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LAN_403

Gregory

Tue Jan 03 2012, 11:29PM

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

Eric pointed out on my blog, too much capacitance causes an impedance mismatch with the coil, reducing the effectiveness of regulation.

This is one thing that I was saying.

I was thinking that it may be possible to measure the vout and feed it back through a op amp circuit to my VCO

Yeah! it will work, but I THINK that you need some derivative and integrative therm to get ir right. The drsstc bridge has very fast transients and the modulator need a very fast response. Compensation on feedback, like a simple capacitor, can be see as a small integral/derivative therm that will make your system get a very better response.

Dr. ISOTOP

Wed Jan 04 2012, 12:31AM

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

How does the math for the impedance matching work?
In particular, how do you calculate the input impedance of the bridge?

Gregory

Wed Jan 04 2012, 03:09AM

Registered Member #2922 Joined: Sun Jun 13 2010, 12:08AM
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In particular, how do you calculate the input impedance of the bridge?

I think that the impedance isn't of the bridge, because she aways is connecting the primary LC to the modulator output so she "isn't here as a load". So, I THINK that the impedance is the impedance of the primary tank side SQRT(L / C).

Goodchild

Wed Jan 04 2012, 03:17AM

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

You know my delta modulator consist of a single comparator and a single 5V supply, there is really no need to put a 100MHz MCU in there.

All you need for a good bang controller is a comparator with a small amount of hysteresis (say 5mV to 20mV). You feed the ref ramp into one input and the output of your feedback divider into the other and they are internal subtracted and squared off for the gate drive. The speed of integration (switching speed) is controlled simply by the LC (mainly the inductance of the L and ESR of the cap), load and delays in the control circuitry including the hysteric window on the comparator. Switching speed can also be manipulated by the addition of a feed-forword capacitor and series resistor before the HV feedback divider.

My system is a little fancier in that it internally generates the integration making switching speed independent of the output filter parasitics and the load. But it still only used one comparator

The use of a fast precision comparator is a must, any switching error in the comparator can result in regulation error and unwanted operation. The LM311 is NOT a precision comparator

It seems like a lot of EE's out there stay away from bang bang control because of the dependance on temp ESR and load dynamics controlling switching speed. But it's a really awesome control topology because of it's simplisity and ability to go from 0 to 100% duty cycle giving it excellent transieant response. No control loop compensation has to be added ether which makes it just all the more awesome.

Dr. ISOTOP

Wed Jan 04 2012, 04:47AM

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

Goodchild wrote ...

You know my delta modulator consist of a single comparator and a single 5V supply, there is really no need to put a 100MHz MCU in there.

I briefly thought about implementing the controller this way, but decided against it. I suppose my excuse for using a fancy micro in the controller is so I can implement "optional" features like overcurrent detection without reworking the entire circuit. I really want to avoid the fault condition where the bridge fails short, which will take out the buck and god knows what other bits of silicon down the line.

Goodchild

Wed Jan 04 2012, 03:12PM

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

Dr. Isotop wrote ...

Goodchild wrote ...

You know my delta modulator consist of a single comparator and a single 5V supply, there is really no need to put a 100MHz MCU in there.

Well even with a MCU you would still have to add some sort of extra circuit to sense the current in the first place. Even at 100MHz and interrupts your OCD response time will probably still be in the hundreds of u second range unless your uC spends all it's time checking the OCD. A comparator is really the better way to go for speed, as you want to get that silicon shut off in <10uS if possible. MBED also runs a rater complex firmware and library architecture that will eat up that 100MHz rather fast making it not much faster than a 8bit 16MHz uC.

In my opinion 100MHz uC is not the right way to go with this sort of controller, it's will be a lot a slower than a single dedicated comparator and a lot more susceptible to noise and such.

If you wanted a configurable option that was fast and had better noise immunity you should look into a FPGA or CPLD. There is a reason uCs are not widely used for SMPS control.

Eric

teravolt

Wed Jan 04 2012, 04:34PM

Registered Member #195 Joined: Fri Feb 17 2006, 08:27PM
Location: Berkeley, ca.
Posts: 1111

I don't know if any body is interested but I found this IC that I stumbled on and thought that this may be pertinent

Goodchild

Wed Jan 04 2012, 06:26PM

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

teravolt wrote ...

I don't know if any body is interested but I found this IC that I stumbled on and thought that this may be pertinent

teravolt that is an awesome little chip, it dose bang bang control a little bit differently than normal seems like it samples rather than running all the time. From my initial glance at the datasheet it looks like all timing is done internally rather than using external integration, as I said earlier this is a wonderful way to improve noise and load immunity on the switching times.

I may have to give one of these a try some time in the future.

Gregory

Wed Jan 04 2012, 06:47PM

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

ou know my delta modulator consist of a single comparator and a single 5V supply

So, are you using a single histeresy comparator? only a comparator with the -input at the feedback and the +input at the reference ramp with some histeresy at comparation? This is my original desing that worked very well.

Goodchild

Wed Jan 04 2012, 07:45PM

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

Gregory wrote ...

ou know my delta modulator consist of a single comparator and a single 5V supply

Basically yep that's it. I however also sum my own integrated ramp back onto the feedback so that the integration is set internally rather than by external LC.

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