What should I do if the power ripple is large
Time:2023-03-09
Views:1201
1、 Generation of power ripple
Our common power supplies include linear power supplies and switching power supplies. Their output DC voltage is obtained by rectifying, filtering and stabilizing the AC voltage. Because the filter is not clean, the clutter signal containing periodic and random components will be attached above the DC level, which will generate ripple.
Under the condition of rated output voltage and current, the peak value of AC voltage in the output DC voltage is usually called ripple voltage.
Ripple is a complex clutter signal. It is a periodic signal that fluctuates up and down around the output DC voltage. However, the period and amplitude are not fixed values, but change with time, and the ripple shape of different power sources is also different.
2、 Damage of ripple
Generally speaking, ripple is harmful without any benefit. The main hazards of ripple are as follows:
Ripples carried by the power supply will generate harmonics on the electrical equipment and reduce the efficiency of the power supply;
Higher ripple may produce surge voltage or current, which may lead to abnormal operation of electrical equipment or accelerate equipment aging;
In digital circuits, the ripple will interfere with the logic relationship of the circuit;
Ripple will also bring noise interference to communication, measuring and measuring instruments and meters, damage the normal measurement and measurement of signals, and even damage equipment.
So when making power supply, we should consider reducing the ripple to less than a few percent. For equipment with high ripple requirements, we should consider reducing the ripple to smaller.
The measurement methods of power ripple are generally divided into two categories, one is the identification of individual power supply, and the other is the commissioning measurement of products.
In the power supply industry and power supply users, it is required to select indoor (about 20 ℃) for power supply identification, and the humidity should be less than 80%. The surrounding mechanical vibration and electromagnetic interference that may affect the measurement, the standard instrument and the tested power supply should be placed in the above test environment for more than 24 hours.
For pure power supply, when measuring the ripple of power supply, it is required to measure at the time of loading, and the applied load should make the output current greater than 80% of the rated output current.
For low-noise pure resistive load or electronic load, the corresponding measurement standard should also be selected. Different standards will produce different measurement results.
Ripple voltage can be expressed by quantity or relative quantity. Generally, the ratio of ripple voltage to DC output voltage is used to evaluate the filtering performance of DC power supply, that is, ripple coefficient.
Ripple coefficient is an important index to evaluate DC power supply. Its calculation method is the effective value of ripple voltage and the percentage of DC output voltage.
3、 Measurement of power ripple
The power supply ripple is generally measured by oscilloscope, and the following three measurement methods are commonly used:
Relying on connection method: use the oscilloscope probe with ground wire ring to contact the probe directly with the positive output pin and the line ring directly with the negative output pin. This is because the loop is as short as possible, so the peak value read from the oscilloscope is the ripple and noise on the output line
When measuring ripple, it should be noted that the upper limit of ripple bandwidth should be clear. Ripple is low-frequency noise, so oscilloscopes that do not exceed the upper limit of ripple bandwidth are generally used.
When measuring, first turn on the bandwidth limit function of the oscilloscope, limit the bandwidth to 20MHz, and directly connect the shielded ground and output ground of the probe to reduce the loop interference caused by the long ground wire.
A small ceramic chip capacitor and a small electrolytic capacitor are connected in parallel at the probe access point to filter out external interference signals and prevent them from entering the oscilloscope.
4、 Ripple suppression method
The power output ripple mainly includes five aspects: low frequency input ripple, high frequency ripple, common mode ripple noise caused by parasitic parameters, and ripple noise caused by closed-loop control.
The common methods to suppress these ripples are to increase the capacitance in the filter circuit, use LC filter circuit, use multi-level filter circuit, use linear power supply instead of switching power supply, and reasonable wiring.
But according to its classification, targeted measures often achieve twice the result with half the effort.
Suppression of high-frequency ripple: high-frequency ripple noise is more than high-frequency power conversion circuit. In the high-frequency power conversion circuit, the input DC voltage is converted by the high-frequency power device and then rectified and filtered to achieve the stabilized output, which generally contains high-frequency ripple with the same frequency as the switch operating frequency.
Its influence on the external circuit is mainly related to the conversion frequency of the switching power supply, the structure and parameters of the output filter. In the design, the working frequency of the power converter should be increased as much as possible to reduce the filtering requirements for high-frequency switching ripple.
Suppression of low-frequency ripple: the size of low-frequency ripple is related to the size of filter capacitance in the output circuit. The capacity of the capacitor cannot be increased indefinitely, which will inevitably result in the residual output low frequency ripple.
The AC ripple is output after being attenuated by the DC/DC converter circuit and belongs to the low frequency noise range. Its size is determined by the gain of the control system and the DC/DC converter circuit.
Because the ripple suppression ability of current mode and voltage mode control DC/DC converter circuit is relatively low, and their output low frequency AC ripple is large. Therefore, it is necessary to take filtering measures to achieve low ripple output of the power supply.
For some power supplies, the closed-loop gain circuit of DC/DC converter can be increased and the pre-voltage stabilizing circuit can be used to enhance the ripple suppression effect. The low-frequency ripple can be suppressed by changing the capacitance of the rectifier filter and adjusting the parameters of the feedback circuit.
Common mode ripple suppression: Common mode ripple noise generally occurs in switching power supply. When the rectangular wave voltage of switching power supply acts on the power device, it interacts with the parasitic capacitance between the power device and the radiator base plate and the primary and secondary sides of the transformer, as well as the parasitic inductance in the wire, resulting in common mode ripple noise. Common mode ripple noise suppression methods include:
Reduce the parasitic capacitance between the control power device, transformer and enclosure ground, and add common mode suppression inductance and capacitance at the output end;
EMI filter can effectively suppress the interference of common mode ripple;
Reduce switch burr amplitude.
Suppression of closed-loop control loop ripple: The cause of closed-loop control loop ripple is generally the improper parameter setting in the loop. When there is a certain fluctuation at the output end, the feedback network will feed back the fluctuation voltage at the output end to the regulator loop, causing the regulator to generate self-excited response, thus generating additional ripple.
The suppression methods mainly include: reasonable selection of the amplification factor of the loop, suppression of the self-excited response of the regulator, stability of the regulator, and LDO filtering at the output end of the power supply. This is an effective method to reduce ripple and noise.
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