What faults can occur when a switching power supply sampling resistor is open?
There are two main types of sampling resistors for switching power supplies.
Parallel in the output terminal, sampling output voltage, as a feedback voltage signal, and TL431 and other precision voltage source comparison, through the feedback feedback to the control terminal, constituting a constant voltage control loop. Usually two resistors are used to divide the output voltage to get the fed signal.
They are upper sampling resistor and lower sampling resistor. If the upper sampling resistor is open circuit, the feedback voltage is 0, and the control terminal may adjust the duty cycle of PWM to the maximum. The control side may adjust the duty cycle of PWM to the maximum. If the upper sampling resistor is open, the feedback voltage will be 0, and the control terminal may adjust the PWM duty cycle to the maximum, which will make the actual output voltage very high and cause load damage.
If the lower sampling resistor is open, the feedback voltage is the un-divided output voltage, the control terminal may adjust the duty cycle of PWM to the minimum, making the actual output off or output very low voltage, the load does not work.
In series with the switching tube, the working current of the switching tube is converted into a voltage signal and input to the switching power supply control chip for detecting the working current of the switching tube and playing the role of It is used to detect the working current of the switching tube and play the role of overcurrent protection. That is to say, when the voltage signal input to the control chip exceeds a certain value, it means that the current flowing through the switching tube is too large, and the load may have overcurrent or short-circuit faults. When the voltage signal of the input control chip exceeds a certain value, it means that the current flowing through the switching tube is too large, and the load may have an overcurrent or short-circuit fault. If the current sampling circuit is open-circuit, the switching tube connected in series with it is also open-circuit, resulting in the switching power supply does not work and there is no output voltage.
