# technical description of the use of FWD



## Ziggythewiz (May 16, 2010)

The primary purpose of Front Wheel Drive is to provide better traction control.


----------



## major (Apr 4, 2008)

bjfreeman said:


> First the FWD primary purpose is to protect the voltage sensitive devices from reverse EMF of a Reactive device they control.
> Most Engineers will place the FWD closet to the Reactive Device to limit excessive current and voltage spike being generated.
> So in the case of Motors which are inductive reactive, the FWD should be placed in a junction box on the motor. This elevates the heavy current caused by the collapse magnetic field of the inductive component.
> The Characteristic of a FWD has a fast reaction time compared to the normal power blocking Diode. maximium voltage from cathode to anode of a FWD is less than 0.7 volts
> ...


Hello bj,

Here is an example of an opinion of yours which would be unwise to apply to motor controllers. I think you see the function of the FWD (free wheeling diode) used in the motor controller the same as the flyback diode used to protect switches controlling inductive devices. There is a big difference and with motor controllers the FWD should be as close to possible to the switch. This is why power semiconductor manufacturers design modules which contain both the switch (IGBT) and FWD (diode). Here is an example: http://www.pwrx.com/pwrx/docs/cm300e3u12h.pdf 

The circuitry used for DC motor controllers in all that are currently commercially available is called a buck converter. It may differ in detail from examples of buck converter circuits you find in some literature because the motor functions as both the output inductor and load. This eliminates the use of an output capacitor between the inductor and load.

A half bridge can be used for a motor controller (or buck converter) and is in fact for some products. Series wound motors are used for the bulk of DC motor controllers used by members of this board. Therefore operation is contained to a single quadrant, forward current and reduced voltage. This restriction means that only one of the 2 switches on the half bridge is used and the diode from the other is used as the FWD. So for applications like DC motor controllers they make modules consisting of one IGBT and a FWD internally connected. Again see the PowerEx module from above. I like to refer to modules like that as a quarter bridge 

Regards,

major


----------



## steven4601 (Nov 11, 2010)

@bj 
Free wheeling diode

assuming your talking about DC motors. Not sure how a free wheel diode inside the motor works well for the motor with current controlled motor controllers. If the motor controller measures the current only while the mosfet/igbt is on, then it may work. If it measures the average over the whole period than it may cause serious overloading of the motor. 

assuming you are talking about AC motors, commercial high power, low impedance motor wire harnesses may be also equipped with extra EMI reduction leads. But mostly the inductive energy is bled of at the mosfet/igbt side. Not the motor side.


----------



## bjfreeman (Dec 7, 2011)

Ziggythewiz said:


> The primary purpose of Front Wheel Drive is to provide better traction control.


Funny.


----------



## bjfreeman (Dec 7, 2011)

major said:


> Hello bj,
> 
> Here is an example of an opinion of yours which would be unwise to apply to motor controllers. I think you see the function of the FWD (free wheeling diode) used in the motor controller the same as the flyback diode used to protect switches controlling inductive devices. There is a big difference and with motor controllers the FWD should be as close to possible to the switch. This is why power semiconductor manufacturers design modules which contain both the switch (IGBT) and FWD (diode). Here is an example: http://www.pwrx.com/pwrx/docs/cm300e3u12h.pdf
> 
> ...


Though the Power semi provide this, a educated engineer will know he does not want spurious Current flowing through circuits when they can stop it..
from a practical point of view, as you pointed out larger cable is necessary to accommodated this getting back to the controller.
There are member that use AC also my description fits both.


----------



## bjfreeman (Dec 7, 2011)

steven4601 said:


> @bj
> Free wheeling diode
> 
> assuming your talking about DC motors. Not sure how a free wheel diode inside the motor works well for the motor with current controlled motor controllers. If the motor controller measures the current only while the mosfet/igbt is on, then it may work. If it measures the average over the whole period than it may cause serious overloading of the motor.


for micro based controllers, this is accomplish for speed of the motor and a table of motor characteristic. This is also used to determine if the motor is operating with in specs. 
Measuring of current from the controller is for overload sensing. Now some economical controller my cut corners.


> assuming you are talking about AC motors, commercial high power, low impedance motor wire harnesses may be also equipped with extra EMI reduction leads. But mostly the inductive energy is bled of at the mosfet/igbt side. Not the motor side.


The EMI do not take care of Current generated by the collapse of the magnetic fields. Yes I know most cut corners and put in the controller.


----------



## major (Apr 4, 2008)

bjfreeman said:


> There are member that use AC also my description fits both.





bjfreeman said:


> So in the case of Motors which are inductive reactive, the FWD should be placed in a junction box on the motor.


So you advocate putting diodes in the junction box of AC motors?


----------



## Tesseract (Sep 27, 2008)

bjfreeman said:


> ...
> The EMI do not take care of Current generated by the collapse of the magnetic fields. Yes I know most cut corners and put in the controller.


You should look up the phrase "unclamped inductance" and report back your findings.


----------



## bjfreeman (Dec 7, 2011)

major said:


> So you advocate putting diodes in the junction box of AC motors?


We been doing this for years on our products. actually we use schotky dioses back to back. as long as the voltage does reach above the rated voltage they have no effect.
Note that a PWM 3 phase AC motor is pulsed only in positive direction


----------



## bjfreeman (Dec 7, 2011)

Tesseract said:


> You should look up the phrase "unclamped inductance" and report back your findings.


http://www.fairchildsemi.com/an/AN/AN-7514.pdf
I describe this condition when I said that voltage sensitive device need to be protected.
was my definition not enough words.


----------



## steven4601 (Nov 11, 2010)

bjfreeman said:


> We been doing this for years on our products. actually we use schotky dioses back to back. as long as the voltage does reach above the rated voltage they have no effect.
> Note that a PWM AC motor is pulsed only in positive direction


What you might have see there are either voltage suppressors, or VDR's.


SRM, PMAC, ACIM etc all use alternating current. Even with a DC source connected to the inverter/controller. It is a matter of reference. Absolute voltage seen from the negative inverter bus terminal on either motor terminal is positive between 0 and DC bus voltage. But if you'd measure (with an appropiate measuring device, scope with trace measuring options would do) you'd see the voltage going negative and positive across the motor windings.


----------



## major (Apr 4, 2008)

bjfreeman said:


> We been doing this for years on our products. actually we use schotky dioses back to back. as long as the voltage does reach above the rated voltage they have no effect.
> Note that a PWM 3 phase AC motor is pulsed only in positive direction


All of the inverter feed (PWM three phase) induction motor drives I have dealt with personally or ever seen in person or in literature use synthesized 3 phase AC sinewave output (to the motor) which swings both ways, positive and negative.

edit:


> as long as the voltage does reach above the rated voltage they have no effect.


 If they have no effect then they do not function as FWD.


----------



## bjfreeman (Dec 7, 2011)

steven4601 said:


> What you might have see there are either voltage suppressors, or VDR's.


Are you familiar what a Schotty Diodes characterists are?
manufacturing wise for ERP, we attempt to break down the components to basics, unless the cost of the assembly over rides.



> SRM, PMAC, ACIM etc all use alternating current. Even with a DC source connected to the inverter/controller. It is a matter of reference. Absolute voltage seen from the negative inverter bus terminal on either motor terminal is positive between 0 and DC bus voltage. But if you'd measure (with an appropiate measuring device, scope with trace measuring options would do) you'd see the voltage going negative and positive across the motor windings.


 apologize was thinking of the signal into the Driver Board. I use 56F803 in a full bridge mode.


----------



## steven4601 (Nov 11, 2010)

Let's say I know a 'little' about diodes. 

you quoted " we attempt to break down the components to basics". I am quite sure that will happen, one way or another. 

Schottky's generally are NOT used with motor drives as free-wheeling diodes at all. The reason is that Schottky's are good for low voltage applications where a minimal forward voltage is desired to improve efficiency. Afaik, schottky doesn't or rarely go above 100V. Putting them back to back doesn't help either to solve the 100V issue.


----------



## bjfreeman (Dec 7, 2011)

steven4601 said:


> Let's say I know a 'little' about diodes.
> 
> you quoted " we attempt to break down the components to basics". I am quite sure that will happen, one way or another.
> 
> Schottky's generally are NOT used with motor drives as free-wheeling diodes at all. The reason is that Schottky's are good for low voltage applications where a minimal forward voltage is desired to improve efficiency. Afaik, schottky doesn't or rarely go above 100V. Putting them back to back doesn't help either to solve the 100V issue.


http://www.littelfuse.com/products/Technology/TVS+Diodes/Peak+Pulse+Current+%28Ipp%29+8x20%26mu%3Bs+%28A%29/10000/Max+Clamp+Voltage+%28Vc%29++%28V%29/625/search.html
http://whites.sdsmt.edu/classes/ee320/notes/320Lecture9.pdf
A flyback diode (sometimes called a snubber diode, freewheeling diode, suppressor diode, or catch diode) is a diode used to eliminate flyback, the sudden voltage spike seen across an inductive load when its supply voltage is suddenly reduced or removed. 
Flyback diodes are used whenever inductive loads are switched off by silicon components: in relay drivers, H-bridge motor drivers, and so on. A switched-mode power supply also exploits this effect, but the energy is not dissipated to heat but used to pump a packet of additional charge into a capacitor, in order to supply power to a load.


----------



## DJBecker (Nov 3, 2010)

You've quoted random textbook material without context.

First, I think you probably mean back-to-back Zener diodes for protection. Although it is better to use similar TVS diodes which are built for overvoltage protection. They have less abrupt characteristics in exchange for a more robust physical structure.

Schottky diodes have a lower forward voltage drop and minimal reverse recovery time/charge. They would be ideal for freewheel diodes except that it is difficult to fabricate them with a high reverse voltage rating. There are some rated at 200V, but they are more fragile with spiking voltage than other diode types.

The wiring from the controller to the motor is part of the inductive loop. If you put the freewheeling diode on the motor terminals, the magnetic field in the wiring has to collapse and rebuild each PWM cycle. That collapse will create a voltage spike at the controller, adding to the voltage stress at turn-off. If you add a capacitor to control the spike, you'll dramatically add to the current surge at turn on. (Perhaps someone could develop a Major Breakthrough to handle that spike at the controller. They might even use a clamping diode. It would have to big enough to handle the full current...)


----------



## bjfreeman (Dec 7, 2011)

DJBecker said:


> You've quoted random textbook material without context.
> 
> First, I think you probably mean back-to-back Zener diodes for protection. Although it is better to use similar TVS diodes which are built for overvoltage protection. They have less abrupt characteristics in exchange for a more robust physical structure.


I provided a data sheet to show the device, which tells me you did read the links.


> Schottky diodes have a lower forward voltage drop and minimal reverse recovery time/charge. They would be ideal for freewheel diodes except that it is difficult to fabricate them with a high reverse voltage rating. There are some rated at 200V, but they are more fragile with spiking voltage than other diode types.


the device I gave a link to is 600v 10,000 amps the only physical difference from a FWD it has a layer that lets not have any effect till a voltage level is reached. as long as that voltage is less than the PIV mosfet/IGBT then is has the same effect as FWD.


> The wiring from the controller to the motor is part of the inductive loop. If you put the freewheeling diode on the motor terminals, the magnetic field in the wiring has to collapse and rebuild each PWM cycle. That collapse will create a voltage spike at the controller, adding to the voltage stress at turn-off.


That is what the FWD in the IGBT is for. and the collapse is a lot less than the extra current sent back.
using your logic the increase in the Current from motor, when it is source, as to the the collapse when the controller current starts, would be just as stressful.


> If you add a capacitor to control the spike, you'll dramatically add to the current surge at turn on. (Perhaps someone could develop a Major Breakthrough to handle that spike at the controller. They might even use a clamping diode. It would have to big enough to handle the full current...)


see my link about the device that handles anything over 600 volts at 10,000 amps.


----------



## DJBecker (Nov 3, 2010)

bjfreeman said:


> I provided a data sheet to show the device, which tells me you did read the links.


You stated in post #10 that you used back-to-back Schottky diodes for protection in a junction box. Schottky diodes are effective small signal clamps, but aren't good at absorbing power spikes.

The TVS you referenced later is a very different kind of device. It can provide brief overvoltage protection. You seem to think that the 10000 amp rating means something during regular operation. It doesn't. It's like the crumple zone in a car. A shock absorber continuously handles millions of tiny potholes and a has a big impact on performance. The crumple zone might let you survive running into a barrier, once. Similarly, that TVS will protect the circuit from the 10000 amp pulse for 20 microseconds, but its going to vaporize while doing so.


----------



## bjfreeman (Dec 7, 2011)

DJBecker said:


> You stated in post #10 that you used back-to-back Schottky diodes for protection in a junction box. Schottky diodes are effective small signal clamps, but aren't good at absorbing power spikes.
> 
> The TVS you referenced later is a very different kind of device. It can provide brief overvoltage protection. You seem to think that the 10000 amp rating means something during regular operation. It doesn't. It's like the crumple zone in a car. A shock absorber continuously handles millions of tiny potholes and a has a big impact on performance. The crumple zone might let you survive running into a barrier, once. Similarly, that TVS will protect the circuit from the 10000 amp pulse for 20 microseconds, but its going to vaporize while doing so.


the reason I put the link to the Schottky description link was to show that Schottky is the basis for all such diodes with fast avalanche. They have been many "marketing names", but they are all based on Schottky Technology.

what do you base your vaporizing on?


----------



## major (Apr 4, 2008)

DJBecker said:


> You stated in post #10 that you used back-to-back Schottky diodes for protection in a junction box. Schottky diodes are effective small signal clamps, but aren't good at absorbing power spikes.
> 
> The TVS you referenced later is a very different kind of device. It can provide brief overvoltage protection. You seem to think that the 10000 amp rating means something during regular operation. It doesn't. It's like the crumple zone in a car. A shock absorber continuously handles millions of tiny potholes and a has a big impact on performance. The crumple zone might let you survive running into a barrier, once. Similarly, that TVS will protect the circuit from the 10000 amp pulse for 20 microseconds, but its going to vaporize while doing so.


Hi DJ,

Maybe you don't realize who you're trying to help here. It is bj; the same guy who calls me a troll after attempting to help him in this thread: http://www.diyelectriccar.com/forums/showthread.php/max-size-wire-motor-68439p12.html bj believes he knows it all and will not listen to reason. 

I attempted to point out his confusion with several posts in this thread. Again he knows he is right and pays no attention to me. I have come to the conclusion that bj does not understand the operation of the DC motor controller and the free wheeling diode. He calls the free wheeling current "spurious" in post #6. He then claims to use back to back Schottky diodes and these TVS devices as FWDs. He says the FWD should be in the motor terminal box. 

Those of us who do understand the FWD in the motor controller know the importance of the free wheeling current and that it is desirable, not spurious, and without it the controller would not function efficiently if at all. We know that often the free wheeling current is well in excess of the main switch current. We know it is unwise to put the FWD in the motor terminal box. Tesseract would have to sell liquid cooled motor terminal boxes with his controllers.

bj won't listen or learn anything new. He is sure he is right. And after you try and try to help him, he calls you a troll.

Anyway DJ, I thought you should know.

major


----------



## DJBecker (Nov 3, 2010)

bjfreeman said:


> the reason I put the link to the Schottky description link was to show that Schottky is the basis for all such diodes with fast avalanche. They have been many "marketing names", but they are all based on Schottky Technology.


Errrkkk? You are claiming that Schottky diodes and TVS diodes are the same? Schottky diodes are metal-semiconductor junctions. Zeners are doped p-n semiconductor-semiconductor junctions. 



bjfreeman said:


> what do you base your vaporizing on?


The 6MW of power that the TVS must handle. It's rated for 20usec. That's 120J. Assuming that the junction is about a milligram (there isn't going to be significant thermal conduction in 20usec), the temperature rise will be 120J / (0.7J/g-K * 0.001g) = glowing plasma.
Glowing plasma is a good conductor, but it doesn't re-form into a diode for the next pulse.


----------



## bjfreeman (Dec 7, 2011)

DJBecker said:


> Errrkkk? You are claiming that Schottky diodes and TVS diodes are the same? Schottky diodes are metal-semiconductor junctions. Zeners are doped p-n semiconductor-semiconductor junctions.


I stand corrected about TVS's



> The 6MW of power that the TVS must handle. It's rated for 20usec. That's 120J. Assuming that the junction is about a milligram (there isn't going to be significant thermal conduction in 20usec), the temperature rise will be 120J / (0.7J/g-K * 0.001g) = glowing plasma.
> Glowing plasma is a good conductor, but it doesn't re-form into a diode for the next pulse.


Please check the link about the little fuse and see if you still have the same opinion


----------



## Gary B (Jun 2, 2011)

major said:


> Hi DJ, - - -
> We know it is unwise to put the FWD in the motor terminal box. Tesseract would have to sell liquid cooled motor terminal boxes with his controllers.- - major


 It seems to me (no builds yet) that the switching power devices AND the FWDs would both need cooling. - That may be done best in the nearly same location, especially with liquid cooling. I was struggling with how to get the FWDs across the motor anyway (mechanically). - It seems to me that the caps, the FWDs, and the power switches should all be in the same "power" box, probably liquid cooled. - Is this common practice? - (I'm using the P & S (Cougar) design guide. - Thanks, Gary B.


----------



## DJBecker (Nov 3, 2010)

Gary B said:


> It seems to me (no builds yet) that the switching power devices AND the FWDs would both need cooling. - That may be done best in the nearly same location, especially with liquid cooling. I was struggling with how to get the FWDs across the motor anyway (mechanically). - It seems to me that the caps, the FWDs, and the power switches should all be in the same "power" box, probably liquid cooled. - Is this common practice? - (I'm using the P & S (Cougar) design guide. - Thanks, Gary B.


The freewheel diodes do need cooling, but the compelling reason to put them next to the switching devices is minimizing the switched inductive loop area.

If you use silicon diodes as the Cougar design does, you prefer them to run a little warm, perhaps 100C. When warm their forward voltage drop decreases, resulting in less power lost. As they get hot their breakdown voltage drops and reverse leakage current rises. If you are already running them close to the breakdown voltage, the heating from leakage current can result in thermal runaway.

MOSFETs, on the other hand, have lower resistance when cool. The better your cooling, the more efficient. So you want to keep the MOSFET part of the controller as cool as possible.

If you use synchronous or active rectification with MOSFETs instead of diodes, you obviously want to follow the rule for MOSFETs.

The Cougar design puts both diodes and MOSFETs on the same heatsink, so you don't get to make separate decisions. Cooling always wins.

Air cooling is feasible at most power levels. High-end video cards move 250W of waste heat with a tiny heatsink and fan (and lots of careful engineering). A CPU cooler with heatpipes can do 500W or more.


----------



## bjfreeman (Dec 7, 2011)

DJBecker said:


> The freewheel diodes do need cooling, but the compelling reason to put them next to the switching devices is minimizing the switched inductive loop area.


So you say that the Voltage and current generated by the motor should be dealt with at the box instead of the motor?
or are you talking of a different inductive loop?


----------



## EVfun (Mar 14, 2010)

Gary B said:


> It seems to me (no builds yet) that the switching power devices AND the FWDs would both need cooling. - That may be done best in the nearly same location, especially with liquid cooling. I was struggling with how to get the FWDs across the motor anyway (mechanically). - It seems to me that the caps, the FWDs, and the power switches should all be in the same "power" box, probably liquid cooled. - Is this common practice? - (I'm using the P & S (Cougar) design guide. - Thanks, Gary B.


1. Correct, the power devices and free wheel diodes both need cooling. They also both need temperature monitoring with current cutback if either get hot. The caps may need temperature monitoring too, as the ripple current can be quite high.

2. Correct, they should be in the same box and typically are in the same box. How to cool them is optional, I really like the hybrid air cooled with water cooling option seen on the Soliton controllers.


----------



## bjfreeman (Dec 7, 2011)

DJBecker said:


> The 6MW of power that the TVS must handle. It's rated for 20usec. That's 120J. Assuming that the junction is about a milligram (there isn't going to be significant thermal conduction in 20usec), the temperature rise will be 120J / (0.7J/g-K * 0.001g) = glowing plasma.
> Glowing plasma is a good conductor, but it doesn't re-form into a diode for the next pulse.


 I agree with your statement, however I take exception to the die size.
I will agree that even a ¼ size plastic package will not handle the 10000 amps for 20us.
If you had read the product sheet the the TVS, you would have seen that it uses a Zener divider to drive a transistor that sinks the current.
The Case has a metal part the Die is heat-sink to. It has 1/8 leads to transfer heat to the PCB which is 2-3 oz copper that is a plane to dissipate heat.
So the short of it is, you can:


used low voltage and low current device like a zener to control devices that can deal with higher voltages and current. the package exhibits the characteristics of the controlling device

That PCB devices can handle reasonable heat dissipation if design properly.


----------

