World's fastest power switch test and measurement

Silicon power devices in regulators and DC-DC power supplies will soon be replaced by GaN FETs. Compared to silicon MOSFETs, they switch much faster and have lower RDS(on). This will enhance the power efficiency of the power supply and bring benefits to everyone. If you are designing a power circuit with a GaN device, you need to know the switching speed of the device. To measure this speed, the oscilloscope, probes, and interconnects must be fast enough to minimize their impact on the measurement.

The question I am most often asked about device performance is, “How fast are they?” I usually answer that they are very fast, but I don’t really know how fast. To find out, I measured them using a 33GHz real-time oscilloscope and high-speed transmission line probes. I will discuss the design constraints that affect device speed and what the future holds. Based on these measurements, I believe we will soon be able to design power supplies that switch at 250MHz.

Figure 1 shows the two evaluation boards used to make the measurements. Both boards are equipped with a gate regulator, a driver, a pulse conditioner, and two eGaN switches. The board on the right is a complete DC-DC converter with a Gen4 monolithic half-bridge (both switches on the same wafer) and an LC output filter. The evaluation board on the left uses individual Gen3 eGaN devices in a half-bridge configuration without an LC output filter. In both cases, an external pulse generator provides the PWM signal through a BNC connector soldered to the pulse width modulation (PWM) input of the test board. I measured the switch rise time on each evaluation board with an input voltage of 5V and 12V.

Figure 1: Here only the half-bridge configuration is configured on the left board, while the board on the right is equipped with a complete DC-DC converter. Banana jacks connect the test board to the electronic load. Connection to an external pulse generator is possible via the BNC connector.

Instrument and probe requirements

To ensure that the instrumentation and probes do not significantly affect the measurement, we can assume that the rise times of the probe, oscilloscope, and half-bridge can be added together using the root sum square method. Although this is not always correct, we can assume this relationship for our initial estimates.

The measured half-bridge rise time includes the oscilloscope’s rise time and the probe’s rise time, and is:

The actual rise time of the half-bridge can be determined as follows:

To limit the measurement error to a certain percentage K, the instrument's rise time can be related to the actual rise time:

Solving for K, the ratio of the instrument rise time to the actual half-bridge rise time is:

Therefore, for these two examples, if we want to measure less than 5% or 10%, the rise time of the oscilloscope and probe needs to be less than 32% or 46% of the FET rise time, respectively. In other words, the instrument rise time should be 3.1 or 2.2 times faster than the FET rise time, respectively.