effect of ave load/dc load variations in buck converter vs. output ripple

[alimjoco]

Dear experts,

If changing the DC/ave load current of the buck converter, does it affect output ripple? Why or why not??

I measured and it doesnt affect the output ripple of a buck converter with changing load current(not ripple current)..please give the best explanantion.

 

[mtwieg]

It can affect ripple, depending on its operating point and mode of operation.

If a fixed-frequency buck converter is operating in CCM, then ideally the duty cycle and ripple only depend on Vin and Vout. But resistances in the converter make this not completely true, and the control circuit will make slight adjustments to duty cycle when the load changes, which will affect ripple. Generally the resistances are low, and thus the changes in D are low as well, as are the changes in ripple.

For a fixed frequency buck converter operating in DCM, changes in loading result in significant changes in duty cycle, which can affect ripple significantly. Also variable frequency buck converters will change frequency with load, also giving rise to variation in ripple.

 

[alimjoco]

That's a good explanation..actually im evaluating forced PWM CCM mode...I think ripple mainly depends on inductor current ripple, ESR, and switching frequency. Even if you changed the DC load(dc offset), the inductor current ripple(depends on vin and vout, inductor value and switching freq) and duty will remain unchanged(if there is , it will be insignificant). and so is the voltage ripple will remained unchanged through different load.. just for curiosity, why does we need to initially set in design stage the inductor current ripple to be 30% of the Iload max??

 

[mtwieg]

just for curiosity, why does we need to initially set in design stage the inductor current ripple to be 30% of the Iload max??

That's just a rule of thumb that some designers use. As far as I know there's not much reasoning behind it (and I've heard everything from 10% to 50% as recommendations). In the end you should decide the ripple yourself based on your requirements for output ripple voltage, efficiency, size, etc.

 

[alimjoco]

Hi mtwieg,

just a thought in measuring ripple with 50 ohm input term of the scope vs. 1M input term for forced PWM CCM buck...Isn't it the ripple is also unaffected of
AC loading(DC loading is disregarded since I'm using a series dc blocker)due to 50 ohm input term because ripple doesnt change that much in regards with differents loads. so iguess ripple amplitude is very consisten through out any terminations? I'm just concern about the ripple...and about the switching noise, a 50 ohm termiation with 50 ohm coax is a must...thanks

 

[Sanchayan]

A higher ripple current will dissipate more power in the capacitor's ESR. So, you might set it to 50% of I Load, but, use a capacitor accordingly.