HomeMade Motor Controllers

[John Gutmann]

Originally Posted by sciguy125

http://en.wikipedia.org/wiki/Field_effect_transistor

A MOSFET is a kind of FET. I'm not well versed in transistor theory, so I'm not sure what you gain my using other FETs over MOSFETS. But the reason MOSFETs are used as opposed to normal bipolar transistors is that you can parallel them. Let's say I have a MOSFET and a bipolar that can handle 10A each. For my device to handle 50A, I would need to parallel 5 of them. Paralleling the MOSFETs is fine becuase when they heat up, their resistance increases. Let's say that MOSFET 1 starts to heat up. Its resistance increases and it carries less current. The other 4 would pick up the slack. As 1 starts to cool down, it will carry more current and another would get a break. Of course, it isn't that linear and it happens very fast, but I think it gives you the idea. Bipolars, on the other hand, will all heat up together. When one of them finally explodes, the others are left with distributing the full 50A load and another will blow even faster than the first one. They will all cascade until you have nothing left but a smoking pile of transistors.

so a MOSFET is like the same thing but only, i guess you would say safer?

[John Gutmann]

what kind of chips or what different chips can I use to interpret the pwm signal for forward and reverse and the correct duty cycles and all that?

[sciguy125]

Originally Posted by sparksandtabs

so a MOSFET is like the same thing but only, i guess you would say safer?

Not exactly, MOSFETs and bipolars each have their uses. They work in slightly differnet ways that would suite different applications. In some applications, you would need to use bipolars because MOSFETs won't work and vice versa. For a high powered motor controller, I think the MOSFETs would be best.

Originally Posted by sparksandtabs

what kind of chips or what different chips can I use to interpret the pwm signal for forward and reverse and the correct duty cycles and all that?

It would depend on what your final application would be. I don't remember the manufaturer/part# off the top of my head, but there's a chip to control a MOSFET h-bridge (there's some subtle timing/voltage issues that need to be taken care of). That specific chip doesn't do any PWM though. For that, you'd probably have to use a PIC (possibly in conjunction with this h-bridge controller). I don't know of any commercial ICs that handle hobby radio PWM to motor control PWM. If you can't find an existing program for a PIC, you'd probably have to write it yourself.

[Generalx5]

All I remember is that these Metal Oxide Semiconductor field effect transistors in those victors are linked together in parallel, they are high current transistors that either has an N substrate or a P substrate. PMOS or NMOS, I also know that they require something called a MOSFET Driver that delivers short bursts of high current 1- 3 amp of 5- 22 volts to the gates of the MOSFETS, these drivers can be found either inverted or non-inverted so that a +5 could either indicate MOSFET conduct or not conduct. There is a helpful guide on the IRF website that explains how to pick your MOSFET and match it with a proper MOS Driver. Its really complicated...

examples of MOSFET DRivers are the ones from Microchip.com, TC4426 (improved version of TC426), TC426. The two listed are dual drivers, they can drive 2 different signals at one time. You can also find single drivers and Quad drivers as well. These drivers tipically require about two miliamps of current in their source. Its important to have a capacitor at their drain and source terminals. The MOSFETs we see in the Victors each can handle about 20 - 30amps? (ballpark figure...maybe more) with it hooked up in parallel, they are able to source large amounts of current.

It is important that you look at the white pages on the MOSFET specific to your project, they will tell you how much current it is able to handle under what temperatures, proper air cooling with sinks may be needed. also as mentioned above, with the rise in die temp, the resistance also increases, but this shouldnt be a problem if you are using a MOSFET driver. Hope that helps.

[Adam Y.]

That's why the Victors can handle such a high current. However, it gets very dangerous to handle such a high current.

Well in reality the only danger that you have at such low voltages is from stuff exploding. You really can't be harmed from shock but even at 36 volts the amount of shrapnel produced by the FETs is amazing when they fail.

Not exactly, MOSFETs and bipolars each have their uses. They work in slightly differnet ways that would suite different applications. In some applications, you would need to use bipolars because MOSFETs won't work and vice versa. For a high powered motor controller, I think the MOSFETs would be best.

Your forgetting about IGBTs which is an odd mix between an MOSFET and a BJT. Essentially, it turns on like a MOSFET but handles current like a BJT. Also Power MOSFETs are different from regular MOSFETs because there is a diode included . If you look at a circuit symbol for a power MOSFET you will see that diode include.

There is a helpful guide on the IRF website that explains how to pick your MOSFET and match it with a proper MOS Driver. Its really complicated...

It's complicated to the point where I really wouldn't recommend such a project if your not out of in college. If your PWM frequency is high enough you may run into transmission line problems. You start running into weird parasitic capacitances/resistances/inductances that cause the gates to ring which will result in the FETs not turning on and off cleanly. The high side drivers can actually latch on which results in the FETs staying on when they shouldn't be. I spent the last four months with people much smarter than myself trying to do essentially what you want to do but we really had problems.

[sciguy125]

Originally Posted by Adam Y.

Also Power MOSFETs are different from regular MOSFETs because there is a diode included . If you look at a circuit symbol for a power MOSFET you will see that diode include.

I'm sure that power MOSFETs do have some differences in construction, but that diode is in all MOSFETs; it's part of the way they're built. This diode is called a parasitic because it exists, but isn't there intentionally. Attached is a drawing of a MOSFET with the parasitic diode in red. Note that the diode only exists when the source and body are connected (as it usually is).

[Qbranch]

Originally Posted by John Gutmann

so a MOSFET is like the same thing but only, i guess you would say safer?

The main difference between your PNP/NPN transistors and P-/N-Channel (MOS)FETs is that standard transistor are "voltage" effect while Field Effect Transistos are more of "current" effect devices.

FET devices function much like variable resistors... their spreading resistance allows them to evenly share currents unlike transistors which, when paralleled, tend to put much more (or all) of the current on a single device while the others sit idle. A FET, since you can think of paralleling FETS like paralleling resistors of equal value, transmits curret equally between all of the devices in parallel.

One disadvantage of FETs is that they have strange drive characteristics... they are difficult to turn on and off. Thankfully FETs have become much more civilized and inideof most FETs many of the suppression diodes that were once external are now internal... (i suggest IR for FETs, they make the most friendly ones).

IR(InternationalRectifier) also makes some drive ICs which can handle driving your gates on and off for you, saving some circuit knowledge.

Also, using a PIC to drive your circuit helps as well. Many 16/18 series PICmicros incorporate ECCP peripherals which can directly drive (in hardware) the four lines for a full H-bridge.

If you want more detail... just let me know. Hope that answers some ?'s.

-q

[Al Skierkiewicz]

Originally Posted by Qbranch

The main difference between your PNP/NPN transistors and P-/N-Channel (MOS)FETs is that standard transistor are "voltage" effect while Field Effect Transistos are more of "current" effect devices.

FET devices function much like variable resistors... their spreading resistance allows them to evenly share currents unlike transistors which, when paralleled, tend to put much more (or all) of the current on a single device while the others sit idle. A FET, since you can think of paralleling FETS like paralleling resistors of equal value, transmits curret equally between all of the devices in parallel.

One disadvantage of FETs is that they have strange drive characteristics... they are difficult to turn on and off. Thankfully FETs have become much more civilized and inideof most FETs many of the suppression diodes that were once external are now internal... (i suggest IR for FETs, they make the most friendly ones).

IR(InternationalRectifier) also makes some drive ICs which can handle driving your gates on and off for you, saving some circuit knowledge.

Also, using a PIC to drive your circuit helps as well. Many 16/18 series PICmicros incorporate ECCP peripherals which can directly drive (in hardware) the four lines for a full H-bridge.

If you want more detail... just let me know. Hope that answers some ?'s.

-q

Alex,
Bipolar transistors are current controlled devices while FET are voltage controlled. I think of the FET as having an insulated gate so no current can flow in a FET. Another reminder is a bipolar transistor must have current flowing through a resistor to bias the base on and cause collector current to flow. FETs do function more like a variable resistance but they are easier to parallel becuase of the very low impedances involved. Both transistors and FETs are best paralleled using a small series resistance in series with one of the output elements to balance the currents in each device. As sciguy Phil, pointed out earlier, the construction technique of power MOSFETs (HEXFETS) causes the diode to be a natural (and usable) byproduct of production.

[John Gutmann]

I asked the questions 2 years ago............But thanks. Even though I have learned the answer in college by now.

-John