Main test points for switching power supplies
When debugging a switching power supply, in addition to using a voltmeter to measure the voltage of the pins of related components in the control circuit, it is more important to use an oscilloscope to observe the relevant voltage waveforms in order to judge whether the switching power supply is in the best working condition. This article mainly explains the selection of oscilloscope test points. For example, when the test point is the output pin of the PWM control chip, an oscilloscope can be used to simultaneously measure the two important parameters of the driving pulse amplitude and duty cycle.
The selection of test points is very important. Reasonable selection of test points can not only ensure the safety of debugging, but also reflect the working status of the switching power supply and simplify the debugging process.
Test point selection of switching power supply The test point TP1 is the drain of the MOSFET power switch tube, TP2 is the source of the switch tube, Rs is the current sampling resistor, and TP3 is the negative pole of the primary high-voltage circuit. We can connect the two test points TP1 and TP2 to the two input channels (CH1 and CH2) of the dual-trace oscilloscope, and observe the voltage waveforms of the two points at the same time. At this time, the ground terminals of the two probes should be connected to the negative pole of the primary input DC circuit at the same time, that is, the TP3 position. During the actual measurement, the ground clip of the probe can be clamped directly on the ground pin of Rs.
From TP1, we can see the drain voltage waveform of the power switch tube. This waveform can reflect information such as drain peak voltage, input DC high voltage, secondary reflection voltage, switch tube conduction voltage drop, and conduction and cut-off time. In a single-ended flyback switching power supply, the drain voltage waveform of the power switch tube
From TP2, you can see the source voltage waveform of the power switch tube. This waveform is the voltage waveform on the sampling resistor Rs, which can reflect information such as drain current, turn-on and turn-off time. The drain current waveform of the power switch tube. This waveform reflects that the switching power supply works in continuous current mode. In each cycle, when the switch tube is turned on, the drain current starts to rise from the minimum initial current. Before the switch tube is turned off, the drain current reaches the maximum value.
The two test points TP1 and TP2 are very critical, and can basically reflect the working status of the switching power supply and whether there is any fault. During debugging, pay special attention to the waveforms of these two test points. When gradually increasing the input AC voltage, if the peak voltage or peak current exceeds the design range, the power supply should be turned off immediately to find out the cause to prevent damage to the power switch tube.
Sometimes, in order to observe the current waveform of the primary winding of the high-frequency transformer, a sampling resistor can also be connected in series with the primary winding. For the sampling circuit of the primary loop current, the measurement state at this time is the "floating" measurement mentioned in the previous article. Theoretically speaking, the sampling resistor can be connected in series at the upper end or lower end of the primary winding; in fact, if it is connected in series at the lower end of the primary winding, see the Rs1 position in the figure, a floating high-voltage pulse will be generated on the oscilloscope ground wire during measurement. It is neither safe to do so, but also produces large measurement interference and errors, and may also affect the normal operation of the switching power supply. The correct method is to connect the sampling resistor in series with the upper end of the primary winding, see the Rs position in the figure, and connect the signal line of the oscilloscope probe at the TP1 end, and connect the grounding clip of the probe at the TP2 end. In this way, although the waveform of the primary loop circuit is of reverse polarity, the interference and error during measurement are minimal, and have no effect on the normal operation of the switching power supply. The primary current waveform of positive polarity can be observed through the reverse polarity (INV) function button of the oscilloscope.
Special note: When observing the primary current waveform of a high-frequency transformer, the ground clip of the oscilloscope probe will be connected to the positive end of the DC high voltage. The probes and ground clips of other channels of the oscilloscope must be disconnected from the relevant circuits, otherwise a short circuit will occur or the components of the circuit will be damaged. That is to say, when observing the primary primary loop current waveform, only one channel can be used, and the other channels must be completely disconnected. This problem does not exist when using oscilloscopes with built-in isolated channel technology.
