The switching power supply will cause the output voltage to drop when the load is short-circuited, and also the output voltage will increase when the load is open or no-load. Overhaul in the general use of false load replacement method to distinguish between the power part of the fault or load circuit fault. On the selection of fake load, the general choice of 40W or 60W bulb for the fake load (large-screen color TV can use more than 100W bulb for fake load), the advantage is intuitive and convenient, according to whether the light bulb and light-emitting brightness of the power supply voltage output And the level of output voltage. But the disadvantages are also obvious, such as 60W bulb thermal resistance of 500Ω, while the cold resistance is only about 50Ω. It can be seen from the following table: Suppose the main voltage output of the power supply is 100V. When the 60W bulb is used as a dummy load, the current of the power supply is 200mA, but the main load current at startup is up to 2A, which is 10 times of the normal working current Therefore, using a bulb as a dummy load makes it difficult to start the power supply. The larger the lamp power is, the smaller the cold resistance is. Therefore, the high-current lamp has a larger starting current and the power supply is more difficult to start. Calculate the starting current and operating current of the power supply, I = U / R can be calculated using this formula: the load current when the power supply is 100V / 50Ω = 2A, the load current when the power supply is 100V / 500Ω = 0.2A However, Yes: The above is the theoretical calculation, there may be discrepancies. In order to reduce the start-up current, a 50W electric iron can be used as a dummy load (900Ω for both hot and cold states) or a 50W / 300Ω resistor, which is more accurate than using a 60W lamp. Some power can be directly connected to the load, some power is not, need specific analysis of the specific problems, the following three categories explain the details. The first category for his excitement switching power supply . For pulse-free synchronization of his exciting power supply (such as Changhong N2918-type color TV), you can disconnect the line load directly connected to the load. For pulse excited and indirect sampling of his excitation type switching power supply (such as the Panda 2928 color TV), directly connected to the load (especially the power of the lamp, such as 150W), the output voltage may drop more or No output, because of such power supply, although the line pulse to join only the role of synchronization and frequency locking, and not involved in oscillation, but the line sync pulse can make the switch turn-on time ahead of time, when the power supply with the strongest load capacity If the line load is disconnected, the line synchronization pulse also loses its function. The power supply will inevitably lose its load capability. In addition, the indirectly regulated power supply voltage sensitivity is lower and the output voltage will inevitably decrease. However, if the power supply of such a power supply adopts direct sampling (the sampling voltage is taken from the secondary side of the switching transformer ), the load can be disconnected and the load can be disconnected even without load due to the high voltage regulation sensitivity. The second category for the line pulse synchronization switching power supply can disconnect the line load directly connected to the load. This switching power supply is purely self-excited switching power supply, switch the base of the introduction of the forward line reverse pulse is intended to make the switch self-oscillation and line pulse synchronization, the switching power supply pulse radiation on the screen of the ramp interference Restricted to the line scan reverse, so the screen can not see the interference. Line switch added to the base of the pulse, just make the switch turn-on early in the deadline, does not constitute a basic auxiliary excitation function, therefore, known as the line pulse synchronization switching power supply. Discrimination whether it is the power of this method is to switch off the line pulse when the switching power supply only appears because of the oscillation frequency becomes low, the output voltage does not drop. Therefore, this power supply can disconnect the line scan circuit, with a dummy load method of maintenance. The third category is the line pulse auxiliary excitation switching power supply . This switching power supply line back pulse, not only to complete the switching power supply self-oscillation frequency synchronization, but also constitutes an integral part of the switch tube feedback network. The working process of this switching power supply is as follows: the switch tube generates self-excited oscillation when the power is turned on, and its feedback network can only make the output end generate a voltage lower than 40% of the normal output under the rated load. The voltage enables the line scanning to be started, The feedback to the switch to assist the incentive, in order to achieve the rated voltage output. This has two purposes: First, a step-down protection, once the line scan circuit fault, regardless of open or short circuit, the switching power supply output voltage is reduced to 60% of the original value, so that the scope of damage reduced. The second is the power and the line scan has a very short soft-start process, reducing the power and line scan failure rate. Such power supply, if you remove the line pulse feedback circuit, then the power output voltage dropped 40% to 60%, or even low output voltage. Obviously, this type of power supply can not be directly disconnected from the line scan with a dummy load method because it is impossible to output the rated voltage even if the power supply circuit is normal at this time. To distinguish between power and line scan circuit failure is to use an external power supply to the line scan circuit alone, if the line scan circuit is working properly, indicating poor switching power supply.
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