Current Regulation on a Buck DC Converter
Current Regulation on a Buck DC Converter
(OP)
I'm an entry-level engineer, and I have a design question for the forum:
I'm designing a multiphase buck DC/DC converter which will convert roughly 65VDC to 52VDC at up to 150A. I'm using 4 phases, so inductors, etc will be designed to about 40A/phase, MOSFETS will be paralleled for less (about 13A)current, but that is unimportant.
The crucial point where this design deviates from the common design is that it is essential that we have current control rather than just a current limit. An external analog comparator signal is to dictate the current setpoint. I'm really wanting to use two Linear LTC3860 controllers, but again they only provide a current limit. Another option is to do a custom design using a PIC microcontroller, but I still need help with how to use the current sense circuit to control the PWM to provide current regulation.
Anyone have any ideas? I attached a rough schematic of one of the stages, with the current sense signals going off-sheet to my theoretical controller.
Thanks for your help!
I'm designing a multiphase buck DC/DC converter which will convert roughly 65VDC to 52VDC at up to 150A. I'm using 4 phases, so inductors, etc will be designed to about 40A/phase, MOSFETS will be paralleled for less (about 13A)current, but that is unimportant.
The crucial point where this design deviates from the common design is that it is essential that we have current control rather than just a current limit. An external analog comparator signal is to dictate the current setpoint. I'm really wanting to use two Linear LTC3860 controllers, but again they only provide a current limit. Another option is to do a custom design using a PIC microcontroller, but I still need help with how to use the current sense circuit to control the PWM to provide current regulation.
Anyone have any ideas? I attached a rough schematic of one of the stages, with the current sense signals going off-sheet to my theoretical controller.
Thanks for your help!





RE: Current Regulation on a Buck DC Converter
150A power supplies are a tall order for a fresh engineer. Good luck.
RE: Current Regulation on a Buck DC Converter
RE: Current Regulation on a Buck DC Converter
Basically this switches on and off as the current through your buck inductor rises and falls between two set thresholds.
Suppose you wanted to set 40 amps average constant load current with 4 amps peak to peak ripple (in one phase). When the current through the flywheel diode and inductor falls to 38 amps, you turn on your FETs.
When the current through the FETs and inductor ramp up to 42 amps, the FETs switch off. The whole thing self oscillates to provide a constant amplitude peak to peak output ripple symmetrical around the desired average dc output current.
Current limiting is very fast and inherent in the strategy. Only difficulty with this might be that the switching frequency will vary considerably with load variations, and that feature may be objectionable.
But ripple regulators are very simple, respond very quickly to sudden line/load voltage changes and offer an interesting alternative.
It probably offers the fastest responding constant current switching supply you can build, because the speed of response is limited only by the stored inductive energy.
A second LC filter stage can reduce the actual final output ripple to well below what the ripple regulator stage itself needs to function.
How to do this with multiple phases is something I have not really thought through.
But it may be possible to phase lock four in quadrature by very slowly shifting the current sensing threshold of three of the phases in the manner of a phase locked loop.
The whole thing would probably briefly break out of quadrature lock to respond to sudden step load changes, but I cannot see that being a serious disadvantage compared to the advantage of speed of response to said step load change.
RE: Current Regulation on a Buck DC Converter
RE: Current Regulation on a Buck DC Converter
Warpspeed, great response - I'm looking at ripple regulation circuits right now. This is an unusual design as well - because output current ripple is not the concern so much as ripple current seen by the source (think battery or fuel cell - current source). So a follow up question is - and maybe this is a dumb question, but is current ripple regulation the same on the input and output of a buck converter? Will regulating the output current to +/-2% also have the same effect on the input side, since the converter is essentially pulling current through to the load?
RE: Current Regulation on a Buck DC Converter
This would be easy if it weren't for the current level.
First off, you don't use the current limiting function of a switchmode regulator chip to regulate current - it's strictly for protecting the switch. You use the error amplifier for this job. Whether you regulate voltage, current or some combination depends on where you get the error signal from in the first place. So if you want to regulate output current you sample the current; if you want to regulate output voltage then, yes, you sample the voltage. Either use a shunt (with a high side amplifier or instrumentation amp, depending on the accuracy needed) or a Hall effect device. If, for example, the error amplifier uses a 2.5V reference then you need to amplify the voltage produced by the shunt (or Hall effect device) to 2.5V when the maximum current is reached.
The buck converter is an intrinsically stable topology so nothing exotic needs to be done to close the loop.
Nothing particularly complicated about multiphasing but you would do well to read this most excellent article on them:
http://www
It explains how input and output ripple is affected by the number of phases.
I STRONGLY suggest you build a single phase prototype first. Also, expect to require 4oz Cu (140uM) FR-4 board to carry this sort of current.
Another name for what Warpspeed suggested is called a "bang-bang" regulator, btw.
NB: I did not even look at your schematic on the assumption it's a hopeless mess filled with errors ;)
RE: Current Regulation on a Buck DC Converter
That is the usual reason for adopting the multi phase approach of having several buck regulators running in parallel out of phase. The peak input ripple will be lower, and of a higher frequency which is much easier to filter.
Far less total input filter capacitance and inductance will be required.
Input filtering of any buck regulator is going to present a serious problem to overcome where conducted emissions on the supply need to be taken into account.
Another topology for you to investigate is the coupled CUK converter. This has the unique property that both input and output ripple can be made to cancel.
Smoker is right, first try building a scaled down model, or one phase first. You will discover that the problems multiply exponentially with the current level. Four forty amp modules, no matter what topology you end up with, is a far more manageable approach.
RE: Current Regulation on a Buck DC Converter
Allegro makes high-current Hall sensors with 120kHz BW (ACS156). If your switching Freq is < ~15kHz these might work. Otherwise, use current-sense transformers (and limit the duty cycle so that the CST's can reset).
Those are just a few of hundreds of tips I can come up with.
RE: Current Regulation on a Buck DC Converter
The UC3842A datasheet is lacking in application detail, so I'll look around for an alternative.
Side question - Any complication to using the synchronous FET configuration with separate drivers? I assume I'd just route the PWM signal to the high side driver, and use an inverting buffer of sorts going to the low side driver. Assuming the timing worked out, I'd never have both on at the same time. Alternative would be using a synchrous driver like the LTC4449, haven't found one with the power ratings yet though...for reference I currently have the MIC4421A tabbed for use.
I'm learning a ton on this project already, thanks all for the comments.
RE: Current Regulation on a Buck DC Converter
As your output voltage is fairly high, diode voltage drop will not form a significant part of the total loss.
Going from 65v to 52v, the off time might be something like only 20% and at 150 amps dc, and with (say) one volt diode drop, that is only maybe 30 watts of diode conduction loss power loss out of 7.8kW of load power.
Just not worth trying to reduce that 30 watt conduction loss with another 150 amps worth of expensive FETS and drivers on the low side.
RE: Current Regulation on a Buck DC Converter
RE: Current Regulation on a Buck DC Converter
You could still use an active rectifier, but you will gain far more efficiency wise, by using more FETS on the high side switch than on the diode side, with your particular design requirements.
I would build up a low power prototype first, for maybe ten amps, and verify your design equations, and test that your control strategy and conducted emissions meets all your goals.
Then scale it up for forty amps. You will discover that physical layout, thermal design, and current sharing is just as important as the schematic and parts selection.
I would then combine four forty amp modules in parallel, rather than try to scale it up even further.
RE: Current Regulation on a Buck DC Converter
Basic question. The datasheet to the UC3842B does show how to use an external clock synchronization, but I am having trouble conceptualizing putting 4 of these 90deg out of phase with each other. In the attached datasheet, Figs 21 and 22 are pertinent.
Does the internal oscillator of the CMC receive this clock input as logic input and only go high if the clk is high, etc.? Again, the phasing is a little beyond my reach right now.
Thanks for the help!
RE: Current Regulation on a Buck DC Converter
RE: Current Regulation on a Buck DC Converter
Each of your 3842,s is then synchronized at one fourth the master clock frequency in ninety degree quadrature.
A two stage twisted ring (Johnson) counter could be used to divide your master clock by four, and the Q and not Q outputs provide four output phases in exact phase quadrature.
Small value capacitors could differentiate the edges, and provide the required narrow synchronizing spikes to firmly lock each 3842 in quadrature.
RE: Current Regulation on a Buck DC Converter
What warpspeed wrote about pysical layout is important. Make an estimate for every parasitic inductance in the power path. Make the copper paths wide, no skinny, 20 mil wide inductors, er I mean traces. In your spice model for example, put 15 or 20 nH into your drain and source and watch the effect on the MOSFET power during turn on/off.
RE: Current Regulation on a Buck DC Converter
Still ahead are dialing in the error amplifier ckt, and reading up on slope compensation for these UC3842s.
Thanks again!
RE: Current Regulation on a Buck DC Converter
But the buck topology does not require slope compensation for stability.
RE: Current Regulation on a Buck DC Converter
http://focus.ti.com/lit/an/slua101/slua101.pdf
where it states the case for bucks over 50% Duty cycle.
RE: Current Regulation on a Buck DC Converter
RE: Current Regulation on a Buck DC Converter
You may also require an additional series diode to prevent the negative going edges of your 100Khz synchronizing clocks from putting a one diode drop negative going kink half way up your timing ramp. This ramp must be very clean and noise free, except for the narrow positive going locking spike right at the very top.
RE: Current Regulation on a Buck DC Converter
1) There are four phase controller IC's available.
2) Forget that this is a switching regulator. It's a "Block" that steps down the input to output voltage. Just as a linear regulator would. Then design a "Current Regulator" vs. a "Voltage Regulator"; the feed back signal is Output Current rather than Output Voltage.
RE: Current Regulation on a Buck DC Converter
I've seen some 4-phase controller ICs, but I haven't come across anything in the 100kHz switching range - thus the more custom design. I'd be glad to hear about any ICs in this range that are good for CMC.
RE: Current Regulation on a Buck DC Converter
You said:
"Make only one error ampiflier, based on the voltage loop. The output of this will feed the internal error amplifiers of the CMC ICs which you will set up as unity-gain followers....Now, all of your converters will switch at the same peak current and the input filter will see the lowest ripple possible."
Can I ask why I would make one common EA and not just feed the Vfb signal back to each IC separately? It seems if I designed the compensation loop around the effective RLC of each stage, it would accomplish the same thing. I believe you're saying sum the C and L values of each stage to design one EA, feed it into the EA on the chip, with the compensation pin tied directly to the inverting input of the EA.
Thanks again
RE: Current Regulation on a Buck DC Converter
Nothing worse than being twelve months into volume production, and then being told that the exotic special purpose chip you designed your whole gizmo around is now becoming all but impossible to source.
That sort of problem is not likely to win a circuit designer any glory from senior management.
RE: Current Regulation on a Buck DC Converter
RE: Current Regulation on a Buck DC Converter
The compensation pin in these, can easily be overdriven from an external op amp.
A single external op amp used as the main error amplifier (for all channels), gives you many advantages, especially a much more user friendly input common mode range, and the availability of both inverting and non inverting inputs.