How to Reduce Ripple of Switching Power Supply?
As to the ripple wave of the switching power supply, it certainly exists both theoretically and practically. Generally speaking, here are some approaches restraining or reducing it.
Reducing EMI - diode back connection inductance
The metal shell can be adopted as the shield which is used to reduce the radiated interference of the external electromagnetic field. In order to reduce the electromagnetic interference of the power line input, the power input terminal is equipped with the EMI power line filter, just as the below diagram shows (EMI filter is also named as the power line filter). This is also a method usually used to restrain the high-frequency noise.
Adopting capacitance with good high-frequency performance and low ESR on the output terminal
It is the best to adopt aluminum or tantalum electrolytic capacitance of high-molecular polymer solid electrolyte as the output capacitance. Its characteristics include small size and big capacitance, low ESR resistance under high frequency, thus allowing high ripple current. It is most appropriate to high-efficiency, low-voltage, high-current decompression-type DC/DC converter and DC/DC module power source as the output capacitance.
For instance, a kind of high-molecular polymer Ta solid electrolytic capacitor is 68μF. Its maximum value of equivalent series resistance (ESR) is 25mΩ in 100kHz when the temperature is 20℃. And the maximum allowed ripple current (in 100 kHz) is 2400mA (rms). Its size is 7.3mm (length) × 4.3mm (width) × 1.8mm (height).
The ripple voltage ΔVOUT is
ΔVOUT = ΔIOUT × ESR
If ΔIOUT = 0.5A, ESR = 25mΩ,
Then ΔVOUT = 12.5mV.
If the ordinary aluminum electrolytic capacitor is used as the output capacitor with the rated voltage of 10V and the rated capacitance of 100μF, the equivalent series resistance in 20℃ and 120Hz is 5.0Ω, and the maximum ripple current is 70mA. It can only work under 10 kHz, but it is unable to work under high frequency (frequency above 100 kHz). It is still invalid if the capacitance is increased, because it has become the inductance characteristics when the frequency exceeds 10 kHz.
Some powers with the switching frequency between 100 kHz to several hundreds of kHz also have good effects in adopting multilayer ceramic capacitance (MLCC) or Ta electrolytic capacitor as the output capacitance. Its price is much lower than that of the high-molecular polymer solid electrolyte capacitance.
Adopting a switching power supply in synchronous frequency with the system
In order to reduce the output noise, the switching power frequency of the power supply should be synchronous with the system frequency. In other words, the switching power supply adopts the frequency of the external synchronous input system, thus enabling the same frequency between the switching frequency and system frequency.
Avoiding the mutual interference among several module power supplies
On the same PCB, there might be several module power supplies working together. If the module power supplies are not shielded and they get very close, they might have mutual interference, so as to increase the output noise voltage. In order to avoid such mutual interference, the shielding measures can be adopted or they can be kept with a certain distance, so as to reduce the mutual interference.
For example, when two K7805-500 switching mode modules are constituted into the ±5V output power supply, if the two modules are very close, the output capacitance C4 and C2 fail to adopt the low-ESR capacitance, and the welding part is relatively away from the output terminal, there is a possibility that the ripple wave and noise voltage increase because of mutual interference, just as the below figure shows.If the same PCB has circuits which might cause noise interference, similar measures should be adopted in designing PCB so as to reduce the mutual interference and influence of the interference circuit to the switching power supply.
Adding LC filter in power supply
The restraining effect of LC filter to noise ripple wave is relatively significant. The appropriate inductance capacitance shall be selected according to the output ripple frequency to constitute the filter circuit. Generally, it can well reduce the ripple wave. In order to reduce the ripple wave and noise of the module power supply, LC filter can be equipped to the input and output terminals of DC/DC module.
Table 1 and table 2 list the capacitance values of the VIN terminal and VOUT terminal of 1W DC/DC module under different output voltages. It is important to note that the capacitance should not be too high, thus avoiding the launching problems. The resonant frequency of LC should stagger the switching frequency, so as to avoid mutual interference. L adopts μH polarity. Its DC resistance must be low, so as to avoid affecting the output voltage accuracy.
|Single output DC/DC module (Table 1)|
|Dual output DC/DC module (Table 2)|
Adding LDO after switching power supply output
After switching power supply output or module power output, one low dropout regulator (LDO) will be equipped, which can dramatically the output noise, so as to satisfy the demand of the circuit having special noise requirement (See the below Figure). The output noise can reach μV grade.
Because the dropout voltage of LDO (difference value between the input and output voltages) is only several hundreds of mV, the standard voltage can be put out if the output of the switching power supply is several hundreds of mV higher than that of LDO, and the loss is not high. As to the switching ripple wave of several hundreds of KHz, the restraining effect of LDO is very good. However, in the high frequency scope, LDO effect is not so ideal.
Adding active EMI filter and active output ripple wave attenuator
Active EMI filter can decay the noise of the common mode and differential mode in 150 kHz ~ 30 MHz, and it is especially effective in decaying the low-frequency noise. In 250 kHz, it can decay 60dB common mode noise and 80dB differential mode noise. In full load, the efficiency can reach 99%. The output ripple attenuator can reduce the power supply output ripple and noise for over 30dB in the scope of 1 ~ 500 kHz, and it can improve the dynamic response and reduce the output capacitance.