N-Channel MOSFET Drain-Source Voltage

[d13]

Hi, I'm messing around with some mosfets to eventually build an H-Bridge, but I can't seem to get the right power to the motor. Currently, I'm using the N-channel RFP30N06LE mosfet (http://www.fairchildsemi.com/ds/RF/RFP30N06LE.pdf).  I'm using a 14.4V DeWalt drill battery (about 15V fully charged) for the circuit.  I have +5V going from a Voltage regulator to the gate and the full 15V going to the drain.  The source pin is only reading 6.8V without any load.  When I put an old 12V cpu fan from source to the negative terminal the source pin voltage drops to 5.3 and is using 25mA.  When I connect the fan directly to the battery it is much more powerful and seems to be using 70mA.  I'm not exactly sure what the battery is rated for, but I'm assuming it's well above what's needed for this circuit.

The mosfet doesn't seem to be getting hot.

I've been trying to understand the datasheet, but 1) I really don't understand it to begin with and 2) whenever I think I understand something it makes me think the setup should work.  For example, Figure 7 looks like 5V (80 microsecond pulse, 0.5% duty cycle) would allow almost 80A as long as the drain is greater than around 4.5V.  Also, in figure 8, it looks like 5V (80 microsecond pulse, 0.5% duty cycle) with 15V on the drain would allow about 80A.  I'm not pulsing the signal, but it seems like that wouldn't be the issue here.

Some other specifications for this mosfet:
V_GS(TH) = min:1V,max2V
V_GS (Gate to Source Absolute Max Rating) = +10V,-8V
I_D (Continuous Drain Current ABS Max) = 30A

I was originally using a radioshack bought IRF510 with similar results.  I switched to the RFP30N06LE because it is supposed to be able to be ran directly from a microcontroller.

This problem (or my lack of understanding) has been driving me crazy and any help would be very appreciated.

Thanks!

[paulstreats]

Ive just seen the graph on page 4, at the top left, this seems to explain the characteristics better than the rest of the datasheet.

[d13]

Ive just seen the graph on page 4, at the top left, this seems to explain the characteristics better than the rest of the datasheet.

It seems like you're referring to "FIGURE 6. UNCLAMPED INDUCTIVE SWITCHING" chart.  To be honest, I am totally confused by that graph.  Any pointers for what to look for?

Also, interestingly, I tried a another mosfet (same RFP30N06LE, just a different physical chip) and I'm only getting 4V on the source pin.

[paulstreats]

do you also have the GND from the voltage regulator connected to the source?

[Soeren]

Hi,

I'm using a 14.4V DeWalt drill battery (about 15V fully charged) for the circuit.  I have +5V going from a Voltage regulator to the gate and the full 15V going to the drain.

No wonder it doesn't work!
You have to connect the source to ground and put the fan between drain and +15V, since the gate has to be positive with respect to source.

[d13]

do you also have the GND from the voltage regulator connected to the source?

No wonder it doesn't work!
You have to connect the source to ground and put the fan between drain and +15V, since the gate has to be positive with respect to source.

Wow!  Thanks guys...  I just took this pic of the circuit http://corruptionexperiment.com/misc/mosfet_problem/mosfet_circuit.jpg, but I rearranged placement of the fan and everything seems to work now!

My only concern is how to go from here to an H-Bridge.  If I have an N-Channel Mosfet on both sides of the fan would the gate still be considered positive with respect to source on both mosfets?  Also, if I use two different power supplies, is correct for me to just connect the negatives together (at some single point)?

Please pardon my ignorance.  I REALLY appreciate everyone's help!!

Oh, and if any can suggest any good books that would help me understand and avoid these types of problems I would definitely in interested.

[Soeren]

Hi,

If you use P-channel for the high side devices and use eg. an NPN transistor for driving the gate (which has to be negative w. resp.to the source, which in this case should be connected to the positive supply), you can drive it like the N-channel devices.

There are a bunch of amateurish N-channel only H-bridges on the net, but to get such a beast to work well, you have to drive the high side from a higher voltage than your motor supply - do it right, make the high side P-channel.

[d13]

If you use P-channel for the high side devices and use eg. an NPN transistor for driving the gate (which has to be negative w. resp.to the source, which in this case should be connected to the positive supply), you can drive it like the N-channel devices.

I just looked at some random P-Channel MOSFET Datasheet and it said the V_gs(th) is -0.8V to -2V.  Does this mean that the gate actually needs negative less than -0.8 to allow current from source to drain or is it that it has to be -0.8V or less relative to the source?  If its the relative to source way, I think I understand how the NPN transistor may fit in: pull-up resistor to the motor + and P-Channel Gate with the NPN between P-Channel Gate and Ground, maybe?

[Soeren]

Hi,

I just looked at some random P-Channel MOSFET Datasheet and it said the V_gs(th) is -0.8V to -2V.  Does this mean that the gate actually needs negative less than -0.8 to allow current from source to drain or is it that it has to be -0.8V or less relative to the source?

First off, the V_gs_th is just the threshold value (doh), not the switching voltage, which should be higher (magnitude wise).
It means that the voltage at the gate has to be negative relative to the source - just like an N-ch. device mirrored around a horizontal line below it, if you get what I mean.

If its the relative to source way, I think I understand how the NPN transistor may fit in: pull-up resistor to the motor + and P-Channel Gate with the NPN between P-Channel Gate and Ground, maybe?

Would say supply + rqathern than "motor +", but I think you got it.
To illustrate it, I just slammed this together:

For some reason this forum shows the graphic at a smaller size. Get a readable schematic here: http://That.Homepage.dk/Img/H.png
Mind you, it's just for the example, since this has no ability to brake the motor (by shunting both sides of the motor to ground or supply + at once).

[d13]

Thanks for the schematic!  You're really helping me begin to develop an understanding here.  One curious question that I think would really solidify the whole thing in my brain: would N-Channel MOSFETs work in place of the transistors?

[Soeren]

Hi,

Sure, but a BjT is cheaper.

[d13]

I happen to have a few extra N-Channels around now that I'm going to use the P-Channel on the high side.    But, I'll probably go with the BJTs anyway just for the experience.

Thanks again for your help!!!