Full schematic of a BLDC motor controller

AlbertHall

Joined Jun 4, 2014
12,345
If HO goes to 12V the voltage on the gate will be 12V. Where is this 1Ω you mention?
The high side supply has a capacitor to maintain the voltage while the high side is conducting. Usually this is charged by the repeated switching but in this circuit there is DC-DC converter to provide the high side voltage.
 

jpanhalt

Joined Jan 18, 2008
11,087
When the mosfet is in the off-state, Vs = 0V and V(ho) pulses between 0-12V. So when V(ho) gets a pulse and rises to 12V, the voltage differential across gate en source can be calculated (47 ohm resistor + gate resistance = 1 ohm). The voltage at the gate is only 0,25V which means the mosfet will not switch.
First, Vgs(th) is the gate-source threshold voltage at which the mosfet starts to turn on. It is not fully on at that voltage.

Second (quoted material), I am not sure of what you are asking, particularly your calculation of gate resistor + gate resistance. Vho is referenced to the supply, So when the mosfet is on, the source is at supply and the gate is supply + some increment depending on the driver.

John
 

jpanhalt

Joined Jan 18, 2008
11,087
The Vgs(th), as I stated, is the threshold. Different devices can have different thresholds. Apparently, the upper range of that threshold for aceptable devices is 4.9V. That does not affect the allowed maximum for Vgs, which is usually about + or - 20V.

John
 

jpanhalt

Joined Jan 18, 2008
11,087
In the mosfet datasheet.
Your need to look at the whole datasheet. Rg refers to the effective resistance that would be in series with an external gate resistor as the gate capacitance is charged. In your example, the 1 Ω is in series with the 47Ω gate resistor. If you were to calculate the maximum gate current at a certain voltage and capacitance, you might need to consider it. For the purposes of your question, I believe it can be ignored.

John
 

AlbertHall

Joined Jun 4, 2014
12,345
JP14 is a two pin header onto which you can put one of those little links. If the link is fitted then errors latch, if not they will automatically reset.
 
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