MOSFET Driver - Vcc=+5v

Thread Starter

nztech

Joined Feb 24, 2009
44
Hi all,

Can someone please assist as I am wanting to power on 3 power MOSFETs not simultaniously to a motor with 3 steps.
what I am wanting to know is how these MOSFET high side drivers work.

I have a source to motor +12V and regulated for the supply to all electronics at +5V 7805.

If I run the MOSFET driver chip at +5V rails will the output voltage output to the MOSFET fed gate be pumped to +10V?
Is this the design of the chip?

If possible a recommendation would be much appreciated to drive these 3 MOSFETs.

Kind Regards
 

mik3

Joined Feb 4, 2008
4,846
You can use the MOS as low side drivers as to eliminate the need of a MOS IC driver. If you need a high side drive then use a MOS IC driver to bootstrap the gate voltage and turn the MOS fully on.

Also, note that most MOS need 10V Vgs to fully turn on expect some designs for logic levels and turn on with 5V.
 

Thread Starter

nztech

Joined Feb 24, 2009
44
Isn't the low-side used for P-Channel to set the gate low?
I need to turn the N-channel on which will require 10V to turn on fully.

Is this what a high-side MOSFET Driver do from a 5V rail supply?

Cheers
 

mik3

Joined Feb 4, 2008
4,846
If you use a logic level MOS and connect its source to ground and the load on its drain and power supply then you will be able to turn it on with 5V.
 

Thread Starter

nztech

Joined Feb 24, 2009
44
The MOSFET's I have are not TTL based hence the driver required. This driver is the issue I am not too sure of.
Would the driver do this with v5 rails?
 

Thread Starter

nztech

Joined Feb 24, 2009
44
Does anyone have the practically on how a MOSFET Driver chip can up the gate voltage?
Is this possible?
 

Thread Starter

nztech

Joined Feb 24, 2009
44
Hi all,

Can someone please assist as I am wanting to power on MOSFETs to a motor with 3 step windings/3 speeds.
what I am wanting to know is how these N-MOSFET high side drivers work.

I have a source to motor +12V and regulated for the supply to all electronics at +5V 7805.

Question:
1. If I run the MOSFET driver chip at +5V rail will the output voltage output to the MOSFET fed gate be pumped to +10V?

2. I need about 10V to turn the MOSFET on fully. MOSFET's supplied are not TTL driven, hence I require a driver circuit.
Is this the design of the chip as I am not too familiar with these?

If possible a recommendation would be much appreciated to drive these 3 MOSFETs.
 

jpanhalt

Joined Jan 18, 2008
7,697
It is explained in the datasheet for the Max621 you suggested in your other post. Simply put, imagine a capacitor that is charged up by a source. It is then electronically flipped and put in series with the source to effectively double the voltage.

John
 

Thread Starter

nztech

Joined Feb 24, 2009
44
A N-channel requires a high side 10V to switch on whereas a P-channel requires a negitive voltage to turn on.
Hence the High-side Mosfet driver is set to drive the n-Channel connected to the GND and switching the Drain +ve.

Cheers
 

Thread Starter

nztech

Joined Feb 24, 2009
44
Cheers, basically a non-inverting op-amp and a inverting op-amp creating a +5v and -5V input gate drive, but reference -5V thus giving a 10V drive.
If this is undrestandable..
Also as per schematic the N-channel from the MAX621 should be ok?
This is to drive a motor.

Regards
 

jpanhalt

Joined Jan 18, 2008
7,697
Your two threads on essentially the same subject are confusing.

From what I gather, you are considering N-channel topside and P-channel on the bottom. That is opposite usual practice. Why do you want to do it that way?

It seems that your PWM supply will be at 5V and you are planning to use the Max621 to generate Vcc+10V or so. For a topside , N-channel mosfet with a drain at 12V, that may not be enough to fully turn it on, unless it has a logic-level gate.

Also, the Max621 puts out only 25 mA, which may cause relatively slow switching for large mosfets. The Max621 datasheet shows an example. You also seem to be considering using a couple of op-amps to generate the PWM, but have not made any mention of dead-time between turning on the top and bottom. Slow switching and no dead-time may lead to shoot-through (i.e., both top and bottom mosfets are on at the same time).

In sum, I think it would help to: 1) Post your schematic; and 2) Stick to one thread on this subject.

John
 

jpanhalt

Joined Jan 18, 2008
7,697
The block diagram looks OK to me based on the Max621 driver. The circuit posted by bertus makes sense too, of course. However, for motor control, I would be concerned about its switching speed. If the Max621 would be current-limited, the bertus circuit would be even more so. The later has the possible advantage of optical isolation.

John
 

Thread Starter

nztech

Joined Feb 24, 2009
44
Thanks for that.
Instead of 12V supply can I use +5V?
This is the supply I am feeding all chips. Will this give me +10V MOSFET Drive.
 
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