# Can these be used for flyback?



## major (Apr 4, 2008)

CaptConan said:


> Then what should I do with G1 so that it is always...OFF.


I have just shorted the G to E terminals on the IGBT module to keep the gate OFF. But Tesseract says you should bias it with a negative 7 or 8 volts (isolated) to make sure it doesn't ever turn on.

major


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## Tesseract (Sep 27, 2008)

major said:


> I have just shorted the G to E terminals on the IGBT module to keep the gate OFF. But Tesseract says you should bias it with a negative 7 or 8 volts (isolated) to make sure it doesn't ever turn on.
> 
> major


To clarify, you need to apply a negative bias to the G-E terminals IF you switch the other IGBT on and off faster than oh, about 500-600ns. If you can live with the leisurely rate (and lower switching frequency that implies) of 1us transitions then you can simply short the G-E terminals.

The specific culprits here are the loop area of the internal G-E wiring and the Miller capacitance (Cres).

If the negative bias supplies in the Soliton1 go missing the IGBTs become difficult/impossible to turn off. Needless to say, that's one of the things we check before doing dyno testing


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## CaptConan (Jul 12, 2010)

I don't see a reason why I couldn't live with a PWM frequency of 20kHz. That equates to a switching time of about 50us. As I understood it, the higher the frequency the more switching losses there are, which means more heat for the poor IGBT. And 20kHz is just a frequency high enough to keep the motor out of the audible range. Just curious, why would one want a higher switching frequency?


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## Tesseract (Sep 27, 2008)

CaptConan said:


> I don't see a reason why I couldn't live with a PWM frequency of 20kHz. That equates to a switching time of about 50us.


Ahem. Switching time is just that, the time it takes to switch from on to off or vice versa. I try to always make the distinction between that and switching _frequency_ clear.


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## CaptConan (Jul 12, 2010)

OK thanks for making that clear. Sounds like shorting G1 and E1 when driving G2 at 20kHz shouldn't give me grief. Appreciate it!


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## samborambo (Aug 27, 2008)

CaptConan said:


> OK thanks for making that clear. Sounds like shorting G1 and E1 when driving G2 at 20kHz shouldn't give me grief. Appreciate it!


Nope. Like Tesseract said, don't confuse the switching period with the turn on + turn off time. I normally work on about 1% of the switching period to maintain reasonable efficiency as long as your configuration allows for this. 1% would be 500ns.

10kHz should be high enough not to be audible.

The free wheeling diode reverse recovery is stated as being 300ns max. I think you're going to end up with induced turn on anyway. Its a pity you can't simply bootstrap a negative gate supply.

Sam.


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## jackbauer (Jan 12, 2008)

I'm running two quite old fuji igbts. One as the switch and one as the flyback with the g-e shorted. Switching frequency is 8khz and so far has not exploded That said those old 1mbi800 parts have a nice long turn off tail.


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## CaptConan (Jul 12, 2010)

Does the car "whine" at that 8kHz?

My next thought was just that, to use the 3rd IGBT just for it's diode


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## Tesseract (Sep 27, 2008)

jackbauer said:


> I'm running two quite old fuji igbts. One as the switch and one as the flyback with the g-e shorted. Switching frequency is 8khz and so far has not exploded That said those old 1mbi800 parts have a nice long turn off tail.


Hate to sound like a broken record here, but it doesn't seem to be registering that how _often_ you switch the IGBTs is not the same as how _fast_ you switch them.

You could be running a switching frequency of 1_Hz_ and still blow those old IGBTs - all you have to is try to switch them on in less time than the freewheeling diode takes to go from conducting to blocking (ie - its reverse recovery time, or t_rr_)). If you attempt to switch the IGBT faster than that time you basically apply a short circuit across the battery pack.

EDIT: 8kHz is usually not audible, or just barely so. Motors make poor speakers, after all. Also, the diode needs to have the same current rating as the switch in an EV motor controller. Indeed, it is often beneficial, especially when running at high pack voltages and low motor voltages, to give the diode a higher current rating than the switch.


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## CaptConan (Jul 12, 2010)

yeah my theory must be off somewhere and I appreciate the insight from everyone. As I understood it, Pulse Width Modulation is a method of sending an AVERAGE voltage to a load. So with a 144v pack and a duty cycle of 50% there'd be an initial drop of 72V across the motor. As the motor speeds up a back EMF is produced at the other end of the motor that will be something less than the voltage applied. When the power is cut and the motor is still spinning a voltage spike will occur at the other end of the motor which can ruin components and other bad things. Flyback diodes allow a path for the current to keep moving with the direction the motor is spinning and keeps this back voltage spike down.
So, just thinking out loud here. Lets say cruising down the road at, oh say, 75% duty cycle. At this cruising speed, and with a PWM frequency fast enough (like 8, 10 or 20kHz), would this flyback diode here in the Fuji IGBTs ever really turn on? Implying that would the back EMF exceed that which is seen at the front of the motor in order to be enough for the diode to conduct?

These diodes have a 300ns reverse recovery time. So looking at a 10kHz PWM frequency: 1/10000= 100us. 300ns/100us means that the PWM frequency is .3% of the reverse recovery time. The switch ON time for the sinking IGBT is on the same order as the built in diode, between 1 and .1 us.

So for summary:
Would the diodes even turn on at normal cruising operation?
and
Would it even matter if they did?



EDIT: What isn't stated in the datasheet is the FORWARD recovery time...


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## jackbauer (Jan 12, 2008)

@CaptConan , the 8khz is only barely audible at low speeds and absent above 10mph at least to me. 

@Tesseract. I'm sure its more luck than judgement that the switching speed on the igbt is compatible with the intrinsic diode of the other. I do understand what you mean regards difference between switching frequency and switching speed. I do think , perhaps mistakenly? , that a higher switching frequency will result in the driver having to chage and discharge the gate capacitance quicker thus having an effect on speed. Perhaps i'm mistaking this for switching losses being higher at higher frequency.


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## major (Apr 4, 2008)

CaptConan said:


> As I understood it, Pulse Width Modulation is a method of sending an AVERAGE voltage to a load........would this flyback diode here in the Fuji IGBTs ever really turn on?


Hey Capt,

Take a look at a buck converter. Four components. Capacitor on the input. Series switch (transistor). Inductor in series with the load. Diode reverse biased across the inductor and load.

For the motor controller, the inductor is common with the load. That is the motor's coils provide enough inductance, typically.

The diode is called the freewheeling diode. I thought this is what you meant saying "flyback". There is also a diode in the transistor module called an antiparallel diode, or intrinsic diode. It is possible to use this intrinsic diode as a freewheeling diode if you prevent the IGBT from ever turning on. I thought that is what you were asking.

Now, as for the freewheeling diode in the buck converter, it must turn on every time the main switch turns off. Or once every pulse.

Again, Tess, the floor is yours 

major


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## Tesseract (Sep 27, 2008)

CaptConan said:


> yeah my theory must be off somewhere...
> Would the diodes even turn on at normal cruising operation?
> and
> Would it even matter if they did?


Every time the switch turns off the FWD turns on, which is necessary because inductors demand that the current flow through them be a continuous function - no step changes allowed. Thus current through the motor ramps up when the switch is on and ramps down (courtesy of the FWD) when the switch is off. If the switch stays off long enough the current will ramp all the way back down to zero - this is called discontinuous mode - but usually the current only has time to ramp down 10-20% because the inductance of the motor is high relative to the switching frequency.



jackbauer said:


> @Tesseract. I'm sure its more luck than judgement that the switching speed on the igbt is compatible with the intrinsic diode of the other. I do understand what you mean regards difference between switching frequency and switching speed. I do think , perhaps mistakenly? , that a higher switching frequency will result in the driver having to chage and discharge the gate capacitance quicker thus having an effect on speed. Perhaps i'm mistaking this for switching losses being higher at higher frequency.


No - switching speed only depends on how much current you can pump into - and out of - the IGBT gate. Switching the gate more often - at a higher frequency - does not affect its speed. 

Now, the more often you switch the more switching losses you incur, but, in general, switching losses go down as switching speed speeds up. Until you switch the IGBT faster than the FWD - or the stray inductance between the two if using discrete components - allows, that is.


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## DJBecker (Nov 3, 2010)

I know that this is responding to a three month old thread, but I thought that this might be clarified for those reading even later.



CaptConan said:


> ... So with a 144v pack and a duty cycle of 50% there'd be an initial drop of 72V across the motor. As the motor speeds up a back EMF is produced at the other end of the motor that will be something less than the voltage applied. When the power is cut and the motor is still spinning a voltage spike will occur at the other end of the motor which can ruin components and other bad things. Flyback diodes allow a path for the current to keep moving with the direction the motor is spinning and keeps this back voltage spike down.
> 
> ...
> So for summary:
> ...


The cross-communication here is that you are thinking of the motor as a motor-generator. In the time frame of a PWM pulse it is essentially a pure inductor.

That means instead of dropping to the back-EMF generator voltage, which would be the same polarity, the motor continues to draw exactly the same current and the driven power lead changes polarity. And as a really good inductor, it will spike the voltage as high as it needs to until it finds a path to draw current from -- hopefully the freewheel/flyback diodes.


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## Anaerin (Feb 4, 2009)

Tesseract said:


> EDIT: 8kHz is usually not audible, or just barely so. Motors make poor speakers, after all. Also, the diode needs to have the same current rating as the switch in an EV motor controller. Indeed, it is often beneficial, especially when running at high pack voltages and low motor voltages, to give the diode a higher current rating than the switch.


IIRC, Curtis controllers switch at 8kHz in the bottom of their range when they're in current limit, which makes for the (in)famous "Curtis whine". Above that they switch at 16kHz, which makes the whine inaudible.


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## Tesseract (Sep 27, 2008)

Anaerin said:


> IIRC, Curtis controllers switch at 8kHz in the bottom of their range when they're in current limit, which makes for the (in)famous "Curtis whine". Above that they switch at 16kHz, which makes the whine inaudible.


The lower switching frequency on the Curtis controllers is 1.5khz, not 8khz.


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