Cascading two different dc-dc converters

[balzee]

Hello everybody,While working on my project I got this doubt...Is it possible to cascade two different types of dc-dc converters like a buck and a boost ,a buck and a buck-boost etc....If so how their avg output voltage can be related to input voltage,and how their effective resistance varies when cascaded....plz help guys...

 

[emontllo]

Hello balzee,

Yes it is possible.
Actually the typical power supply is a boost (operating as PFC controler) plus a flyback.

You may consider each converter as a separated block and calculate the output voltage as a multiplication of different converter ""gains"". This is an approximate aproach.

However there are a few considerations to take into account (and provably more, depending on the objective of the project):
- The buck, is voltage input current output converter while the boost is a current input, voltage output converter. The buck-boost is voltage-voltage. It is recommended not to connect voltage to voltage nor current to current converters toguether. I.E. Boost - buck is ok, buck-buckboost ok, buck - buck ok, but boost - buck it is not recommended.
- Minimize the number of cascaded converters for simplicity, efficiency and stability
- The overal control may become more complex
- Intermediate filtering may be requiered (capacitors or inductors), considere syncronizing switching frequency of different DC/DC

Hope it helps!

Ernest

 

[balzee]

Thanks for u r reply....and u r correct multiplying gains of each converter to get the o.p voltage of cascaded converters is not good....so could u specify any
formulae that can be used to calculate o.p voltage and effective resistance of buck&buck-boost cascade connection.....or please specify any link or document that mentions that.....

 

[jumpjack]

Digging up this old thread because I have a DC/DC cascading issue too:
I need a DC/DC converter capable of turning 30-70V into 60V constant... but with up to 100A output! (It's for an electrich vehicle).
I can't find any suitable ready-made circuit for such an application.
Can small DC/DC converters be connected in series and in parallel to reach high currents and voltages?

The use?
Converting the variable voltage of a supercapacitor into the constant voltage needed by the vehicle.

 

[FvM]

Efficiency objectives suggest a synchronous buck-boost topology:
half-bridge, inductor, half-bridge

For low current ripple, multi-phase might be an option.

I don't believe that you'll find off-the-shelf converters with suitable specification.

 

[jumpjack]

I found something very interesting from Linear Technologies!
https://www.edaboard.com/threads/295163/

 

[FvM]

Yes, I'm suggesting the buck-boost toplogy that LT8705 is designed for. I assume that the gate drivers are not well suited for 100 A FETs, so you would need to supplement it by respective strong gate drivers.

 

[jumpjack]

Yes, I'm suggesting the buck-boost toplogy that LT8705 is designed for. I assume that the gate drivers are not well suited for 100 A FETs, so you would need to supplement it by respective strong gate drivers.

Could you please be more specific?
I don't understand from datasheet if, to reach 100A, I need multiple DC1924 developer boards in parallel, or if adding multiple mosfets in parallel is enough.
I also can't understand if the board and/or the chip are bidirectional as I need: if I get 60V from 3V supercaps, then how do I send 60V back to supercap to recharge it when it is depleted? Do I need two DC1924, or can I switch it using a microcontroller?
Finally, how can I change the demo board from 48V to 60V?

 

[FvM]

If I understand right, the evaluation board is designed for 5A. Handling 100 A by paralleling 20 boards doesn't sound like a practical suggestion. I was assuming high current FETs (may be multiple FETs in parallel) and boost gate drivers.

It's the first time that you mention bidirectional converter operation. Although the buck-boost topology is well suited for it, LT8705 seems not to be prepared for it. I'm neither aware of another controller chip ready to use as bidirectional converter. I presume, you need to design your own controller that implements the respective control algorithms.