Switching power supply dummy load

The main purpose of the dummy load of the switching power supply is to overcome the instability of the no-load or light-load output. The output is unstable, the primary D is too small, the energy cannot be turned on, that is, the secondary has overshoot voltage or the feedback current is too large, forcing the IC to decrease D; therefore, at this time, the secondary shunt resistor can be cut off. The voltage overshoot reduces the feedback current so that D can be turned on. As for the selection parameters, one is that the resistance power is sufficient, and the other is the resistance value, that is, the resistance value that can make the output stable is small, and too small will increase the no-load loss, affecting the efficiency, and too much will not work.

The switching power supply will cause the output voltage to decrease when the load is short-circuited, and the output voltage will also rise when the load is open or no load. In the overhaul, a dummy load replacement method is generally used to distinguish whether the power supply part is faulty or the load circuit is faulty. Regarding the selection of the dummy load, a 40W or 60W bulb is generally selected as a dummy load (a large-screen color TV can use a bulb of more than 100W for a dummy load). The advantage is that it is intuitive and convenient. According to whether the bulb is illuminated or illuminated, it is known whether the power supply has a voltage output. And the level of the output voltage. But the shortcomings are also obvious. For example, a 60W bulb has a thermal resistance of 500Ω, while a cold resistance is only about 50Ω. According to the following table, it can be seen that if the mains voltage output of the power supply is 100V, when the 60W bulb is used as a dummy load, the current when the power supply is working is 200mA, but the main load current at startup is 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. Since the power of the bulb is larger, the cold resistance is smaller. Therefore, the starting current of the high-power bulb is larger, and the power source is more difficult to start.

When calculating the starting current and working current of the power supply, you can use the formula I=U/R to calculate: the load current is 100V/50Ω=2A when the power is turned on, and the load current is 100V/500Ω=0.2A when the power supply is working, but you need to pay attention to it. Yes: The above is theoretical calculations, and there may be discrepancies. In order to reduce the starting current, a 50W soldering iron can be used as a dummy load (cold state resistance is 900Ω) or 50W/300Ω resistor, which is more accurate than using a 60W bulb.

Some power supplies can be directly connected to the dummy load, and some power supplies are not available. Specific analysis is required. The following is a detailed description of the three types of conditions.

The first type is his exciting switching power supply. For the non-pulse-synchronized other power supply (such as Changhong N2918 color TV), the line load can be disconnected directly from the dummy load. For the excitation switching power supply (such as Panda 2928 color TV) with pulse pulse frequency locking and indirect sampling, the output voltage may drop more when directly connecting the dummy load (especially the bulb with high power such as 150W). No output, because this type of power supply, although the addition of the line pulse only plays the role of synchronization and frequency locking, and does not participate in the oscillation, but the line synchronization pulse can make the switching tube conduction time advance, when the power supply has the strongest load capacity. If the line load is disconnected, the line sync pulse will lose its effect, and the power supply with load capacity will inevitably decrease. The sensitivity of the power supply with the inter-sampling sampling is lower, and the output voltage is also necessarily reduced. However, if the power supply circuit adopts direct sampling (sampling voltage is taken from the secondary of the switching transformer), the voltage can be disconnected from the line load and can be directly connected to the dummy load for maintenance.

The second type of switching power supply with line pulse synchronization can disconnect the line load and directly receive the dummy load. The switching power supply is purely a self-excited switching power supply. The purpose of introducing a forward line reverse pulse at the base of the switching tube is to synchronize the self-oscillation and the line pulse of the switching tube, and the pulse radiation of the switching power supply interferes with the diagonal of the screen. Limited to the line scan reverse, so no interference is seen on the screen. The line pulse applied to the base of the switch tube only makes the switch tube turn on in advance during the cut-off period, and basically does not constitute an auxiliary excitation function. Therefore, it is called a line-pulse synchronous switching power supply. The method for judging whether it is such a power source is that when the line reverse pulse is disconnected, only the sound is generated by the switching power supply (because the oscillation frequency becomes low), and the output voltage does not decrease. Therefore, this power supply can be disconnected from the line scan circuit and repaired by the dummy load method.

The third type is a switching power supply with line pulse assisted excitation. The reverse pulse of the switching power supply not only completes the synchronization of the self-oscillation frequency of the switching power supply, but also forms an indispensable part of the switching tube feedback network. The working process of the switching power supply is: after the power is turned on, the switching tube generates self-oscillation. Under the rated load, the feedback network can only make the output terminal generate a voltage lower than 40% of the normal output. This voltage enables the line scan to start, and the line pulse The feedback is given to the switch tube to assist the excitation to achieve the rated voltage output. This has two purposes: First, there is a step-down protection function. Once the line scanning circuit is faulty, whether it is open or short circuit, the output voltage of the switching power supply is reduced to 60% of the original value, so that the damage range is reduced. Second, both power and line scan have a very short soft start process, reducing the power and line scan failure rates. For such a power supply, if the feedback line pulse circuit is removed, the power supply output voltage drops by 40% to 60% at this time, and even the output voltage is low. Obviously, this power supply cannot be directly disconnected from the line scan by the dummy load method, because even if the power supply circuit is normal, it is impossible to output the rated voltage. The method for distinguishing between the power supply and the line scan circuit is to separately supply the line scan circuit with an external power supply. If the line scan circuit works normally, the switch power supply is defective.