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Tips and Ticks for AC-DC Module Layout

Recently, our new hardware engineer Li Yun studies IWATT’s AC-DC solution. He finds out that the basic theories of all solutions are the same, and the different performance due to using different electronic components. Among all the AC-DC solutions, there are two important signals that control the performance of output – ISENSE and VSENSE signals. ISENSE shall be placed near the switch in order to avoid the signal interference, meanwhile, pins shall be placed near the ISENSE. To improve the quality of feedback signals, VSENSE shall be placed near the transformer. Except that, all bypass capacitors shall be placed near their own pins to improve the performance of transmission.

To reduce the EMI, the switch loop circuit must be minimized. The main loop circuits include:

  1. First loop circuit comprises large input capacitor, primary winding, switch, and RISENSE.
  2. Second loop circuit comprises output diode, output capacitor, and secondary winding.
  3. Third loop circuit comprises assist winding, rectifier diode, and VCC.

To improve the performance, the lines between AC power supply and transformer shall adopt low-impedance lines and you must make sure that the line should not go through the GND pin of IC. Secondary power supply shall keep a distance from the AC power.
In the power switch, there are some ground signals. They are power ground, switch ground, and control logic ground. These signals shall adopt the three-phrase wye-wye connection, and they must be as short as possible. Besides, it is useful to reduce the switch consumption when the square of switch connecting to ground enlarges.