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# Waveform at the blocking MOSFET transistor gate
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# Waveform at the blocking MOSFET transistor gate
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The somewhat weird waveform at the gate of the driving MOSFET in the
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blocking output stage is shown below. Instead of what one would await (a
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rectangular pulse), the shape seems to indicate some capacitors that
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trickle in at one point.
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Reading application note
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[AN11158](http://www.nxp.com/documents/application_note/AN11158.pdf)
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from NXP, the answer is revealed in Section 2.6.2.1, where the three
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transistor charges are presented. The answer lies in the Miller
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capacitance. The flat region on both rising and falling edges of the
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gate voltage is called the Miller plateau and is due to the MOSFET's
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capacitance changing on the threshold voltage. This change in
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capacitance causes the voltage to stay constant. After some time, the
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change in capacitance stops and the gate voltage increases further.
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The length in time of the Miller plateau is highly dependent on the
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circuit, as both the drain current, the drain-to-source voltage and the
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transistor load (in our case, inductive), affect its shape.
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The same section of the NXP application note offers pointers on how to
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decrease this Miller plateau. By decreasing the drain-to-source voltage
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the length of the plateau is shortened, and by decreasing the drain
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current the threshold voltage is
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lowered.
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![](/uploads/a163ab35a266562c4db9bd10359c0da4/mosfet-gate.png)
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![](/uploads/a163ab35a266562c4db9bd10359c0da4/mosfet-gate.png)
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-----
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Theodor-Adrian Stana, Jul. 2014
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### Files
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### Files
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