The role of the starting resistor of the regulated power supply
The selection of resistors in the 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, the resistance value is very small, and the maximum voltage drop does not exceed 2V. It seems that it is not necessary to use a high-power resistor in terms of power consumption. However, 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 very important. If it is opened by the current impact during work, a pulse high voltage equal to the power supply voltage plus the reverse peak voltage will be generated between two points on the printed circuit board where the resistor is located. For this reason, the resistors are generally 2W metal film resistors. In some switching power supplies, 2-4 1W resistors are connected in parallel, not to increase power dissipation, but to provide reliability. Even if one resistor is damaged occasionally, there are several other resistors to avoid open circuits. In the same way, the sampling resistor of the output voltage of the switching power supply is also very important. Once the resistor is open, 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. In addition, there are current limiting resistors of optocouplers (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 of the resistors, but to improve the ability of the resistors to withstand peak voltages. In general, resistors don't pay much attention to their withstand voltage. In fact, resistors with different power and resistance values have the index of maximum working voltage. When it is at the highest operating voltage, the power dissipation does not exceed the rated value due to the extremely large resistance, but the resistance will also break down. The reason is that the resistance of various thin film resistors is controlled by the thickness of the film. For high-resistance resistors, after the film is sintered, the length of the film is extended by grooves. The larger the resistance value, the greater the density of the grooves. When used in high-voltage circuits, ignition and discharge between the grooves will cause damage to the resistor. Therefore, in switching power supplies, sometimes several resistors are deliberately connected in series to prevent this phenomenon from happening. For example, the start-up bias resistor in the common self-excited switching power supply, the resistance of the switching tube connected to the DCR absorption circuit in various switching power supplies, and the high-voltage part application resistor in the metal halide lamp ballast, etc.
PTC and NTC are heat-sensitive performance components. PTC has a large positive temperature coefficient, and NTC, on the contrary, has a large negative temperature coefficient. Its resistance value and temperature characteristics, volt-ampere characteristics, and current-time relationship are completely different from ordinary resistors. In switching power supplies, PTC resistors with positive temperature coefficients 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 it is turned on, its low resistance value provides the starting current to the driving integrated circuit. After the integrated circuit establishes an output pulse, it is powered by the rectified voltage of the switching circuit. During this process, the PTC automatically closes the starting circuit due to the temperature rise and the resistance value increasing through the starting current. NTC negative temperature characteristic resistors are widely used in instantaneous input current limiting resistors of switching power supplies to replace traditional cement resistors, which 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 high, and the NTC heats up rapidly. After the capacitor charging peak value passes, the resistance of the NTC resistor decreases due to the temperature rise, and maintains its low resistance value under normal operating current conditions, which greatly reduces the power consumption of the whole machine.
In addition, zinc oxide varistors are also commonly used in switching power supply lines. The zinc oxide varistor has a very fast peak voltage absorption function. The biggest feature of the varistor is that when the voltage applied to it is lower than its threshold value, the current flowing through it is extremely small, which is equivalent to a closed valve. When the voltage exceeds the threshold value, the current flowing through it increases sharply, which is equivalent to the valve opening. Utilizing this function, it is possible to suppress the abnormal overvoltage that often occurs in the circuit and protect the circuit from damage caused by overvoltage. The varistor is generally connected to the mains input terminal of the switching power supply, which can absorb the lightning high voltage induced by the power grid and play a protective role when the mains voltage is too high.
