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LAN_403

Goodchild

Fri Jan 06 2012, 06:07AM

Registered Member #2292 Joined: Fri Aug 14 2009, 05:33PM
Location: The Wild West AKA Arizona
Posts: 795

Gregory wrote ...

Eric

I tryed to make some simulation in LTspice with a RC integrator and I get only sheet, hehehe.

I tried a simples quantizer/comparator (amp with ramp at +input and feedback at -input) , the output of the comparator driven the switch and at the same output the RC that summing at the feedback (-input).

Check for silly errors, it could just be that it's phased wrong.

Dr. ISOTOP

Fri Jan 06 2012, 06:52AM

Registered Member #2919 Joined: Fri Jun 11 2010, 06:30PM
Location: Cambridge, MA
Posts: 652

I *think* (not sure on this) if your comparator wasn't set up with hysteresis, the controller will not oscillate.

teravolt wrote ...

hi Dr. Isotop that's a nice package lots of stuff on it. it seems that arm is prity popular to. I like MBED and Arduino because there is a large base of users. when it comes to software I'm prity much a newbie but when it comes to hardware I think of myself as not to bad. micros have a lot of potential.

Gregory, I kind of have the idea but do you have a schematic

The Mbed is cute and all, but I don't trust their library to be performance-optimized. Overhead on a micro is a no-no, IMHO.
Plus, it gets a tad costly after you kill two or three

Gregory

Fri Jan 06 2012, 04:12PM

Registered Member #2922 Joined: Sun Jun 13 2010, 12:08AM
Location:
Posts: 226

I *think* (not sure on this) if your comparator wasn't set up with hysteresis, the controller will not oscillate.

,

I tried some simple histerese using a RC delayer at the quantizer output, and a only get more ripple, normal. The histeresys is here only to make the thing not oscilate and make the comparation more stable. I think the problem is the RC constant that I need to calculate

Gregory, I kind of have the idea but do you have a schematic

Goodchild

Fri Jan 06 2012, 06:17PM

Registered Member #2292 Joined: Fri Aug 14 2009, 05:33PM
Location: The Wild West AKA Arizona
Posts: 795

Gregory wrote ...

I *think* (not sure on this) if your comparator wasn't set up with hysteresis, the controller will not oscillate.

Gregory, I kind of have the idea but do you have a schematic

Sounds like your not feeding enough amplitude from the RC back onto the feedback, the RC ripple must be larger than the ripple from the feedback for the RC to take effect.

Dr. ISOTOP

Sat Jan 07 2012, 07:56AM

Registered Member #2919 Joined: Fri Jun 11 2010, 06:30PM
Location: Cambridge, MA
Posts: 652

I'm not sure I understand how the internal integrator works; since you're not taking feedback directly from the bus voltage, won't it fail to regulate sometimes?
Also, I simulated the LTC1041 and it seems to work decently:

For some reason the LTC1041 doesn't like it when the divider resistors are too high (at least the simulation says that).

Goodchild

Sat Jan 07 2012, 03:42PM

Registered Member #2292 Joined: Fri Aug 14 2009, 05:33PM
Location: The Wild West AKA Arizona
Posts: 795

Well yes and no; I do sample from the bus voltage, but not the bus voltage of the buck, rather the output of the buck which also happens to be the bus of the inverter.

Your using a bang bang controller there, that is a totally different story than what I was talking about with my internal integration. I use a single comparator and not a full BB controller. It also has the integration set internal using the delta pin.

As a result switching will be determined mainly by the delta set point and the sampling rate. I would suggest reading the datasheet, all of this info and more are already in it.

Gregory

Sat Jan 07 2012, 03:43PM

Registered Member #2922 Joined: Sun Jun 13 2010, 12:08AM
Location:
Posts: 226

For some reason the LTC1041 doesn't like it when the divider resistors are too high (at least the simulation says that).

I think that with a higher resistor values you get less high frequency gain. Try increase the resistor and use a feed-foward capacitor at your divider, it will "derivate" and will increase the gain for the high frequency (ripple). Some 270p capacitor in parallel with the upper resistor of the divider will make the job.

Dr. ISOTOP

Sun Jan 08 2012, 01:35AM

Registered Member #2919 Joined: Fri Jun 11 2010, 06:30PM
Location: Cambridge, MA
Posts: 652

Goodchild wrote ...

Well yes and no; I do sample from the bus voltage, but not the bus voltage of the buck, rather the output of the buck which also happens to be the bus of the inverter.

Oops, that's what I meant by "bus voltage".

Gregory wrote ...

For some reason the LTC1041 doesn't like it when the divider resistors are too high (at least the simulation says that).

And what do you know, the feed-forward capacitor works magic

Though not in the way I expected it too.
Turns out I missed the bit on the datasheet about charging the input capacitor in the LTC1041

Mmm I'm liking that waveform...hopefully it works in real life!

Gregory

Sun Jan 08 2012, 02:15AM

Registered Member #2922 Joined: Sun Jun 13 2010, 12:08AM
Location:
Posts: 226

I think that you can increase even more the resistances of the divider, I think 100k -> 1k is a better value.

Dr. ISOTOP

Sun Jan 08 2012, 03:39AM

Registered Member #2919 Joined: Fri Jun 11 2010, 06:30PM
Location: Cambridge, MA
Posts: 652

Gregory wrote ...

I think that you can increase even more the resistances of the divider, I think 100k -> 1k is a better value.

The LTC1041 datasheet says 10K is the highest input resistance at which the input capacitor can fully charge. It still works at 600K:10K, but the voltage at the feedback pin starts misbehaving. The datasheet says this can be worked around with a bypass cap, but I'm afraid the integrating effect of the capacitor will give me poor transient response.
10K@300V is 9W dissipated in the divider, which is a bit hot but is only a tiny fraction of the output power.

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