Have a large transformer,bridge and 60000uf cap feeding my buck, question, does the ripple off the buck have anything to do with the ripple off the main power supply. Does one benefit from less ripple going into the buck. Thanks

 

Yes, there is some ripple on the output of a voltage regulator caused by ripple on the input to the regulator. It is usually quite small. that is, the output ripple is probably less than 1/1000 of the input ripple.

The data sheet for the regulator will have a specification for this. What regulator are you using?

By the way, the circuit layout can make the ripple voltage on the output of the regulator worse by coupling the ripple current into the reference or feedback circuits of the regulator.

 

Thanks very much, the buck I bought is a dps5020.

https://www.aliexpress.com/item/RD-...-Voltage-converter-color-LCD/32702714880.html.

Have 3 of em, just to play with.

 

It depends a lot on the design of the buck converter. A converter that uses "current mode" control (i.e. an inner loop that controls the current in the switch & inductor with a voltage control outer loop) does a very good job of rejecting input ripple, even if the voltage control loop isn't very good.

Even where current mode control isn't used, the voltage error amplifier generally has very high gain at the input ripple frequency, so it is able to reject a large amount of ripple. If the bandwidth of the error amp has been rolled off to a low frequency for some reason, then you can get ripple on the input showing up on the output. Of greater concern with low bandwidth is overshoot and undershoot of the output voltage if the load is abruptly decreased or increased - poor dynamic respsonse.

It isn't too common anymore, but occasionally you see "feedforward" added to a switcher. The purpose of this is to directly change the duty cycle in inverse proportion to the change in input voltage. Perfectly tweaked, feedforward can make the switcher almost totally reject variation in the input voltage. Current mode control is actually effectively a feedforward method.

 

It depends a lot on the design of the buck converter. A converter that uses "current mode" control (i.e. an inner loop that controls the current in the switch & inductor with a voltage control outer loop) does a very good job of rejecting input ripple, even if the voltage control loop isn't very good.

Even where current mode control isn't used, the voltage error amplifier generally has very high gain at the input ripple frequency, so it is able to reject a large amount of ripple. If the bandwidth of the error amp has been rolled off to a low frequency for some reason, then you can get ripple on the input showing up on the output. Of greater concern with low bandwidth is overshoot and undershoot of the output voltage if the load is abruptly decreased or increased - poor dynamic respsonse.

It isn't too common anymore, but occasionally you see "feedforward" added to a switcher. The purpose of this is to directly change the duty cycle in inverse proportion to the change in input voltage. Perfectly tweaked, feedforward can make the switcher almost totally reject variation in the input voltage. Current mode control is actually effectively a feedforward method.

Thank you for the feed back, this stuff all looks good on paper till you get down to play with it.