Accurate energy loss measurements are one of the key objectives of double pulse testing. Eliminating the timing skew between voltage and current probes is a critical step in making accurate power and energy measurements on an oscilloscope.

The double pulse testing software (WBG-DPT), available on the 4 Series, 5 Series and 6 Series MSOs, includes a new deskew technology designed specifically for double pulse testing.  This novel approach is very different from the traditional approach and significantly faster. It can shave hours off of testing.

The technique works for power converters that use either FETs or IGBTs. For the purpose of this post, we will use FET terminology to keep things simple.

Why Deskew?

Energy loss during switching must be minimized in the design of any power converter. This energy loss can be measured using an oscilloscope. The general approach is to multiply concurrent voltage and current samples to generate a power waveform. 

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

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

E = ∫p(t)dt

For these energy loss measurements to be accurate, the transitions of current and voltage waveforms should align in time. So, in order to achieve meaningful energy loss measurements, designers must correct for different delays introduced by test fixtures and probes.

Traditionally, the skew between probes is computed before starting any measurement on the test setup. For low-voltage applications, a function generator and deskew adapter (Tektronix P/N 067-1686-03) can be used for alignment. However, this approach is not optimal for high-voltage and high-current applications.

The traditional technique for aligning higher power, low side drain-to-source- voltage (V_DS) and drain current (I_D) measurements requires rewiring the test setup. The load inductor must be removed and replaced with a resistor.  Then measurements are made and the V_DS and I_D measurements are aligned. This procedure can take an hour or more.

Figure 1. The traditional approach to deskew involves removal of the load inductor and replacing it with a resistor.

 

A New Deskew Approach

The Tektronix WBG-DPT solution’s industry-first software based deskew technique eliminates the need for rewiring and is performed after double pulse measurements have been taken. In the new method, the drain current (I_D) is acquired and used as a reference waveform. During turn-on, a low-side V_DS alignment waveform is calculated using a parametric model of the test circuit.  The alignment waveform references the I_D waveform and will have zero skew with respect to I_D. The deskew algorithm determines the skew between the calculated V_DS alignment waveform and the measured V_DS waveform. It then applies the skew correction to the V_DS measurement channel.

Figure 2. With the new approach, deskew is performed after the test. Parameters are specified in the Deskew menu.

The Deskew Procedure

As noted above, the deskew can be performed after the measurements are taken. One can start a double pulse test without worrying about skew between VDS and ID, later select the Deskew settings and provide the following parameters:

• Probe resistance - assumed to be a current viewing resistor (CVR) or shunt resistor in this post
• Effective “loop” inductance
• Bias Voltage (the mean V_DS across the low-side FET when it is off)
• Differential Order (filter order used by the model for smoothing)

Figure 3. The equivalent circuit used to build the V_{DS_low} alignment waveform. This circuit assumes a current viewing resistor is being used to measure I_D.

The parameters entered in the deskew menu are used to build the V_DS alignment waveform. The waveform is built using Kirchhoff’s Voltage Law:

Where:

V_DD - V_{DS_high} represents the power rail voltage and drop across the high-side FET. Note that during turn-on, this will be constant since V_DD is fixed and V_{DS_high} is the voltage across the high-side FET’s body diode.

R_shunt is the resistance of the current shunt.

 I_D is the measured drain current, based on the 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.

During turn-on, V_DD-V_{DS_high} will be practically constant as noted above. R_shunt and L_eff are also constant.  This means the modelled V_{DS_low} alignment waveform is a function of I_D.

After configuring the parameters, the user presses the WBG deskew button. The system generates a mathematical model of V_DS, based on the specified parameters and the drain current.  This alignment waveform is displayed on the screen.

Figure 4. The V_DS alignment waveform, calculated from I_D, is compared to the measured V_DS waveform. Skew is the time difference between the alignment and measured waveforms. Once calculated, the skew can be removed from the I_D waveform.

The effective inductance, L_eff, is a “lumped” element that considers the entire loop as shown above. Thus, L_eff often unknown and getting it dialed in will be an iterative process. The deskew process can easily be run repeatedly, and adjustments can be made to L_eff, until the calculated alignment waveform and the measured V_DS waveform have the same shape. If there are differences in the shapes of the modeled V_DS alignment waveform and the measured V_DS waveform, the parameters can be adjusted and the deskew can be run again.

Once the parameters accurately represent the system, the modeled alignment waveform will have the same shape and the system can determine and correct the skew. The skew value shows up in the Deskew settings and is automatically applied to the channel to which the V_DS signal is connected.

This new process accurately accounts for skew and reduces the deskew time from an hour or more to just 5 to 10 minutes. 

To learn more about double pulse testing, visit the Double Pulse Testing web page.