Table 1. Design Parameters

9.2.2.1 Timing Resistor and Capacitor Selection

Generally, a higher switching frequency results in a smaller size but has higher switching losses. Operation at 750 kHz is used in this example as a reasonable compromise between size and efficiency. The values for timing resistance (R_T) and timing capacitance (C_T) are selected for the 1.5-MHz oscillator based on Figure 1.

9.2.2.2 Turns Ratio Selection

The maximum primary-to-secondary turns ratio (N_MAX) can be determined with the target output voltage, minimum input voltage, and the estimated maximum duty cycle. D_LIM = 0.35 is used for this example. N_MAX can be calculated using Equation 1.

Equation 1.

Rounding N_MAX down to the next lowest integer results in a turns ratio of N = 5.

9.2.2.3 Inductor Selection

The maximum inductor ripple current occurs at the maximum input voltage. Typically, 20% to 40% of the full load current ripple is a good compromise between core loss and copper loss of the inductor. Higher ripple current allows for smaller inductor size, but places more burden on the output capacitor to smooth the ripple voltage on the output. In this example a ripple current of 25% of 10 A is used. The inductor value can be calculated with Equation 2.

Equation 2.

The closest standard value of 0.8 µH is chosen for L_O, this step is necessary if the chosen L_O differs significantly from the value calculated in Equation 2. The actual ΔI_L is based upon this selected inductor which must be calculated with Equation 3.

Equation 3.

9.2.2.4 Rectifier Diode Selection

A rectifier diode must always possess low-forward voltage drop. When used in high-frequency switching applications, however, the diode must also posses a short recovery time. Schottky diodes meet both requirements and TI recommends their use for push-pull converter designs.

9.2.2.5 Snubber Components Selection

A resistor-capacitor snubber network crossing the low-side MOSFET reduces ringing and spikes at the switching node. Excessive ringing and spikes can cause erratic operation and may couple noise to the output voltage. Selecting the values for the snubber is best accomplished through empirical methods. First, make sure the lead lengths for the snubber connections are very short. Start with a resistor value between 5 Ω and 50 Ω. Increasing the value of the snubber capacitor results in more damping, but higher snubber losses. Select a minimum value for the snubber capacitor that provides adequate damping of the spikes on the switch node waveform at heavy load. A snubber may not be necessary with an optimized layout.

9.2.2.6 VCC and VC Capacitor Selection

The primary purpose of the VCC and VC capacitor is to supply the peak transient currents of the drivers as well as provide stability for the VCC and VC regulator. These peak currents can be several amperes. The value of the VCC and VC capacitor must at least 0.47 µF, and must be a good quality, low ESR, ceramic capacitor. The VCC and VC capacitor must be placed at the pins of the IC to minimize potentially damaging voltage transients caused by trace inductance. A value of 4.7 µF is used in this design.

9.2.2.7 Output Capacitor Selection

The output capacitors smooth the output voltage ripple caused by inductor ripple current and provide a source of charge during load transient conditions. In this design example, a 6-µF capacitor is selected as the main output capacitor.

9.2.2.8 Input Capacitor Selection

The input supply voltage typically has high source impedance at the switching frequency. Good quality input capacitors are necessary to limit the ripple voltage at the VIN pin while supplying most of the switch current during the on-time. The input capacitor must be selected for RMS current rating and minimum ripple voltage. In this example, a 4.7-µF capacitor is used. With ceramic capacitors, the input ripple voltage is triangular.