Switching power supply starting resistor effect
The selection of resistors in switching 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 a switch MOS transistor, which is connected in series between the switch MOS transistor and the ground. Generally, this resistance value is very small, and the maximum voltage drop does not exceed 2V. It seems unnecessary to use a high-power resistor based on power consumption. However, considering the ability to withstand the maximum peak current of the switch MOS transistor, the current amplitude is much larger than the normal value at the moment of startup. 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 back 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, it will also break down the integrated circuit IC of the overcurrent protection circuit. For this reason, usually a 2W metal film resistor is selected for this resistor. Some switching power supplies use 2-4 1W resistors in parallel, not to increase dissipation power, but to provide reliability. Even if one resistor is occasionally damaged, there are several others to avoid the occurrence of open circuits in the circuit. Similarly, the sampling resistance of the output voltage of the switching power supply is also crucial. Once the resistance is open, the sampling voltage is zero volts, and the PWM chip output pulse reaches 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 switching power supplies, the use of resistors in series is common, not to increase the power consumption or resistance value of resistors, but to improve the resistance's ability to withstand peak voltage. In general, resistors do not pay much attention to their withstand voltage. 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 high resistance, the power consumption does not exceed the rated value, but the resistance can also break down. The reason is that various thin film resistors control their resistance values based on the thickness of the film. For high resistance resistors, after the film is sintered, the length of the film is extended by grooving. The higher the resistance value, the higher the grooving density. When used in high-voltage circuits, spark discharge occurs between the grooves, causing resistance damage. Therefore, in switching power supplies, sometimes several resistors are intentionally connected in series to prevent this phenomenon from occurring. For example, the starting bias resistance in common self-excited switching power supplies, the resistance of switching tubes connected to DCR absorption circuits in various switching power supplies, and the application resistance in the high-voltage part of metal halide lamp ballasts.
In switching power supplies, the use of resistors in series is common, not to increase the power consumption or resistance value of resistors, but to improve the resistance's ability to withstand peak voltage. In general, resistors do not pay much attention to their withstand voltage. 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 high resistance, the power consumption does not exceed the rated value, but the resistance can also break down. The reason is that various thin film resistors control their resistance values based on the thickness of the film. For high resistance resistors, after the film is sintered, the length of the film is extended by grooving. The higher the resistance value, the higher the grooving density. When used in high-voltage circuits, spark discharge occurs between the grooves, causing resistance damage. Therefore, in switching power supplies, sometimes several resistors are intentionally connected in series to prevent this phenomenon from occurring. For example, the starting bias resistance in common self-excited switching power supplies, the resistance of switching tubes connected to DCR absorption circuits in various switching power supplies, and the application resistance in the high-voltage part of metal halide lamp ballasts.
PTC and NTC belong to thermal 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 relationship are completely different from ordinary resistors. In switching power supplies, PTC resistors with a positive temperature coefficient are commonly used in circuits that require instantaneous power supply. For example, the PTC used in its excitation driving integrated circuit power supply circuit provides starting current to the driving integrated circuit with its low resistance value at the moment of startup. After the integrated circuit establishes an output pulse, it is then supplied with rectified voltage by the switch circuit. During this process, PTC automatically closes the starting circuit due to an increase in temperature and resistance through the starting current. NTC negative temperature characteristic resistors are widely used as instantaneous input current limiting resistors in switching power supplies, replacing traditional cement resistors. They not only save energy but also reduce internal temperature rise. At the moment of turning on the switching power supply, the initial charging current of the filter capacitor is extremely high, and the NTC rapidly heats up. After the peak charging of the capacitor, the NTC resistance decreases due to the temperature increase. Under normal working current conditions, it maintains its low resistance value, greatly reducing the power consumption of the entire machine.
In addition, zinc oxide varistors are also commonly used in switching power supply circuits. Zinc oxide varistors have an extremely fast peak voltage absorption function. The biggest characteristic of varistors is that when the voltage applied to them is below its threshold, the current flowing through them is extremely small, equivalent to a closed valve. When the voltage exceeds the threshold, the current flowing through it surges, equivalent to a valve opening. By utilizing this function, abnormal overvoltage that often occurs in the circuit can be suppressed and the circuit can be protected from overvoltage damage. Varistors are generally connected to the mains input of switching power supplies and can absorb lightning induced high voltage from the power grid, providing protection when the mains voltage is too high.
