Analysis of how much information can be derived based on a MOS tube waveform

The DS voltage waveform on a digital oscilloscope gives us valuable insights into the operation of the power stage. By analyzing the waveform, and knowing that the transformer inductance is 1mH, we can determine several key parameters through measurement and calculation.

First, we can estimate the AC input voltage by observing the DC value. If there’s no PFC circuit, the AC input voltage is roughly equal to the DC input divided by √2. Next, we can identify the reflected voltage from the secondary side to the primary, which helps in determining the turns ratio of the transformer.

The duty cycle can be calculated using volt-second balance. By measuring the on-time (Ton) and the time when the reflected voltage is present (T1), we can derive the duty cycle as D = Ton / (Ton + T1 + T2). This is crucial for understanding the converter’s efficiency and control strategy.

Leakage inductance and total stray capacitance can also be estimated from the oscillation frequency seen in the waveform. The resonance between the leakage inductance and the stray capacitance provides a way to calculate these values. Similarly, the magnetizing inductance and MOSFET’s Coss (output capacitance) can be analyzed during the second oscillation phase.

The energy transferred by the transformer can be determined by calculating the peak current in the primary winding. Using the formula E = ½ * Lp * Ip², where Lp is the magnetizing inductance (1mH), we can estimate the energy per cycle. This is particularly useful in flyback converters operating in discontinuous conduction mode (DCM).

The waveform also shows the transient behavior of the MOSFET. Initially, the leakage inductance causes oscillations, followed by the resonance between the magnetizing inductance and the stray capacitance. These oscillations help us understand the switching losses and the overall performance of the system.

By carefully examining the waveform, we can gather a wealth of information about the converter’s operation, including voltage levels, timing, energy transfer, and component characteristics. This analysis is essential for troubleshooting, design optimization, and performance evaluation in power electronics.