[Basic knowledge about power supply] The measurement method I only mentioned after being asked many times!
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Let's take a look at the LED driver. The LED driver is a more realistic product. For example, the lamp is basically LED, and it works 24 hours a day, or at least for several hours a day. Unlike a mobile phone fast charger, it may charge for five minutes or ten minutes and then stop working. Therefore, the requirements of the LED driver for electrolytic capacitors are relatively more and higher than those of the charger fast charger.
Let's look at how it's done. When the ambient temperature is 90 degrees, the required lifespan is 50,000 hours. What does this ambient temperature of 90 degrees mean? For example, inside a light bulb, the outer shell is basically above 60 degrees. It can reach 70 degrees if it's a little bit lower. So when converted to the temperature inside the bulb cavity, 90 degrees is basically very common and reasonable. The specification of our product is 400V 10UF, the RMS value of the ripple current is 350mA, and the corresponding frequency is 100K. So we can know its coefficient at low frequency, and we measure it and do high and low frequency decomposition,
The left side is low frequency and the right side is high frequency.
It is easy to calculate at low frequencies, and a relatively friendly value can be obtained through the calculation of the oscilloscope itself. Then the low-frequency and high-frequency decomposition here actually uses a relationship similar to trigonometric functions to calculate.
This coefficient corresponds to the requirement of 50,000 hours.
You see, at 120Hz, it becomes 0.4. So at low frequencies, the ripple of the entire electrolytic capacitor is greatly affected.
Let's take a look at how the output capacitor filtering is calculated. In fact, the same algorithm is used for all our buck and flyback output capacitors.
We still calculate it this way, it's still 90 degrees, it's still at the output end, the size is like this, the capacitance value is like this, and the effective value of the entire ripple is like this, in the case of 100KHz.
After a long calculation, we concluded that the life span is only 28,000 hours, which cannot meet the requirements. What should we do? We can only replace the capacitor, for example, replace it with a 130-degree capacitor, or connect it in parallel. Of course, in fact, when the volume is limited, we basically directly choose a 130-degree capacitor.
Summarize:
1. [Used to calculate capacitor heating] DC/DC power supplies also have multiple switching frequency components and ripple currents with complex high-order harmonic frequency components.
2. In such cases, it is actually difficult to calculate the effective value of the ripple current from the waveform and find the self-heating value of the capacitor. The frequency correction factor is used to convert the effective value of the ripple current at each frequency measured in the FFT analysis to the specified frequency of the capacitor's ripple current (120Hz or 100kHz, etc., depending on the product series).
3. Synthesize the frequency-corrected ripple current RMS value. The heating value is calculated using the rated ripple current and the frequency-corrected ripple current measurement.
4. In general, it is common sense that the electrolytic capacitor determines the life of the entire power supply. We need to pay attention to the current waveform flowing through it and use it to calculate its life. It can be seen that due to the selected topology, the current waveform on the output electrolytic is a mixed wave of high frequency and low frequency. The actual calculation of the electrolytic life is generally decomposed, and different manufacturers have slightly different calculation formulas for the electrolytic life. This is mainly due to the differences in electrolyte formulas, processes, design capabilities, etc. of different manufacturers.
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