1. Output ripple of switching power supply
①The output ripple of the switching power supply is the fluctuation of the output DC voltage, which is related to the switching action of the switching power supply. During each on and off process, electrical energy is "pumped" from the input end to the output end, forming a charging and discharging process, which causes the output voltage to fluctuate at the same frequency as the switching frequency.
② The reason is that the current ripple of the switching power supply acts on the ESR of the capacitor.
③ Ripple voltage is the peak-to-peak value between the peak and trough of the ripple, and its size is related to the capacity and quality of the input capacitance and output capacitance of the switching power supply.
2. Output noise of switching power supply
①The output noise of the switching power supply refers to the AC amount superimposed on the output voltage under the full bandwidth.
② One of the reasons is the switching power supply itself. The other is the interference of external electromagnetic fields (EMI), which can enter the switching power supply through radiation or enter the switching power supply through the power line.
③ The noise generated by the switching power supply itself is a high-frequency pulse train, which is caused by the sharp pulses generated at the moment when the switch is turned on and off. The frequency of the noise burst is much higher than the switching frequency, and the noise voltage is its peak-to-peak value. The amplitude of the noise voltage is largely related to the topology of the switching power supply, the parasitic states in the circuit, and the design of the PCB.
3. Measures to reduce ripple and noise voltage
①Reduce EMI interference
The metal shell is used as a shield to reduce the radiation interference of the external electromagnetic field. In order to reduce the electromagnetic interference input from the power line, add an EMI filter to the power input end.
② Use capacitors with good high frequency performance and low ESR at the output end
Aluminum or tantalum electrolytic capacitors with polymer solid electrolyte are the best output capacitors, which are characterized by small size and large capacitance, low ESR impedance at high frequencies, and large allowable ripple current. It is most suitable for high-efficiency, low-voltage, high-current step-down DC/DC converters and DC/DC module power supplies as output capacitors.
③ Adopt frequency synchronization with the product system
In order to reduce the output noise, the switching frequency of the power supply should be synchronized with the frequency in the system, that is, the switching power supply adopts the frequency of the external synchronization input system, so that the switching frequency is the same as the frequency of the system.
④ Avoid mutual interference between multiple module power supplies
. There may be multiple module power supplies working together on the same PCB. If the module power supplies are unshielded and close together, they may interfere with each other and increase the output noise voltage. In order to avoid this mutual interference, shielding measures can be adopted or it can be properly kept away to reduce the mutual interference.
⑤Add LC filter
In order to reduce the ripple and noise of the module power supply, LC filters can be added to the input and output ends of the DC/DC module.
⑥Increase LDO
Adding a low dropout linear regulator (LDO) after the output of the switching power supply or module power supply can greatly reduce the output noise to meet the needs of circuits with special requirements for noise, and the output noise can reach μV level.
Since the voltage drop of the LDO (the difference between the input and output voltage) is only a few hundred mV, the standard voltage can be output when the output of the switching power supply is a few hundred mV higher than that of the LDO, and its loss is not large.
⑦ Add active EMI filter and active output ripple attenuator
Active EMI filter can attenuate common-mode and differential-mode noise between 150kHz and 30MHz, and is particularly effective at attenuating low-frequency noise. At 250kHz, it can attenuate 60dB common mode noise and 80dB differential mode noise, and the efficiency can reach 99% at full load.
The output ripple attenuator can reduce the power supply output ripple and noise by more than 30dB in the range of 1~500kHz, and can improve the dynamic response and reduce the output capacitance.