Starting resistor function of the switching power supply
The selection of resistors in switch mode power supply circuits not only considers the power consumption caused by the average current value in the circuit, but also the ability to withstand the maximum peak current. A typical example is the power sampling resistor of the switch MOS transistor, which is connected in series between the switch MOS transistor and ground. Generally, this resistance value is very small, and the maximum voltage drop does not exceed 2V. It seems unnecessary to use high-power resistors based on power consumption, but considering the ability to withstand the maximum peak current of the switch MOS transistor, the current amplitude at the moment of startup is much larger than the normal value. At the same time, the reliability of the resistor is also extremely important. If it is open circuit due to current impact during operation, a pulse high voltage equal to the supply voltage plus the anti peak voltage will be generated between the two points on the printed circuit board where the resistor is located, and it will be broken down. At the same time, the integrated circuit IC of the overcurrent protection circuit will also be broken down. For this reason, generally a 2W metal film resistor is selected for this resistor. In some switch mode power supplies, 2-4 1W resistors are connected in parallel, not to increase dissipated power, but to provide reliability. Even if one resistor is occasionally damaged, there are several others to avoid open circuit in the circuit. Similarly, the sampling resistor for the output voltage of a switching power supply is also crucial. Once the resistor opens, the sampling voltage is zero volts, and the PWM chip output pulse rises to its maximum value, causing a sharp increase in the output voltage of the switching power supply. In addition, there are current limiting resistors for optocouplers (optocouplers) and so on.
In switch mode power supplies, the series connection of resistors is common, not to increase the power consumption or resistance of the resistors, but to improve their ability to withstand peak voltage. In general, the withstand voltage of resistors is not very important. In fact, resistors with different power and resistance values have the highest operating voltage as an indicator. When at the highest operating voltage, due to the extremely high resistance, its power consumption does not exceed the rated value, but the resistance will also break down. The reason is that various thin film resistors control their resistance value based on the thickness of the film. For high resistance resistors, after the film is sintered, the length of the film is extended by grooves. The higher the resistance value, the higher the groove density. When used in high-voltage circuits, sparks and discharges occur between grooves, causing damage to the resistor. Therefore, in switch mode power supplies, sometimes several resistors are intentionally connected in series to prevent this phenomenon from occurring. For example, the starting bias resistor in common self-excited switching power supplies, the resistor connecting the switch tube to the DCR absorption circuit in various switching power supplies, and the high-voltage part application resistor in metal halide lamp ballasts, etc.
PTC and NTC are thermal sensitive components. PTC has a large positive temperature coefficient, while NTC has the opposite, with a large negative temperature coefficient. Its resistance and temperature characteristics, volt ampere characteristics, and current time relationship are completely different from ordinary resistors. In switch mode power supplies, PTC resistors with positive temperature coefficient are commonly used in circuits that require instantaneous power supply. For example, it drives the PTC used in the power supply circuit of the integrated circuit. When the power is turned on, its low resistance value provides a starting current to the driving integrated circuit. After the integrated circuit establishes an output pulse, the switch circuit rectifies the voltage and supplies power. During this process, PTC automatically shuts off the starting circuit due to the increase in starting current temperature and resistance. NTC negative temperature characteristic resistors are widely used as current limiting resistors for instant input in switch mode power supplies, replacing traditional cement resistors. They not only save energy but also reduce internal temperature rise. At the moment of turning on the switch power supply, the initial charging current of the filtering capacitor is extremely high, and the NTC quickly heats up. After the peak charging of the capacitor, the resistance of the NTC resistor decreases due to the increase in temperature, and it maintains its low resistance value under normal working current state, greatly reducing the power consumption of the whole machine.
In addition, zinc oxide varistors are also commonly used in switch power supply circuits. Zinc oxide varistors have an extremely fast peak voltage absorption function. The biggest feature of varistors is that when the voltage applied to it is below its threshold, the current flowing through it is extremely small, equivalent to a closed valve. When the voltage exceeds the threshold, the current flowing through it surges, equivalent to the valve opening. By utilizing this function, it is possible to suppress the frequent occurrence of abnormal overvoltage in the circuit and protect the circuit from damage caused by overvoltage. Varistors are generally connected to the mains input of switching power supplies, which can absorb the high voltage induced by lightning in the power grid and provide protection when the mains voltage is too high.
