Reg:Gate Resistor For MOSFET

**sakthivel.eee** · 22-12-09, 10:28

Dear All,
What are all the factors should be considered for selecting gate resistor for MOSFETS? Is there any formula which relates rise & Fall times and gate resistor & capacitor?How do i calculate Rise and fall times for perticular gate reisitance?

Thanks in advance,

Thanks & Regards,
Sakthivel.S

**IanP** · 22-12-09, 10:46

goto:
http://www.edaboard.com/ftopic75684.html

rgds,
ianp

**sakthivel.eee** · 10-01-10, 10:44

Hi,
Is there any application notes which explain about relation between gate resistance and rise time,turn on delay etc.,

How to choose gate reisitance for perticular application?

Sakthi

**btbass** · 10-01-10, 11:14

The main factors affecting switching time is the 'Total Gate Charge' which is usually given on the data sheet in Nana Coulomb's, and the 'Turn-on/Turn-off delay times. But there are variations in these parameters from device to device.
Below is an extract from a switching loss calculation.

Calculating the mosfet switching loss is difficult because it depends on many hard-to-quantify and typically unspecified factors that influence both turn-on and turn-off. We can use a rough approximation and verify performance on the test bench.
From the data sheet, the gate charge of the Si9945AEY mosfet is 2nC. By using a 2K2 gate resistor to supply 2mA gate current, the mosfet will switch in 2nC / 2mA = 1uS.

**Tahmid** · 10-01-10, 16:18

A gate resistor should be placed in series with the gate of the MOSFET in order to reduce the ringing. The ringing is caused by the MOSFET's parasitic capacitance and inductance. These parasitic capacitances and inductances for a resonant tank circuit, which causes oscillation or ringing.
To dampen the ringing, the gate resistor has to be added. Choosing an ideal gate resistor value for the MOSFETs can be difficult.
There are tradeoffs for the different values of resistance. A higher value of resistance will reduce the ringing of the MOSFET but the switching efficiency will also be reduced. The efficiency is affected because the resistance causes the MOSFET's gate voltage to rise slower as opposed to a MOSFET with no gate resistance. With the slower rising gate voltage the MOSFET's switching efficiency will decrease.
According to Fairchild Semiconductor Application Bulletin AB-9, I think:
For gate resistance, Change in efficiency:
0 0(nominal)
3.3 -0.40%
4.7 -0.80%
10 -2.90%
20 -5.60%

Keeping a balance to maintain efficiency and dampening ringing, usually gate resistors are used in the order of 10R to 22R.
All my designs use 10R.

**btbass** · 11-01-10, 01:04

If we take Tahmid's gate resistor of 10R.
A typical Total gate charge for a medium power device is 20nC.
If we assume he is using 5V to drive the mosfet, then the drive current will be 5/10 = 500mA.
If you divide nC by mA the result is in uS.
So the turn on/off time is 20/500 = 0.04uS.
So the device will be trying to switch in 40nS.
We need to look at the data sheet to see if the turn on/off delay times can meet this switching frequency.
Next, we build the circuit and use a scope to verify the switching times.

**sakthivel.eee** · 11-01-10, 08:09

Hi all,
Thanks for the reply. Do we have any application note to understand this clearly?

Sakthi

**btbass** · 11-01-10, 08:22

International Rectifier Technical papers.

http://www.irf.com/technical-info/appnotes/mosfet.pdf

**Tahmid** · 11-01-10, 18:34

Hi,
Take a look at the Fairchild Application Bulletin 9 (like I mentioned earlier). Here's the link:
http://hmin.tripod.com/als/ccs/docs/...child_AB-9.pdf