Power supply ripple and ripple factor
The main function of the power supply is to provide electrical energy for electronic products, but it will inevitably introduce ripples, noise, etc. during the power supply, which will reduce the stability and reliability of the electronic system and even the entire product.
Voltage ripple can greatly affect various circuits of the power supply, such as A/D conversion circuit, operational amplifier circuit, rectifier filter circuit, etc. Common applications have the following hazards:
Unexpected harmonics are generated to cause accidents caused by overvoltage or overcurrent; increase additional loss and reduce the efficiency and utilization of electrical equipment;
Make the equipment run abnormally, accelerate aging, and shorten the service life; make relay protection, automatic devices, computer systems and other equipment run abnormally or fail to operate normally;
Make measuring and measuring instruments deviate; interfere with communication systems, reduce signal transmission quality, and even damage communication equipment.
Therefore, when designing electronic products, it is necessary to accurately measure the ripple and suppress the ripple within a certain range.
1 Power supply ripple and ripple factor
Strictly speaking, the stabilized power supply includes four parts: power transformer, rectifier circuit, filter circuit and voltage stabilizer circuit. Since DC-DC can also be regarded as a stabilized power supply, the rectifier circuit, filter circuit and stabilized voltage circuit are regarded as the necessary three parts of the stabilized power supply [1].
The rectifier circuit uses unidirectional conductive devices to convert alternating current into pulsating direct current. The pulsating direct current is not smooth and contains a large amount of alternating current.
The filter circuit uses the energy storage element to convert the pulsating direct current into a relatively flat direct current. Due to the different performance of the filter circuit, although most of the alternating current components can be filtered out, they cannot be completely filtered out.
The voltage stabilizing circuit after rectification and filtering uses the adjustment function of the circuit to stabilize the output voltage and reduce the AC component to a minimum. This AC component that cannot be completely filtered out with the stable voltage output is called ripple voltage.
In order to characterize the filter performance of DC regulated power supply, the concept of ripple coefficient is introduced [2-3]. Define the ripple coefficient ψ as the percentage value of the ripple voltage effective value Vr and the DC output voltage Vo, namely:
Ripple coefficient is an important index to evaluate the stable and pure output of DC power supply. According to the above formula, it can be seen that the ripple voltage needs to be measured to find the ripple coefficient.
2 Measurement of power supply ripple
Accurate measurement of power supply ripple generally requires two instruments, namely electronic load (Electronic Load) and digital storage oscilloscope (Digital Storage Oscilloscope, DSO).
The electronic load is convenient to adjust the current, and it is generally set in the constant resistance mode (CR); the digital storage oscilloscope can directly capture the entire ripple waveform, store and amplify it, and read out the ripple value. Substitute the oscilloscope reading into the formula to get the ripple factor.
When measuring, you must pay attention to the following two points (these two points are especially important for the accuracy of the measurement results):
(1) The probe ground wire of the digital storage oscilloscope must be unplugged and replaced with the ground spring pin in the probe assembly. It can prevent the ground loop coupling into EMI noise and make the measurement result inaccurate.
The ground wire of the probe is too long and the loop area is too large, forming a receiving antenna, high-frequency clutter or EMI noise will be coupled into the measured signal.
(2) The digital storage oscilloscope itself needs to adjust the settings.
The digital storage oscilloscope needs to have a good grounding to further filter out the clutter added from the power supply; use the AC coupling of the digital storage oscilloscope to block the direct current, making the ripple test more intuitive and accurate;
The general ripple test requires the frequency to be limited below 20MHz, so the digital storage oscilloscope should open the 20MHz bandwidth limit to isolate high-frequency noise.
3 Methods to suppress power supply ripple
There are generally four methods to suppress the ripple of the output voltage of the regulated power supply: RLC filtering method, common mode filtering method, ferrite magnetic ring filtering method and a combination of the three methods.
The filter circuit for suppressing DC-DC power supply ripple is demonstrated through experimental verification. In the verification experiment, a 100W DC-DC power supply with 48V input and 5V output is selected, and the model is SD-100C-5 of Meanwell.
The digital storage oscilloscope chooses GDS-1072B of GWINSTEK, the bandwidth is 70MHz, the sampling rate is 1GSa/s, and the storage depth of each channel is 10M.
The electronic load is PEL-3021 from GWINSTEK, the voltage range is 1.5V~150V, the current range is 0~35A, and the power is 175W.
According to this calculation, the current in the circuit is 20A. Figure 3 is the connection block diagram of the power ripple test.
In order to make the effect of suppressing the power supply ripple more intuitive and obvious, the filter circuit of SD-100C-5 is short-circuited first, and the ripple of its output voltage is measured. It can be obtained that the power supply ripple is roughly 85.6mVpp, and the effective value is 48.2mVrms.
