PWM translates to movement?

[intellec7]

Actually, there are some applications that switching between full-forward and full-reverse (Locked-Anti phase) are more advantageous than full-forward/reverse and float/brake (Signed-Magnitude). It normally offers more linearity in motor RPM vs Duty Cycle. This is probably because the motor coasts for different periods of time depending on the shaft's velocity prior to turning off power to the motor and this relation is not linear.

[sciguy125]

Originally Posted by Generalx5

Also, theres something else im not really clear, is that, if a mosfet had already been turned on so that it conducts, and current is then applied at the source after. will the mosfet heat up? From what I've read, its the point when the mosfet conducts (rise/fall) when current is present at the source that heats up the mosfet. But say it the current wasnt there when it conducts, will that heat up the mosfet? The other thing is, will it even work if there is no current present at the source terminal?

I'm not sure I competely understand your question.

The transistor will only heat up when there is current through it (just like anything else). P=IV; no current, not heat. In a MOSFET, all current is between the drain and source. So, Idrain=Isource. No source current means no drain current, which means no current at all.

To turn the transistor on, it must be biased correctly. The main part is that Vgs (gate to source voltage) must be above the threshold voltage for the transistor (usually a few volts). However, Vds (drain to source voltage) also needs to be higher than Vgs. I'd have to think about it a little more, but I don't think it's possible to properly bias it while also cutting the current.

[ebarker]

At risk of showing my age.....

Think of a MOSFET as being a Voltage Controlled Resistor.

The MOSFET's 'transfer function' sez you apply a voltage to the gate. That will control the resistance of the MOSFET, somewhere between completely open and completely closed.

BTW, a vacuum tube has a transfer function a lot like a MOSFET.

In your application you are operating this thing complete like an on/off switch and just varying the duty cycle. When the switch is complete on, the resistance doesn't go completely to zero, but a very low value. The better the MOSFET, the lower the value.

If you motor stalled or shorted, or (even if it isn't shorted) there is a voltage drop, and I squared R loss across the device, that turns into heat. It you can dissipate the heat and stay within the operating limits of the device then it will be fine. You just have to move the heat off the silicon.

That is pretty much the trick for reliability. Keep all you signals, spikes, heat loads, etc well withing the device parameters. Just like you would design a building or a bridge (no pun intended).

FYI,

MOSFETs have a 'transfer function'. It is a mathematical thing (often times graphed) that shows the relation between the input and output of the device.

If you operate this thing over it's 'linear region' you can build an amplifier. Nearly all guitar, PA, stereo amplifiers are built around mosfets. Broadcast transmitters now use mosfets in all stages on most transmitters or drive final tube stages on the largest transmitters.

Years ago mosfets (the gates failed) were pretty fragile but modern mosfets are amazingly tough. These broadcast transmitters have to be because they get really banged around with those big lightning seeking iron sticks in the backyard.

Ed

[Al Skierkiewicz]

General,
Sorry for the late reply. Brushless DC motors first. These fall into a couple of categories. One is used in fans, and effectively uses switched DC as an AC simulator for an AC motor. Another is a multiphase driver. This type uses several coils spaced around the motor rotor and current is switched on and off simulating a multiphase AC supply. This is the type that is used in VCR video head motors where coils are mounted directly to the circuit board. Often this type of motor has the drive circuitry and power devices integrated inside the motor and need only an error voltage and power supply to operate. Stepper motors are very similar to this type of motor and can be manufactured to be very precise in their rotational position and torque.
The rest of your questions likely come from the article you read about MOSFET behavior in a particular application. Modern CPUs generate heat even though they use MOS technology. The reason is that the devices pass through a linear region as they switch from high to low. It is during this transition that the device is neither open nor shorted (source to drain) and some power is lost as heat. Since these transistions occur rapidly in a CPU and in a very confined space (think GHz and millions of transistors), there is significant heat buildup. This will occur no matter how the device is used. As I explained earlier, the IRL MOSFET device has an "ON" resistance that is very low. At high current some heat develops due to the current through this low resistance.
As others have pointed out, MOSFETs are used in a variety of devices but a designer must still take into account the benefits vs. hazards when using any device. Many semiconductors are tailored for a particular application. The MOSFETs used in the VHF transmitter I work on are already becoming inefficient at the high end of the VHF band (Channel 11) and have significantly less gain at 200 MHZ than at 100 MHz or less. Although they are 300 watt devices, they are not very useful at UHF. The type of modulation is also a big factor and reguires different techniques for different modes. Video (both analog and digital reguire high linearity and low distortion) transmitters use class AB1 while analog audio (TV and FM) can use class C and don't require a device that is extremely linear. AM broadcasting is another application altogether. Although these devices have become very robust over the years (a static discharge from an operator stroking their hair could kill a MOSFET in the first few years) lightning discharge must be handled outside the transmitter for best results.