Here is my sim file from LTSpice
Attachments
-
2.4 KB Views: 1
Oooops, that's outside my analog knowledge. I sort of understand what you mean by using the transformer as filter, and MOSFET gate-driver stuff. But that's about it.Presumably, at 625Hz, you will be using a custom-made transformer.
Drive the transformer primary directly with the PWM pulse-train - you'll need some form of power driver - a MOSFET gate-driver IC might do the job (it depends on your supply voltage)
Then measure the leakage inductance referred to the secondary (short out the primary and measure the inductance at the secondary terminals).
Then use the transformer as the filter by connecting a capacitor across the secondary
\(C= \frac{1}{(2 \pi f)^2 L_{leak}}\)
where f is the cutoff frequency of the filter.
Choose
\(f=\sqrt{f_{PWM}\cdot f_{out}}\)
because there's nothing happening at that frequency, so changes in the output load which affect the Q won't excite the LC resonance.
If it's a custom transformer, you can get it made with just the right amount of leakage inductance.
That is way too powerful for what I needJust use one of these
https://www.pjrc.com/store/teensy40.html
Unfortunately not. I can take a guess at what it would be for a split-bobbin mains transformer, but that's no help to you!Is there a way to calculate an ideal transformers leakage inductance so I can simulate it on LTSpice? I googled, but all they talk about are how to MEASURE it, not calculate it.
The technique is used extensively on pure sinewave inverters. For a 3kW inverter the -3dB frequency of the filter is 900Hz and the capacitor is around 3.3uF. You will have a higher frequency, so that would make it smaller, but a lower voltage, with fewer transformer turns so that would tend to make it bigger. I'd guess it would be around a microfarad.Base on my experience of dc/dc converter, I got a feeling that for this low frequency, my output cap need to be very very very big to filter out to make it looks like a sine wave?
5V is what I am aiming for.You haven't even said what the output voltage will be!
Unfortunately, transformer knowledge is what I don't have. (I do know a bit magnetic stuff, left hand rules and right hand rules something basic)It's probably worth pursuing, because it's efficient and requires very few components, but it does require a bit of transformer knowledge.
The link is broken.I followed this idea. Calculated the resistors myself and probably still have the Excel file somewhere around.
What's the advantage of this over DDS with PWM DAC? I can see it doesn't need a PWM, but require a fast ADC I guess.Let's assume that you have selected a PIC16F15243, datasheet.
PIC16F152xx family has ADC, PWM but no DAC.
Here is something I would try but have not done before. I would implement a 1-bit delta modulation algorithm. All it takes is one digital output pin and one analog input pin.
The output pin goes to an op-amp integrator circuit. The ADC reads the voltage output of the integrator.
The software has a sine wave in a look-up table and loops through each entry of the table. Each value is compared with the ADC reading. The GPIO pin is set high or low in order to reduce the error.
View attachment 243675
The alternative is to go the PWM route which does not need the ADC feedback.
It will be handy if I don't have a PWM module and ADC onlyI don't have an answer.
One solution uses ADC module. It does not have to be fast. 625Hz is slow, 1.6ms period. 16 samples per cycle requires a sample every 100μs which is a slow for an ADC.
The other solution uses PWM module.
If you need an output transformer, then you're going to have to acquire some transformer knowledge anyway, because it's a weird frequency, whatever method you use to generate the sinewave.Unfortunately, transformer knowledge is what I don't have. (I do know a bit magnetic stuff, left hand rules and right hand rules something basic)
Yes you are correct, I guess I will have no choice.If you need an output transformer, then you're going to have to acquire some transformer knowledge anyway, because it's a weird frequency, whatever method you use to generate the sinewave.
If you calculate the waveform beforehand and store it in memory, you can output it using the SPI peripheral. Some will even do 16 bits at a time.I would implement a 1-bit delta modulation algorithm.
I found a couple of transformers in the shed, both were 18V output, so I worked it back to 5VYes you are correct, I guess I will have no choice.
At that price, ( USD 20 )That is way too powerful for what I need
How much of a load is on the transformer? You will need to drive the transformer sufficiently based on the load. What peak to peak voltage do you want? Also, for transformer make sure you have no DC offset. I think after you send it through the LPF you will need to send it also through a HPF with cutoff of say 10Hz so the transformer does not see any DC offset, which will driver more current into the transformer. What is the DC resistance of the transformer?I am not sure, that's the spec I was given. I am guessing it's a transformer isolated output, the input side of the transformer need to be constantly changing for the transformer to work the best?