Switching power supplies are rarely used in RF or low-frequency analog circuit designs, as they can introduce noise and interference. However, there are specific scenarios where a switching power supply becomes necessary to meet system performance requirements.
1. When the input voltage is lower than what the system requires, a boost chip is needed. If the power supply is part of a sensitive analog or RF signal path, it's better to use an LC filter or a primary LDO to reduce the voltage with minimal ripple. This helps maintain signal integrity and reduces noise.
2. If the system needs a negative voltage, a switching power supply can convert the positive voltage into a negative one. Again, for sensitive circuits, using an LC filter or a dedicated negative voltage LDO can help achieve a cleaner output with less ripple.
3. When the input voltage is significantly higher than what the system needs, and the current demand is high, using an LDO or a three-terminal regulator may lead to excessive power loss and heat generation. In such cases, it’s more efficient to first step down the voltage using a switching power supply and then use an LDO for final regulation, improving both efficiency and thermal management.
There are several important considerations when using switching power supplies:
1. Recently, we encountered an issue with a positive-to-negative voltage converter where the output negative voltage was insufficient. When the load was disconnected, the voltage returned to normal. Initially, we suspected a problem in the downstream circuit, but after stabilizing the power supply, we found that the negative voltage had no current limit. Testing with a multimeter showed only 50 mA of current. After removing a large inductor connected in series with the positive output of the chip, the output functioned properly. This shows that adding a large inductor on the input line can interfere with the switching power supply’s operation, even if it separates DC components to some extent.
2. Although switching power supplies are highly efficient, they are not 100% efficient. Their thermal resistance is relatively low, so proper heat dissipation is crucial. It’s essential to calculate the system's required voltage and current, estimate the power consumption, and determine the losses based on the efficiency curve. Then, using the thermal resistance, check whether the chip temperature will exceed its maximum allowable value. Always include a safety margin when selecting the component to ensure long-term reliability and stability.
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