AN892 Datasheet, PDF (11/16 Pages) Silicon Laboratories – DESIGN GUIDE FOR ISOLATED DC/DC USING THE

Figure 7 shows the magnitude Bode plot of the loop in PI mode.
AN892
Figure 7. Simplified Transfer Function PI Mode
4. Design Example
Consider the desired requirements listed in Table 1.
Table 1. Design Requirements
Parameter
Input Voltage
Output Voltage
Input Voltage
Ripple
Output Voltage
Ripple
Maximum Output
Current
Value
5.0 V  10%
5.0 V
 150 mV
 50 mV
400 mA
4.1. Transformer Design
The design of a transformer for the asymmetric half bridge flyback is very similar to the design of a flyback
transformer operating in continuous current mode. This section provides a starting point for the transformer design
which is often an interative process.
Equation 13 establishes the relationship between turns ratio n and duty cycle D. For this design, D=0.25 was
chosen. The formulas derived in the DC steady state section are based on ideal elements. In practice, a larger duty
cycle is expected due to losses in the parasitics. Accounting for forward voltage drop across D1 of 0.5V and solving
Equation 13 for turns ratio:
Rev. 0.1
11