Prince Charming
Member level 2
Hello, I've designed an SMPS to power an Arduino with few sensors. I aimed for 9V and 2A max. I thought the Arduino has its own regulator so I wasn't too nervous about my design. This is the first time got into a circuit that has closed loop control and transformers. That's why I wanted to get professional thoughts about my design before I buy the parts. The schematic is given below:
First of all, for the EMI design I followed the video:
and I found my max x caps as 327 nF and y caps as 1.4 nF. I chose a very popular choke (Uu9.8) but I am open to suggestions. My snubber circuit which consists of R9,C4 and D6 is a copy of another SMPS I found on internet. The target output wasn't the same but I ignorantly thought these values would do the job. Also I added the C12 capacitor to dampen the voltage spikes during switching but I chose the capacitance arbitrarily so I am open to suggestions.
The transformer was the most difficult one for me. It has 25:1 ratio and according to the design in this website: https://techweb.rohm.com/knowledge/acdc/acdc_pwm/acdc_pwm01/940 it has 8.18 Amps of secondary peak current, 13 uH secondary inductance, 8mH primary inductance, minimum 173 turns at primary and 7 turns at secondary. I have never bought a transformer before and they are hard to choose. I know that there must be some kind of an air gap in flyback transformers but I don't have access to many options so it would be great if you could give me some very popular transformers that I might have access to.
Also I added an inductor filter to the output contrary to the general structure of a flyback converter. I saw it at somebody else's design and I thought as long as the inductance isn't too large a current filter wouldn't harm. But I am open to suggestions on why that inductance should/shouldn't be there and how I should choose the value of it. (AS far as i know current ripple depends on transformer inductance so this inductance seems like an extra to me)
I've had hard time choosing the output capacitor values as the formula for the minimum C is: "C_min = D/((Vo_ripple/Vo)*R*f)" and the resistance of Arduino doesn't make sense. I would be glad if I could know how to chose output caps for this kind of loads.
I used an IC for PWM control and basically I supplied 5V to negative side and divided Vo to the positive side of the error amplifier of TL494 as the datasheet suggests –0.3 V to (VCC – 2V) input to error amplifiers. If my observation is correct the PWM comparator in TL494 has constant 0.7V at PWM comparator as below:
And the purpose of that constant voltage is to prevent instability that would occur when zero signal comes from error amplifier, basically creates an acceptable threshold for zero error. If I am mistaken I'd be glad to be corrected. I didn't use deadtime comparator as it is irrelevant to my design so I connected that leg to ground.
I chose to use one MOSFET at the converter and kept the D7 diode in my schematic to make the design simple that's why I didn't need two outputs from the IC and I shorted the emitters and collectors as well as I shorted pin13 to ground. I learned how this IC behaves from the datasheet and it doesn't explain fully so I hope I planned and connected the IC in the right way. However, if you claim that that diode would waste energy I can replace it with a MOSFET. I think Vforward * 2Amps is at most 2 Watts. So does it really matter?
The way I applied the control signal to the MOSFET is purely my idea and I am not too confident about it. Normally people use optocouplers to separate high and low voltage but I was scared of being faced with extra problems and I thought adding a protection diode would be sufficient. If not, I would like to know the reason.
ULTIMATE QUESTION:
The TL494 IC in my design is fed with 12V voltage. How can this supply voltage be obtained? There are numerous designs in the internet and everybody connects an 12V power supply to the IC. That is impossible if this is the only power supply in a device and nobody considers this possibility, I am very surprised.
Thanks.
First of all, for the EMI design I followed the video:
The transformer was the most difficult one for me. It has 25:1 ratio and according to the design in this website: https://techweb.rohm.com/knowledge/acdc/acdc_pwm/acdc_pwm01/940 it has 8.18 Amps of secondary peak current, 13 uH secondary inductance, 8mH primary inductance, minimum 173 turns at primary and 7 turns at secondary. I have never bought a transformer before and they are hard to choose. I know that there must be some kind of an air gap in flyback transformers but I don't have access to many options so it would be great if you could give me some very popular transformers that I might have access to.
Also I added an inductor filter to the output contrary to the general structure of a flyback converter. I saw it at somebody else's design and I thought as long as the inductance isn't too large a current filter wouldn't harm. But I am open to suggestions on why that inductance should/shouldn't be there and how I should choose the value of it. (AS far as i know current ripple depends on transformer inductance so this inductance seems like an extra to me)
I've had hard time choosing the output capacitor values as the formula for the minimum C is: "C_min = D/((Vo_ripple/Vo)*R*f)" and the resistance of Arduino doesn't make sense. I would be glad if I could know how to chose output caps for this kind of loads.
I used an IC for PWM control and basically I supplied 5V to negative side and divided Vo to the positive side of the error amplifier of TL494 as the datasheet suggests –0.3 V to (VCC – 2V) input to error amplifiers. If my observation is correct the PWM comparator in TL494 has constant 0.7V at PWM comparator as below:
And the purpose of that constant voltage is to prevent instability that would occur when zero signal comes from error amplifier, basically creates an acceptable threshold for zero error. If I am mistaken I'd be glad to be corrected. I didn't use deadtime comparator as it is irrelevant to my design so I connected that leg to ground.
I chose to use one MOSFET at the converter and kept the D7 diode in my schematic to make the design simple that's why I didn't need two outputs from the IC and I shorted the emitters and collectors as well as I shorted pin13 to ground. I learned how this IC behaves from the datasheet and it doesn't explain fully so I hope I planned and connected the IC in the right way. However, if you claim that that diode would waste energy I can replace it with a MOSFET. I think Vforward * 2Amps is at most 2 Watts. So does it really matter?
The way I applied the control signal to the MOSFET is purely my idea and I am not too confident about it. Normally people use optocouplers to separate high and low voltage but I was scared of being faced with extra problems and I thought adding a protection diode would be sufficient. If not, I would like to know the reason.
ULTIMATE QUESTION:
The TL494 IC in my design is fed with 12V voltage. How can this supply voltage be obtained? There are numerous designs in the internet and everybody connects an 12V power supply to the IC. That is impossible if this is the only power supply in a device and nobody considers this possibility, I am very surprised.
Thanks.
