# Determine the value of inductor in DC-DC (boost) converter

Discussion in 'The Projects Forum' started by hbklau, Jan 18, 2010.

1. ### hbklau Thread Starter New Member

Nov 4, 2009
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Hi all,

I am currently building a DC to DC boost converter and I have problem identifying the value of the inductor in the circuit. Can someone guide me for this? I am using a frequency of 5kHz and when the duty cycle is 0.75, it should have an output of 20V (5V as input).

I tried using different values of inductor along the way and found out the following:

Duty cycle of 0.75
4mH = 14V
0.220uH = 30V+
1mH = 15-16V

2. ### t_n_k AAC Fanatic!

Mar 6, 2009
5,448
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It would probably help if you posted the full schematic for the circuit you are analyzing.

Jul 17, 2007
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Jan 28, 2005
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hgmjr

5. ### k7elp60 AAC Fanatic!

Nov 4, 2008
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The load current is the major determining factor in the value of the inductor.

6. ### hbklau Thread Starter New Member

Nov 4, 2009
9
0
Hi,

Sorry for the late reply. Pls kindly see attached. In fact, I have build the PWM for the Mosfet but now is the inductor value to be determine.

Best Regards

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7. ### SgtWookie Expert

Jul 17, 2007
22,199
1,801
It appears you have no provisions for feedback from the output to adjust your PWM percentage.

5kHz is a rather low frequency for a boost-type DC-DC converter. You will likely need a large inductor that is very resistant to saturation.

8. ### hbklau Thread Starter New Member

Nov 4, 2009
9
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Hi sir,

thanks for the reply. Sorry that I did not mention about the feedback circuit. As my professor told me, he want me to work on the open loop circuit first before working on the feedback. (Getting 20V for output with 0.75 for duty cycle) He had already given me the formula for the feedback and I will be working on that after determining the basic element value.

But I don't understand why we need larger inductance as what I learn from the formula of the inductor is V = L (di/dt). If the value of inductor is small, then the rate of change of current will be fast. As mentioned earlier, I tested the open loop circuit with 2 inductors, 220uH and 4mH and it turns out the smaller value of inductance gave a higher output. I suppose i might need a inductance value between 220uH and 4mH in order to achieve the 20V (Duty cycle:0.75).

Please advise me if I have misunderstand the theory behind it. Thanks a lot.

9. ### hbklau Thread Starter New Member

Nov 4, 2009
9
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hi sir, can you elaborate more on this? Thanks a lot.

10. ### mufcforever7 New Member

Feb 14, 2011
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i think the inductor value is L = 7.5mH