[Tektronix Application Sharing] A new software timing deviation calibration method accelerates the double-pulse test process

An important goal in double-pulse testing is to accurately measure energy loss. For accurate power and energy testing in an oscilloscope, the key step is to calibrate the voltage probe and current probe to eliminate timing deviations.

Double-pulse test software (WBG-DPT) can be used on 4-series, 5-series and 6-series oscilloscopes. The software includes a new calibration technology designed for double-pulse testing to eliminate timing deviation (deskew). This new method is very different from the traditional method, and the test speed is significantly accelerated and the test time is shortened.

This technology is suitable for power converters using field effect transistors or IGBTs. In this article, we will use FET terminology to keep the description simple and clear.

Why should we eliminate timing skew ( deskew )?

When designing any kind of power converter, it is necessary to minimize the energy loss during the switching process. This energy loss can be measured using an oscilloscope. The general method is to multiply the voltage and current samples at the same time to generate a power waveform.

p(t) = v(t)*i(t)

Since the power waveform represents energy consumption over time, the energy can be determined by integrating the power waveform:

E = ∫p(t)dt

To accurately measure energy loss, the transitions in current and voltage waveforms should be consistent in time. Therefore, to make accurate energy loss measurements, designers must correct for delays caused by test fixtures and probes.

Generally speaking, the deviation between probes is calculated before starting any measurement on the test setup. For low-voltage applications, calibration can be performed using a function generator and a timing skew calibration fixture (deskew fixture) (Tektronix P/N 067-1686-03). However, this approach is not optimal for high voltage and high current applications.

In order to match the measurement of low-voltage drain-source voltage (VDS) and drain current (ID) at higher powers, traditional techniques require rewiring the test setup. This requires removing the load inductor and replacing it with a resistor. Next, the measurements are taken and the VDS and ID measurements need to be matched. This process may take an hour or more.

A new timing skew calibration ( deskew ) method

Tektronix WBG-DPT solution is the industry's first software-based timing deviation calibration (deskew) technology that does not require rewiring and can be performed only after double-pulse measurement. In the new method, the drain current (ID) is collected and used as a reference waveform. During the turn-on period, the low-voltage side VDS alignment waveform is calculated using the parameter model of the test circuit. The calculated waveform refers to the ID waveform and has no timing offset relative to the ID. The timing skew elimination algorithm determines the timing skew between the calculated VDS waveform and the measured VDS waveform. The deskew calibrated data is then corrected to the VDS measurement channel.

Timing Skew Calibration Process

As mentioned above, timing offset calibration can be performed after measurement. There is no need to worry about the deviation between VDS and ID when starting a double pulse test, then select the deskew setting and provide the following parameters:

• Probe Impedance - assumed in this article to be the current sense resistor (CVR) or shunt resistor

• Effective "loop" inductance

• Bias voltage (average VDS across low-side FET when turned off)

• Difference order (the order of the filter used by the model for smoothing)

Figure 3. Equivalent circuit used to create VDS_low aligned waveforms. This circuit assumes the use of a current-sighting resistor to measure ID .

The parameters entered in the deskew menu are used to construct the VDS corresponding waveform. The waveform is built using Kirchhoff's voltage law:

in

V _DD - V _{DS_high} represents the supply rail voltage and the voltage drop across the high side field effect transistor FET. It should be noted that during turn-on, this quantity is constant since V _DD is fixed and V _{DS_high is the voltage on the body diode of the high-side field effect transistor FET.}

• R _shunt is the shunt resistor.

• ID _is the drain current measured from the voltage drop across R _{shunt .}

• dI _D /dt is the measured rate of change of drain current.

• L _eff is the effective inductance of the entire power loop.

As mentioned above, during turn-on, V _DD - V _{DS_high} is effectively constant. R _shunt and L _eff are also constant. This means that the simulated V _{DS_low} trace waveform is a function of _ID .

After configuring the parameters, the user presses the WBG's deskew button. The system will generate a mathematical model of VDS based on the specified parameters and drain current. The waveform will be displayed on the screen.

Figure 4. _Calculated V _DS alignment waveform based on ID compared to measured V _DS waveform. Offset is the time difference between the aligned waveform and the measured waveform. Once the skew is calculated, it can be removed from the _ID waveform .

As shown in the figure above, the effective inductance L _eff takes into account the "superposition" of the entire loop. Therefore, L _eff is usually unknown, and this parameter needs to be adjusted repeatedly. Simply run the correction process over and over again and adjust L _eff until the calculated alignment waveform and measured V _DS waveform have the same shape. If there is a difference in shape between the calculated V _DS alignment waveform and the measured V _{DS waveform, you can adjust the parameters and run the calibration time offset again.}

Once the parameters are set accurately, the alignment and measurement waveforms will have the same shape, allowing the system to identify and correct skew. The skew value is displayed in the Deskew setting and is automatically applied to the channel connected to the V _DS signal.

This new process accurately calculates skew values ​​and reduces timing skew calibration time from an hour or more to 5 to 10 minutes.