U1 during normal operating
conditions. Capacitor C7 applies drive to the optocoupler during supply startup to reduce
output voltage overshoot. Capacitor C21 and R29 provide frequency compensation for
error amplifier U3.
5.5 Output OV Protection
Q2 is positioned across the 30 V output winding; upon OV on the 5 V, detected by VR10,
Q2 will be triggered ON to short the 30 V winding. The PS will go into auto-restart mode
until the OV fault is removed.
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