® Design Idea DI-24 ® DPA-Switch 30 W DC-DC Converter Application Device Power Output Input Voltage Output Voltage Topology DC-DC Converter DPA424R 30 W 36-75 VDC 5V Forward Design Highlights • • • • • • • • Resistor R3 programs the internal DPA424R current limit to 85% of nominal, just above the level needed at full load, limiting overload power. This feature also allows a larger DPA-Switch to be selected, without requiring any other circuit changes. A larger DPA-Switch reduces conduction losses, raising efficiency. Extremely low component count High efficiency – 85% using Schottky rectifiers No current sense resistor or current transformer required Output overload, open loop and thermal protection Accurate input under/over voltage meets ETSI standards Operates to zero load 400 kHz operation minimizes size of magnetics Available for prototyping in DAK-21 Capacitors C8 and C9 provide transformer core reset; C8 also limits the leakage inductance spike on the DRAIN. Resistor R5 in series with C9 damps ringing. Zener diode VR1 provides a hard voltage clamp to limit DRAIN voltage, but is only active during transients and overload conditions. Operation DPA-Switch greatly simplifies the design compared to a discrete implementation. Resistor R1 programs the input under/over voltages to 33 V and 86 V, respectively, and linearly reduces the maximum duty cycle with input voltage to prevent core saturation during load transients. These thresholds have tolerances that guarantee the converter is operational at 36 V, without the cost of additional line sense components. C7 1 nF R14 1.5 kV 10 Ω +VIN 36-75 VDC L1 1 µH 2.5 A 1 C8 47 pF 200 V R1 619 kΩ 1% The bias supply for U1 is provided from an auxiliary winding on output inductor L2. This gives higher efficiency than a transformer winding, since it provides a fixed voltage independent of input voltage. Pre-load R13 maintains the bias voltage ≥8 V at zero load. On the secondary, a soft finish network, C13, D3 and R7, eliminates output turn-on overshoot. The remaining components provide output voltage regulation and loop compensation. C9 2.2 nF R5 50 V 1.0 Ω T1 L2 8 µH 6 A 5,6 7,8 4 C2 1 µF 100 V 5,6 D2 MBRB2545CT U1 DPA424R S VR1 SMBJ 150 U2 L CONTROL X F C R3 8.25 kΩ C5 1% 220 nF -VIN R4 1.0 Ω C6 68 µF 10 V C12 1 µF 10 V 5 V, 6 A RTN U2 R7 10 kΩ D3 BAV19WS R6 150 Ω C13 10 µF 10 V U3 LM431AIM3 R10 10.0 kΩ 1% C16 100 nF R12 5.1 Ω R9 220 Ω C14 1 µF R11 10.0 kΩ 1% PI-2993-090602 Figure 1. DPA-Switch 30 W DC-DC Converter. DI-24 D1 BAV19WS DPA-Switch D R13 160 Ω 1 C4 4.7 µF 20 V C3 1 µF 100 V C11 100 µF 10 V 7,8 3 2 C1 1 µF 100 V C10 100 µF 10 V www.powerint.com September 2002 DI-24 Key Design Points • For nominal under-voltage set point VUV: R1 = (VUV-2.35 V)/50 µA. VOV = (R1×135 µA)+2.5 V. • Select C9 such that the core resets at VUV and the DRAIN voltage ≤170 V at VOV. To reduce leakage spike, C8 may be added, adjusting C9 accordingly. • Zener VR1 safely limits the DRAIN voltage below BVDSS and guarantees transformer reset. • Opto U2 should have a CTR of between 100% and 200% for optimum loop stability. • At zero load, maximum input voltage, the bias voltage across C4 should be ≥8 V (12 V to 15 V under nominal conditions). Efficiency (%) 85 TRANSFORMER PARAMETERS Core PR1408 Siemens N87 material, ungapped Bobbin P1408 8 pin (B&B B-096 or equivalent) Winding Order (pin numbers) Primary: 7T + 8T, 27 AWG Secondary: 4T x 27 AWG Primary (4-3), tape, Secondary (5,6-7,8), Tape, Primary (3-1),tape Inductance Primary: 450 µH ± 25%, Leakage 1 µH (max) Primary Resonant Frequency 3.8 MHz (minimum) Winding Details PI-2995-073002 90 • Good layout practices should be followed: - Locate C5, C6 and R4 close to U1, with grounds returned to the SOURCE pin. - Primary return should be connected to the DPA-Switch tab, not the SOURCE pin. - Minimize the primary and secondary loop areas to reduce parasitic leakage inductance. • Consult AN-31 and EPR-21 for additional design tips and information. 80 75 Table 1. Transformer Construction Information. 70 VIN = 36 V VIN = 48 V VIN = 60 V VIN = 72 V 65 OUTPUT INDUCTOR PARAMETERS 60 0 10 20 30 PR1408 Siemens N87 material Gap for AL of 163 nH/T2 Core 40 Bobbin P1408 8 pin (B&B B-096 or equivalent) Winding Details Bias: 18T, 32 AWG, Main: 7T, 2x24 AWG Winding Order (pin numbers) Inductance Pins 5,6-7,8 Output Power (W) Figure 2. Efficiency vs. Output Power. Bias (1-2), tape, Main winding (7,8-5,6), tape 8 µH ± 10% Table 2. Output Inductor Construction Information. For the latest updates, visit our Web site: www.powerint.com Power Integrations reserves the right to make changes to its products at any time to improve reliability or manufacturability. Power Integrations does not assume any liability arising from the use of any device or circuit described herein, nor does it convey any license under its patent rights or the rights of others. 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