fet suppliers

[eP]

So by how many STP FETs parallel we can load the crystalyte controler driver without any impact for rise and fall times ?

Have you any proofs for your original theories ?

Regards

If you insist...is International Rectifier a reputable enough source?

From AN-944, top of page 3:

"The required gate drive current is derived by simply dividing the gate charge, 15 X 10-9, by the required switching time, 100 X
10-9, giving 150 mA. From this calculation, the designer can further arrive at the drive circuit impedance. If the drive circuit
applies 14 volts to the gate, for instance, then a drive impedance of about 50 ohms would be required. Note that throughout the
“flat” part of the switching period (Figure 3), the gate voltage is constant at about 7 volts. The difference between the applied 14
volts and 7 volts is what is available to drive the required current through the drive circuit resistance."

http://www.irf.com/technical-info/appnotes/an-944.pdf
...
Dan

So what driver current we need for 68ns time and 2 STP parallel load ?

Would you like calculate it ?

What is the right-driver cost ?

 

[cadstarsucks]

So what driver current we need for 68ns time and 2 STP parallel load ?

Would you like calculate it ?

What is the right-driver cost ?

Ummm...the right driver cost is whatever you can afford that will do the job fast enough...

I=Qgt/t

233nC*2/68nS=7A there actually are 8A gate drive chips these days. Since the 4110 Qg is slightly less than that of the STP it would be around 3A.

Are you looking to switch a brushless at 100KHz or just looking to minimize losses?

Dan

 

[eP]

So what driver current we need for 68ns time and 2 STP parallel load ?

Would you like calculate it ?

What is the right-driver cost ?

Ummm...the right driver cost is whatever you can afford that will do the job fast enough...

I=Qgt/t

233nC*2/68nS=7A there actually are 8A gate drive chips these days. Since the 4110 Qg is slightly less than that of the STP it would be around 3A.

7A at what slew rate ?

How much time the gate need to get these 7amps ?
0 ns ?

Are you looking to switch a brushless at 100KHz or just looking to minimize losses?
Dan

I'am looking for really reliable solution, not one based on wrong calculations.

Regards

 

[cadstarsucks]

7A at what slew rate ?

How much time the gate need to get these 7amps ?
0 ns ?

Are you looking to switch a brushless at 100KHz or just looking to minimize losses?
Dan

I'am looking for really reliable solution, not one based on wrong calculations.

Regards

7A gate drive reliably switches in the 68nS you requested, there is no slew rate in that number. Driver slew rates are a bit misleading as they tell you have fast a given driver output slews at a given load capacitance.

Many specs are actually related to how they do the characterization. What I think you are really asking is how do you limit the actual switching time? Looking at your favorite graph the delay time is before the plateau and the charge required during the plateau is the actual slew time, the plateau being caused by the miller charge. Unfortunately, they do not give that graph for various operating points since that is the one that gives the most real information, but it varies all over the place with the operating point.

That said, higher drive voltage and lower gate resistance both increase switching speed since the difference between the plateau voltage and the drive voltage divided by the resistance gives you the drive current at the plateau.

Alas, since changing most of this is beyond the expertise of most controller modifiers they will be limited to switching FETs

Dan

 

[TylerDurden]

Alas, since changing most of this is beyond the expertise of most controller modifiers they will be limited to switching FETs

True, that. But some of our members are keen to design & build better stuff than what's on the shelf.

Slackers like myself get a free education and better bike parts by paying attention.

8)

 

[eP]

7A at what slew rate ?

How much time the gate need to get these 7amps ?
0 ns ?

Are you looking to switch a brushless at 100KHz or just looking to minimize losses?
Dan

I'am looking for really reliable solution, not one based on wrong calculations.

Regards

7A gate drive reliably switches in the 68nS you requested, there is no slew rate in that number. Driver slew rates are a bit misleading as they tell you have fast a given driver output slews at a given load capacitance.

No way Dan. Slew rate tell us how fast temporary value can reach max. value.

You can't forget parasitic parameters values.
Parasitic inductance of traces (driver to gate) especially.

Electronic circuit can't work out of physics laws.

Current pulse is not any more rectangle at that level of amplitude and scale of time.

If you are unable understand that obvious fact give up funy advicing and cripled calculations and try do better other things.

Many specs are actually related to how they do the characterization. What I think you are really asking is how do you limit the actual switching time?

IC you have no idea what i'm talking about.

Things are not so simple as you are able to understand.
So right calcs are little more complex than you are able to do.

Sorry Dan, don't waste your time if you don't have idea what you talking about.

Regards.

 

[fechter]

Now that you guys have figured out how much switching losses there are, perhaps you could enlighten me on something related.

Several people have mentioned how poor the body diodes in a typical FET (like a IRFB4110). As I recall, the reason is they switch too slow to efficiently conduct the current pulse from the collapsing magnetic field in the windings.

According to the datasheet, the intrinsic turn on time is next to zero, limited by the inductance of the leads. About as fast as a piece of solid wire.

So what am I missing here? The recovery time is slower, but still pretty fast at 50nS. When the diode is recovering, the dv/dt of the voltage is relatively slow compared to when it needs to turn on.

Below is the part of the datasheet for the 4110 body diode.

 

[eP]

Several people have mentioned how poor the body diodes in a typical FET (like a IRFB4110).
...
So what am I missing here? The recovery time is slower, but still pretty fast at 50nS. When the diode is recovering, the dv/dt of the voltage is relatively slow compared to when it needs to turn on.

I'm sure you missing there:

152ns and 760nC for STP120NF10.

IRFB4110 is not a typcal FET
It is very fast and efficient FET in its class.

 

[cadstarsucks]

IC you have no idea what i'm talking about.

Things are not so simple as you are able to understand.
So right calcs are little more complex than you are able to do.

Sorry Dan, don't waste your time if you don't have idea what you talking about.

Regards.

Actually I was intentionally NOT dealing with parasitic anomalies. Good layout all but eliminates what you are talking about.

While it is easy to stick the gate drive chip right on the gate of the FET on low voltage circuits and have proper decoupling right on the chip, which limits gate inductance to nano if not pico henry levels, I do not kid myself as to the layout quality of these controllers.

Realizing that most of the readers are not electronic engineers, I also did not go into the intricate details of the gate resistors job-to slow down the switch. That protects the gate (often with the additional gate zener) and prevents parasitic ringing which can cause as much if not more dissipation that the actual switching transient.

If there is ringing on the output you could easily be looking at triple the switching dissipation.

Dan

 

[TylerDurden]

Realizing that most of the readers are not electronic engineers, I also did not go into the intricate details of the gate resistors job-to slow down the switch.

I'm not an engineer, but I'd wager most the readers of this thread are; considering the OP was about FET suppliers and the topic has continued to increase in technicality.

Regardless, the information you contribute will enlighten present and future readers, even if it takes time for some of us to fully understand all the details.

Game on.

 

[fechter]

I actually have no clue what they are talking about, but it is interesting.

OK, here' another question:

Does the gate charge depend on load current?

I've also observed by o'scope that the output seems to switch faster than the gate. What we really want to know is how fast the output swings, not how fast the gate swings. I'll assume the losses in the gate drive circuit are negligible compared to the switching losses in the FET.

 

[NeilP]

Any updates for suppliers of 4110's at the moeent? any one seen a good deal? Only need 6 but will take a few more if need be

 

[scriewy]

daamn shame they didn't continue their argument with fragments and shrapnel of excellent information flying in all directions !! hitting us innocent jaw dropping bystanders been collateral damage of INFORMATION ! !

nailP NEVER buy only 6, always get extra ontop of extra, never know when shit will hit the fan