Misunderstanding of signal frequency in bridge circuits of power devices: Switching frequency vs. signal bandwidth
When engineers analyze bridge circuits with power semiconductors, misunderstandings often arise regarding the signal frequency. A common statement is: "There is no high frequency here – the gate driver frequency is only a few tens of kilohertz."
This interpretation overlooks an important concept: the distinction between switching frequency and signal frequency components. High frequency in this context does not refer to the switching repetition rate of the device. Instead, it refers to the high-frequency spectral component within the signal itself, often driven by fast switching edges.
Modern power semiconductor technology has rapidly evolved towards higher voltage ratings, lower conduction losses, and ultrafast switching performance. These improvements are particularly evident in wide-bandgap devices such as SiC MOSFETs and GaN transistors. By reducing switching losses, the voltage rise rate (dv/dt) of Vds or Vce increases dramatically, enabling switching operations to occur in just a few nanoseconds.
Therefore, bridge circuits generate considerable high-frequency energy in the middle, even at relatively low switching frequencies. This behavior leads to high-frequency common-mode interference at high voltages, especially when measuring sensitive signals such as Vgs or Vge on high-side instruments.
It is clear that the signal frequency refers to the internal high-frequency components of a waveform, not the switching frequency of the device. Even if a power supply switches at 1 Hz, the waveform can contain very high-frequency components due to the rapid rise and fall times. If the measurement system does not have sufficient bandwidth, these components may remain hidden, leading to incomplete or misleading observations.
This limitation was demonstrated by a customer in the power semiconductor industry. When testing IGBTs, a differential probe with a 5 MHz bandwidth limit was used to measure the gate-emitter voltage (Vge). The oscilloscope displayed smooth and clean waveforms, which initially seemed ideal. However, this setup inadvertently masked high-frequency spikes and ringing, posing a potential risk to production quality and the long-term reliability of the components.
In another case, engineers at a power device laboratory in Dongguan were validating GaN transistors and suspected abnormal high-frequency behavior that could not be detected with conventional high-voltage differential probes on the high side. When they replaced the probe with a 500 MHz bandwidth optical isolation probe, they observed oscillations of up to 580 MHz in the high-side Vgs signal. These oscillations were attributed to parasitics on the circuit board and interactions between components. Further analysis revealed that an optically isolated probe with a 1 GHz bandwidth would be required for accurate characterization.
These examples illustrate an important conclusion: The switching frequency describes how often a power device switches on and off, while the signal frequency describes the high-frequency component caused by rapid transitions and parasitic effects. As power devices switch ever faster, the frequency spectrum of signals from bridge circuits expands considerably.
Therefore, when testing high-side signals in SiC and GaN bridge circuits, engineers must select probes with high bandwidth and excellent common-mode rejection, such as optical isolation probes. Without sufficient bandwidth, critical oscillations, overshoots, or spikes can be filtered out, leading to incorrect conclusions about the circuit's behavior.
To correctly interpret the signal behavior in bridge circuits of power devices, it is important to distinguish between switching frequency and signal frequency components. Understanding this difference enables you to ensure accurate measurements, reliable device validation, and robust power electronics design.
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Misunderstandings regarding signal frequency in power component bridge circuits: switching frequency versus signal bandwidth