Controller for bi-dir synchronous buck with current mode? - Page 1
Forum section: electronics / projects

author: Siwastaja
author_rank: Super Contributor
timestamp: August 24, 2015, 09:14:17 am
content: Hi,Late last year, I designed a non-dissipative battery cell cycle test equipment. It ended up working just fine, even though it was my first real DC/DC production project, but I really had to do it the hard way.I simply couldn't find a proper controller IC, so I had to do the control from scratch.I ended up designing a self-oscillating current-mode controller with slow secondary voltage control on MCU that exists anyway for data logging and control. This is just fine for batteries, but the BOM is really long and the controller is still lacking protection features, and I had to kludge some in at a later point.I use a high-side shunt amplifier to give a measured current signal. Then I use dual fast comparator to compare to two reference levels, positive and negative overcurrent. The two overcurrent signals and two enable signal (buck (charge) and boost (discharge) enable) go to a quad NOR gate that provide synchronous control signals for high and low side switches. A bootstrap MOSFET driver follows.There's nothing really wrong here, it works just fine; the voltage regulation response is hugely slow (done by MCU by changing the current setpoint, and the current setpoints are made with PWM RC DAC) - but that's fine with battery charging and discharging.So the architecture measures the actual inductor current waveform and keeps the inductor current ripple constant (using the comparator hysteresis). This is quite nice. If anyone is interested, here's the (almost full) schematic:http://www.students.tut.fi/~alhonena/zyklon.pngAlthough this works fine for the application, the limitations are quite severe:- It's limited to single phase as it's self oscillating - syncing would complicate the BOM even more; it would then be a full DC/DC controller implementation, and there is a reason they are IC's.- Current shunt amplifier, comparator speed etc. limit the switching frequency to < 100 kHz. I would like to go a bit higher for obvious reasons.- BOM is too big - in addition to the MCU and current shunt amplifier that are needed in any case, there are: a few opamps, comparator, logic gates, and dozens of passives, mostly resistors.- Having quick voltage control - for any other application than battery testing - would complicate the BOM even more. Sure, that would be doable with an extra dual comparator and some extra logic!So I REALLY wanted to use a DC/DC controller IC with the following features:- Syncronous bidirectional buck- Proper current limit -- in two directions!- Multiphase operation -- preferably multiple phases (2 is just fine) in one chip, or at least a sync pin.- Pulse by pulse current limiting, of course.But I couldn't find any. Was I just blind?The good old TL494 has two error amplifiers that allow easy CC-CV (current limit and voltage setting). Why I'm not seeing the same in modern chips - only crude current limits to protect switches from capacitance and shorts? Or am I looking this in a wrong way?Another thing is that I need to sense negative currents - the controller needs to be able to run in the boost mode. If this just means forcing CCM, it is just fine. Bidirectional operation is childishly easy and comes naturally from a synchronous buck on a lab bench with only a bootstrap FET driver with a single input and an adjustable PWM function generator.Any suggestions for integrated controllers are welcome.

author: dmills
author_rank: Super Contributor
timestamp: August 24, 2015, 10:01:19 am
content: Linear tech IIRC have some current mode controller parts that will do this, but it is never a feature that they make a big deal out of, and you have to strap the part into forced CCM to make it work (Also watch the input range of the current sense pins).Sort of a pity that so few parts are well specified in this area, because as you say it is more or less inherent in the architecture of a sync buck converter and is sometimes just what you need.The other approach here is a phase shift half bridge with sync rectification, TI do the appropriate sand that can be run either way at reasonable bandwidth (Their example is a slow drain modulator for a radio transmitter).Regards, Dan.

author: MagicSmoker
author_rank: Super Contributor
timestamp: August 24, 2015, 11:52:47 am
content: Quote from: dmills on August 24, 2015, 10:01:19 amWhile a synchronous buck (or boost) converter is inherently capable of bidirectional operation, one huge problem that a lot of the academic papers overlook or ignore is that the same frequency compensation can't be used for both modes (ie - buck and boost). More specifically, the frequency response needs to be rolled off well before the right half plane zero that is present in the boost (and buck-boost) converter, which cripples the transient response of the buck.Also, synchronous converters always operate in CCM because they allow the inductor current to reverse during the off time of the main switch (in other words, during the on time of the sync. switch). This doesn't usually cause any problems analytically, but it does reduce efficiency at light load.

author: johansen
author_rank: Super Contributor
timestamp: August 24, 2015, 02:29:49 pm
content: Quote from: MagicSmoker on August 24, 2015, 11:52:47 amno i'm pretty sure the frequency responce is the same regardless of the direction the current is flowing..unless you are operating in discontinuous boost mode.forced CCM shouldn't have this problem.also yes i've been looking for the same thing OP is, but for a 4 switch buck converter.