Switching power supply startup resistor effect
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 a switching MOSFET, which is connected in series between the switching MOSFET and ground. Generally, this resistance value is extremely small, and the maximum voltage drop does not exceed 2V. It seems unnecessary to use a high-power resistor based on power consumption, but considering the ability to withstand the maximum peak current of the switching MOSFET, 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 opened 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, the integrated circuit IC of the overcurrent protection circuit will also be broken down. For this reason, a 2W metal film resistor is generally selected for this resistor. In some switching power supplies, 2-4 1W resistors are used in parallel, not to increase dissipated 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 a switching power supply is also crucial. Once the resistance is open, the sampling voltage is zero volts, and the PWM chip outputs a pulse that 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 ability of resistors to withstand peak voltage. In general, resistors are not very careful about 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, its power consumption does not exceed the rated value, but the resistance will also breakdown. The reason is that various thin film resistors control their resistance values based on the thickness of the thin film. For high resistance resistors, after the thin 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 switch mode 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 switch mode 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 the power supply circuit of the excitation drive integrated circuit provides a starting current to the drive 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, the PTC automatically closes the starting circuit due to the increase in temperature and resistance caused by the starting current. The NTC negative temperature characteristic resistor is widely used as a current limiting resistor for instantaneous input in switching power supplies, replacing traditional cement resistors. It not only saves energy but also reduces the temperature rise inside the machine. At the moment when the switching power supply is turned on, the initial charging current of the filtering capacitor is extremely high, and the NTC rapidly heats up. After the peak charging of the capacitor, the NTC resistance decreases due to the increase in temperature. 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 switch mode 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 increases sharply, equivalent to the valve opening. By utilizing this function, abnormal overvoltage that often occurs in the circuit can be suppressed, protecting the circuit from overvoltage damage. Varistors are generally connected to the mains input of switching power supplies and can absorb lightning high voltage induced by the power grid. When the mains voltage is too high, they play a protective role.
