[SOLVED] MOSFET gate current calculation

ismu · Nov 19, 2012

Can any body help to How can i Calculate MOSFET gate current if i use :

Totally 8 MOSFETs IRF8055 connected parrallely .
Frequency : 50Hz square wave.
Vgs=12V
so CAN I USE ANY TRANSISTOR TO DRIVE THIS GATE WHICH HAVE MAX: CURRENT CARRIYING CAPPACITY IS 500mA.
IF NOT Y?

Junus2012 · Nov 19, 2012

Hello
MOSFET has a very high input impedance because the gate is isolated by the oxide, therefore the input current in the gate is assumed to be zero except for the leakage current which is small. it is right when you connect 8 transistors in parallel it will have more considerable value but still you have to think that the control of the MOS transistor is by voltage (Voltage controlled current device) not by current.
it mean you have to calculate the needed gate voltage not the current

ALERTLINKS · Nov 19, 2012

you have to calculate the needed gate voltage not the current

What about input capacitance of gate which has to be charged and discharged and hence draw current. It is directly proportional to input frequency. For 50HZ, it is low. A gate resistance of 120 ohm will do. See datasheet Are you sure it is IRF8055.
http://www.analogzone.com/pwrt1208.pdf
**broken link removed**

See gate charge,
http://www.irf.com/technical-info/appnotes/mosfet.pdf

Junus2012 · Nov 19, 2012

right but if the frequency is very small like this case of 50 Hz then I think you no need to care much about the current

ALERTLINKS said:
What about input capacitance of gate which has to be charged and discharged and hence draw current. It is directly proportional to input frequency. For 50HZ, it is low. A gate resistance of 120 ohm will do. See datasheet Are you sure it is IRF8055.
http://www.analogzone.com/pwrt1208.pdf
**broken link removed**

See gate charge,
http://www.irf.com/technical-info/appnotes/mosfet.pdf

ismu · Nov 19, 2012

Junus2012 said:
Hello
MOSFET has a very high input impedance because the gate is isolated by the oxide, therefore the input current in the gate is assumed to be zero except for the leakage current which is small. it is right when you connect 8 transistors in parallel it will have more considerable value but still you have to think that the control of the MOS transistor is by voltage (Voltage controlled current device) not by current.
it mean you have to calculate the needed gate voltage not the current

Hi thanks your replay

But why we use MOSFET driver? and why can't we drive the MOSFET directly from Microcontroller output pin [assume Vgs=5V].
MCU current soursing per pin is 20mA . So can we drive directly using this PIN? ;-)

Junus2012 · Nov 19, 2012

Obviously I dont see any problem with driving the MOS directly, nowadays even it is the preferable switch driver because the gate current is negligible and hence it will not load the µC, While if you use the bipolar transistor then you need to drive the base with a current equal to (IE/B). any way as ALERTLINKS said: for the high frequency application you need to consider the input capacitance of the MOS because it will limit the maximum speed (slew rate) of the switch and for this case only we consider the current of the MOS gate to be enough.

jeffrey samuel · Nov 19, 2012

You don't need current at the gate for all the frequency but you need to set an Voltage there so we go for drivers

Junus2012 · Nov 19, 2012

we need just enough current at the gate to charge the input capacitance of the MOS with sufficient rate to follow the dynamic operation(frequency) otherwise the driving control signal is the voltage

jeffrey samuel said:
You don't need current at the gate for all the frequency but you need to set an Voltage there so we go for drivers

ALERTLINKS · Nov 19, 2012

Most FETs need 10-12V to switch on their gate. Microcontroller run with 5V. That is why a driver or level converter is used. There are logic level FETs which swtch with logic level (5V) and can be directly driven with MCU.

Junus2012 · Nov 19, 2012

it depend on the transistor type voltage, not all working with 10-12V, you can find with 5V or even less

ALERTLINKS said:
Most FETs need 10-12V to switch on their gate. Microcontroller run with 5V. That is why a driver or level converter is used. There are logic level FETs which swtch with logic level (5V) and can be directly driven with MCU.

picgak · Nov 20, 2012

ismailpp2008 said:
Can any body help to How can i Calculate MOSFET gate current if i use :

Totally 8 MOSFETs IRF8055 connected parrallely .
Frequency : 50Hz square wave.
Vgs=12V
so CAN I USE ANY TRANSISTOR TO DRIVE THIS GATE WHICH HAVE MAX: CURRENT CARRIYING CAPPACITY IS 500mA.
IF NOT Y?

No you cannot use 500ma .During switching the gate capacitance causes a heavy inrush of current for 8 nos of mosfets it may require atleast 2A mosfet driver chip

ALERTLINKS · Nov 20, 2012

Current carrying capacity is much higher for pulse. See data sheet and calculate pulse duration to charge gate capacitance and compare with transistor parameters. It is always safer to use components with higher ratings.

Junus2012 · Nov 20, 2012

Obioisly he doesn't need, but there is very small leakage current at the gate because of the tunneling effect and he want to calculate it
the other leakage current source is the subthreshold voltage current

jeffrey samuel said:
You don't need current at the gate for all the frequency but you need to set an Voltage there so we go for drivers

KonstanU · Nov 20, 2012

How much MOSFET draws gate current depends on how fast do you want switch MOSFET.
Basic equation is Qtotal = I * tswitch. Qtotal you could find in datasheet.
From here you can see that if you need speed up MOSFET transient process you need increase gate current.
Main rule is higher switching speed means less dynamic power losses (In most cases)

Junus2012 · Nov 20, 2012

high speed mean more dynamic power because the dynamic power is related to the frequency of operation that is limited by charging time of the capacitor which need more current to make it faster and hence more power

KonstanU said:
How much MOSFET draws gate current depends on how fast do you want switch MOSFET.
Basic equation is Qtotal = I * tswitch. Qtotal you could find in datasheet.
From here you can see that if you need speed up MOSFET transient process you need increase gate current.
Main rule is higher switching speed means less dynamic power losses (In most cases)

dick_freebird · Nov 20, 2012

You can calculate the time averaged current from the
Qgg and frequency (presuming your driven levels are
consistent with the spec test condition - drain voltage
swing being the dominant variable I expect).

But it is critical that you have a surplus of current
capability on the gate transitions, so that you can
get through the switching regions quickly and not
cook the device. You may want to have excessively
low MOSFET driver resistance, so that you can add
a selected gate resistor to obtain the drain dV/dt
that your MOSFETs can tolerate. Some FET drivers
offer separate source and sink legs to let you set
rising and falling slopes with separate resistors; if
not then you see R / D+R schemes employed.

Point being, don't just get as far as figuring the
average current and think you're sone.

KonstanU · Nov 20, 2012

Junus2012 said:
high speed mean more dynamic power because the dynamic power is related to the frequency of operation that is limited by charging time of the capacitor which need more current to make it faster and hence more power

That is not what I said. I meant that if you have less time on switching you have less dynamic power dissipation under the same frequency

rohitkhanna · Dec 13, 2012

ismailpp2008 said:
Can any body help to How can i Calculate MOSFET gate current if i use :

Totally 8 MOSFETs IRF8055 connected parrallely .
Frequency : 50Hz square wave.
Vgs=12V
so CAN I USE ANY TRANSISTOR TO DRIVE THIS GATE WHICH HAVE MAX: CURRENT CARRIYING CAPPACITY IS 500mA.
IF NOT Y?

We assume that you want the MOSFET to have minimal switching time - to limit the power dissipation. Yes ?

What you need is to find out the "Total Gate Charge' value from the spec-sheet. I could not find spec sheet for irf8055 ??

Then --

using gate charge you can easily calculate the amount of current required from the drive circuit to switch the device on in a desired length of time because Q = CV and I = C.dv/dt, the Q = Time x current. For example, a device with a gate charge of 20nC can be turned on in 20msec if 1ma is supplied to the gate or it can turn on in 20nsec if the gate current is increased to 1A.

These calculations would not have been possible with input capacitance values alone.

Conversely - if you have a limited current drive capability, then you can find out the fastest switching time you can achieve.

ismu · Dec 13, 2012

rohitkhanna said:
We assume that you want the MOSFET to have minimal switching time - to limit the power dissipation. Yes ?

What you need is to find out the "Total Gate Charge' value from the spec-sheet. I could not find spec sheet for irf8055 ??

Then --

using gate charge you can easily calculate the amount of current required from the drive circuit to switch the device on in a desired length of time because Q = CV and I = C.dv/dt, the Q = Time x current. For example, a device with a gate charge of 20nC can be turned on in 20msec if 1ma is supplied to the gate or it can turn on in 20nsec if the gate current is increased to 1A.

These calculations would not have been possible with input capacitance values alone.

Conversely - if you have a limited current drive capability, then you can find out the fastest switching time you can achieve.

-------------------------

Thanks rohitkhanna and all others
i got the complete solution. So here we must care about that Gate capacitance and switching speed only .
Thanks again to all

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