How to plan a multi-phase switch-mode regulator? - Page 1
Forum section: electronics / projects

author: technix
author_rank: Super Contributor
timestamp: October 09, 2017, 05:32:29 am
content: I have two projects that may need a multi-phase buck converter. Here are some basic questions about planning them:1. How to calculate the required phase count based on input, output and MOSFET current? For example can I generate 10V/5A from a 20V/3.2A source using a 2-phase or 3-phase converter and 25V/8A MOSFETs? Is there anything else I need to be aware of?2. How to implement voltage-mode or current-mode control? How to link those to the control loop? How does the multi-phase control scheme affect those?3. How to properly implement high side current sensing with normal rail-to-rail op amps like TLC2274? The last time I did it it didn’t work.One of the projects is a network-controlled soldering station with proper PID temperature control loop. The heater element have a cold resistance of about 2 ohms and an built in thermal couple.

author: T3sl4co1l
author_rank: Super Contributor
timestamp: October 09, 2017, 12:21:41 pm
content: 1. Arbitrary. You'd have to complete a design for each number of phases you're testing, and compare costs. The savings is in capacitors and filtering, and maybe knock-on savings from higher quantities of other components; the costs are in more components, especially components that may not be necessary for the single phase version (like a separate current sense amp).2. Same as any current mode circuit: outer voltage loop, inner current loop. Each phase is its own independent (except for the synchronous clock), voltage-in, current-out (transconductance) amplifier. Simply connect them in parallel and there you go!3. Diff amp sensing is tricky because you need resistors matched at least as well as the CMRR you're expecting. You also need an amp that's immune to the high frequency ripple that appears on the resistor, and that's fast enough to amplify the current ramp waveform with good fidelity and low time delay / phase shift.I don't know if that helps answer why your TLC2274 didn't work.Tim

author: technix
author_rank: Super Contributor
timestamp: October 09, 2017, 12:59:13 pm
content: Quote from: T3sl4co1l on October 09, 2017, 12:21:41 pmThere should be a minimum phase count for scenario where continuous output current is greater than input current (or not?)

author: dmills
author_rank: Super Contributor
timestamp: October 09, 2017, 02:55:58 pm
content: Not really, and I would note that more phases is not automatically more efficient, particularly at light loads where switching losses can dominate.Current mode is MUCH less of a pain in the arse the voltage mode for this sort of thing, you usually pretty much just tie the 'comp' pins together, diddle the current loop gain to be 1/Nphases for each converter, set up the clocks correctly and it just works.Well, there is a little more to it then that, but you get the idea....Regards, Dan.

author: Siwastaja
author_rank: Super Contributor
timestamp: October 09, 2017, 04:12:27 pm
content: 1:Imagine a synchronous buck.Draw the FET drive waveform for each phase, like this, with relative current:1 phase only:3330000000000000Sum taken from the input caps:33300000000000003 phases:111000000000000000000111000000000000000000111000Sum taken from the input caps:1110011100111000Then you'll notice that the lower the duty cycle, the more discontinuous the input current is. The input capacitor is only delivering energy in short pulses, hence it needs higher ripple rating. (The input caps in the buck converter are often much more difficult than people expect.)So, if you drop 12V to 1V, you'd need 12 phases to completely fill in the voids in the input capacitor waveform! OTOH, if you drop 12V to 9V, adding a lot of phases offers much smaller improvement.This is why modern motherboard CPU buck converters need to have so many phases - a lot of power, very low output resistance, and the input:output ratio is not very good. For a 1V, 100A output, the input cap (at 12V) would need to provide 100A pulses, producing huge ESR losses and, most likely, due to parasitic inductance, it could simply not deliver the required powerat all. But divide it to 10 phases, and each takes only 10A pulses from the same capacitor bank, and the pulses form an almost continuous flow.2: Completely forget the classical "voltage mode" where measured voltage controls the PWM through an error amp. It makes absolutely no sense to implement in 2017, is clearly worse in all aspects. Sadly, it's still all over the place. The issue is that the inductor stores energy beyond the switching cycles, so at the beginning of each new cycle, it has an energy state. Voltage feedback cannot know this state at all, and you can only try to do your best to compensate, basically forming a PID controller over this physical phenomenon.But instead of controlling a difficult phenomenon, don't create that situation at all!In the very first "switch mode basics 101", you see the inductor current triangle wave. Then comes ten courses about feedback loops, compensation etc. how you try to achieve it.Guess what? With current mode, youdirectlycreate that triangle wave, and it's extremely simple to do so! Just measure the current quickly - it's easy today - and stop increasing it when it's where you want it to be!You still need to understand the basic concepts of feedback loops, compensation, i.e., PID parameters to build the outer voltage control loop - but the parameters will now be much easier to get right, and even if you don't get them right, it won'tblow up-- especially in a software-based controller, it's totally trivial to write hard limits for the current setpoint: if(current > 10A) current = 10A. A current mode controller will automatically respect this current, and the current control willneverbe unstable, even if your voltage control loop is.

author: technix
author_rank: Super Contributor
timestamp: October 09, 2017, 04:40:09 pm
content: Quote from: dmills on October 09, 2017, 02:55:58 pmI may need the phases for current handling. The input supply rail have a continuous rating of 3.2A but I may need up to 5A at lower voltage.

author: technix
author_rank: Super Contributor
timestamp: October 09, 2017, 05:00:17 pm
content: Quote from: Siwastaja on October 09, 2017, 04:12:27 pmIs there any rules of thumb of phases versus voltage or current ratio? Since I am using a software-driven SMPS anyway I might as well turn phases on or off as needed.Quote from: Siwastaja on October 09, 2017, 04:12:27 pmHow to nest the control loops? There are four inputs per phase: inductor current (each phase,) total output current, total output voltage and a shared auxiliary feedback input (can be used to implement remote sensing for a PSU, or thermal couple input for a soldering station. I am reusing the same basic board design for two projects, swapping a plug-in module and part of the firmware.) For the PSU it need to provide CV or CC output, maybe making use of the remote sensing input; for the soldering station application it need to feedback on the temperature sensor while the output current is limited not to damage the heating element.