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DC-DC converter and his capacitor filters

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jano2358

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hi!

I'm making a buck-boost converter with three parts: +12V, +5V and -12V (all at 1A), right now it's working but I want to increase the efficiency, particularly in the negative side where the measurements show 84% at 1A and the IC wants to rise up to 100ºC+
in this one, for the output filter I choose a parallel of three low esr tantalum (22u) and two ceramic caps (10u X5R, which becomes 3u for 12V)
in the datasheet and simulators the suggested compensation network is type III in case of ceramic and type II in case of tantalum, which one should I use?
it's a good choice to mix capacitor types at the output? I did that to have more current capacity and save some cost
I have 43% of duty cycle in the coil, it's is a good number? it's possible to cut this a bit somehow?
Thanks in advance!
 

it's a good choice to mix capacitor types at the output?

Hi jano2358

using different kind of capacitors in parallel together is a regular method . for instance film capacitors and electrolytic capacitors .

All the matter is , decreasing ESL and ESR . as a suggestion , try to read about Y2X2 capacitors .


Best Wishes
Goldsmith
 

Hi! goldsmith

Thanks for the suggestion, but the film capacitors I found are much more expensive (50x), probably because all the design is smd.
I managed to have a decent temperature now in the converter, the problem was not the power dissipated by the IC but the inductor, seems like we where reaching saturation, luckily when I buyed the inductors I choose I also brought some others for test purposes and one with a higher saturation current can stand in the regime (75º@1A)
Also I make new simulations and seems like 84% for a negative buck-boost is a good figure and the simulator also says 43% will be my duty cycle, they also say I will not be able to draw 1A from this chip :)
I still have some questions tho, they recommend about 20k bandwidth for 610k of fsw, why this numbers? I put a network calculated with 95k of bandwidth and when connecting/disconnecting the load seems stable, can this be a problem somehow?
 

Hi
about capacitors that was an example . i wanted to tell you you most consider equivalent circuit of a real capacitor (not an ideal one ) . then you'll face with several issues e.g ESR or ESL or leakage . you must try to reduce em . how ? several different ways are available . one of them is making some capacitors in parallel together ( electrolytic and mkt or polyester caps in parallel together or ... ) it would do the job .
I put a network calculated with 95k of bandwidth and when connecting/disconnecting the load seems stable, can this be a problem somehow?
it can cause some problem in different loads . you tested only one type of load isn't it ? a safe design isn't walking on borders !

Best Wishes
Goldsmith
 

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