SMPS inverter voltage feedback methods

How about this parts intensive idea? Do it FM radio style :P

Vin>VCO(1Ghz)>xformer>FM demod>Vout isolated

RF Power Detector

xformer small ferrite xformer

VCO Crystek Corporation (Yikes...around $25.00 for ~1Ghz)

Dunno if this has been done before.. Just made it up.. :P

Propagation time ? delta V to delta V response time ? EMI ? Phase response? Nonlinearity ?

D from BC British Columbia Canada.

This would be a good way to go since the output is not isolated from what I can see.

Also:

If you make a bandpass filter that takes the voltage directly from the switches, you can improve the loop responce. It gives you something very like the "D" part for your PID controller.

On Jan 26, 9:44 am, D from BC wrote: [...]

If you go down to much lower frequencies, the idea works quite well. At 1MHz or so, the VCO doesn't take very many parts.

The receiver side can be a PLL that uses a tri-state phase detector built into your PCLD or FPGA or whatever and a second VCO like the first.

Cool... :)

D from BC British Columbia Canada.

Nah. Solenoid on the sensed AC acting against a spring. Throw in a dashpot to damp it if necessary. Connect to a suitable potentiometer via a bellcrank type linkage.

$$$ ... Ouch!

To monitor the waveform? Why not just sample it? Send sampling pulse through toroid xfmr, send some AC there if you can't feed the HV side circuitry locally (toroid xfmr or something), sample, then send sample across another toroid xfmr back to LV side. I use little #43 ferrites for those jobs, sometimes #77. Mostly #43 because there always seems to come a time when someone asks for even more bandwidth later.

Could also use LAN xfmrs. They are often rated 1500VAC but mostly not agency approved for this kind of job. If they don't feature a split bobbin don't use them.

How about an electric heater boiling water to push a piston.

Nah. A pump, waters a plant, plant grows, weight increases, click.

You're making this way too difficult. You can do it with all off-the- shelf parts. Just hardwire a fluke meter to the AC side, then use a webcam on the sensing side connected to a laptop running computer vision software.

Better make it a backlit meter. Oh --and a monkey to change the batteries from time to time.

This is the way I'd like to do the voltage feedback, with a VCO feeding a transformer, and then the PLL circuit, but I think that part is too slow to react, the maximum VCO modulation bandwidth of the LTC6900 is

cheers, Jamie

Is this the same as what Moosefet is talking about with the VCO on the transformer primary? Can the sampling be tied into the PWM signals sent to the SMPS fets? So that the modulation and demodulation is done by these signals, not sure if this would work with variable dutycycle PWM?

cheers, Jamie

The Fluke 45 plugs in the wall. Its display it lit so you don't need the monjey.

The servo feedback needs a bandwith much higher than 60Hz but not higher than the 200KHz. For the AC feedback, you can just use a transformer drivien with the output signal. The DC feedback is just to ensure that you don't have a DC component in the output.

The PWM dutycycle needs to be set each cycle, using the voltage and current feedback, I'm using small inductors so it can change quite rapidly. Would you be able to help me design this modulator/demodulator ready to feed into the 200kHz ADC if I put it in the public domain and pay you or at least send you some chocolates? :D

I already am going to use a bipolar ADC floating with the 120VAC and SPI optocoupled, but its good to have a backup, as I'm not sure if I can get

cheers, Jamie

ha, what about Calibration certification when changing the batteries? Who's going to do that ?

From my reading of it, it isn't but it is worthy of consideration. A balanced modulator and balanced demodulator will bring DC across with no trouble. If you pick the right sort of running frequency, it will be quite insensitive to all the switching noise.

With a bit of trichery, you could get any imbalance between the two sides to come across on the same transformer. The second channel could be phase or an even harmonic.

think Nyquist. no matter how hard you try, you cant organise closed-loop voltage or current control beyond 100kHz. Fsmps/10 is about as fast as you can go.

If its a current-mode controller then you get cycle-by-cycle current limiting, and the voltage EA generates a (slowly varying) current setpoint.

Cheers Terry

That isn't quite true. Depending on the sort of PWM modulator you use, you get two samples per cycle because both edges of the on time can be moved by the input.

This still ends up about the right limit. When you move both edges in a PWM, you usually end up with a giant noise spike right at Nyquist.

The is true for moderately large values of "slowly". Since the OP is trying to construct a 60Hz sine wave, he has a lower limit on the voltage feedback frequency responce of perhaps 1KHz or so.

If he has a microcontroller in the system, he can also use the information from this point in the previous cycle to improve his waveform for a constant load.

Yep, works like a synchronous detector. Most ECG units operate that way since there usually cannot be any conductive path to the patient.

Which frequency it is tied to doesn't matter. But if the SMPS FETs don't consistently switch in excess of 500kHz Jamie isn't going to yield his desired 200kHz signal bandwidth. So it might be better to send sample pulses across. It is also quite common to use this same sample pulse transmission to get some energy across to feed circuitry on the isolated side. Saves a toroid.

Pick your frequency carefully and don't get into trouble with the FCC because something was too leaky.