Here to share with you some of the cattle DIY power supply design experience, I hope to help everyone design power.
DCM discontinuous mode: A triangular wave whose current rises from zero.
CCM continuous mode: A side trapezoidal wave whose current rises from a non-zero value.
DCM mode: At the beginning of the load hour, the secondary side currents are rising triangle and falling triangle wave respectively. If the switching power is fixed, its switching power supply is not turned on when the secondary magnetic energy is released. At this time, the primary and secondary switching devices are When the switch is turned off, the coil and the parasitic capacitance generate attenuated oscillation. The voltage across the coil is lower than the output voltage, the secondary diode is turned off, and the oscillation of the coil is slowed down when the primary and secondary are turned off. Although the voltage is high, the current is small. Until the switch is turned on again, the cycle continues. Because the coil inductance is involved in the oscillation, not only the leakage inductance, but the oscillation frequency is lower (the peak frequency of the oscillation is much lower than the peak of the switching tube. The peak of the switching tube is the leakage inductance and the high frequency attenuation caused by the distributed capacitance. oscillation). If it is a self-excited power supply, the secondary magnetic energy is immediately transferred to the conduction phase of the switch tube after the release of the secondary magnetic energy, and there is no damped oscillation process in which both sides are turned off.
CCM mode: If it is a power supply with a fixed switching frequency, when the load is large, the voltage control must keep the output voltage constant, the duty ratio is increased, and the load current is also large. After the switch is turned off, the secondary The current through the diode is large. Because the output voltage is constant, the slope of the output current is constant. When the output current has not dropped to 0, the switch is turned on again, that is, the magnetic energy of the coil is not released. The excitation current is not reset to 0. The switching tube current starts to rise again on the basis of this exciting current. Since the power supply voltage is constant, the slope of the switching tube current rises unchanged. That is, the slope of the primary current rise and the secondary current decrease are constant, but the starting point and the end point of the primary current rise are both raised, and the start point and the end point of the subsequent stage decrease are also raised. Thus the primary input energy is increased and the secondary output energy is increased. There is no damped oscillation process in which the primary and secondary are turned off (ie, there is no oscillating wave portion at the rear of the waveform shown on the 8th floor). If it is a self-excited switching power supply, immediately after the magnetic energy is released, the steering tube is turned on, and the excitation current is reset to zero. That is to say, the self-excited switching power supply will not work in CCM mode.
The oscillation generated at 0.15 MHz is the interference caused by the 3rd harmonic of the switching frequency.
The oscillation generated at 0.2 MHz is the interference caused by the 4th harmonic of the switching frequency and the superposition of the Mosfet oscillation 2 (190.5KHz) fundamental wave, so this part is stronger.
The oscillation generated at 0.25 MHz is the interference caused by the 5th harmonic of the switching frequency;
The oscillation generated at 0.35 MHz is the interference caused by the 7th harmonic of the switching frequency;
The oscillation generated at 0.39 MHz is the interference caused by the 8th harmonic of the switching frequency and the superposition of the Mosfet oscillation 2 (190.5KHz) fundamental wave;
The oscillation generated at 1.31 MHz is the interference caused by the fundamental wave of Diode oscillation 1 (1.31 MHz);
The oscillation generated at 3.3 MHz is the interference caused by the fundamental wave of Mosfet oscillation 1 (3.3 MHz);
The oscillation of the switching tube and the rectifier diode generates strong interference.
Electrolytic capacitor life analysis:
The following are the reasons for the long life found by many LED power supply manufacturers. This article gives a brief description.
We say that the rated life of an electrolysis is an actual life in the same working environment as its rated parameters. It is also the design life.
The main factors affecting the life of electrolytic capacitors are the following: ambient temperature, voltage, ripple current, and frequency.
1. Frequency: First, please make sure that the electrolytic capacitor used is a high-frequency electrolytic capacitor to ensure that the frequency does not affect the actual working frequency of your power supply.
2. Ripple current: This parameter can find the rated ripple current in the electrolysis specification, and select the appropriate electrolysis according to the ripple current of the power supply itself.
The above two items should consider the margin of the parameter, which is generally sufficient to calculate 1.5 times.
The following are the main parameters that affect life:
3, the ambient temperature: According to the current most common method of estimating the life of the capacitor, the actual operating temperature is 10 degrees lower than the rated temperature of the capacitor, and the life expectancy is doubled. The rated temperature is 105 degrees, and the measured temperature is 65 degrees 105-65 = 40 degrees is also increased by 4 times. We use electrolytic capacitors rated at 10,000 hours, that is, 20,000 hours at 95 degrees, 40,000 hours at 85 degrees, 80,000 hours at 75 degrees, and 160,000 hours at 65 degrees. This 160,000 hours is temporarily recorded here.
4, working voltage: we choose the electrolytic rating of 63V, the actual work of 37.2V, we can be sure that the life is longer than the rated, as for how long, we do not care.
Then analyze the performance attenuation characteristics of electrolytic capacitors:
We say that the life of an electrolytic capacitor is over. In fact, not all functions fail, but start to decay until the effect of electrolysis in the circuit is not met. Then we have to look at the role of electrolysis in the actual circuit, I will first talk about two kinds of uses, one is in the PFC circuit, the other is used for filtering at the output of the power supply, when the electrolytic performance is attenuated, the PF value will Reduced, but even if it is reduced to 0.5 (without PFC circuit), the power supply is the same, the output current and voltage are not affected at all. The same is true for the processing of ripple at the output, except that the output ripple is constantly increasing, and this ripple does have a large effect on the LED, but it does not immediately invalidate the LED.
Therefore, in summary, we have to do the following two things to do the power supply:
1. Select electrolytic capacitors of famous brands;
2. When designing the circuit, fully consider the balance between the actual working parameters and the electrolysis parameters.
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