# Gate resistor calculations

Discussion in 'General Electronics Chat' started by DumbDummy77, Feb 13, 2010.

1. ### DumbDummy77 Thread Starter Member

Jan 21, 2010
43
0
Hi

Since I am going to connect two N-channel MOSFET to the IR2151 (Self oscillating IC), I need to connect gate resistor on each gate. I cannot find any formula or calculation on IR2151's datasheet. How can I calculate the gate resistor?

I would be appreicate if example calculation carries out which will be great for me.

Regards

DumbDummy77

Oct 2, 2009
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3. ### DumbDummy77 Thread Starter Member

Jan 21, 2010
43
0

Thank you for this link and I want to point out two things which are a bit unclear to me:

According to (e) on the last page, the IClamp is 500mA and why 0.500mA is applied in the fifth term of equation instead of 500mA

ITOT = 1.10mA + 0.840mA + 0.175mA + 0.180mA + 0.500mA = 2.795mA

Secondly, why this application note use kW instead of kOhm?

R1 = (167V - 15.4V)/2.795mA = 54kW

Thirdly, I cannot find any equation which enables me to calculate RGATE for high side and low side respectively.

Please highlight the equation in the design tip application note me?

Regards

DumbDummy77

4. ### DumbDummy77 Thread Starter Member

Jan 21, 2010
43
0
Hello guys

I would be very grateful to get some helps from you guys

Regards

DumbDummy77

5. ### DumbDummy77 Thread Starter Member

Jan 21, 2010
43
0
I am struggling here...

DumbDummy77

6. ### beenthere Retired Moderator

Apr 20, 2004
15,815
290
A bit of familiarity with Ohm's law helps. R = E/I. Since we have a resistance value being calculated, and a voltage being divided by a current, we may see that 54KW is a misprint, as 54,000 ohms is what the relationship solves for.

Looks like another sloppy proofreading. That current level is supposed to be just enough to keep an internal zener in regulation. Half a milliamp is probably enough, 500 milliamps is certainly enough to fry the device, which will hardly be able to dissipate significant power.

Knowing and applying some basics is very useful.

7. ### DumbDummy77 Thread Starter Member

Jan 21, 2010
43
0
Thank you for letting me know about it. I am glad that I am not only one who spotted errors in the application note.

Let move to the gate resistor, I have been looking on internet for hours and hours in attempt to find the formula or design example which allows me to calculate the gate resistor for high side and low side respectively which connect between IR2151 and IGBT/MOSET. I would be very very grateful if you can provide any assistance to calculate the gate resistor.

Regards

DumbDummy77

8. ### Papabravo Expert

Feb 24, 2006
11,083
2,159
There is no "formula" for the gate resistor. This is because the gate of a FET is not a constant impedance. As the gate voltage rises it reaches a plateau where it will remain for a period of time before proceeding to the final value. Simulation can help you see this effect.

In practice a large value will lead to long switching times and excessive power dissipation before the FET is turned on or off completely. It also makes it difficult to discharge the gate when you want to turn it off. Too small a value is just a waste because the turnon and turnoff times cannot be reduced below some minimum value.

You just have to know the part.

Last edited: Feb 18, 2010
9. ### DumbDummy77 Thread Starter Member

Jan 21, 2010
43
0
Okay but why many application notes and project advise to use gate resistor to remove some sort of parastc and I cannot remember in great details. Since you mentioned the effect of using gate resistors, what if I don't use gate resistor, will the MOSFETs be affected?

Do you know any good simulation allows me to analyse on the IR2151, IR2110, IR2113 or IR2184?

DumbDummy77

10. ### shortbus AAC Fanatic!

Sep 30, 2009
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Last edited: Feb 18, 2010
11. ### Papabravo Expert

Feb 24, 2006
11,083
2,159
The purpose of the gate resistor is to limit the current. Yes it also provides the damping required of a high impedance input. No you can't just ignore it. With any given MOSFET some experimentation is required, to find out how the gate capacitance can be charged and discharged. This is why they pay engineers who understand these things the big bucks!

Mar 8, 2009
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13. ### Papabravo Expert

Feb 24, 2006
11,083
2,159
I can recommend LTSPICE, a free download from Linear Technology. The LTSPICE group on Yahoo is also an excellent resource.

14. ### Audioguru AAC Fanatic!

Dec 20, 2007
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Who pays engineers big bucks?
Lawyers, politicians and doctors make big bucks.

15. ### DumbDummy77 Thread Starter Member

Jan 21, 2010
43
0

Does this simulation allow me to analyse the IR2110, IR2113 or IR2184?

DumbDummy77

16. ### Papabravo Expert

Feb 24, 2006
11,083
2,159
You need to find models (small files that describe the particular devices) for each device you wish to analyze. In many cases these are available from the manufacturers. You can start by googling the following
"IR2110 model"
"IR2113 model"
"IR2184 model"
They guys on the LTSpice yahoo group are also a good source of information.

17. ### Papabravo Expert

Feb 24, 2006
11,083
2,159
That's a joke -- son!