Switching Power Supply Starting Resistor Role
Switching power supply circuit in the choice of resistance, not only consider the average current value in the circuit caused by power consumption, but also consider the ability to withstand the maximum peak current. Typical examples of power sampling resistors for switching MOS tubes, switching MOS tubes in series between the ground sampling resistor, the general value of this resistance is very small, the maximum voltage drop of no more than 2V, according to the power consumption to calculate the seemingly unnecessary to use a high-power resistor, but taking into account the ability to withstand the maximum peak current of switching MOS tubes in the power-on moment the current amplitude is much larger than the normal value. At the same time, the reliability of the resistor is also extremely important, if the work of the current impact and open-circuit, the resistor is located in the printed circuit board between the two points will produce a voltage equal to the supply voltage plus the counter-peak voltage of the pulse high voltage and was broken, but also the overcurrent protection circuit IC breakdown. For this reason, the general selection of the resistor are 2W metal film resistor. Some switching power supply with 2-4 1W resistors in parallel, not to increase the power dissipation, but to provide reliability, even if a resistor is occasionally damaged, there are several other to avoid the circuit open-circuit phenomenon occurs. By the same token, the sampling resistor for the output voltage of the switching power supply is also critical. Once the resistor is open, the sampling voltage is zero volts, the PWM chip output pulse rises to its maximum value, and the output voltage of the switching power supply rises sharply. There are also photocoupler (optocoupler) current limiting resistors and so on.
In the switching power supply, 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 ability of the resistor to withstand peak voltage. Resistors in general, the withstand voltage is not too much attention, in fact, the power and resistance value of different resistors is the maximum operating voltage of this indicator. When in the highest operating voltage, due to the great resistance, its power consumption does not exceed the rated value, but the resistor will also breakdown. The reason is that a variety of film resistors is the thickness of the film to control the resistance value, the high resistance value of the resistor is also sintered in the film after the groove to extend the length of the film in the form of grooves, the greater the resistance value, groove density is also large, when used in high-voltage circuits, grooves between the occurrence of firing discharges resulting in damage to the resistor. Therefore, switching power supply, sometimes deliberately composed of several resistors in series to prevent this phenomenon. For example, the common self-excited switching power supply in the start-up bias resistor, a variety of switching power supply switching tube access to the DCR absorption circuit resistance, as well as metal halide lamp ballasts in the high-voltage part of the application of the resistance and so on.
PTC and NTC belong to the thermal performance components, PTC has a large positive temperature coefficient, NTC is the opposite, has a large negative temperature coefficient, its resistance value and temperature characteristics, volt-ampere characteristics and current and time relationships are completely different from ordinary resistors. In the switching power supply, positive temperature coefficient of the PTC resistor is commonly used in circuits that require instantaneous power supply. For example, it is excited to drive the integrated circuit power supply circuit using PTC, when the power on instantaneous low resistance to drive the integrated circuit to provide start-up current, to be established after the integrated circuit output pulse, and then by the switching circuit rectifier voltage supply. During this process, the PTC automatically shuts down the start-up circuit due to the temperature rise of the start-up current and the increase of the resistance value.The NTC negative temperature characteristic resistor is widely used in the instantaneous input current limiting resistor of switching power supply to replace the traditional cement resistor, which not only saves energy, but also reduces the temperature rise inside the machine. Switching power supply at the moment of switching on, the initial charging current of the filter capacitor is very large, the NTC quickly warmed up, to be after the capacitor charging peak, the NTC resistor resistance due to temperature rise decreases in the normal operating current state to maintain its low resistance, so that the power consumption of the whole machine is greatly reduced.
In addition, zinc oxide varistors are also commonly used in switching power supply lines. Zinc oxide varistor has a very fast spike voltage absorption function, the biggest feature of the varistor is when the voltage added above it is lower than its threshold, the current flowing through it is very small, equivalent to a shut-off valve, when the voltage exceeds the threshold, the current flowing through it surge, equivalent to the valve open. The use of this function, you can inhibit the circuit often abnormal overvoltage, protect the circuit from overvoltage damage. Varistor is generally connected to the switching power supply utility input, can absorb the grid induction lightning high voltage, in the utility voltage is too high, play a protective role.
