Abstract:The MAX5074 circuit serves as a reference design for a 5V, 3A, DC-DC step-down power converter with isolated output operating from a 36V to 72V source. Included in this application note are a detailed schematic, complete component list with manufacturer's part numbers, and a planar transformer design example. Conversion efficiency exceeds 84.5% over a 1.5A to 2.5A load range at 48V input. Response to a 1A load transient is below 20mV and settles in 100µs. Ripple is 8mVP-P at a 2.5A load and 60V input. Soft-start provides a 5ms turn on time.
General Description
This Reference Design is based on the MAX5074 evaluation kit, originally designed for supplying 12V, 1.2A from a 36V to 72V input-voltage range. While keeping the simple nonsynchronous architecture, the efficiency at 5V is above 84% at 48V within most of the output-current range. Please refer to the MAX5074 Evaluation Kit data sheet for the warnings appropriate to dangerous voltages and for the general description of the board.
This document includes the Component List for the 5V output application. Performance data are shown. A basic 5-layer planar transformer was built, the layout of which is given for reference.
A 100µF electrolytic capacitor (C19) is added at the input. While its value could be decreased to 33µF, it is important to meet a low ESR to reduce losses due to input ripple current. The tested component (BC Components 2222 13669101) is rated for an ESR < 0.15Ω.
The transformer is based on an ER23 ferrite set, available from several manufacturers. The ferrite material is 3F3, N49, or equivalent, showing low core loss at and above 250KHz.
Windings are realized with five layers of 70µm copper separated by 80µm dielectric layers. Only five layers are used, the auxiliary winding return (pin 6) is connected by an enamel wire strap on the TOP layer (see Figure 3).
The primary has 11 turns, with a measured ESR of 225mΩ ; the secondary has 4 turns with an ESR of 25mΩ ; the auxiliary winding has 4 turns and a required ESR < 1Ω.
Primary inductance measures 340µH, while primary leakage inductance is only 430nH, a direct benefit of the planar construction. Parasitic capacitance between primary and secondary measures 110pF.
The output filter is modified for a lower output voltage: inductor L1 is now 10.2µH and the total output capacitance is 300µF.
The integrating fault protection delay is increased, with C3 now equal to 1µF.
Clamping diodes D1 and D2 are replaced by SSH10 (or equivalent) 100V 1A devices, so forward instantaneous voltage is reduced in case of abnormal loads or transformer defects.
Output voltage is set to 5V by R1 = 30.9K and R2 = 10.2K.
A 180pF capacitor (C21) is added across R1 for a faster reaction of the closed-loop circuits.
A 7.5K resistor (R15) is inserted with C13 to set high-frequency error amplifier gain = 1.
C20, 1nF, replaces R8 to eliminate high-frequency residuals.
R3 is decreased to 180W to set maximum optocoupler LED current to 21mA.
Compensation poles are shifted to accommodate these modifications, so C3, C13, and C14 are now 1µF, 1µF, and 2,2µF, respectively.
Output rectifiers D3 and D4 are replaced by high-current 40V Schottky devices in SMB case.
If a transformer with higher parasitic inductances is used, the output rectifiers must be protected against voltage overshoots.
With a 72V maximum input voltage, only 26V reflected voltage is seen by the rectifiers. This headroom allows rectifier protection by connecting 33V ±5% zeners across them, with appropriate polarities.
As the original Evaluation Kit was not meant to handle high output currents, a heatsink is glued on top of U1 with thermally-conductive glue. This allows a 0.6% gain in efficiency, compared to a no-heatsink situation.
U1 is offered in a 20-lead TSSOP package with exposed pad. Consequently the heatsink could be omitted by a careful PCB layout that allows heat to spread through large power planes and numerous vias.
Efficiency Measurements
Figure 1. Efficiency function of output current at various input-voltag values.
For Larger Images Figure 2. Schematic of the MAX5074 5V, 3A power supply.
Figure 3. Example of transformer realisation with planar technology.
Figure 4. Response to a 1A peak-to-peak transient.
Figure 5. Output-voltage ripple at I = 2.5A, VIN = 60V, BW = 20MHz, 50W adapted.
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