Explain in detail why a switching power supply should be connected to an equivalent load.
The switching power supply causes output voltage to drop when load is short-circuited and output voltage to increase when load is open or not present.
In maintenance, dummy load replacement method is commonly used to determine if part of power supply is faulty or load circuit is faulty. Regarding choice of dummy load, usually choose a 40W or 60W light bulb as dummy load (large screen color TV can use over 100W light bulb as dummy load) and output voltage level.
But disadvantages are also obvious: for example, resistance of a 60 W light bulb in a hot state is 500 ohms, and in a cold state it is only about 50 ohms. The table below shows that under condition that main output voltage of power supply is 100V, when using a 60W incandescent bulb as an equivalent load, current during operation of power supply is 200mA, but main load current at startup reaches 2A, which is 10 times more than normal operating current, so using a light bulb as a dummy load can easily make it difficult to start power supply, because greater power of light bulb, less resistance to cold, therefore, greater starting current of a powerful light bulb, more difficult it is to start power supply.
When calculating starting current and running current of power supply, you can use formula I=U/R to calculate: load current when power supply starts is 100V/50Ω=2A, and load current when power supply is running is 100V/500 Ohm = 0.2 A. It should be noted that above is a theoretical calculation, and actual one may differ. To reduce inrush current, a 50W electric soldering iron (both cold and hot resistance is 900Ω) or a 50W/300Ω resistor can be used as a dummy load, which is more accurate than using a 60W incandescent light bulb. Some power supplies can be directly connected to a dummy load, and some cannot. Specific problems need to be analyzed in detail, and following three types of situations will be explained in detail. The first type is a switching power supply with excitation. For a separately driven power supply without line pulse synchronization (such as Changhong N2918 color TV), line load can be disconnected and directly connected to a dummy load. For a switching power supply with independent drive, horizontal clock and indirect sampling (such as Panda 2928 color TV), when a dummy load is connected directly (especially with a large lamp power such as 150 W), output voltage may drop significantly. or no output, due to this type of power supply, although addition of horizontal pulses is intendedonly for frequency synchronization and blocking, and does not participate in oscillation, but horizontal synchronization pulse can increase switch on time, and power supply has largest load capacity at this time, if line load is disconnected, line synchronization pulse will lose its effect, and power supply's ability to carry load will inevitably decrease. In addition, sensitivity of sampling power supply indirect regulation is low, and output voltage must also decrease. However, if voltage regulation circuit of this type of power supply uses direct sampling (the sampling voltage is taken from secondary winding of switching transformer), due to high sensitivity of voltage regulation, it is possible to turn off linear load and dummy load. can be connected directly, even no-load maintenance can be done. The second type is a switching synchronous switching power supply, which can turn off linear load and directly connect to dummy load. This switching power supply is an exclusively self-excited switching power supply. The purpose of introducing forward and reverse pulses at base of switching tube is to synchronize self-excited oscillations of switching tube with horizontal pulse, and to transmit pulsed radiation of switching power supply to screen. The slant band interference is limited to reverse horizontal scanning, so interference is not visible on screen. The horizontal pulse added to base of switching tube only ensures conduction of switching tube until cutoff period, and is basically not an auxiliary drive function, so it is called horizontal pulse synchronous switching power supply. The way to judge whether it belongs to this kind of supply is that when flyback pulse is turned off, switching power supply only sounds (because oscillation frequency becomes lower), and output voltage does not drop. Therefore, this power supply can be disconnected from horizontal scanning circuit and repaired using a dummy load method. The third type is a switching power supply for auxiliary excitation of a line pulse. The retrograde pulse of this switching power supply not only completes synchronization of electrical excitation oscillation frequency when switching, but is also an integral part of switching tube feedback network. The working process of this type of switching power supply is as follows: switching tube produces self-excited oscillations after starting, and its feedback circuit can only make output terminal generate a voltage below 40% of normal output at rated load. Feedback with switching tube for auxiliary excitation to achieve rated outputth voltage. It has two purposes: first, to have a low protection function. If line scanning circuit fails, whether it is an open circuit or a short circuit, output voltage of switching power supply will drop to 60% of original. value, reducing range of destruction. Secondly, both power supply and line scanning have a very short soft start process, which reduces failure rate of power supply and line scanning. For this type of power supply, if feedback line pulse circuit is removed, output voltage of power supply will drop by 40-60%, or even output voltage will be very low. Obviously, such a power supply cannot be directly disconnected and repaired by dummy load method, because even if power circuit is good at this time, it is impossible to deliver rated voltage. The way to distinguish between a power supply failure and a horizontal scanning circuit failure is to use an external power supply to supply power to horizontal scanning circuit only.
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