Misunderstanding the signal frequency in bridge circuits of power devices: switching frequency vs. signal bandwidth
When engineers analyse bridge circuits with power semiconductors, there are often misunderstandings about 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 in the signal itself, which is often driven by fast switching edges.
Modern power semiconductor technology has evolved rapidly towards higher voltage levels, lower conduction losses and ultra-fast switching performance. These improvements are particularly evident in wide bandgap devices such as SiC MOSFETs and GaN transistors. By reducing switching losses, the voltage slew rate (dv/dt) of Vds or Vce increases dramatically, so that switching operations take place in just a few nanoseconds.
Therefore, bridge circuits generate considerable high-frequency energy in the centre, even if the switching frequency is relatively low. This behaviour leads to high-frequency common mode interference at high voltages, especially when measuring sensitive signals such as Vgs or Vge on high-side devices.
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 may contain very high frequency components due to the fast rise and fall times. If the measurement system does not have sufficient bandwidth, these components may remain hidden, resulting in incomplete or misleading observations.
This limitation was demonstrated by a customer in the power semiconductor industry. When testing IGBTs, a differential probe with a bandwidth limitation of 5 MHz was used to measure the gate-emitter voltage (Vge). The oscilloscope displayed smooth and clean waveforms, which initially seemed ideal. However, this arrangement unintentionally masked high-frequency spikes and ringing, posing a potential risk to production quality and long-term component reliability.
In another case, engineers at a power device lab in Dongguan were validating GaN transistors and suspected abnormal high-frequency behaviour that could not be detected with conventional high-voltage differential probes on the high-side. When they replaced the probe with an optical isolation probe with 500 MHz bandwidth, they observed oscillations of up to 580 MHz in the high-side Vgs signal. These oscillations were due to parasitics on the circuit board and interactions between the components. Further analysis showed that an optically isolated probe with a bandwidth of 1 GHz would be required for accurate characterisation.
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 fast transitions and parasitic effects. As power devices switch faster and 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, e.g. optical isolation probes. Without sufficient bandwidth, critical oscillations, overshoots or peaks can be filtered out, leading to incorrect conclusions about the circuit behaviour.
In order to correctly interpret the signal behaviour in bridge circuits of power devices, it is important to distinguish between switching frequency and signal frequency components. Understanding this difference will ensure accurate measurements, reliable device validation and robust power electronics design.
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Misunderstanding Signal Frequency in Power Device Bridge Circuits: Switching Frequency vs. Signal Bandwidth