Board Layout and Summary

＜AC-DC Design Examples of Non-isolated Buck Converters＞

1. Overview of Design Examples of AC-DC Non-isolated Buck Converters

   Key Points

   ・The design of non-isolated AC-DC converters is described.

   ・An example circuit known as a diode-rectified or asynchronous-rectified buck converter is considered.

2. Basic Operation of Buck Converters and Discontinuous Mode vs. Continuous Mode

   Key Points

   ・Step-down converter operation is in either a continuous mode or a discontinuous mode.

   ・In DC-DC conversion, the continuous mode is generally used, but in AC-DC conversion at about 60 W, the discontinuous mode is often used.

3. Selection of Power Supply ICs and Design Examples

   Key Points

   ・Design begins with selection of a power supply IC that satisfies the power supply specifications.

   ・It is important to understand the differences with isolated-type circuits.

4. Selecting Critical Components: Input Capacitor C1 and VCC Capacitor C2

   Key Points

   ・The rated voltage of the input capacitor is selected considering that a voltage equal to 1.41 times the maximum input voltage is applied.

   ・It should be remembered that in addition to stabilizing VCC, the VCC capacitor also determines the startup time.

5. Selecting Critical Components: Inductor L1

   Key Points

   ・The inductor should be set such that the operating mode is discontinuous mode

   ・The inductance is determined from the VIN minimum condition and the maximum value of ton.

   ・The inductor current is determined from the VIN maximum condition and the minimum on-time.

6. Selecting Critical Components:　Current Sense Resistor R1

   Key Points

   ・The switching current limiting resistor R1 required in the example circuit is determined.

   ・In calculating R1, the numerical values used when calculating the inductance L1 are necessary.

7. Selecting Critical Components: Output Capacitor C5

   Key Points

   ・An output capacitor is selected based on the ripple current and capacitor impedance, so as to satisfy the output ripple voltage design target.

   ・Aluminum electrolytic capacitors are components with limited lifetimes, and the lifetime is shortened by large ripple currents.

8. Selecting Critical Components: Output Rectifying Diode D4

   Key Points

   ・Normally, a device capable of fast switching is used as the output rectifying diode. Here, a fast recovery diode is used.

   ・An output rectifying diode is in essence selected based on consideration of the rated voltage and losses.

9. EMI Countermeasures

   Key Points

   ・EMI countermeasures involve adding input filters, capacitors at switches (across drain and source), and snubbers at output rectifying diodes.

   ・LC filters are added to the output to address output noise.

   ・The board layout also has a considerable impact, and should be studied as well.

10. Board Layout and Summary

    Key Points

    ・The board layout has a great influence on performance and noise in switching power supply design, regardless of whether the design is for AC-DC or for DC-DC conversion.