# Using logic-level Mosfets in a Half Bridge.

#### ore51

Joined Nov 26, 2012
4
Hey all,

I have been wanting to design a simple but effective 3 phase mosfet half bridge with logic level mosfets, in hopes to drive a small 3 phase brushless DC motor. the motor will not be using any more than 2A at 3v ~ 3.7v.

one such mosfet is 785-1004-1-ND found at digikey
http://www.digikey.com/product-detail/en/AO3404A/785-1004-1-ND/1855946

I am no expert when it comes to mosfets and properly driving them, but im hopping some of the community here can help shed some light on this information with me.

my questions to you guys are,

1 do i need a specified high and low side driver for this application, or will this part suffice in creating the 3 half bridges?

2 can these logic level mosfets be driven by an 3v regulated IC with a 3.7v drain? will it be saturated enough to get to get the ideal minimum "on" resistance from drain to source?

3 should it matter if the two mosfet forming the half bridge both be N or P channel? or do they need to be opposite of each other to be effectively driven?

4 if the IC cannot fully drive the mosfet, what would the appropriate driver for an 3v regulated IC look like?

Thank you for your time, i hope to here back from someone. i will gladly share the schematic and design if help arrives.

#### tubeguy

Joined Nov 3, 2012
1,157
2 can these logic level mosfets be driven by an 3v regulated IC with a 3.7v drain? will it be saturated enough to get to get the ideal minimum "on" resistance from drain to source?
I don't quite understand the 3.7v drain. Do you have a higher supply voltage available ?

If you look on the datasheet pg 3 you will see that these mosfets really need about 4-4..5 volts to be turned on sufficiently. They have a sharp increase in resistance below that voltage.

Simplest to have a higher voltage supply or create one from 3 volts with a boost regulator.
Look at Linear Technology site.
http://www.linear.com/products/switching_regulator
I learned about it from this forum.

Using P and N Mosfets makes for easy driving.

Could you post any info you have on 3-phase bridge setup.
Is it wye or delta ??

#### ore51

Joined Nov 26, 2012
4
Hey tubeguy, that's for some info.

As for the setup I'm goin for, it is a 3.7v Lipo battery that will be driving the bridges. While a smps buck converter will be regulating a microcontroller and wireless communication devices at ~3.0v.

I was hoping to find out more information on how to properly drive a fet, a logic level fet, from a regulated microcontroller. As well as if all n or p channel fets would suffice or a combination of n and p channel fets are needed. I would like to get as much out of these motors as I possibly can.

As for the motor I do believe they are delta motors as they do not have any other indication. But you can find it here in the link below. ADH30S 6100kv
http://www.hobbyking.com/hobbyking/store/uh_viewItem.asp?idProduct=9996

Iv also attached a crude and simplistic drawing of how the mosfets would attach and control a 3 phase brushless dc motor. This drawing is by no means a final design so please don't judge haha.

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#### tubeguy

Joined Nov 3, 2012
1,157
Interesting little motor.
If you look on the datasheet pg 3 you will see that these mosfets really need about 4-4..5 volts to be turned on sufficiently. They have a sharp increase in resistance below that voltage.
Using P and N MosFets makes for easy driving.
The above Mosfet is not 'Logic Level'
Edit: Its considered logic kevel but not usable 'til 4-4.5v. You need something that has a very low on resistance at 3 volts
because you can't have much drop due to the low voltages available.
The one below is usable at lower voltage levels. It contains an N and P in a small surface mount package - can you work with surface mount ??
I think you could use three of these with the P on 'Top' connected to +3.7.

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#### ore51

Joined Nov 26, 2012
4
Interesting little motor.
The above Mosfet is not 'Logic Level'
Edit: Its considered logic kevel but not usable 'til 4-4.5v. You need something that has a very low on resistance at 3 volts
because you can't have much drop due to the low voltages available.
Agreed..

The one below is usable at lower voltage levels. It contains an N and P in a small surface mount package - can you work with surface mount ??
I think you could use three of these with the P on 'Top' connected to +3.7.
Yes I can easily work with smd

Although this sparks a few more questions.

1 would any other circuitry be needed to drive a logic level fet, such as a switching transistor, pull up or pull down resistors, or would a series resistor from the micro to gate be enough control?

2 would over driving the gate voltage be harmful to the fet and or micro?

#### tubeguy

Joined Nov 3, 2012
1,157
The Si5517DU data sheet has this Absolute Maximum for Gate to source drive voltageas
Gate-Source Voltage VGS ± 8.
So you can safely drive the gate at 3-4 volts.

You should be able to drive the gates without pull-up pr pull-down resistor if the MCU can drive close to both V+ and Gnd. If the battery is 3.7v I think you should simply find an MCU that works well at 3.7 volts.

I'm not familiar with the many new MCU's, so I can't recommend but I would think you should use one that can drive LED's directly ~ 20-25ma source and sink current or the highest drive current you can find.
I was taught use a small value gate resistor, 47 ohms for example to prevent oscillations. But I'm not sure if its absolutely required.

#### tubeguy

Joined Nov 3, 2012
1,157
Just to clarify...
Unlike a BJT you can drive the gate of a Mosfet with a gate-source voltage up to it's maximum rating which varies by the particular device. Protection from over voltage is frequently provided. by an appropriate zener diode along with a gate resistor. Whereas a BJT requires a current limiting base resistor
because the base-emitter voltage 'Stops' at ~ 0.7vdc.