Summary With the ever-increasing switching frequency of DC/DC converters, the effect of puncturing noise becomes more and more obvious. Now, although the observation of puncture noise has become simple due to the continuous improvement of the performance of the oscilloscope, the exact size of the puncture noise and the reasons for the analysis are still related to many factors, such as the use of the correct test method, external use The right choice of devices, the rational layout of circuit boards and so on.
The causes of puncture noise are mainly divided into two categories:
1. Test method When testing, different connection methods of the oscilloscope probe have a great influence on the measurement of puncture noise.
2. The correct choice of peripheral devices and the influence of the circuit layout When the DC/DC converter controls the switching operation, the reverse leakage current flows through the Schottky diode at the moment of turning on and off, and the switch turns on and off. There is a large current flowing on the instant substrate, resulting in a large potential difference between different devices on the substrate; in addition, the influence of the electromagnetic field of the electromagnetic wave of the coil also leads to a greater puncture noise.
Improvement of test method When the DC/DC converter is working, it can be observed by an oscilloscope that several hundred millivolts of puncture noise signal will be generated in the output voltage at the moment of switch closing and opening, and during the test, With the change of direction of the probe test, the size of the noise signal will also change greatly. The noise in the actual output voltage is affected by other factors besides the correct test method. Therefore, to accurately measure the size of the puncture noise component in the output voltage, it is necessary to avoid increasing the test probe due to the external environment. Part of the noise signal, the specific operation is as follows:
A: This common test probe is longer than the antenna because of the ground line.
B: The grounding of the probe and the shielding portion of the probe are easily removed by the external environment.
C: Then directly connect the metal end of the probe to the output end. The ground part of the probe is connected to the ground of the circuit board. This can effectively prevent external noise interference.
Similarly, the selection of the ground test reference point also has a great influence on the size of the puncture noise. The capacitance of the output capacitor at the back end of the DC/DC converter also has an important influence on the actual value of the puncture noise.
Application countermeasures:
There are many places that need attention in the actual application process. Here are some examples for reference:
1. When using the input capacitor, use a capacitor with a small inductance, or a 0.01μF ceramic capacitor in parallel, as close to the switching transistor and diode as possible.
2. The equivalent resistance ESR of the output capacitor is a very important factor that affects the size of the puncture noise (VRIPPLE = RESR * DIL), so the correct choice of a capacitor requires a comprehensive consideration of the price of the capacitor, the size and volume of the equivalent impedance Relationship between values. Because the ceramic capacitor has very good high-frequency characteristics and the ESR of the equivalent resistance is also very small, the DC/DC conversion chip that can use the ceramic capacitor is selected as far as possible in the practical application circuit;
3. Add a 5-50 milliohm resistor between the G terminal of the FET switch and DC/DC;
4. An RC buffer circuit is connected in parallel across the switch and the Schottky diode;
5. Select a Schottky diode with reverse conduction recovery time characteristics to reduce leakage current flow;
6. When there is a non-continuous large current in the circuit, the wire between the external switch and the coil should be as short as possible, and as far away from the output ground;
7. Choose to use a closed magnetic type circuit inductance coil;
8. Add a filter circuit at the output (low-pass LC & RC loop);
9. Add a ferrite magnet device in the circuit to filter out high-frequency clutter signals.
note:
When there is a malfunction or a low efficiency conversion, first check the part of the line with bypass capacitor and ground connection.
The causes of puncture noise are mainly divided into two categories:
1. Test method When testing, different connection methods of the oscilloscope probe have a great influence on the measurement of puncture noise.
2. The correct choice of peripheral devices and the influence of the circuit layout When the DC/DC converter controls the switching operation, the reverse leakage current flows through the Schottky diode at the moment of turning on and off, and the switch turns on and off. There is a large current flowing on the instant substrate, resulting in a large potential difference between different devices on the substrate; in addition, the influence of the electromagnetic field of the electromagnetic wave of the coil also leads to a greater puncture noise.
Improvement of test method When the DC/DC converter is working, it can be observed by an oscilloscope that several hundred millivolts of puncture noise signal will be generated in the output voltage at the moment of switch closing and opening, and during the test, With the change of direction of the probe test, the size of the noise signal will also change greatly. The noise in the actual output voltage is affected by other factors besides the correct test method. Therefore, to accurately measure the size of the puncture noise component in the output voltage, it is necessary to avoid increasing the test probe due to the external environment. Part of the noise signal, the specific operation is as follows:
B: The grounding of the probe and the shielding portion of the probe are easily removed by the external environment.
C: Then directly connect the metal end of the probe to the output end. The ground part of the probe is connected to the ground of the circuit board. This can effectively prevent external noise interference.
Similarly, the selection of the ground test reference point also has a great influence on the size of the puncture noise. The capacitance of the output capacitor at the back end of the DC/DC converter also has an important influence on the actual value of the puncture noise.
Application countermeasures:
There are many places that need attention in the actual application process. Here are some examples for reference:
1. When using the input capacitor, use a capacitor with a small inductance, or a 0.01μF ceramic capacitor in parallel, as close to the switching transistor and diode as possible.
2. The equivalent resistance ESR of the output capacitor is a very important factor that affects the size of the puncture noise (VRIPPLE = RESR * DIL), so the correct choice of a capacitor requires a comprehensive consideration of the price of the capacitor, the size and volume of the equivalent impedance Relationship between values. Because the ceramic capacitor has very good high-frequency characteristics and the ESR of the equivalent resistance is also very small, the DC/DC conversion chip that can use the ceramic capacitor is selected as far as possible in the practical application circuit;
3. Add a 5-50 milliohm resistor between the G terminal of the FET switch and DC/DC;
4. An RC buffer circuit is connected in parallel across the switch and the Schottky diode;
5. Select a Schottky diode with reverse conduction recovery time characteristics to reduce leakage current flow;
6. When there is a non-continuous large current in the circuit, the wire between the external switch and the coil should be as short as possible, and as far away from the output ground;
7. Choose to use a closed magnetic type circuit inductance coil;
8. Add a filter circuit at the output (low-pass LC & RC loop);
9. Add a ferrite magnet device in the circuit to filter out high-frequency clutter signals.
note:
When there is a malfunction or a low efficiency conversion, first check the part of the line with bypass capacitor and ground connection.
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