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How to select the Output Capacitor and TVS when designing Power Supply

The power supply is one of the core components in the system, and the stability of output voltage is particularly important. The output capacitor can effectively reduce the output voltage ripple, and the TVS can effectively suppress the transient overvoltage in the circuit.

  1. Why the Output Capacitor and TVS play important roles in the design or application of switching power supply?
  2. How to select the Output Capacitor when designing a power peripheral circuit?
  3. How to select the TVS when designing a power peripheral circuit?

You can find the answers from this article.

  1. Why the Output Capacitor and TVS play important roles in the design or application of switching power supply?

① C2 and C3 are electrolytic capacitors, used as output rectifier filters, and provide protection against over-voltage surge.

② C4 is a ceramic capacitor to remove high-frequency noise, with a general value from 0.1 to 4.7uF.

Mornsun Design
Overall Design of the Peripheral of DIY Power LS Series (AC/DC)

③ TVS can effectively protect against overvoltage surge caused by lightning, load switch, and other operation errors.

  1. How to select the Output Capacitor when designing a power peripheral circuit?

Output electrolytic capacitors are commonly used as filtering when applied in switching power supplies. Under low temperature conditions, liquid aluminum electrolytic capacitors will suffer from significant capacity reduction or even failure due to electrolyte freezing.

The product is divided into external capacitor type (such as MORNSUN LS series) and build-in capacitor type (such as MORNSUN LD / LH series) according to whether capacitor is built-in or not. The output capacitor shall not be too large, its maximum capacitance value shall not exceed the maximum capacitive load value of the power product (refer to the datasheet/specifications), otherwise, the product may not start up normally.

① External capacitor type

Figure 1 is an ACDC power supply with an external electrolytic capacitor (such as MORNSUN LS series). C3 is an output filter electrolytic capacitor (must be connected externally). It forms a Pi-type filter circuit with C2 and L1. C2 is recommended to use high-frequency and low-resistance electrolytic capacitors (ESR ≤ 1.1 Ω at – 40 ℃) or solid capacitors. If it is only used in normal-temperature and high-temperature environments, an electrolytic capacitor can be used as C2. For capacity and rated ripple current of the electrolytic capacitor, please refer to the technical specifications provided by the manufacturer. If the selected capacitance value is too low, it will lead to problems such as low output voltage accuracy and large output ripple.

② Built-in capacitor type

Figure 2 is an ACDC power supply with a built-in electrolytic capacitor. The output electrolytic capacitor C2 is mainly used for filtering. Generally, 10uF ~ 47uF can be selected. For details, please refer to the recommended values in the product datasheet/technical manual.

Mornsun LD Series
Fig 2: Recommended Circuit Diagram of Power Module Output Filter & Output Protection (MORNSUN LD Series)

C1 in Figure 2 and C4 in Figure 1 are ceramic capacitors used to filter high-frequency noise, with the general value from 0.1 to 4.7uF. For details, please refer to the product technical manual. It is recommend to consider the derating of capacitor withstand voltage, actual withstand voltage=80% rated voltage, and the following standard values shall be preferred for the rated voltage: 6.3V, 10V, 16V, 25V, 50V, 100V, 250V, etc.

  1. How to select the TVS when designing a power peripheral circuit?

TVS can effectively suppress overvoltage pulse interference at output side of Power supply which caused by load swiching or coupling input side surge. In general, the power supply itself does not need additional TVS for protection. It is recommended to use TVS when the output line is long (interference will be introduced) or when there is inductive load (voltage spike will be generated). It should close to the circuit that requires protection. Especially when the back-end load is MCU, TVS can effectively suppress overvoltage Pulse interference. The SMAJ, SMBJ, SMCJ, and SMDJ series of common chip packages of TVS represent peak pulse power of 400W, 600W, 1500W, and 3000W respectively.

① The specification selection is generally selected according to (breakdown voltage VBR) ≥ 1.2 times the output voltage.

② Selection of pulse peak current IPP: select the appropriate model according to the maximum surge current that may appear on the circuit.

③ For example, in the LD20-23B15R2 output circuit (Vo: 15V), SMBJ20A is recommended, and the breakdown voltage is 20V (VBR = 1.33Vo).

  1. Summary

The information age is swarmed with a power supply. With the development of science and technology, the requirements for the safety and stability of the power supply become increasingly higher. Adding protective circuits, filtering and other measures to the output of the switching power supply can greatly improve the safety, reliability, and stability of the power supply. Having been deeply involved in eight industries, MORNSUN has tens of thousands of product models and is able to provide related peripheral recommended circuits when applied to different products and environments.

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