DC operatring frequencies?

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Hi,

I got some MC34063A chips that can do DC-to-DC voltage step up or down. And I found the tool that helps me to get the correct values of resistors and caps in order to get the correct output voltage. The tool is at http://www.nixiefunkuhr.de/index... . As you know it asks for minimum freq (Fmin), which I am bit puzzled on what would be a good freq typical value, say, for a 5V DC output? Also there is a value called Vripple. Don't know what that is as well. (How the output voltage is calculated based on the input values, I have no idea, although would love to find out.)

Thanks

Last Edited: Wed. Oct 14, 2009 - 06:44 PM
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That circuit is a fixed pulse width, variable frequency. Hardly ever used any more.

Rather than fixing a frequency, first look at the device spec sheet to see what the recommended frequency range is. Then choose a frequency in that range and see what the component values are. If the values are not very desirable, choose a different frequency. For ripple, choose something relatively small compared to your desired output. For 5V out, you might choose 50mV or less.

By the way, I could not make the link work. I had to go back to the home page and search for "Tools" from there. Not knowing German doesn't help!

Jim

Jim Wagner Oregon Research Electronics, Consulting Div. Tangent, OR, USA http://www.orelectronics.net

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ka7ehk wrote:
That circuit is a fixed pulse width, variable frequency. Hardly ever used any more.

Rather than fixing a frequency, first look at the device spec sheet to see what the recommended frequency range is. Then choose a frequency in that range and see what the component values are. If the values are not very desirable, choose a different frequency. For ripple, choose something relatively small compared to your desired output. For 5V out, you might choose 50mV or less.

By the way, I could not make the link work. I had to go back to the home page and search for "Tools" from there. Not knowing German doesn't help!

Jim

Thanks for the response.

-The chip operates max at 100Khz. One of the many links (THANKS TO GOOGLE!) to the spec is at http://www.biltek.tubitak.gov.tr...

-The correct link to the tool is http://www.nomad.ee/micros/mc340... (How did I get the wrong link in the first place. :oops: )

-Also found a spreadsheet that does the calculation for the Vout. http://www.jiggerjuice.net/elect...

Last Edited: Wed. Oct 14, 2009 - 06:43 PM
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unebonnevie wrote:
ka7ehk wrote:
That circuit is a fixed pulse width, variable frequency. Hardly ever used any more.

Rather than fixing a frequency, first look at the device spec sheet to see what the recommended frequency range is. Then choose a frequency in that range and see what the component values are. If the values are not very desirable, choose a different frequency. For ripple, choose something relatively small compared to your desired output. For 5V out, you might choose 50mV or less.

By the way, I could not make the link work. I had to go back to the home page and search for "Tools" from there. Not knowing German doesn't help!

Jim

Thanks for the response.

-The chip operates max at 100Khz. One of the many links (THANKS TO GOOGLE!) to the spec is at http://www.biltek.tubitak.gov.tr... .

-The correct link to the tool is http://www.nomad.ee/micros/mc340... . (How did I get the wrong link in the first place. :oops: )

-Also found a spreadsheet that does the calculation for the Vout. http://www.jiggerjuice.net/elect...

Did a little more research and the MAX1674EUA IC is even better, since it can take a Vin as small as 0.7V. But, the MC34063A takes a min Vin of 3V. That explains why I was able to get 30 pcs for $7. Still not a bad deal. :-)

Last Edited: Mon. Oct 19, 2009 - 07:10 AM
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Quote:
The chip operates max at 100Khz.

This is a switching frequency. So, I got more data sheet from various vendors, such as, TI, ON Semiconductor, etc. TI suggests typically, this freq should be 33kHz.

Could someone explain what the switching frequency is? And Vripple? Their purposes? Things to watchout for (besides consulting the spec for things to watch out for.) I know I could google and read more, but the docs tend to be too technical or theorectical. So, I prefer an AVRfreak answer. My guess is that it is the freq that switches between the Vin and Vout.

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The switching frequency is whatever you want it to be (within the specs of the chip). The higher the frequency, the more critical the layout, the smaller the inductor and your choice of filter capacitor becomes more critical. Vripple is the amount of ripple on the output. Since the basic operation of this converter is to switch current on and off and the inductor tries to smooth out the voltage ripple along with the filter capacitor so you get to tradeoff vripple vs inductor and capacitor size. A vripple of 100mV is not uncommon. For less critical applications, you might be able to tolerate more vripple. If you're running nixies, then 100mV of ripple is not going to upset them. The design is an iterative process - plug in a few values and see what that takes you.

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Switching frequency selection is a compromise between efficiency and ease of design.

Everytime the switching element (MOSFET) switches, there are some losses between the full-on and full-off positions. In a conventional modern switching regulator, these losses make up for the majority of power losses throughout the circuit. Thus, the lowest possible switching frequency is desirable to obtain the highest efficiency (circuit switches less often, so less loss over the same period of time).

However, the lower you go in terms of switching frequency, the bigger and more expensive the filter components become. So ultimately you have to sacrifice some efficiency to get a reasonably sized and priced filter to follow it.

Ripple voltage is yet another parameter that will constrain your choice of switching frequency. For example if you can live with a 1V ripple (noise) on the output, you can use less beefy components on the filter, which in turn allows you to lower the switching frequency a bit more for better efficiency.

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unebonnevie,

Please - Do not include periods within links to end a sentence - results in Object not found! message.

You've done that in your first and second posts above.

Also, you don't seem to have noticed my PM of 14 hours ago about this.

Stan

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sbennett wrote:
unebonnevie,

Please - Do not include periods within links to end a sentence - results in Object not found! message.

You've done that in your first and second posts above.

Also, you don't seem to have noticed my PM of 14 hours ago about this.

Stan

Can't you tell it's not me that put the period in as part of the URLs? :-) Unfortunately, I ended the sentences with periods and the posting include them as part of URLs.

Point noted.