Hello. I'm building a buck dc/dc power supply with the UC3823a pwm controller. How do I control the Pwm signal? I don't understand how i should calculate the value of my resistors and capacitors. Can anyone help me understanding the pwm controller.
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Hello. I'm building a buck dc/dc power supply with the UC3823a pwm controller. How do I control the Pwm signal? I don't understand how i should calculate the value of my resistors and capacitors. Can anyone help me understanding the pwm controller.
Morton,
I'm not certain where your inputs and output are in your schematic so I will start by pointing out some obvious errors.
On the extreme left. The NPN transistor needs to be inverted. Swap the emitter and collector.
Same with the FET. The way it is configured the body diode will be conducting all the time.
You have both outputs connected with Shottky diodes which are then shorted. Save yourself the components or put them from ground to the outputs to prevent the outputs from going below ground in operation.
Your voltage sensing is being done off the collector of a PNP transistor. Not a good idea. Go to http://www.ti.com/general/docs/lit/getliterature.tsp?literatureNumber=slua125&fileType=pdf and look at fig 17 for an idea on how to do the feedback.
Hello
Thank you for your answer.
I still don't understand how to chose the resistor values to get the voltage that I want. I want an output voltage of 14 V (I'm going to charge a battery). The input voltage varies a lot. It is the rectified voltage from an alternator on a motorcycle. I think the voltage will vary from 20-180 V
About the sensing. Do you think i should drop the PNP transistor? This design is copied from a buck power supply for motorcycle, but i don't understand how its working. Why is such a bad idea?
This is the schematic so far. I have corrected the errors.
Morten,
The circuit is configured as a shunt regulator. Far from the most efficient configuration. You might want to review designs for flyback regulators.
If you want to measure and control the Vout voltage with respect to ground, connect the R8/R10 junction to the C1/L1 junction and remove R5, R6, D2, T1 and R2.
The voltage at the R10-R11 junction when Vout is in regulation should equal the voltage on the error amp non-inverting terminal (5.1V) inside theUCC2823A.
Regards.
John
Morten,
The regulation is done by the current through what you have listed as Rload. What voltage are you regulating with respect to what other voltage?
The resistors and caps you questioned are part of the feedback loop poles and zeros.
Take a look at the attached paper.
Are you in university or industry?
John
I'm in university. And I'm not having so much experience with electronics. (clearly). But I'm trying to learn this.
This mignt help:
“How to Design a Discontinuous Flyback Converter”
Pages 74-79: http://www.bodospower.com/restricted/downloads/bp_2012_05.pdf
Why do you need frequency compensation, regarding c3,r8,c4,c6 and r9? Is it necessary? I don't think i understand the meaning of it? Could I just skip c3, r8 c4 and c6.
Morten,
The attached should give you some idea about feedback loop. I think you should be discussing these issues with your Prof. That's what he is being paid to do.
John
Morten,
Your gate voltage has to be higher than the source to have the FET "ON" and the drive impedance (100 ohms) you are using looks to be to much.
John
The gate voltage is going from 0 to 30 V for switching the mosfet on and off. The schematic is working, until the mosfet get to hot. I Tried up to 150 VAc in today, and the problem is that the mosfet gets superhot and breaks. There is obliviously too much heat loss in the mosfet. And i dont under stand why. The gate signal was better than the picture from my last post. Would this design work at all? Could it be an idea to put the mosfet between ground and regulate diffident ground planes?
Morten
Hello again.
The data sheet says that there should be an 1 mF on Vref. Isn't 1 mF a lot? Why is it so high?
Morten,
The Terminal Function Table on page 3 says the pin should be bypassed by 0.1uF ceramic LOW ESR LOW ESL cap with minimal trace length.
Figure 14 page 11 shows it as 0.1uF.
I suspect that the 1 - mF on page 10 is the difference in the type font where the "m" of the "mF" is not "symbol" font.
I would expect that a 1 uF cap would be acceptable even with the other two places specifying 0.1 uF
Regards,
John
Hello
Now when I'm testing, I gets some problems. The the duty cycle gets low very fast. It goes from about 60% to about 5% when the input voltage goes from 22-30V. When the duty cycle get to low, the regulator stops. It seems that the mosfet does not get enough time to switch on and off, when the duty cylce get very low. I'm running the duty cycle at 40KHz, and tried down to 10 kHz, but I'm still having the same problem.
Also the source voltage when the mosfet is off, gets a lot of ringing at a ~50 % duty cylce. Why? Do I need to change something in my buck circuit to improve this? See the picture. I'm also attaching the circuit as it is know
Ringing:. BLUE (Source voltage on mosfet.)