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DC - DC Converter inductor&capacitor value

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s_cihan_tek

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Hi all,

I want to build a simple 0-20V variable output DC-DC converter with a max current rating of 1A. It's output voltage will be adjusted according to its output current by using a look-up table.
Since the converter will supply low currents, it'll operate in discountinuous conduction mode.

I have 2 questions:

1 - I don't know how to calculate the output inductor and capacitor value when the output voltage isn't constant and operation mode is discontinuous.
All of the calculations i've seen use the minimum output voltage and ripple for continuous conduction mode as input.

2- What's the best way to regulate the output voltage according to a I-V lookup table? Would using a microcontroller,DSP etc. to create a PWM and vary the duty cycle according to the current and voltage values coming from its ADC suffice? The response time shouldn't be very long.

Thanks in advance.
 

Hello,

I understand, that you want to drive the power switch directly from uC PWM output. That should basically work. Regarding the inductance, I would use the highest inductance that is available for the intended current range, to minimize peak current and increase on time for a given output current. Otherwise, it can be an issue to set the on time precisely enough.

Regards,
Frank
 

FvM said:
Hello,

I understand, that you want to drive the power switch directly from uC PWM output. That should basically work. Regarding the inductance, I would use the highest inductance that is available for the intended current range, to minimize peak current and increase on time for a given output current. Otherwise, it can be an issue to set the on time precisely enough.

Regards,
Frank

Thanks for the response.

What do you mean by "Highest inductance that is available for the intended current range"? The inductance value could be as high as i want for any current range, i can't see any limiting factors here. Or am i missing something?

Regards
 

Hello,

The inductance value could be as high as i want for any current range, i can't see any limiting factors here.

If so, current flow wouldn't be discontinuous, as you assumed, cause discontinuous operation is a result of inductance being lower than necessary. Actually size is a limiting factor. You may read highest inductance available as highest inductance fitting the design.

The point is rather simple. Usual design rule could be e.g. have ripple = 1/3 DC current. For variable current, this may become impossible, if current drops to zero, current flow is discontinuous anyway. My first approach is: Try to extend continuous mode as far as possible.

But there could be other criteria: Fast response (short current risetime) could require to limit inductance, also different tradeoffs losses versus output ripple are possible.

Regards
Frank
 

FvM said:
The point is rather simple. Usual design rule could be e.g. have ripple = 1/3 DC current. For variable current, this may become impossible, if current drops to zero, current flow is discontinuous anyway. My first approach is: Try to extend continuous mode as far as possible.

Hello again,

I understood the things you said but it's the math that makes me stuck.
Suppose that i try to extend continuous mode as far as possible and therefore want select an inductance value which will make the current continuous above 10mA in my case. The basic formula i know for the critical inductance value is
L=Vo*D*(1-D)/2*I*f
Here, Vo is both adjustable and dependent on D and that's what frustrating me.
I can't find out how to find a initial inductance value for testing in the actual circuit and fine tuning it. It seems like the equation above can't give me a value even for continuous cond. mode.

Regards
 

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