Switching power supplies are not commonly used in RF or low-frequency analog circuit designs due to their potential for noise and interference. However, there are specific scenarios where they become essential to meet the system's performance requirements.
1. When the input voltage is lower than what the system requires, a boost chip is necessary. If the power supply is part of a sensitive analog or RF signal path, it’s advisable to use an LC filter or a primary LDO to reduce the voltage and achieve a low-noise output.
2. If the system needs a negative voltage, a switching power supply can convert a positive voltage into a negative one. Again, if this is part of a sensitive signal path, using an LC network or a negative voltage LDO can help maintain a stable and clean output.
3. When the input voltage is significantly higher than the required system voltage, and the current demand is high, using an LDO or a three-terminal regulator may result in excessive power loss and heat generation. In such cases, it’s better to first step down the voltage with a switching power supply and then use an LDO for further regulation.
There are also some common issues to watch out for when using switching power supplies:
1. Recently, when using a positive-to-negative power supply chip, it was observed that the output negative voltage was insufficient. When the load was disconnected, the voltage returned to normal. Initially, it was suspected that the issue was in the later stages of the circuit, but after stabilizing the power supply, the negative voltage had no current limit. Testing with a multimeter showed only 50 mA of current. Later, removing a large inductor connected in series with the positive output of the chip restored normal operation. This highlights how adding a large inductor on the input line can interfere with the switching power supply’s operation, even though it helps separate DC components.
2. Although switching power supplies are efficient, they are not 100% efficient. Their thermal resistance is relatively low, which means they don’t dissipate heat as effectively as other types of regulators. Therefore, it's important to calculate the system's voltage and current requirements, estimate the power consumption based on the efficiency curve, and determine the power loss on the chip. Using the thermal resistance, you can check whether the chip temperature will exceed its safe operating range. It’s always wise to include a safety margin when selecting the appropriate chip for your design.
By carefully considering these factors, you can ensure that your switching power supply design is both effective and reliable, while minimizing noise and maximizing performance.
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