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buck converter-question about the mosfet size

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nelly1

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Hello everyone!

I'm trying to design a buck converter with pmos as switch and nmos as diode by using TSMC 65um.
Vin=3.6V Vout=1.2V Iout=150mV Vripple<50mV f=1MHZ
I caculated : L=8uH C=0.25uF R=8Ω

I have two questions now and hope someone can help me:

1.For the pmos as switch, i caculated the power loss(conduction loss and switch loss)to get the optimum W/L value.But for the diode which using the nmos i don't know how to determine the W/L value of the mosfet in order to get the lowest power loss.

2.because there is impossible to set the diode voltage to zero, the Vl is increased so that the Il as well as Ir are also increased. Should i change the Rload value to get Vout=1.2V?(now the design is only a open loop without PWM and so on).

Any advices will be appreciated!!
 

Hello everyone!

I'm trying to design a buck converter with pmos as switch and nmos as diode by using TSMC 65um.
Vin=3.6V Vout=1.2V Iout=150mV Vripple<50mV f=1MHZ
I caculated : L=8uH C=0.25uF R=8Ω

I have two questions now and hope someone can help me:

1.For the pmos as switch, i caculated the power loss(conduction loss and switch loss)to get the optimum W/L value.But for the diode which using the nmos i don't know how to determine the W/L value of the mosfet in order to get the lowest power loss.

To minimize power losses, the idea is to use a mosfet with the lowest On-resistance.

However you're operating at 3.6V, which can be a problem with mosfets which will not turn on fully when 3.6V is applied to the gate...

However there may be a type for operating at that low volt level. The name may be digital mosfet. Don't know much else.

2.because there is impossible to set the diode voltage to zero, the Vl is increased so that the Il as well as Ir are also increased. Should i change the Rload value to get Vout=1.2V?(now the design is only a open loop without PWM and so on).

Any advices will be appreciated!!

If you only change the load... then the greater the load, the lower the volts on it.

Try different loads to see what you get.

Or reduce duty cycle. Etc.
 

However you're operating at 3.6V, which can be a problem with mosfets which will not turn on fully when 3.6V is applied to the gate...
I don't understand it very much.. when mosfet works as a diode, it should be always in saturation region cuz vDG >= -VT
Do u mean Vth can be also minus?????:?:
 

I don't understand it very much.. when mosfet works as a diode, it should be always in saturation region cuz vDG >= -VT
Do u mean Vth can be also minus?????:?:

The body diode will do this to a certain extent under the right circumstances. However you must watch whether current stays under a certain amount to be safe.

If you are biasing the mosfet normally, the gate needs to reach a certain volt level before it turns on the mosfet sufficiently, depending on the type of mosfet.

See the performance graph (1/3 down the page) at:

https://en.wikipedia.org/wiki/MOSFET
 

not quite sure about this questions because i m not sure about the type of mosfet i used...may ask my tutor later. whatever thx a lot!
 

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