| Electronics > Projects, Designs, and Technical Stuff |
| Turning a fixed SMPS into a variable power supply. |
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| Yamin:
Just an update - I changed the ZD9 to a diode which has a Zener voltage of 6V (it was 12V before). I changed R73 and with the 1K VR - I get resistance from about ~6K to 1K. I get the output voltage variation from 10V to about 14V. Not what I really wanted. 1. Does the reference pin of the 431 always have to have 2.5V 2. If so how can I design a feedback which can provide 2.5V for the reference pin and also have a large variable option for the output? Really need help on how to design a feedback. I don't mind the output going from roughly 12V to 24V. This project is more about learning :) |
| T3sl4co1l:
What's the rest of the SMPS? If it's self-powered, it's not going to take more than about +/-30% range. More and you overvolt the controller on the high end, and undervolt it (so it browns out and hiccups) at the low end. If it has an aux supply, it should be able to go down to zero, but mind particulars about compensation and parasitics and minimum pulse width (min/max duty cycle, especially if bootstrap drivers are involved somewhere). Tim |
| xavier60:
--- Quote from: Yamin on November 28, 2018, 08:07:59 pm ---Just an update - I changed the ZD9 to a diode which has a Zener voltage of 6V (it was 12V before). I changed R73 and with the 1K VR - I get resistance from about ~6K to 1K. I get the output voltage variation from 10V to about 14V. Not what I really wanted. 1. Does the reference pin of the 431 always have to have 2.5V 2. If so how can I design a feedback which can provide 2.5V for the reference pin and also have a large variable option for the output? Really need help on how to design a feedback. I don't mind the output going from roughly 12V to 24V. This project is more about learning :) --- End quote --- Most of the things that you are blindly trying such as removing ZD9 and R74 are disabling the voltage regulation loop causing the controller to go to full duty cycle, limited to something less than 50% usually for Forward converters. The output goes over-voltage causing the OVP circuitry to repeatedly shutdown the controller. Forward converters generally tolerate output over-voltage. R74 needs to be gradually deceased while monitoring C34's voltage if you have the equipment to do it safely. |
| Conrad Hoffman:
It can be hard to make a SMPS cover a wide range without some performance hiccup. This is pretty stone age, but I'd just put a big linear regulator after it and adjust that. Job done. |
| xavier60:
--- Quote from: strawberry on November 28, 2018, 12:19:22 pm ---to regulate output voltage from 0..100%and at full current. and not run ML4800 into UVLO , power that chip from external power source but dont run output voltages too high , othervise MOSFET in your FORWARD topology will overvoltage and blow --- End quote --- The presence of C7 means that the Reset winding has the same turns number as the Primary winding. The MOSFET's Drain will be clamped at double HVDC voltage. At something less than 800V. . |
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