Switching power supply starting resistor function
The selection of resistors in a switching power supply circuit not only considers the power consumption caused by the average current value in the circuit, but also considers the ability to withstand the maximum peak current. A typical example is the power sampling resistor of the switching MOS tube. The sampling resistor is connected in series between the switching MOS tube and the ground. Generally, this resistance value is very small, and the maximum voltage drop does not exceed 2V. Calculated in terms of power consumption, it seems that there is no need to use a high-power resistor. , but considering the ability to withstand the maximum peak current of the switching MOS tube, the current amplitude is much larger than the normal value at the moment of power-on. At the same time, the reliability of the resistor is also extremely important. If it is open-circuited due to current impact during operation, a pulse high voltage equal to the supply voltage plus the reverse peak voltage will be generated between two points on the printed circuit board where the resistor is located. is broken down, and at the same time, the integrated circuit IC of the overcurrent protection circuit is broken down. For this reason, 2W metal film resistors are generally used for this resistor. Some switching power supplies use 2-4 1W resistors in parallel, not to increase power dissipation, but to provide reliability. Even if one resistor is occasionally damaged, there are several others to avoid open circuits in the circuit. In the same way, the sampling resistor of the output voltage of the switching power supply is also crucial. Once the resistor is opened, the sampling voltage is zero volts, the output pulse of the PWM chip rises to the maximum value, and the output voltage of the switching power supply rises sharply. There are also current-limiting resistors for photocouplers (optocouplers) and so on.
In switching power supplies, the use of resistors in series is very common. The purpose is not to increase the power consumption or resistance value of the resistor, but to improve the resistor's ability to withstand peak voltages. Under normal circumstances, resistors don't pay much attention to their withstand voltage. In fact, resistors with different power and resistance values have the highest working voltage as an indicator. When it is at the highest operating voltage, its power consumption does not exceed the rated value due to the extremely large resistance, but the resistance will also break down. The reason is that in addition to controlling the resistance value of various thin film resistors based on the thickness of the film, for high resistance value resistors, the length of the film is extended by notching grooves after the film is sintered. The greater the resistance value, the greater the groove density. , when used in high-voltage circuits, sparking discharge occurs between the grooves, causing damage to the resistor. Therefore, in switching power supplies, several resistors are sometimes deliberately connected in series to prevent this phenomenon from occurring. For example, the starting bias resistor in the common self-excited switching power supply, the resistance of the switching tube connected to the DCR absorption loop in various switching power supplies, and the high-voltage application resistance in the metal halide lamp ballast, etc.
PTC and NTC are heat-sensitive performance components. PTC has a large positive temperature coefficient, while NTC has a large negative temperature coefficient. Its resistance and temperature characteristics, volt-ampere characteristics and current and time relationships are completely different from ordinary resistors. In switching power supplies, positive temperature coefficient PTC resistors are often used in circuits that require instantaneous power supply. For example, it stimulates the PTC used in the power supply circuit of the driving integrated circuit. When the power is turned on, its low resistance value provides starting current to the driving integrated circuit. After the integrated circuit establishes an output pulse, the power is supplied by the rectified voltage of the switching circuit. During this process, the PTC automatically closes the starting circuit because the temperature of the starting current increases and the resistance increases. NTC negative temperature characteristic resistors are widely used in instantaneous input current-limiting resistors of switching power supplies to replace traditional cement resistors. They not only save energy, but also reduce the temperature rise inside the machine. When the switching power supply is turned on, the initial charging current of the filter capacitor is extremely large, and the NTC heats up rapidly. After the charging peak of the capacitor passes, the resistance of the NTC resistor decreases due to the increase in temperature, and it maintains its low resistance value under normal operating current conditions. The power consumption of the whole machine is greatly reduced.
In addition, zinc oxide varistors are also commonly used in switching power supply circuits. Zinc oxide varistor has an extremely fast peak voltage absorption function. The biggest feature of the varistor is that when the voltage applied to it is lower than its threshold, the current flowing through it is extremely small, which is equivalent to a dead switch. When the voltage exceeds the threshold of a valve, the current flowing through it surges, which is equivalent to the valve opening. Using this function, abnormal overvoltage that often occurs in the circuit can be suppressed and the circuit can be protected from overvoltage damage. The varistor is generally connected to the mains input end of the switching power supply, which can absorb the high voltage of lightning induced by the power grid and play a protective role when the mains voltage is extremely high.
