The Role of LDOs in Switching Power SuppliesThe Role of LDOs in Switching Power Supplies
1. Simplifying switching power supply design
Switching power supply multiple outputs are generally realised by adding a high-frequency transformer feedback terminal, which makes the switching power supply increase the designer's workload during the design process. The application of LDO as the output terminal of switching power supply can greatly simplify the design of switching power supply and shorten the development cycle.
2. Improve the load regulation rate of switching power supply
LDO is a special chip to stabilise the power supply voltage, and there are many companies designing LDOs with very small load regulation rate. The application of LDO can significantly reduce the load regulation rate of switching power supply.
3. Effectively filter out the electromagnetic interference of the switching power supply, reduce the ripple output
Switching power supply is the outstanding disadvantage of generating strong EMI. EMI signals have both a wide frequency range, but also a certain amplitude, by conduction and radiation will contaminate the electromagnetic environment, communications equipment and electronic products cause interference. If not properly handled, the switching power supply itself will become a source of interference. LDO has a high power supply rejection ratio, and LDO is a low noise device, so the application of LDO can effectively filter the switching power supply EMI, reduce the ripple output.
4. Provide overcurrent protection for switching power supplies
Although many PWM control chips themselves have overcurrent protection, but the overcurrent protection function of the LDO can enhance the safety factor of the switching power supply.
Test Analysis
The following two experiments are conducted to verify the feasibility of the scheme:
1. Measuring load regulation ratio
The experimental circuit is shown in Figure 3. The current from 0mA to 400mA is pulled out sequentially by the electronic load, and the output voltage of the switching power supply is recorded at each load point. The test data is processed to produce the graph shown in Figure 4. The graph amply demonstrates that the excellent load tuning ratio of the LDO has been fully transplanted to the switching power supply. In other words, the LDO has greatly improved the load regulation rate of the switching power supply.
2. Measurement of Output Ripple
Connecting an oscilloscope to the LDO input and output of the switching power supply respectively, the waveforms shown in Figure 5 can be derived. Where Ch1 is the output waveform at the entrance of the LDO, and Ch2 is the output waveform at the exit of the LDO, the final output waveform of the switching power supply.
As can be seen from the above figure, the LDO effectively filters out the switching power supply EMI signals, as opposed to building conventional EMI filters, the application of LDO is more simple and reliable.
