NCV8876 | PartQuest™ Explore

NCV8876 - Overcurrent Protection

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Member of the PartQuest Explore Development Team. Focused on modeling and simulation of analog, mixed-signal and multi-discipline systems covering a broad range of applications, including power electronics, controls and mechatronic systems.

Description

This example demonstrates the overcurrent protection capabilities of the ON Semiconductor NCV8876. It complements the design: https://www.systemvision.com/design/ncv8876-automotive-start-stop-boost-controller, which shows the performance of the device in normal operation. But in this design, the inductor element proves inadequate for boost operation under a heavy load condition. Inductance collapse, caused by core saturation, could lead to damaging peak current levels in multiple electronic components, were it not for the “hiccup-mode” overcurrent protection feature of the NCV8876. Simulation results show that the boost output voltage (dark blue waveform) falls to approximately 5V when the battery voltage (orange waveform) drops to 6V. This is in contrast to the nominal circuit operation, in which the NCV8876 would enter boost control mode and maintains a 6.8V boost output level. But because of the low drop-out voltage of the NCV59302 VLDO, the 5V regulated output voltage is only slightly reduced. The current through the 2.3 Ohm load resistor is maintained at just under 2.2A throughout the entire operation.While this load level is within the current rating of the inductor (2.3A RMS Max.), it is approaching the saturation inductance “cliff” for this traditional (i.e. non-soft-saturating) part. So when the NCV8876 activates boost control, it switches on the Power MOSFET briefly, effectively grounding the low side of the inductor through the small current sense resistance (0.03 Ohms). With the nominal 3.3uH inductance value, this would result in a slow current build-up (V/L = di/dt = 6V/3.3uH = 1.8A/us). But in this case the inductance collapses with further current increase, and the di/dt value becomes very large. The inductor current spikes to well over 10A (light blue waveform), and the corresponding inductance crashes to a small fraction of its nominal value (green waveform).The current spikes could have gone much higher, possibly resulting in damage to the NVGS3130 Power MOSFET, which has a rated pulse current maximum of 19A. If you zoom in on the magenta waveform, you'll see the Ids current rising, slowly at first while the inductance is intact, but then rapidly rising to over 11A as the inductance collapses, all in just 100ns! Fortunately the gate voltage (brown waveform) is cut off to prevent further current rise. This is thanks to the overcurrent protection feature of the NCV8876. When an overcurrent condition is detected, the device immediately goes into “hiccup-mode”, in which the gate drive is turned off and remains off for a count of 1024 clock cycles. After the mandatory hiccup period, the NCV8876 reattempts boost operation, but with continued overcurrent monitoring. Note that the current spikes are repeated with just over a 2ms period, because the clock is programmed to just over 2 us period (450 kHz switching frequency) in this design.To learn more about the capabilities of the ON Semiconductor NCV8876, click here: http://www.onsemi.com/pub_link/Collateral/NCV8876-D.PDFTo learn more about the benefits of Coilcraft's molded core soft saturation technology for reducing problems with current spikes, see page 4 of this document: http://www.coilcraft.com/pdfs/Doc1140_Beyond_the_data_sheet_Part1.pdf

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