Electronic Design

Isolated DC-DC Converter Sports Dual Outputs

Presented here is an isolated 6-W dc-dc converter optimized to provide low-cost and efficient isolated power across a 2500-V ac potential barrier. The converter, which incorporates inexpensive, readily available components, can provide either two isolated 3-W outputs (Fig. 1a) or one isolated 6-W output (Fig. 1b).

The CD4001, a CMOS quad NOR gate, is configured as an RC relaxation oscillator. The frequency is set at approximately:

FOSC = 0.6/(R2 × C1)
      = 0.6/(10 kΩ × 220 pF)
      = 273 kHz

Actual measurement of the circuit came out to 250 kHz. Resistor R3 has a value that may vary and might need to be terminated to VDD or ground. R3 is inserted to adjust the oscillator trip point so that a 50% duty-cycle waveform is available at the output of U1a and its complement is available at U1b.

This square wave and its complement are connected to the INA and INB inputs of the IXDD404SI, a 4-A dual-gate driver. The outputs, OUTA and OUTB of U2, are then applied to the primary of T1, a VAC #T60403-F4025-X142 transformer that has a single primary and 2500-V ac isolated, dual-secondary windings.

The converter is designed to function over a 13.5- to 25-V VCC power range. C5, which blocks dc from T1's primary, and C6 and C7, which filter the floating dc outputs, are all low-voltage, low equivalent-series resistance and equivalent-series inductance, multilayer ceramic capacitors. D1 and D2, which are DDS10-40BA 1-A Schottky diodes, half-wave rectify the resulting square-wave output from T1.

D1 and D2 are connected in an anti-parallel configuration, minimizing T1 saturation at high currents. For a single isolated output, the portion of the converter after T1's secondary is modified as shown in Figure 1b, where D1 and D2 form a push-pull output. Because of the wide VCC supply-voltage variation, a 15-V zener diode (Z1) is used to limit the voltage supplied for U1 to +15 V or less.

Total manufacturing volume costs for the isolated dc-dc converters are less than $3, or about $0.50/W. The total volume of the solution is about 0.25-in.3

Figure 2 shows both VOUT1 and VOUT2 loaded equally with RL1 = RL2. Figure 3 plots converter efficiency, (POUT/PIN) × 100%, for various power outputs and VCC voltages where:

POUT = (VFloat1)2/RL1 + (VFloat2)2/RL2,

and PIN = VCC × ICC

Note that efficiency is over 80% for total VOUT1 and VOUT2 power levels of 6 W, or 200 mA each for VOUT1 and VOUT2 set at 15 V.

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