Could anybody advise me on MOSFET driving circuit ?

[ayeong]

MOSFETS Basic

Hi,

Could anybody advise me on MOSFET driving circuit ?

Based on datasheet, it is true that the higher the VGS for a N-Channel MOSFET will result in a lower RDS(on) ?

And if that's the case, isn't it will be much easier to operate a P-Channel MOSFET than N-Channel since it only require a low on the gate ?

Where do we usually find the minimum voltage to turn on a N-Channel or P-Channel MOSFET ?

For a P-Channel circuit, why do designer usually attached a huge resistor (in KOhm to MOhm range) between the gate and source ?

For the above MOSFETS, I'm referring to the following MOSFET

Vishay - Si1413DH (P-Channel)
https://www.vishay.com/product?docid=71878&query=si1413

Vishay - Si1406DH (N-Channel)
https://www.vishay.com/product?docid=70684&query=si1406


Thanks

Cheers,

 

[E-design]

Re: MOSFETS Basic

Hi,

Could anybody advise me on MOSFET driving circuit ?

The push/pull driver is very popular using a PNP and NPN transistor with bases tied together as the input and emitters tied together as the output. A low value of resistor around 22-100Ohms is conected from this output to the gate to prevent parasitic oscillations when switching. The 2 collectors are tied to the supply rails according to their proper polarities. Some times a backward diode is connected over the gate resistor to help speed-up the turn off cycle.

Based on datasheet, it is true that the higher the VGS for a N-Channel MOSFET will result in a lower RDS(on) ?

There's rarely ever a need to switch above 9V or so on the gate as any voltage above this will only make a very marginal improvement upon the on-resistance, and also remember that you need to discharge the input and that takes time and current that rapidly increases with switching frequency. The total capacitance you have to charge-discharge on the gate will also increase with the Miller capacitance reflected back into the gate which again depends on the voltage swing on the output of your device. This switching off period is very important with inductive loads.

And if that's the case, isn't it will be much easier to operate a P-Channel MOSFET than N-Channel since it only require a low on the gate ?

This device operates on a different polarity, low voltage to turn on and high to turn off. The drain connects -ve with respect to the source. P-Chan devices normally have higher on-resistances for the same die size.

Where do we usually find the minimum voltage to turn on a N-Channel or P-Channel MOSFET ?

There's normally a curve of VGS/V against gate charge QG/nC on the datasheet. The part where the curve starts to be horizontal the device starts turning on properly. On average around 3-4V

For a P-Channel circuit, why do designer usually attached a huge resistor (in KOhm to MOhm range) between the gate and source ?

This will prevent the device from accidently turning on by keeping the gate discharged with no drive connected.

 

[Hughes]

Re: MOSFETS Basic

And if that's the case, isn't it will be much easier to operate a P-Channel MOSFET than N-Channel since it only require a low on the gate ?

No. P-MOSFET only requires a low on the gate, but it requires a high on the source.

 

[ayeong]

Re: MOSFETS Basic

Thanks to E-Design & Hughes.

Btw, does anybody happen to know of any MOSFET that have the lowest ON resistance (RDS ON) for about 10V & less than 3A application (preferably SOT-23 package) ?

Thanks

Cheers,

 

[opamp741]

Re: MOSFETS Basic

a simple inverter (( with appropriate p/n ratio) is a good driver

 

[ayeong]

Re: MOSFETS Basic

Hi All,

Found some good links for MOSFETS :

Fairchild - Introduction to Power MSOFETS & Applications
**broken link removed**

Fairchild - MOSFETs Basic
**broken link removed**

International Rectifier - MOSFET
**broken link removed**

Motorola - Interfacing Power MOSFETs to Logic Devices
**broken link removed**

Overall, that website is very informative for all EEs
**broken link removed**

Hope it helps

Cheers,

 

[alphi]

Re: MOSFETS Basic

All mosfet company have mosffet basic application note.it is basic but also useful.

 

[rauol]

Re: MOSFETS Basic

8) Hi ayeong,
the reason for putting a high value resistor from gate to source is to avoid false switching of the fet, because the input impedence is very high a small amount of voltage induced on th gate gan turn it on.

generally the mosfet will turn on after its gate voltage threshold is reached.
Zetex is also manufacturing small mosfets.
www.zetex.com

 

[hylas]

Re: MOSFETS Basic

The reason that gate resistor for PMOS is larger than NMOS is that, for the same RDS(on), normally PMOS will have much larger gate area and Qgd than NMOS one. The large gate resistor can slow down the transient change of the drain voltage and improve the switch on time.

U will find that NMOS is usually used as it is usually much more inexpensive, especially high power MOSFET

 

[ayeong]

Re: MOSFETS Basic

Hi All,

thanks for the reply.

By connecting a resistor between the Gate & Source for a P-Channel to prevent accidental switching, so how do we size that resistor ?

Based on some calculation or simply just a rule of thumb ?

Thanks

Cheers,

 

[monnoliv]

Re: MOSFETS Basic

If you have a push-pull driver (say MAX4426 for example), you don't need resistor between gate and source. You only need a right resistor between driver output and MOSFET gate.