Gate capacitance of Power p-channel MOS Device

[g.s.javed]

HI .

I am designing a MOS device to drive 600mA (0.6 A).
The switching frequency desired for this device is 200K Hz.
The technology used is 0.6um.
So, I have the idea that the gate capacitance of the device has to be charged and discharged for switching operation.
The device size needed is (my approx calculation. Please correct me if I am wrong)
W = 1357um L = 3um.
This value was obtained for Id=600mA at VGS = 15V.

Also, the books say at saturation , Cgs=CGSO*Weff + 2/3*Cox*Leff*Weff

the device details give CGSO = 2e-10 tox = 4.1e-8

For these values, after calculation I got the CGS = 2.258pF

Now my question is , Is this calculation right ??
I am having serious doubts, as the device size is very large and the switching frequency also is very high.

I could use with some brainstorming.
I would pick each one of yours brains for it.

Thank you
Regards

Javed

P.S. : It is infact 0.6A and not a typo error.

 

[keith1200rs]

You didn't mention the ON resistance you were trying to achieve, but those values don't seem unreasonable to me. I have designed some high current transistors on 0.35um and they were 275mm wide (yes, mm) and the capacitance was nearly 200pF.

Keith.

 

[solidstate]

The L cannot be smaller? (0.6um?)

 

[g.s.javed]

@keith1200rs

I did it using the above said formulae. The reason I used a longer L , is to have better control over the current.

Oh Sorry! The ON resistance needs to be <3 ohm.

And about the capacitance, I will need the device to switch at 150 to 200KHz.

The logic i used was, the amount of gate charge needed to charge the gate capacitance Cgs determines how fast the device switches.
It came around 85pC.
using i = q/t ; i=85pC / 2us = 42.5uA

This is the amount of current it needs to charge the capacitance.
For discharging , provide a discharge path.

Am I on the right path , or I have missed something very fundamental here.

Kindly help.

 

[keith1200rs]

I am not sure why you don't use minimum L. What do you mean by "better control of current"? Is it being used as a current source or a switch? In my case I had to use 1um on 0.35um process because I was using high voltage transistors.

You are correct about the fact that you need more current to charge/discharge the larger capacitance, but 2us seems rather slow if you want to run at 200kHz - the transistor will spend most of its time in transition and therefore dissipating significant power. It is not uncommon to need to put several hundred mA into the gate of large transistors to get them to switch quickly.

Without knowing exactly what you are trying to do and what the design criteria are, it is difficult to advise many specifics, but I would have expected to be using minimum width and pumping quite a bit more current in to the gate - at least several mA. Bear in mind if you need to allow for device variations in sizing the transistor. Also, 3 ohms and 0.6A is over 1W. Remember to check the saturation current if you haven't already done so.

Keith.

 

[g.s.javed]

Thank you Keith

I am actually having a PWM block preceding the output driver. The switching frequency of the PWM is around 200KHz. The transistor we speak about is an output transistor. It should switch according to the switching of the Transistor.

The ON and OFF time of the driver unit is 5us max. So I tried to design is to have less than 3us. Also, the period and duty cycle of the PWM also matter here, I suppose.

The switching frequency is the upper end. Typically, the device might operate at 100KHz to 120KHz.

The Device IS working in a high voltage atmosphere. It is in the range of 16 to 32 V. SO sourcing the current might not matter.

If I use 1um instead of 3um, wont the channel length get affected during high current operation?
I just verified that I may not be able to operate it at the short channel length (0.6um). But 1um , I need a clarification on it.

I shall look into the saturation current also. Thank you for reminding.

Javed

 

[keith1200rs]

If I use 1um instead of 3um, wont the channel length get affected during high current operation?

I am not sure what you mean here.

Don't forget power dissipation.

Keith.

 

[erikl]

Also, the books say at saturation , Cgs=CGSO*Weff + 2/3*Cox*Leff*Weff

the device details give CGSO = 2e-10 tox = 4.1e-8

For these values, after calculation I got the CGS = 2.258pF

Now my question is , Is this calculation right ??
I am having serious doubts, ...

Me too: your CGS result seems too low: Cox(0.6µm process) = 2.1 fF/µm². Let's say Leff(min)=2.8µm , then alone 2/3*Cox*Leff*Weff = (2/3) * 2.1 fF/µm² * ( 1357 * 2.8 )µm² = 5.3pF .

 

[g.s.javed]

Any other suggestions to approach the problem.

Thank you.