# Boost converter

Discussion in 'Power Electronics' started by Graeme Kelly, Mar 22, 2018.

1. ### Graeme Kelly

14
0
Mar 22, 2018
Hello i have designed a boost converter that works on matlab but i am trying to simulate on multisim before i build it but i can seem to get this working i am looking for some assistance . the boost converter is meant to be boosting 3.75 V to 12.8 V to charge a battery

my parameters are
Duty cycle = 73 %
Switching frequency = 200000
Inductor= 0.8 x10-4
capacitor= 370x10-6
mosfet irf520n

Any help would be appreciated

2. ### Harald KappModeratorModerator

11,307
2,588
Nov 17, 2011
Post a schematic. A list of components alone is worthless.
And tell us exactly where the trouble lies. "Not working" is way too general.

3. ### Graeme Kelly

14
0
Mar 22, 2018
i have posted the schematic in this post. I am only getting 4.85 volts , when i was carrying out my duty cycle and components calculation for an output voltage of 12.8 v

File size:
16 KB
Views:
108
4. ### Graeme Kelly

14
0
Mar 22, 2018
Any assistance would be greatly appreciated

5. ### Harald KappModeratorModerator

11,307
2,588
Nov 17, 2011
How long is your (virtual) simulation time? Has the ciruit reached a stable state?
What's the 12 V battery V2 doing at the output? Remove it.
What's the voltage of U1? Is it high enough to completely turn Q1 on and off?

6. ### duke37

5,364
770
Jan 9, 2011
Are you running at 20kHz or 200kHz?
Are you turning on Q1 fully.

7. ### Graeme Kelly

14
0
Mar 22, 2018
thanks for your response i really appreciate it . The simulation doesn't have a stop time so runs continuously. I put the battery in as i am trying to simulate charging of a battery. when i take the battery out the output voltage goes to 99 volts. i have attached a picture so you can see. V1 is 3.75 volts that is the voltage that i am trying to boost up to 12.8 so i can charge a 12 volt battery.

I am running it a 20 kHz

File size:
16.1 KB
Views:
106
8. ### Harald KappModeratorModerator

11,307
2,588
Nov 17, 2011
That won't work.
Use a large capacitor instead. It is not going to be the same as a real battery but lets you observe the charging process.

9. ### Graeme Kelly

14
0
Mar 22, 2018
I have put in a 3x-6F capacitor and it is still giving the same out put voltage . can you suggest anything as i don't know what to try as i have exhausted any ideas i have. i have attached a picture so you can see.

Thanks again for all your help

File size:
15.7 KB
Views:
118
10. ### Graeme Kelly

14
0
Mar 22, 2018
as i am looking for a output voltage of 12.8 and i seem to be getting 99.412 V

I have done all the calculation for duty cycle, inductor and capacitor.

could it be the mosfet i am using ?

11. ### BobK

7,682
1,688
Jan 5, 2010
Change the battery to a resistor that would drop 12.8V at the desired charging current.

Also, the IRF520 required 10V to turn on completely. What is the voltage of your pulse source? For a real circuit, running off a 3.7V battery you will have to use not just a logic level MOSFET but one with an unusually low threshold voltage.

Bob

12. ### BobK

7,682
1,688
Jan 5, 2010
Here is a simulation, using all your parameters, a logic level MOSFET and a 12.8Ω load to simulate approximately 1A of charging current. Looks pretty good to me.

Bob

Arouse1973 and Harald Kapp like this.
13. ### Graeme Kelly

14
0
Mar 22, 2018
hello bob thanks for your reply . see if i was to build this circuit would i put the 12.8 volt resistor in series with the battery to keep the voltage to 12.8 v ?

14. ### Graeme Kelly

14
0
Mar 22, 2018
i have put a 12.8 ohm resistor in and i am only getting 9.3 volts. i have attached the picture of the simulation.

File size:
16.4 KB
Views:
56

7,682
1,688
Jan 5, 2010

Bob

16. ### Alec_t

2,940
799
Jul 7, 2015
As Bob said, the IRF520 needs about 10V Vgs to turn on fully.

17. ### Graeme Kelly

14
0
Mar 22, 2018
Just seen it there . Thank you very much for all your help

18. ### (*steve*)¡sǝpodᴉʇuɐ ǝɥʇ ɹɐǝɥdModerator

25,482
2,830
Jan 21, 2010
Also, your circuit's output, without load will rise to a high voltage. It will be limited by leakage, breakdown, etc.

I would have recommended a 12V zener as a simulation load. That way you could estimate the current into the battery.

Because nothing (other than the battery voltage) will limit the output voltage, this circuit can easily overcharge the battery.

19. ### duke37

5,364
770
Jan 9, 2011
I am interested in the simulation in #12 by BobK.
The output overshoots on switch on and eventually settles down. Since a certain level of charge is transferred each time, I would expect the voltage to rise gradually to its final value.

What is the origin of this apparent oscillation?

20. ### Graeme Kelly

14
0
Mar 22, 2018
Thanks for all your responses i really appreciate them , i have one more question. i need a maximum charge charging current of 1.5 amps for the battery that i am using but on my simulation i am getting a charge current of 3.6 Amps. does anyone have any suggestions ? i have attached a picture of my simulation.

thanks again for the time

File size:
21 KB
Views:
62