Current Regulation on a Buck DC Converter 1

[joelg11]

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!

 

[Warpspeed]

o/k, I was assuming you would be measuring the average output current with a conventional single proportional/integral feedback loop. The transient step response may not be as good, as with current mode control, but you would have far fewer noise and stability problems.

 

[Warpspeed]

Your schematic looks fine, but not sure what the 100 nF capacitors C72, C73, C74, and C75 are for ?

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.

 

[sreid]

Some thoughs:

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.

 

[joelg11]

sreid -

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.

 

[joelg11]

hgldr (or anyone else)-

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

 

[Warpspeed]

I would steer clear of any really obscure four phase regulator chips, and stick with a combination of very common easily sourced simple parts.

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.

 

[DHambley]

I noticed that the PWM IC that you're using doesn't have the + pin of the error amp available so you can't make a unity gain follower if the + pin is missing. You do have to watch your voltage regulation though. If it is an integrator the part with the highest internal 2.5V ref will have the highest error amp output and thus, the highest current. What are all those 0.1uf caps on the NAND gates?

 

[Warpspeed]

Most of these PWM chips now use a transconductance error amplifier with a quite high output impedance.
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.