Multichannel Measurement for SiC Inverter Voltage Waveforms

Introduction

Research and development using silicon carbide (SiC) power devices is proving to be immensely important in the design and creation of significant energy saving instruments.

Now that it has progressed from the R&D stage to practical use, engineers are adopting SiC power devices in the production of motor drive inverters for railways and EVs.

To best utilize SiC devices for improved energy efficiency in equipment, it is important to optimize the internal device peripheral circuits in the inverter according to the device characteristics. In both the R&D and evaluation stages, this means accurately measuring surge voltage, switching time, and high-speed changing voltage signals at multiple locations is the priority.
 

Mitigating Potential Measurement and Design Challenges

There are multiple benefits to employing SiC devices in the production process, including incredibly fast switching and improved energy efficiency. Even so, there are several measurement and design challenges that must be considered.
 

Figure 1. Motor drive inverter circuit

Check the gate-source voltage

A SiC power device greatly reduces switching loss, as the switching speed is very high. However, rapid changes in voltage can generate surge due to stray inductance and capacitive components on the circuit. These rapid changes in the current cause electromagnetic noise, as a result of alterations in magnetic flux. If adequate measures are not taken, the equipment may malfunction or encounter durability problems because of the stress applied to part of the circuit.
 

Figure 2. Switching devices

Optimize deadtime between the high side and low side

If devices at the high and low side are turned on simultaneously, a short circuit current occurs that causes serious harm to devices, making dead time essential. To reduce switching loss, it is necessary to employ dead time, though minimally. As each device has variations, consider delay and jitter due to transient characteristics.

Measurement at multiple channels simultaneously

A three-phase motor inverter consists of six switching devices. A typical four-channel oscilloscope cannot simultaneously measure them, therefore more channels are required.

Figure 3. Output from switching devices measured with an oscilloscope

Remove the effects of common mode noise

Because the potential on the high side of a bridge circuit moves, a differential probe or an isolated input measuring instrument are required. To accurately measure floating signals like an inverter, high CMRR performance at high frequencies is needed.

Sic Inverter Measurement Example

Of the six switching devices (SiC MOSFET) in the threephase inverter, a pair of upper and lower arms for the gatedrive signal and drain-source voltage signal is measured.

Figure 4. Test setup with switching devices being measured via oscilloscope

Figure 5. Zoomed-in waveforms of high and low gate-source and drain-source

Energy Efficiency Solutions

High-speed, simultaneous multichannel measurement of floating voltage signals

Combining the eight-channel, high voltage DLM5000 Mixed Signal Oscilloscope with high-frequency differential probes produces accurate simultaneous measurements for each voltage signal of the inverter internal circuit. This results in the ability to more efficiently check surge voltage and switching operation timing.
 

Figure 6. Differential probes connected to the high and low side of the drain-source, control signal, and gate-source

Increase the number of channels with DLMsync

When eight channels are not enough, two DLM5000s can be connected via DLMsync to enable synchronous measurement of up to 16 analog channels.

Increased efficiency of variability evaluation

Using the History function on the DLM5000, users can save up to 100,000 previously captured waveforms in the acquisition memory. In addition, the DLM5000 enables statistical analysis of repetitive waveform parameters and is effective for jitter measurement and level fluctuation.

DLM5000 Series Mixed Signal Oscilloscope

• Eight analog channels, up to 32 bits logic input in one unit
• 2.5 GS/s for all eight analog channels
• 350 MHz/500 MHz frequency bandwidth
• Maximum 500 Mpoints-long memory
• USB 3.0

Figure 7. Adaptable mixed signal oscilloscopes allow for more efficient and effectibe measurement and analysis of electrical signals

701927 150 MHz High-Voltage Differential Probe

• Frequency band: DC to 150 MHz (-3 dB)
• Maximum allowed differential voltage: ±1400 V (DC + Acpeak) (500:1)
• CMRR (typical): -80 dB (60 Hz), -50 dB (1 MHz)
• Input attenuation ratio: 50:1 and 500:1, user-selectable
• Power supply: Yokogawa interface

Figure 8. High-voltage differential probes enable the detection and floating measurement of the difference between non-grounded signals