January 29, 2025
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Today, let’s discuss the topic of IGBT drive power supplies. For a long time, most of us have believed that IGBT drive power supplies require very high precision and must be designed with great care; otherwise, it could easily lead to IGBT module failure. So, why are many IGBT drive power supplies designed as open-loop?
First, let’s look at a few examples of commercial IGBT drivers: the commercially available IGBT driver boards we can see or purchase are all open-loop. I have yet to come across a driver board with a closed-loop drive power supply.
So, why do many people think that IGBT drive power supplies have high requirements? It might be because IGBTs are too valuable, and poor drive design can cause them to fail. However, in reality, this line of thinking is problematic. Actually, the requirements for IGBT drive power supplies are quite low, and there is no need for extremely high precision—open-loop is sufficient.
IGBTs operate in high-frequency switching states (from several kHz to tens of kHz). Their turn-on and turn-off processes are extremely fast (tens to hundreds of nanoseconds). During these transient periods, the drive circuit needs to provide or sink a significant peak current.
Disadvantages of Closed-Loop: If a closed-loop system (e.g., a DC-DC converter with voltage feedback) is used, the feedback loop itself requires time to process and respond. This response time (even if on the microsecond scale) is too slow compared to the IGBT’s switching transient. When the drive voltage momentarily sags due to a sudden load change (such as charging the Miller capacitance), the closed-loop system hasn’t had time to respond before the switching event is over. Worse still, if the loop is not properly designed, it can cause oscillations, leading to fluctuations in the gate voltage. This seriously threatens the IGBT’s safety and can even cause shoot-through failures.
Advantages of Open-Loop: Open-loop power supplies have no feedback loop, thus avoiding issues related to delay and stability. When the IGBT gate requires a large current, the open-loop power supply can provide it immediately (primarily supplied by the energy storage capacitors on the secondary side in practice), ensuring a fast and deterministic switching process.
The load of an IGBT driver is relatively fixed and low power, typically requiring only around 2W-5W. It consists mainly of the gate resistor and the IGBT’s gate-emitter capacitance (Cge) and Miller capacitance (Cgc).
During each switching event, the load variation pattern is repetitive and predictable: charge the capacitor -> maintain -> discharge the capacitor.
Drive power supply designers can precisely calculate the worst-case voltage sag in advance (caused by the power supply’s internal resistance and peak current). As long as sufficient margin is designed in—selecting a low enough internal resistance and large enough energy storage capacitors—it can be ensured that even under the worst conditions, the drive voltage remains within the required range for the IGBT (e.g., guaranteeing it does not fall below 13V).
Since the load is predictable and initial good design can meet the requirements, introducing a complex and potentially risky closed-loop feedback becomes unnecessary.
In fact, IGBT drives have neither dynamic response requirements nor high precision demands for the power supply. Slightly higher or lower gate voltage generally has little significant impact, as long as thermal management is adequately assessed, issues are typically minimal. This stems primarily from the inherent characteristic of the load being fixed and unchanging.
IGBT drive power supplies usually need to provide high-low voltage isolation to ensure safety. An open-loop design can achieve electrical isolation through components like transformers without the need for complex isolated feedback circuits, simplifying the isolation design.
Reliability is mentioned again here mainly because the open-loop design, compared to the closed-loop design, has fewer components, a simpler structure, lower cost, and reduces potential failure points by over 50%. Consequently, it offers stronger noise immunity and environmental adaptability, along with advantages in cost and size.
Of course, closed-loop isolated drive power supplies are not excluded. For example, in many low-power inverters, to save costs, a single switching transformer is used to power multiple outputs, including 4 or 6 channels of IGBT drive power supplies, ±15V, 24V, etc. However, the primary purpose in such cases is often to stabilize the other power rails, rather than specifically the IGBT drive supply. That’s all for today’s sharing. If you have different opinions, please feel free to leave a message for discussion.
