Power supply ripple and ripple coefficient
The main function of a power supply is to provide electrical energy for electronic products, but power supply will inevitably introduce ripples, noise, etc., 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 circuits, operational amplifier circuits, rectifier filter circuits, etc. Common applications have the following hazards:
Generate undesired harmonics, causing overvoltage or overcurrent to cause accidents; increase additional losses and reduce the efficiency and utilization of electrical equipment;
Cause equipment to operate abnormally, accelerate aging, and shorten life; cause relay protection, automatic devices, computer systems and other equipment to operate abnormally or fail to operate normally;
It can cause deviations in measurement and metering instruments; 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 coefficient
Strictly speaking, the regulated power supply includes four parts: power transformer, rectifier circuit, filter circuit and voltage stabilizing circuit. Since DC-DC can also be regarded as a regulated power supply, the rectifier circuit, filter circuit and voltage stabilizing circuit are regarded as the three necessary parts of the regulated power supply [1].
The rectifier circuit uses one-way 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 components.
The filter circuit uses energy storage components to convert pulsating DC power into a relatively flat DC power. Due to the different performance of the filter circuit, although it can filter out most of the AC components, it cannot completely filter it 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 along with the stable voltage output is called ripple voltage.
In order to characterize the performance of DC regulated power supply filtering, the concept of ripple coefficient is introduced [2-3]. The ripple coefficient ψ is defined as the percentage value of the effective value of the ripple voltage Vr and the DC output voltage Vo, that is:
The ripple coefficient is an important indicator for evaluating the stable and pure output of a 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 (DSO).
Electronic loads facilitate current adjustment and are generally set in constant resistance mode (CR); digital storage oscilloscopes can directly capture the entire ripple waveform, store, amplify and read out the ripple value. Substitute the oscilloscope reading into the formula to get the ripple coefficient.
When measuring, you must pay attention to the following two points (these two points are particularly important for the accuracy of the measurement results):
(1) The probe ground wire of the digital storage oscilloscope must be unplugged and the ground spring pin in the probe assembly must be used instead. It can prevent ground loops from coupling into EMI noise, making measurement results inaccurate.
The probe ground wire is too long and the loop area is too large, forming a receiving antenna and causing high-frequency clutter or EMI noise to couple into the measured signal.
(2) The digital storage oscilloscope itself needs to adjust its settings.
The digital storage oscilloscope needs to be well grounded to further filter out the noise added from the power supply end; use the AC coupling of the digital storage oscilloscope to block DC, making the ripple test more intuitive and accurate;
General ripple testing 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
To suppress the ripple of the output voltage of a regulated power supply, the following four methods are generally adopted: RLC filtering method, common mode filtering method, ferrite magnetic ring filtering method, and a combination of the three methods.
The filter circuit that suppresses DC-DC power supply ripple is demonstrated through experimental verification. In the verification experiment, a 100W DC-DC power supply, 48V input, 5V output, Model SD-100C-5 from Meanwell was selected.
The digital storage oscilloscope chooses GWINSTEK's GDS-1072B, with a bandwidth of 70MHz, a sampling rate of 1GSa/s, and a storage depth of 10M per channel.
The electronic load is PEL-3021 from GWINSTEK, with a voltage range of 1.5V~150V, a current range of 0~35A, and a power of 175W.
According to this calculation, the current in the circuit is 20A. Figure 3 shows the connection block diagram of the power supply ripple test.
In order to make the effect of suppressing power supply ripple more intuitive and obvious, first short-circuit the filter circuit of SD-100C-5 and measure the ripple of its output voltage. From this, it can be seen that the power supply ripple is approximately 85.6mVpp and the effective value is 48.2mVrms.
