EPR-71 Datasheet, PDF (6/28 Pages) Power Integrations, Inc. – Engineering Prototype Report for EP 71 - 6.6 W DC-DC Converter Using DPA Switch- (DPA423G)

EP-71 3.3 V, 2 A DC-DC Power Supply
19-Jul-2005
4 Circuit Description
The schematic in Figure 2 shows a DC input flyback converter using the DPA423G
device operating at 400 kHz. The circuit is designed for the standard nominal 48 V
telecom input voltage range of 36-72 VDC. Using the Flyback topology, circuit board
size, parts count and cost are minimized, while attaining excellent operating efficiency
across the input voltage range.
4.1 Input Filtering
An input pi filter formed by C1, L1 and C2 reduces the input ripple current and high
frequency noise. However additional external filtering may be required depending on
applicable standards and specific application.
4.2 DPA-Switch Primary
The DPA423G IC (U1) provides startup, PWM control, under-voltage lock out,
overvoltage shutdown and over-temperature protection functions. The integrated 220 V
MOSFET has excellent switching characteristics at the selected 400 kHz operating
frequency. This together with the minimal power consumption of the control enables a
typical operating efficiency of 75% to 80% across the operating input voltage range (see
Figure 7).
The DC input voltage is applied to the primary winding of T1. The other side of the
transformer primary is driven by the integrated MOSFET in U1. Zener diode VR1 and C3
clamp leakage spikes generated when the MOSFET in U1 turns off. Under normal
operation, VR1 does not conduct but limits the maximum drain voltage under input
overvoltage and output overload conditions.
Resistor R5 programs the typical input under-voltage on-threshold to 33 VDC and the
protective overvoltage shutdown to 90 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 bias winding provides CONTROL pin current after start-up. Diode D2
rectifies the bias winding, while components R5 and C11 reduce the high frequency
switching noise and reduce peak charging of the bias voltage.
The DPA423G operates well within the recommended junction temperature limits
(110 °C) at an elevated ambient of 50 °C, in a free-convection cooled environment (see
Section 10).
4.3 Output Rectification
Schottky output diode D1 enables low loss rectification of the secondary winding voltage.
Low ESR tantalum output capacitors, C7 to C9, reduce switching ripple and minimize
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