What is power supply ripple, how to measure their magnitude and how to suppress?
1. Power Supply Ripple Generation
Our common power supplies include linear power supply and switching power supply. The DC output voltage is obtained by rectifying, filtering and stabilizing AC voltage. Since filtering is not pure, interference signals containing periodic and random components will be tied to DC level, which creates ripple. In case of nominal output voltage and current, peak value of AC voltage in DC output voltage is usually referred to as ripple voltage. Ripple is a complex interference signal, which is a periodic signal that fluctuates up and down around DC output voltage, but period and amplitude are not fixed values, but change with time, and ripple of different power supplies. The waveforms are also different. .
Secondly, harm of Ripple
Generally speaking, pulsation is harmful, but not beneficial. The main dangers of ripple are as follows:
a. Ripple in power supply will generate harmonics in electrical appliances, reducing efficiency of power supply;
b. Higher ripple may cause a voltage or current surge, causing electrical equipment to malfunction or speed up.
c. Ripple in digital circuits will interfere with circuit logic;
d. Ripple will also create noise interference to communications equipment, measurement and measurement instruments and tools, disrupting normal measurement and measurement of signals, and even damaging equipment.
So when manufacturing a power supply, we should consider reducing ripple to less than a few percent, and devices with high ripple requirements should consider reducing ripple to an even smaller size.
Methods for measuring power supply ripple are generally divided into two categories: one is identification of a single power supply, and other is debugging and measuring products.
In power supply industry and power supply users, to evaluate power supply, it is necessary to select a room (about 20 ° C), humidity must be less than 80%, and ambient mechanical vibration and electromagnetic interference may affect measurement is minimal. The standard instrument and power supply under test It must be placed in above test environment for more than 24 hours.
For a clean power supply, when measuring ripple of power supply, it is necessary to measure it when it is loaded, and added load must make output current more than 80% of rated output current.
For low-noise, purely resistive loads or electronic loads, an appropriate measurement standard must also be selected. Different standards give different measurement results
Ripple voltage can be expressed in absolute or relative terms. Typically, ratio of ripple voltage to DC output voltage is used to evaluate filter performance of a DC power supply, that is, ripple factor. Ripple is an important metric for evaluating a DC power supply, and its calculation method is percentage of effective voltage ripple relative to DC output voltage.
3. Measuring power supply ripple
An oscilloscope is typically used to measure ripple of a power supply. There are three commonly used measurement methods:
1) Rely on relationships
Use an oscilloscope probe with a ground loop, touch probe directly to positive output terminal, and loop of wire directly touches negative output terminal. Peaks represent ripple and noise on the output line. As shown below
2) Direct method
Connect ground ring directly to negative output terminal and use probe's ground ring to test output.
3) String Method
The output pin is connected to a twisted pair followed by a capacitor, and two ends of capacitor are measured with an oscilloscope.
When measuring ripple, keep in mind that upper limit of ripple bandwidth must be clear, and ripple is low-frequency noise, so it is common to use an oscilloscope that does not exceed upper limit of ripple bandwidth. too much.
When measuring, first enable oscilloscope's bandwidth limiting function and limit bandwidth to 20 MHz.Connect probe shield ground directly to output ground to reduce occurrence of excessively long ground wires.
Connect a small ceramic capacitor and a small electrolytic capacitor in parallel to probe's access point to filter out external noise signals and prevent them from entering oscilloscope.
Fourth ripple suppression method
Ripple at output of a power supply is mainly related to five aspects: low-frequency input ripple, high-frequency ripple, common-mode ripple caused by parasitic parameters, and ripple caused by feedback control.
The usual methods to suppress this ripple are: increase capacitance in filter circuit, use an LC filter circuit, use a multi-stage filter circuit, change switching power supply to a linear power supply, rational connection, etc. But, according to his classification, targeted measures often achieve twice result with half effort.
1. High frequency ripple suppression
High frequency ripple noise is mainly due to high frequency power conversion circuits. In high-frequency power conversion circuit, input DC voltage is converted by a high-frequency power device, and then rectified and filtered to achieve a regulated output. Generally, high-frequency ripple will have same frequency as switching frequency and its effect on external circuit The size is mainly related to switching power supply conversion frequency, output filter structure and parameters.When designing, try to increase operating frequency of power converter to reduce high-frequency switching ripple filtering requirements.
2. Low frequency ripple suppression
The amount of low-frequency ripple is related to capacitance of filter capacitor in output circuit. Capacitor capacitance cannot be increased indefinitely, which will inevitably cause residual low-frequency ripple at output. AC ripple is output after being attenuated by DC/DC conversion circuit, which belongs to low-frequency noise range, and its magnitude is determined by gain of control system and DC/DC conversion circuit. Because pulsation suppression capabilitycircuits for converting DC to current mode and voltage mode is relatively low, and low-frequency AC ripple at their output terminals is relatively large. Therefore, measures must be taken to filter low frequency ripple of power supply to ensure low ripple at output of power supply.
For some power supplies, DC/DC converter's feedback gain circuit can be increased and a preconditioning circuit can be used to enhance ripple suppression effect. changing capacitance of rectifier filter and adjusting parameters of feedback loop serve to suppress low-frequency ripples.
3. Common mode rejection
Sinusoids usually occur in switching power supplies. When a square-wave switching power supply is applied to a power device, parasitic inductors interact to create common mode ripple. Methods for suppressing common mode noise ripple:
1) Reduce parasitic capacitance between power management device, transformer, and chassis ground, and add inductance and capacitance to suppress output common mode;
2) The EMI filter can effectively suppress common mode ripple interference;
3) Reduce amplitude of switching noise.
4. Closed Loop Ripple Suppression
The cause of ripple in a closed control loop is usually that parameters in loop are not set properly. When there is a certain fluctuation in output, feedback network will return a fluctuating output voltage. output end of regulator circuit, causing regulator to self-excite, resulting in additional ripple.
The suppression methods mainly include: suppression of self-excited response of regulator, reasonable selection of loop gain, stability of regulator, and LDO filtering at output of power supply. is most effective way to reduce ripple and noise.
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