Buck converter, mosfet choise

Hello to all.
Sorry for my poor english first.
I would like to make a BUCK,I need to a mosfet to use as a switch.
My switching frequency is 200Khz and the power I want to transfer is about 300w.
Is IRFB4110 good for this, I don't know what frequency it can switch.
Another question, Does it need to Heatsink?If you have better suggestion, tell me please.
thanks.

leoncorleone:
Hello to all.
Sorry for my poor english first.
I would like to make a BUCK,I need to a mosfet to use as a switch.
My switching frequency is 200Khz and the power I want to transfer is about 300w.
Is IRFB4110 good for this, I don't know what frequency it can switch.
Another question, Does it need to Heatsink?If you have better suggestion, tell me please.
thanks.

Hi,

You should really specify your input and output voltage too, and i guess you mean it is not synchronous.

  1. What are you using to control the MOSFET?
    1a) What voltage does that controller put out? The IRFB4110 is a standard voltage MOSFET, ie, you're supposed to put ~12v on the gate. If you want to ocntrol it with something that only puts out 5v, you need to use a logic level MOSFET (or use additional components like a gate driver or other transistors).

  2. What input voltage?

  3. What output voltage?

  4. What output current?

If you do not know the answer to any of these questions, you need to figure that out before selecting a MOSFET.

MrAl:
You should really specify your input and output voltage too, and i guess you mean it is not synchronous.

The input voltage of BUCK is 80 volts and output is variable between 0 till 80 volts.
The maximum current is about 5 Amps.

MrAl:
What are you using to control the MOSFET?

I want to use it to control speed of Brushless DC Motor.
tnx.

Okay, so you’re doing 80V in, up to 80V out, and 5A… is that maximum continuous current or peak current? Try to find the other current.

A brushless DC motor requires a much more complicated drive scheme - multiple sets of fets to switch the multiple windings on and off at the appropriate moments while monitoring the speed and position of the shaft in it’s rotation. You vary the speed by changing the timing and duty cycle of these fets - not by adjusting the supply voltage. If you’re working with a brushless DC motor, you need to do some more basic research to understand what one is, and the big picture about how to drive them.

An adjustable DC voltage is suitable for controlling the speed of a normal DC motor, but not a brushless one… (also - often you can adjust speed of a normal DC motor by just PWMing the power to it, without bothering with a buck converter…)

Also - you misread my question - I asked what you were using to control the mosfet (ie, the thing that is driving the gate of the MOSFET and setting the duty cycle and all that), not what the buck converter would be powering ((though, per above, what you said was indeed very important since it calls your whole project into question!)

My general impression is that you have no freaking clue what you’re doing - so some more general information would be a good idea so that we can give you some more general advice. Maybe a link to the motor you’re using? Description of what you’re doing?

DrAzzy:
. is that maximum continuous current or peak current…

That is the maximum continuous current.
Actually I want to use for below circuit.
Thanks again.

Hi,
OPs diagram.
1cb0ee60fbb1b84a19cb6da6a71cc98fa2972429.png
Tom… :slight_smile:

At that switching rate and power you should really be thinking GaNFETs, not silicon devices.

You might be better dropping to 100kHz or lower where switching losses won't cripple you.

80V is also high enough to cause a lot of issues with protection circuitry, but low enough that
IGBT's are rather inefficient (and they run slower too).

If you have a history in switch mode supply design, you'll have a chance of getting this working,
otherwise expect a learning exercise and buy plenty of spare parts (and wear eye protection,
MOSFETs at high power fail by exploding).

You will need gate drivers for fast switching, get high-low MOSFET drivers that have dead-time
enforcement and protection built it, and add zener's across gate/source of the MOSFETs. Use
two switching devices and synchronous rectification, not one switching device and one diode,
I reckon.

Hang on a minute, you are using this to provide analog voltage control for a motor???

That's crazy. Just PWM the motor like the rest of the world.

@5A,
with an RDSON of 4 mohms,

PJunction = I2*RDSON
= (5A)2*0.004 ohms
= 0.1 W (100 mW)

Thermal resistance junction to ambient is 62 degrees C /W so 62 * 0.1W =6.2 degrees

Heatsink is not indicated.

CHOICE (not choise) (not that anyone cares)

choice
CHois/Submit
noun
1.
an act of selecting or making a decision when faced with two or more possibilities.
“the choice between good and evil”
synonyms: option, alternative, possible course of action
“you have no other choice”
adjective
1.
(especially of food) of very good quality.
“he picked some choice early plums”
synonyms: superior, first-class, first-rate, prime, premier, grade A, best, finest, excellent, select, quality, high-quality, top, top-quality, high-grade, prize, fine, special; More
2.
(of words, phrases, or language) rude and abusive.
“he had a few choice words at his command”

Hello again,

I have to second the nomination to go to 100kHz instead of 200kHz. Much more workable.

Also, either way, switching losses are to be considered, and in these fast designs gate dissipation is also usually a consideration because the mosfets gates have to be driven very hard to meet the rise and fall times.

Good mosfets drivers are a must.
Short source leads to minimize ringing which can cause the device to turn on and off several times per actual switch command which of course raises power dissipation.

Part of this also depends on how much of an overkill you want to apply to the mosfet selection. A production design would require careful consideration of cost while a one-off home built unit can go for the gusto and buy a nice hefty device that can easily handle the power :slight_smile:

raschemmel:
Heatsink is not indicated.

Heatsink it very necessary at those speeds, the switching losses will be hard to contain to less than 2%
of the load dissipation I reckon.

200kHz = 400k transitions/second, say 50ns switching time at 25% of load dissipation = 8W

load dissipation

The heatsink is for device power dissipation not load dissipation. At 4 mohm, the RDSON is very low and if he plans to use 80 V then the current would be less than 5A. I don’t have a lot of experience calculating heatsink power so I could be way off, but maybe you can elaborate a little.

raschemmel:
The heatsink is for device power dissipation not load dissipation.

Do you understand switching losses? Switching losses depend on the load dissipation and
fraction of time spent in the switching transients.

Ok. That makes sense.