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balzee

Newbie level 3
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...

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

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.....

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).
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.

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.

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.

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?

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.

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