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Choosing MOSFET for my boost converter

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cpkarthiksai

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Hi

I am doing an undergraduate project of designing a boost converter for a MPPT system. I have a basic circuit diagram for reference which i have attached but i need some help choosing my MOSFET switch. My output voltage should be 24v. I want to know what should the current rating of my MOSFET be if my input supply is variable voltage and current is around 0.3A


boost.JPG
 

The main things are:

ON resistance, since that affects efficiency. This is generally related to the current rating.
The Vgs voltage required to obtain that ON resistance (typically 10V but can be 5V or less for logic-level MOSFETS).
The Vds maximum voltage rating which, for a 24V output, should be at least 50v.
The current rating, which depends upon the input current, which is determined by the ratio of input voltage to output voltage.

Another consideration is the gate charge since the gate driver has to provide that charge to turn the transistor on and off. Smaller is better.

Switching speed is also important but that doesn't vary much between MOSFETS of similar current, ON resistance, and voltage ratings.
 
Thanks a lot.. my circuit has a variable input supply and output as 24v.. so how do i calculate my current rating. also can you give me a formula for on resistance since i need to document stuff
 

If output current is 0.3A at 24V output voltage, the output power is 24*0.3=7.2W

Peak current = (5.5 * Power output)/(Minimum input voltage)

So, you should choose a MOSFET with a current rating higher than the peak current (you should keep a significant margin), with a low enough RDS(on) to minimize losses.
Conduction loss in the MOSFET = (I^2)*R. R here would be RDS(on), I would be current flowing through the MOSFET.

Hope this helps.
Tahmid.
 
ya it helped a lot.. thanks.. i will choose a mosfet and tell you which one.. it would help if someone just verified that it matches my need..
 

i have a chosen a mosfet.. I am attaching the datasheet for reference.. pls tell me if its ok for my application.. by the way i considered minimum input voltage as 5 volts.. the strange part is my input is a solar panel which is supposed to be a 12V 20W panel but its giving me an output of 24 v in the sun.. can anyone explain why?? Also i need a driver IC for my mosfet.. any suggestions how i choose one.. thank you
 

Attachments

  • NTD3055-094 datasheet.pdf
    76.6 KB · Views: 339

Check the solar panel specifications for the open-circuit voltage. This is the voltage you get with the panel not connected to a load. Typical value for a 12V panel is around 21V, so you're getting 24V (slightly higher than the typical value). When you load the panel, the voltage starts dropping depending on the load (and also the light intensity).

If your solar panel is rated for 20W, I guess that's the maximum power you're going to use the MOSFET for, since you can't get any higher power.
So, for 12V 20W application, this MOSFET is good. The VDS(max) rating is 60V. This is a large enough margin. The current rating is 10A (at 100°C). Also large enough margin. RDS(on) (maximum) is 94mΩ, which is good for the current you're using it for. The switching times and gate charge are also low. So, that's good.

Hope this helps.
Tahmid.
 
Thanks a lot.. I will also need some advice as to how to choose my resistors, capacitor and inductor for my circuit. The circuit diagram is in the first post. I have a general idea how to but i need some confirmation. Thanks a lot again Tahmid.

Sai
 

I couldnt get the MOSFET that i had chosen earlier so i had to settle for this one.. IRF510. Please let me know if its applicable
 

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  • IRF510 datasheet.pdf
    56.1 KB · Views: 251

You don't expect much higher than 24V and IRF510 is rated for 100V VDS. About 4X. Lots of margin. So, in terms of voltage, it's okay.

Let's say you assume a max current in the circuit, of 2A (it should be lower as -> 20W, 12V). The max current of IRF510 is 5.6A. However, since it's a boost converter, you may have peak currents as much as 5X average current. So, you should select a MOSFET with higher current capacity.

1 common MOSFET that you can easily find and that can be used in your design with more than enough margin is IRF540. If you can find IRF510, I'm pretty sure you can find IRF540.

It has a high max VDS -> 100V, high max current handling capacity ID -> 33A, low RDS(on) -> 44mΩ. It should also be pretty cheap.

Hope this helps.
Tahmid.
 
ok thanks i will.. also i want to know how to select my capacitance and inductance values.. as well as any other components that will be needed in my actual circuit layout..
 

well when i design my actual circuit i will be using extra capacitances for filtering noise etc right?? actually i still have no idea how to actually interfacer the gate driver and mosfet.. I have the IR2110 driver but i am trying to get the lm5114.. can u suggest which will be better for the mosfet u suggested..
 

Attachments

  • lm5114 mosfet driver.pdf
    1.2 MB · Views: 245
  • ir2110 datasheet.pdf
    328.6 KB · Views: 193

i saw the link u sent me.. it was helpful.. but i need to know how to calculate the switching frequency.. i am using a PIC16f877A to generate PWM for my driver. my PIC uses a 20MHz clock
 

You can select a frequency deciding on the trade-offs. Let's say you start with 30kHz. Then, decide if it's worth going higher or lower, ie what are the advantages and disadvantages of going for a higher or lower frequency. Are the disadvantages too great, etc.

High frequency reduces your inductor and capacitor sizes but you have higher switching losses. One of these is because you need to drive the MOSFET gate 'harder' - with more current in a shorter time. This current is just used to get the MOSFET fully conducting, so you will have higher switching loss than for a lower frequency. Another issue to keep in mind is the ESR of capacitors. And if you go for high frequencies, can you find capacitors with a low enough ESR at a reasonable price. You should read on these trade-offs in detail. You can google it.

In terms of making an educated guess, 50kHz is a good start.

Let's see with 50kHz -> PR2 = 99. [Calculate with formula in page 69 of datasheet].
So, you can control the duty cycle in 100 steps. If that's okay for you, you can use 50kHz. If you need more detailed control, you can use a lower frequency, which will give you a higher number of steps of control.

Hope this helps.
Tahmid.
 
ok thanks that was useful.. and about my output current of the circuit.. say i m going to charge a 12v 1.2A battery.. so will my output current be 1.2A?
 

No. If you're charging a battery, you need to decide at what rate you want to charge it. A common rate is C/8. At this rate, charging is slow but battery is not damaged (provided you don't over-charge it). So, if you choose C/8, the average current will be (1.2/8)=0.15A. That will be your average current. However, as you're using boost converter, you may have peak currents in the converter as high as 5.5*0.15 = 0.825A

Hope this helps.
Tahmid.
 
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