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AP63203WU-7 layout review

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k0cka:
Hello everyone!

I just finished my first buck converter layout and I would really appreciate some feedback. I wasn't sure if I should post this in the beginners category or here, so I hope I chose the right place.
I decided to use the AP63203WU-7 because it seemed like the best and easiest option for me. My input voltage will be 12V or 24V, output voltage 3.3V and the maximum output current 1A. I'm not sure about the GND vias below the inductor, I added them for heat dissipation but I don't know if they are necessary or if I should remove them.

Components I used in the layout:

Thermal fuse - https://www.lcsc.com/product-detail/Resettable-Fuses_BHFUSE-BSMD1206-100-30V_C5358568.html (U1)

Protection diode - https://www.lcsc.com/product-detail/Diodes-General-Purpose_TWGMC-1N4001_C727079.html (D2)

Input capacitors - https://www.lcsc.com/product-detail/Aluminum-Electrolytic-Capacitors-SMD_ROQANG-CK1A470M-CRC54_C3001223.html (C5),
                        - https://www.lcsc.com/product-detail/Multilayer-Ceramic-Capacitors-MLCC-SMD-SMT_Samsung-Electro-Mechanics-CL31A106KBHNNNE_C13585.html (C1)

Buck IC - https://www.lcsc.com/product-detail/DC-DC-Converters_Diodes-Incorporated-AP63203WU-7_C780769.html (U3)

BST capacitor - https://www.lcsc.com/product-detail/Multilayer-Ceramic-Capacitors-MLCC-SMD-SMT_Samsung-Electro-Mechanics-CL10B104KB8NNNC_C1591.html (C3)

Inductor - https://www.lcsc.com/product-detail/Power-Inductors_cjiang-Changjiang-Microelectronics-Tech-FXL0530-4R7-M_C177246.html (L2)

Output capacitors - https://www.lcsc.com/product-detail/Multilayer-Ceramic-Capacitors-MLCC-SMD-SMT_Samsung-Electro-Mechanics-CL31A226KOHNNNE_C90146.html (C2,C4)

mariush:
Negative  on C5 ...  47uF 10v rated from a noname chinese brand ... first of all it's lower voltage rating than 12v or 24v, so your be damaged right away.. second its specs are crap

Try something like https://www.lcsc.com/product-detail/Solid-Capacitors_AISHI-Aihua-Group-SPZ1VM101E08O00RAXXX_C122240.html  or https://www.lcsc.com/product-detail/Solid-Capacitors_AISHI-Aihua-Group-SPZ1HM101F09O00RAXXX_C171434.html  or if you insist on surface mount, this apaq 47uF 35v rated one that's slightly bigger at D6.3xL5.8mm  : https://www.lcsc.com/product-detail/Solid-Capacitors_APAQ-Tech-350AVCA470M0606E38_C494513.html

The fuse is kinda high current... I mean .. 3.3v at 1A is 3.3w ... 3.3v/12v = 0.275a ... so probably even a 0.5A fuse would do, but if you want to support wider input voltage range I guess it's fine.

The rest ... some people say it's best to not have ground copper pour under the inductor (on the other side of the pcb is fine) ... I don't think it matters that much but I wouldn't put vias right under the inductor.

The output capacitors are good enough.  Consider adding a through hole footprint just in case you'd want to add a small through hole polymer cap there... it's free, you have space, and you can just leave it unpopulated if you don't feel it's needed.
 

k0cka:

--- Quote ---Negative  on C5 ...  47uF 10v rated from a noname chinese brand ... first of all it's lower voltage rating than 12v or 24v, so your be damaged right away.. second its specs are crap
--- End quote ---
I must have overseen it, I thought it's 100V. Thank you for pointing it out! I will go with the third option. Can I ask, how do you select components like capacitors? (except from voltage rating and capacitance)


--- Quote ---The fuse is kinda high current... I mean .. 3.3v at 1A is 3.3w ... 3.3v/12v = 0.275a ... so probably even a 0.5A fuse would do, but if you want to support wider input voltage range I guess it's fine.
--- End quote ---
Oh of course, I didn't consider the voltage drop, so the 1A fuse would be more suitable for the output side right? But I think I will just replace the one on the input for 0.5A as you mentioned.


--- Quote ---The rest ... some people say it's best to not have ground copper pour under the inductor (on the other side of the pcb is fine) ... I don't think it matters that much but I wouldn't put vias right under the inductor.
--- End quote ---
Ok I will remove the vias, thank you for clarifying.


--- Quote ---Consider adding a through hole footprint just in case you'd want to add a small through hole polymer cap there...
--- End quote ---
Yeah I can add the footprint for sure -  could you explain why is it needed though?

mariush:

I must have overseen it, I thought it's 100V. Thank you for pointing it out! I will go with the third option. Can I ask, how do you select components like capacitors? (except from voltage rating and capacitance)

I'm not an expert on capacitor selection. Basically what I do is carefully read datasheets and try to understand what they're saying, and by what I learned from repairing electronics, seeing other designers do pcb layouts for switching regulators etc etc 

Your 10uF ceramic capacitors satisfy the minimum input capacitance requirement of the regulator, but they're all very low ESR and when you power such circuits through long leads the inductance of such leads can cause issues, so it's a good practice to add a capacitor with higher ESR.  The polymer capacitors I recommended are not ideal, because it's still quite low ESR at 40-50 mOhm (but still 25-50x higher than ceramic capacitors), I think around 100-200 mOhm would be better ... but to get that with electrolytic capacitors you'd need to go up to around 100-270uF 35v and those get kinda big... I feel what I suggested is fine. 

See also the notes in the datasheet at page 13: https://www.diodes.com/assets/Datasheets/AP63200-AP63201-AP63203-AP63205.pdf


--- Quote ---"The input capacitor reduces the surge current drawn from the input supply as well as the switching noise from the device. The input capacitor has
to sustain the ripple current produced during the on time of Q1. It must have a low ESR to minimize the losses.
The RMS current rating of the input capacitor is a critical parameter and must be higher than the RMS input current. As a rule of thumb, select an
input capacitor which has an RMS rating greater than half of the maximum load current.
--- End quote ---

The ceramic capacitor has the low esr to minimize losses and is adequate. The electrolytic or polymer is in parallel, and can have higher ESR to absorb what the ceramics can't handle, but they must also have that RMS current rating high enough .. as the comment says should be greater than half the load current - yours would be 1A, so the RMS current should be at least 500mA or better ... so your capacitor, even if it was rated for 50v or 100v, is really lousy at RMS current. Polymers are much better in that department , the apaq is good for up to 2.1A ripple current, that's quite a lot.



--- Quote ---The fuse is kinda high current... I mean .. 3.3v at 1A is 3.3w ... 3.3v/12v = 0.275a ... so probably even a 0.5A fuse would do, but if you want to support wider input voltage range I guess it's fine.
--- End quote ---
Oh of course, I didn't consider the voltage drop, so the 1A fuse would be more suitable for the output side right? But I think I will just replace the one on the input for 0.5A as you mentioned.

Your switching regulator is not 100% efficient and may consume current in bursts, so the way I calculated is overly simplified... I was going by your comment that will be powered by 12v or 24v but 1A setting at the input is reasonable ... you may want to use a 7.5v or 9v adapter and then you'd have higher input currents. 

I'd say leave it at 1A, it's a good choice.



--- Quote ---Consider adding a through hole footprint just in case you'd want to add a small through hole polymer cap there...
--- End quote ---
Yeah I can add the footprint for sure -  could you explain why is it needed though?

Well if you look at the datasheet again at page 14, it says

--- Quote ---The output capacitor keeps the output voltage ripple small, ensures feedback loop stability, and reduces the overshoot/undershoot of the output
voltage during load transients. During the first few milliseconds of a load transient, the output capacitor supplies the current to the load. The
converter recognizes the load transient and sets the duty cycle to maximum but the current slope is limited by the inductor value.
..
An output capacitor with large capacitance and low ESR is the best option. For most applications, a 22μF to 68μF ceramic capacitor is sufficient.

--- End quote ---

So 22-68uF is SUFFICIENT,  as in you'll satisfy the minimum recommended of 22uF with 2 ceramic capacitors in parallel (because the actual capacitance varies with voltage and temperature in ceramic capacitors) and the voltage fluctuations on output will be minimal, good enough, but there's no rule against adding a bit of capacitance to get extra smooth output voltage, less overshoot/undershoot if whatever you have connected to the regulator pulls sudden bursts of power from the regulator. IF you find you don't need it, you can simply leave the footprint unpopulated.


k0cka:
Thank you for your detailed reply, I appreciate it!

Based on your suggestions, I replaced the C5 with 350AVCA470M0606E38, added footprint for C6 polymer capacitor and removed the vias below the inductor as shown in the attachment.

One more thing I wanted to ask about is the reverse polarity protection in my circuit - on another forum I was told that the diode I used is not the right fit for my purpose and that I should use P-channel mosfet in combination with zener diode. I went for the solution with the 1N4001 diode because I tried to keep it as small as possible and I found it mentioned on another forum. Do you think I should go for the mosfet or can I stick with the diode?

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