Using a Digital Oscilloscope to Measure Switching Power Supply Dissipation
Due to the power dissipated within the switch mode power supply device, the overall efficiency of the power supply thermal effect is determined. Therefore, measuring the power loss of the switch mode power supply device and inductor/transformer is an extremely important measurement task. This measurement can determine power efficiency and heat dissipation.
Measurement and analysis of power loss
Testing equipment required for power loss measurement
Simplified circuit for switch transformation. MOSFET field-effect power transistor controls the current under the excitation of a 40kHz clock. The MOSFET in Figure 1 is not connected to the AC feed line ground or circuit output ground, i.e. isolated from ground. Therefore, it is not possible to perform simple ground reference voltage measurements using an oscilloscope, as connecting the probe's ground wire to any terminal of the MOSFET will cause a short circuit between that point and ground through the oscilloscope.
In this case, differential measurement is an excellent method for measuring the voltage waveform of M0SFET. By differential measurement, you can determine VDS, which is the voltage at the drain and source terminals of MOSFETs. VDS can fluctuate above voltage, with a voltage range of tens to hundreds of volts depending on the voltage range of the power supply device. You can measure VDS through several methods:
·The chassis ground wire of the floating oscilloscope. It is recommended not to use it, as it is extremely harmful and poses a danger to the user, the device being tested, and the oscilloscope.
·Use two conventional single ended passive probes, connect their grounding wires together, and then measure using the channel calculation function of the oscilloscope. This measurement method is called quasi differential measurement. However, although passive probes can be used in conjunction with amplifiers in oscilloscopes, they lack the "Common Mode Rejection Ratio" (CMRR) function that can appropriately prevent any common mode voltage. This setting cannot accurately measure voltage, but existing probes can be used.
·Use the probe isolator available in the store to isolate the oscilloscope chassis and ground it. The grounding wire of the probe will no longer have the main grounding potential and the probe can be directly connected to a testing point. Probe isolators are an effective solution, but they are relatively expensive, with a cost of two to five bits for differential probes.
·Use a true differential probe on a broadband oscilloscope. You can measure VDS using a differential probe, which is also a good method.
When measuring current through MOSFET, first clamp the current probe and then fine tune the measurement system. Many differential probes are equipped with built-in DC offset fine-tuning capacitors. Turn off the device under test and wait for the oscilloscope and probe to warm up before setting the average values of the voltage and current waveforms measured by the oscilloscope. The sensitivity setting should use the values actually measured. In the absence of a signal, adjust the fine-tuning capacitor to adjust the zero position average of each waveform to 0V. This step can greatly reduce measurement errors caused by static voltage and current in the measurement system.
