# Choosing optimal voltage and current ripple for boost converter

#### zazas321

Joined Nov 29, 2015
936
Hello. Could someone give me advice on how to choose optimal values for ripple voltage and current. Based on the given requirements of the boost converter I am not able to determine what ripples should I use. The only given specifications of the system is :

Vin=20V
Pin=20W

Vout=60V

I need to design and select optimal values of L and C for a boost converter. Any assumptions are allowed!!.

I have derived Inductance and Capacitance formulas, but in order to calculate the values, I am missing current and voltage ripple.

L= (Vin*D) / I(ripple)*F(sw)

C= (Iout * Ton) / Vripple

#### BobTPH

Joined Jun 5, 2013
8,661
There is not an optimum ripple. The larger inductors and capacitors you use, the smaller the ripple will be. What you have to do is choose a level of ripple that is acceptable, then it will give you the sizes of the parts you need.

The acceptable ripple depends on what you are using the output for. If it is for an incandescent light, for example the ripple could be very high, even 50%. For sensitive electronics you probably want it under 1%.

Bob

#### bertus

Joined Apr 5, 2008
22,264
Hello,

You might want to read the attached article.

Bertus

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#### zazas321

Joined Nov 29, 2015
936
I am using the boost converters for a hybrid power system (wind/solar). I have calculated the Inductance and capacitance based on this article - http://www.ti.com/lit/an/slva372c/slva372c.pdf.

I have assumed the ripple current to be 30% and calculated Voltage ripple to be 1.75%.

Calculated values for L=1.3mH
and value for C=2uF.
I have chosen frequency to be 100kHz.

I am trying to simulate the design and I am not quite sure what should I do with the load and input resistances? Do I need to use them?

The results I get are clearly wrong on the simulation:
Voltage input and output are equal and current=0

#### crutschow

Joined Mar 14, 2008
34,044
The picture is too small and blurry to see any detail.
Post a larger and clearer one.

#### zazas321

Joined Nov 29, 2015
936

#### MisterBill2

Joined Jan 23, 2018
17,791
With a wind generation system you should already have AC power at a changing frequency, so a simple transformer could provide the desired voltage boost. Solar power is a different issue completely.
But my question is why 60 volts?? And why is ripple an issue? Also, how much power is needed?
In addition, simulations produce no useful power, tghat only comes from actual hardware.
To get a good education about switchmode voltage boosters check out the TI website, as well as a number of others that some of the folks here can suggest. You will discover that starting with a simulation is not the best place to start.

#### kubeek

Joined Sep 20, 2005
5,793
What are you doing with the output? Charging a battery? IIRC an AGM battery called for less than 1% current ripple when charging to achieve long service life.

#### BobTPH

Joined Jun 5, 2013
8,661

Bob

#### MisterBill2

Joined Jan 23, 2018
17,791
The hardware does not exist. This is a theoretical problem in an incomplete simulation. A small portion of a switch mode supply. Another thread by the same poster asks questions that can't be answered.

#### BobTPH

Joined Jun 5, 2013
8,661
Here is a simulation with the inductor and capacitor you calculated:

I get a voltage and current ripple of about 1.7%. Your inductor is way larger than needed, I get essentially the same results with a 100uH inductor.

Edit: Raising the capacitor can give you much lower ripple if needed.
Bob

#### MisterBill2

Joined Jan 23, 2018
17,791
Once again, the partial power supply is running OPEN LOOP, which does not bear any relationship to how an actual power supply operates. In an actual supply, the feedback plays a big part in eliminating ripple and variations.

#### kubeek

Joined Sep 20, 2005
5,793
Once again, the partial power supply is running OPEN LOOP, which does not bear any relationship to how an actual power supply operates. In an actual supply, the feedback plays a big part in eliminating ripple and variations.
Are you sure? The ripple comes from the basic opertion of the buck converter, and I don´t think any form of feedback can change that. Load regulation, overshoot on load change etc. definitely, but it can´t get below the basic ripple.

#### zazas321

Joined Nov 29, 2015
936
As misterbill mentioned, there is no hardware. I just have to simulate and come up with a concept. I need two different boost converters with two different load ratios. I am trying to do it one by one. First I need to figure out how to design a simple boost converter.

#### zazas321

Joined Nov 29, 2015
936
With a wind generation system you should already have AC power at a changing frequency, so a simple transformer could provide the desired voltage boost. Solar power is a different issue completely.
But my question is why 60 volts?? And why is ripple an issue? Also, how much power is needed?
In addition, simulations produce no useful power, tghat only comes from actual hardware.
To get a good education about switchmode voltage boosters check out the TI website, as well as a number of others that some of the folks here can suggest. You will discover that starting with a simulation is not the best place to start.
There is no difference if its 60 or 10 for me. I am just trying to understand the boost converters and how to design it. If I can design it with one Voltage I should be able to apply the knowledge to design it for the other one.

#### zazas321

Joined Nov 29, 2015
936

Bob
Yes I didint have a load on my simulation cause I was not sure what load should I use. It is not specified in the specifications what kind of load I should use. The main aim of the project is to design two boost converters to control two power supplies with different load ratios. I do not seem to get the same response even if I add 180 ohm resistance as you did.

#### zazas321

Joined Nov 29, 2015
936
What are you doing with the output? Charging a battery? IIRC an AGM battery called for less than 1% current ripple when charging to achieve long service life.

It is one of the options. I can make an assumption that it is if it does make it any easier?

#### kubeek

Joined Sep 20, 2005
5,793
Sorry for me making a confusion, the ripple current in the inductor will not have significant impact on the charge ripple current of the battery, if the output capacitance (and impedance) is good enough to keep the voltage ripple low at the output.

Anyway, you either have to make a guesstimate, assumption, or pick a number by throwing dice, but without a number for the allowed ripple you won´t get anywhere, even for the back of the envelope design. I think that somewhere between 1-10% is reasonable, but still it depends on what the load is and how nice power it deserves or requires to have. After that, and after choosing your operating frequency you will arrive at what size inductor and capacitor you need and how much will they cost, and then you can do a second round of thinking to see if you can tweak some of the parameters to suit your preference better if not all was hunky dory.

#### crutschow

Joined Mar 14, 2008
34,044
You show a bipolar transistor with no base resistor to limit the current.
That definitely will now work.
And why is the drive voltage 100V?