EP68 3.3 V, 2 A DC-DC Power Supply
19-Jul-2005
4 Circuit Description
The Flyback topology was used to minimize circuit board size, parts count and cost, while
attaining excellent operating efficiency across the input voltage range.
4.1 DPA-Switch Primary
The DPA423G IC provides PWM control, startup, feedback, under/over voltage and over-
temperature protection functions. The integrated 220 V MOSFET provides excellent
switching characteristics at the selected 400 kHz operating frequency. The MOSFET and
controller consume minimal power, enabling a typical operating efficiency of 74%-78%
across the operating input voltage range (see Figure 7).
R1 provides a 25 kΩ input impedance, consistent with PoE Class 0 requirements. Above
30 V, Zener VR1 conducts, allowing n-channel MOSFET Q1 to turn on. Zener diode VR2
protects the gate of Q1 from overvoltage damage. Resistors R2 and R3 provide
repeatable on and off timing.
Resistor R5 programs the typical input under-voltage ON threshold to 33 VDC. Resistors
R4 and R6 program the internal device current limit to reduce with increasing input
voltage. Maximum output (overload) current varies less than 5% across the operating
voltage range. The reduction in overload output current reduces secondary transformer
leakage spikes and allows the use of a 30 V Schottky diode, for the output rectifier D1.
The primary-side Zener clamp, VR3, provides protection of the DPA423G drain under
input surge and overvoltage conditions. Zener diode VR3 does not conduct under normal
operating conditions.
The primary bias winding provides CONTROL pin current after start-up. Diode D2
rectifies the bias winding, while components R8 and C11 reduce the high frequency
switching noise and reduce the peak charging of the bias voltage.
The DPA423G operates well within the recommended junction temperature limits
(100 °C) at an elevated ambient of 50 °C, in a free-convection cooled environment (see
Section 11).
4.2 Output Rectification
Schottky output diode D1 enables low-loss rectification of the secondary winding voltage.
Low ESR tantalum output capacitors, C7-9, reduce switching ripple and minimize losses.
Secondary output choke L1 and ceramic output capacitor C10 reduce high frequency
noise and ripple at the output.
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