Buck-Boost Switch relations

[autx790]

Hello,

I'm trying to do a digitally controlled 4 switch Buck-Boost converter. I've done both a buck and a boost, but I'm not quite sure how the pwm signals relate with respect to the buck control side and the boost control side. Do you operate the controls such that if you're in buck operations, you run the control for the buck switches as normal and with the boost side switches, you just drive the highside on 100% and leave the low side off. And the same concept when the converter is to operate in boost mode. When the input is to be the same as the output, do you just drive the two high side switches full on, and the low side off?

 

[mtwieg]

Linear technology recently started making control ICs for this topology. Reading through their documentation should answer your questions. For example: http://cds.linear.com/docs/Datasheet/3780fe.pdf

They use three modes of operation: buck, boost, and buckboost. For switching between the three modes they have some sophisticated state machine and hysteresis. This is to prevent what they call "motorboating," which is when the device tries to rapidly switch between modes of operation.

 

[autx790]

Thanks, I've looked at this line of products but it appears that you dont have control over the output current. You can limit it but I need to be able to regulate it while also fixing the output voltage. I'm trying to build a max power point tracking solar panel battery charger. I'm currently trying to use National/TI's solar magic line of products but was also looking to replace these and go to a uController based solution. So my problem would be how to code the 4 PWM signals. I know how to do it in buck and boost, but not sure how the 4 relate in a buck-boost or if their independent and you either work in buck or in boost.

 

[mtwieg]

Oh, you didn't say you were regulating current, not output voltage. Are you regulating input or output current? Anyways, the same general rules should still apply. The operating mode of the converter will still depend on the input and output voltages, not the current, and you'll use the same three modes of operation. The output current, and your feedback loop, will determine duty cycle. The feedback loop design will be tricky because the transfer function of the converter will change greatly between the three modes of operation.

 

[autx790]

Thanks, sorry if i wasn't clear. I will be using this to charge a battery so i need a fixed voltage on the output at 16.8V and then regulate the current to maximize the solar panel output. My trouble is understaning the pwm signals. I'll have four...2 for the buck switches and 2 for the boost. So when the input is higher than the output, i know how to control a buck converter, but I dont quite understand what to do with the two switches on the output side of the converter (the boost switches). During a buck mode of operation, do you just drive the boost highside mosfet to full on and operate the converter as a buck, or do you still have pulses going to the boost side switches?

 

[mtwieg]

Right, when operating in either buck or boost mode you only operate one of the FET pairs at a time, while the high side FET of the other bridge is always on. The only time you PWM all four switches at once is when you are in buckboost mode.

Also keep in mind that you can't simultaneously regulate both voltage and current for a given source/load impedance. You do one or the other. Normally in battery charging you first regulate output current, then as it reaches its capacity you switch to constant voltage output to "top it off."