12V-24/20A DC-DC converter

[srikanthvajjala]

dc dc convertor 20a 12 24

Hi all,

I would like to design DC-DC converter with following specifications.
Input : 12V from the car battery
Output:24V/20A
Topology: Boost converter
Voltage ripple:0.1%
Current ripple:4A (Iin become 40A as per calculation )
Operating frequency:100KHz
Delta Il taken as 10% of Iin

As per the calculation i got an inductor value of 66UH and capcitance value 4200UF. Can anybody verify my design. I need to know how to select ripple current with unknown input current and operating frequency. If anybody comes forward with design idea please send a mail to gs.srikanth(at)rediffmail.com

Advance Thanks,
sri

 

[max0412]

power torid inductor

This site will give you an idea of your power stage requirements.



The battery in your car will vary it's not 12V constantly.

 

[srikanthvajjala]

tps40210 .pdf

Dear Max,

Thank you for your reply. As per my calculation the inductor value should be 66UH for an inductor ripple current of 4A. But the link given by you if i calculated that is 3.6UH (17A inductor ripple current). Could you tell me please how much the Inductor ripple current has to be taken generally. Either higher side or lower side. Waiting for your earliest reply.

Regards,
SRI

 

[max0412]

dc 12v/24

Here are some data sheets that will answer all your ? rather then me typing it out again. Basically you select inductor size so the converter will stay in CCM under worst case. You can read here why you want to insure that.

https://www.daycounter.com/LabBook/BoostConverter/Boost-Converter-Equations.phtml

This is a controller I’ve used for boost converters it contains a detailed design of a 48W boost . The same in and output voltage as your desired design but at 1/10th the power.

12V nominal input,24V output 2A output current. You may want to start out a this power level or lower until you understand its operation. Use a torrid you can easily adjust the inductance and observe the effect under different line and load conditions.

https://focus.ti.com/lit/ds/symlink/tps40210.pdf

Here is another datasheet with an example

**broken link removed**

You also want to be sure your cars alternator wiring etc. Can handle the extra power assuming 90% efficiency about 533W you are asking of it. This converter will draw large peak currents. At your power level you will probably want a Synchronous controller.

 

[srikanthvajjala]

12 to 24 dc dc converter design

Dear Max,

Thank you for your reply. My question is simple. Is the ripple current of the inductor has to be more or less ? Please reply me back.

Regards,
SRI

Added after 15 minutes:

Dear Max,

Thank you no need to reply my earlier question. I got the information sent by you. Thanks for your replies.

Regards,
SRI

 

[max0412]

dc/dc converter 12 to 24

Hi srikanthvajjala

Did you not take the time to read through the links I gave you. There is no set in stone ripple it depends on your design requirements.

Here is a cut and paste from the daycounter link.


"The main question when designing a converter is what sort of inductor should be used. In most designs the input voltage, output voltage and load current are all dictated by the requirements of the design, whereas, the Inductance and ripple current are the only free parameters. It can be seen form Equation 1, that the inductance is inversely proportional to the ripple current. In other words, if you want to reduce the ripple, then use a larger inductor. Thus, in practice a ripple current is decided upon which will give a reasonable inductance."

"There are tradeoffs with low and high ripple current. Large ripple current means that the peak current is ipk greater, and the greater likelihood of saturation of the inductor, and more stress on the transistor."

"So when choosing an inductor make sure that the saturation current of the inductor is greater than ipk. Likewise, the transistor should be able to handle peak current greater than ipk. The inductor should also be chosen such that the it can handle the appropriate rms current."

This is why you typically increase your calculated inductance to insure you stay in CCM

"It should be noted that when there is a light load the circuit can slip into discontinuous mode, where the inductor becomes fully discharged of it's current each cycle. When a load is reapplied the inductor needs to recharge, and so the transistor's duty cycle increases pulling the inductor towards ground, and because of the increased duty cycle Vout decreases when we really want it to increase. This causes an instability, which is well known for boost converters, and not a problem with buck converters."

You can use this to select your ripple as a guideline,it really depends on your load and the other factors already mentioned.Trying to find an ferrite inductor for your converter may be a bit of a challenge and probably wont be cheap and will likely be the determining factor in your ripple.

"One way to combat this instability is to choose a large enough inductor so that the ripple current is greater than twice the minimum load current. When this condition is met then the inductor is always in continuous mode. "

The rest is here including equations.

https://www.daycounter.com/LabBook/BoostConverter/Boost-Converter-Equations.phtml

 

[srikanthvajjala]

dc/dc converters 20a output

Dear Max,

Thank you for spending the time for my querry.

Regards,
SRI

 

[nareshgtr]

Please some one explain how to select "Capacitor value for boost converter"
Thanks Advance
V. Naresh Kumar
Advay Solutions Pvt. Ltd