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[SOLVED] Gate drive Requirements in Active free-wheeling BLDC motor drive

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gonespa

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Hi,

The Background: Trying to understand how a BLDC motor controller works. The system would comprise a microcontroller generating the PWM signals (6 of them, one for each MOSFET), which would be fed to three IR2110 MOSFET drivers using a synchronous rectification (active freewheeling) drive schema. (the microcontrolles handles the deadtime problem). It would be using high and low N-Channel MOSFET.

I have been struggling to understand how is it possible, for a MOSFET driver IC (IR2110 for instance) to effectively activate a low-side N-Channel MOSFET whenever it is handling the freewheeling current from the motor. As I understand, MOSFETs can conduct from Drain to Source as well as from Source to Drain provided the device is "on", ok, but... in this particular moment the source of the transistor would be higher than ground so, in order for the IR2110 to provide enough gate voltage, it would have to generate voltages greater that the Vcc (Low side supply), isn´t it?. I can´t see how can that happen, what am I missing?.

I understand the need to recharge the charge pump capacitor in order to be able to switch on the High side MOSFET. What I do not understand is, when driving inductive loads as a BLDC motor, how is it possible to switch on the Low side one (in the same Phase which is being driven by the controller at a particular time) to conduct the freewheeling current that the inductor is generating.

At that moment the high side mosfet is switched off, So the normal path for the current to go is interrupted. The inductor reverses the Voltage at its terminals, and to allow the current to go somewhere we turn on the Low side MOSFET (the one located in the same phase), so it can free-wheel through it. But in that situation the Source of the MOSFET is above the Drain (otherwise the current would not flow...). Since the Gate has to be above the Source in order to open de MOSFET, and the IC is apparently unable to do so (not in the Low side), How can this work?
 

FvM

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Don't understand which problem you are talking about.

A MOSFET can be always switched on with 6 - 12 V Vgs, indepedendent of Vds. Respectively supplying the IR2110 with Vcc of 12 V will handle all possible situations. Review a MOSFET datasheet in case of doubt.
 

gonespa

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Don't understand which problem you are talking about.

A MOSFET can be always switched on with 6 - 12 V Vgs, indepedendent of Vds. Respectively supplying the IR2110 with Vcc of 12 V will handle all possible situations. Review a MOSFET datasheet in case of doubt.

I am refering to the situation depicted in the following picture ActiveFW .png

Specifically to the "Freewheel current" blue arrow. The PWM for the top "M1" MOSFET if off, but the low "M6" is kept ON, at the same moment the "M2" Lower MOSFET from the same "leg" as the M1 one os switched on.
This opens the path to the freewheel current to move through M2, the motor and M6. In that situation, since the current is flowing in the inverse way through M2, isn´t its Source terminal at a voltage level higher than its Drain Terminal?, and if so, what happens with the Gate voltage? It should be higher than the Source Voltage, that is now somewhere over ground, isn´t it?
 

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isn´t its Source terminal at a voltage level higher than its Drain Terminal?, and if so, what happens with the Gate voltage? It should be higher than the Source Voltage, that is now somewhere over ground, isn´t it?
No. In inverse operation Vds is negative. f changed at all, the effective channel control voltage is even slightly increased. But because Vds is limited to a diode forward voltage, there's no significant effect on the gate control voltage.
 

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No. In inverse operation Vds is negative. f changed at all, the effective channel control voltage is even slightly increased. But because Vds is limited to a diode forward voltage, there's no significant effect on the gate control voltage.


I am not sure if I understood what you ment (aside the fact that Vds is negative). So, you say a 12V signal for example to the gate of the low MOSFET from the IR2110 (refered to the COM / VSS pin) at this point would switch on the MOSFET? at which voltages would be the source pin?. It´s difficult for me to see the full picture...
 

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Obviously the source pin is connected to ground all the time, with or without current flowing through the MOSFET. So we should expect 0 V at the pin, isn't it?
 
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