Question about cascaded SMPS operation

[pinoy]

Hi,

Is it posible that the output capacitor of the first SPMPS (12V to 5V) and at the same time as the input capacitor of the 2nd SMPS (5V to 3.3V)? [Please refer to the attached picture]

Thanks

 

[pinoy]

Cascaded SMPS

up

 

[raka200]

Re: Cascaded SMPS

HI !!!

You could do that, but you should be carefull. There could be some conflict between the 2 SMPS. One critical point in a SMPS is the startup : The ouput voltage is equal to zero, and the current is equal to zero. The SMPS will try to level the output voltage to the correct value, so there will be a very high inrush current.
Since the input of the second is the output of the first, there could be some oscillation, and may be no startup at all.
You could either delay the start up of the second SMPS until the first has reach its permanent state, or add an inductor between the 2 smps to smooth the inrush current. Some SMPS have a soft start configuration to limit the current.
bye!

 

[pinoy]

Cascaded SMPS

Can the 2nd SMPS input oscillate? The 2nd SMPS have no Input Caps so no one can filter the 2nd SMPS input noise due to switching