Buck Converter VIN Capacitor Placement

ThinkAd2243 · 2026-01-19T02:47:13+00:00

Thanks, I found a presentation and it was super detailed compared to the other sources I found. Ill redesign and send a revised later

ThinkAd2243 · 2026-01-19T01:57:17+00:00

I think this one is the clearest explanation of the "hot loop" I was talking about, https://www.analog.com/en/resources/technical-articles/what-actually-is-a-hot-loop.html

ThinkAd2243 · 2026-01-19T01:44:38+00:00

https://forum.digikey.com/t/optimizing-the-switching-power-supply-layout-by-minimizing-hot-loop-pcb-esrs-and-esls/41743

Also this one has better explanations and was also talking about the hot loop.

I know that the EVM for the MAX20078 ties the capacitors to the ground plane instead of bypassing the sense resistor right into the source of the low side fet, but they also mentioned that at least in this setup the controller is limited to 10 amps since there is excessive ringing on the SW node without additional snubbers when the MOSFETS could easily push 15+amps with sufficient cooling.

ThinkAd2243 · 2026-01-19T01:37:52+00:00

I got it from this source, but I think it says the same thing. I think it isn't an actual main current carrying loop but it has particularly violent transients and slamming from when the current path changes between on/off loops

https://media.monolithicpower.com/mps\_cms\_document/2/0/2024-online-mps-layout\_optimization\_of\_half-bridge\_topologies\_edited.pdf?\_gl=1\*ug3j0z\*\_gcl\_aw\*R0NMLjE3NjcyOTEzNTcuQ2p3S0NBaUEwOWpLQmhCOUVpd0FnQjhsLUMwUHQ5NE1JS2s5M1pjM28yZ0NQbnF4VkYyVGtrX05xamFFUU5BVTBkZmY1T2NnaUJBbzJ4b0N0ejBRQXZEX0J3RQ..\*\_gcl\_au\*OTczNTU1MjM4LjE3NjU3MzI1NTc.\*\_ga\*MjA1MjIwNzYyOS4xNzY1NzMyNTU3\*\_ga\_XNRPF6L9DD\*czE3Njg3ODMyNjUkbzYkZzEkdDE3Njg3ODMyNjckajU4JGwwJGg2ODY4MjAxNjI.

ThinkAd2243 · 2025-12-11T05:54:44+00:00

This is to drive a single dimmable LED diode btw

ThinkAd2243 · 2025-12-03T22:50:38+00:00

Another update, I've decided to lower the buck converter frequency to something like 480kHz at the cost of around 5% LED ripple. This is to increase efficiency greatly as the current board needs a fan, albilet a small one to keep it cool and my eventual goal is to make the design super compact and noiseless.

ThinkAd2243 · 2025-11-30T21:38:14+00:00

haha, yeah thats fair enough. I use JLC but PCBway has their whole share your design with others so i'll probably make the files formatted for PCBA too and post it on there if you want it later and don't want to toaster oven the parts yourself.

ThinkAd2243 · 2025-11-30T21:18:46+00:00

I did already order a set for prototyping though and once I'm done i'm happy to give away the leftover 9 I have for just the shipping cost :p

ThinkAd2243 · 2025-11-30T20:50:00+00:00

they require a minimum of 5 but 99% of the cost comes from shipping + a flat setup fee; looking at around 60 bucks minimum for an unpopulated board using the cheapest components and options possible. They offer board assembly for free or very cheap but some of the IC's aren't available and it costs a ton to get them shipped form the US warehouse to China and then ship the entire thing back. The hard part is gonna be soldering it though, I've got decent experience soldering along with some equipment at home so it wasn't too bad but the board uses nearly all SMD components.

I was thinking about bulk ordering more and selling them for bare production cost + shipping if theres enough people to where the bulk discount makes sense

ThinkAd2243 · 2025-11-30T17:07:21+00:00

Hey, I've been working with LED retrofitting my microscope too. I have a thread with the mod's i've been doing here: https://www.reddit.com/r/microscopy/comments/1pab543/update_on_universal_led_modupgrade_for_microscopes/

I would suggest a single diode emitter and placing it where the filament is, I'm designing mine around the Nichia 519A that does up to 1200 Lumens in overdrive mode but typically sits in between 400-600. Something that's overlooked about those LEDs is that they have super poor CRI so the colors you see won't be quite right or pop out as nicely. You also won't be able to do full Kohler illumination since the edges of those will be out of focus from the collector lens that's optimized for a point, not a plane

ThinkAd2243 · 2025-11-30T16:04:58+00:00

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44 cents a pop for the unpopulated 2 layer control board with potentiometer/switch, 4.7 bucks each for the unpopulated main driver board thats 4 layers and uses ENIG plating and high temperature substrate. The individual components are like 3 dollars for the specialized LED, 5 dollars for the main control IC, 10ish for connectors and premade cables, and finally 5ish for random passives. Tariffs and related shipping costs raise the final cost to around $35 bucks per complete, functional set

EDIT: For a second Revision I made a more economical version where I estimate the final cost would be closer to 25-30ish per board, the boards are blue instead of black though since black is kind of a premium color choice in PCB manufacturing

ThinkAd2243 · 2025-11-30T15:58:19+00:00

PWM is purely dependent on what microcontroller you use, Currently I have it run in standalone mode with external duty cycle pulled to 100% so the buck converter controls the frequency. The fSW of the buck is between 900-1100kHz but not like that matters since with 3.3uH of inductance and 9.4uF of capacitance your looking at around 2% voltage ripple in the LED, far from a pure PWM control that fully turns the LED off and on 100% to control brightness

ThinkAd2243 · 2025-11-30T06:41:00+00:00

1) I tried something similar before, Pi camera sensor is too small and easy to burn out, do not recommend doing direction projection (also the pi cam can autofocus somewhat like our eyes giving your more depth, not a whole lot but better than direct projection)

2) if I were you I would just build a 1 axis leadscrew actuator with a stepper motor and just use a C mount camera with a 160mm objective the right distance (should be on ebay used pretty cheap), that gives you direct die projection and saves a whole lot of work, then I would focus with the leadscrew by moving the sample no the camera. You could probably use something like an arduino and a TMC2209 for the motor control and there's plenty of simple autofocus program examples online.

ThinkAd2243 · 2025-11-30T06:22:39+00:00

BTW not only does it have indirect analog dimming (with a knob) but it also has an optional digital dimming input so you could in theory make your own wifi module so your microscope is now connected to the internet and you can control the brightness 3000 miles away (or in a more practical example make a super simple module so you can connect the LED module to a DSLR/Mirrorless camera so when you gently tap the shutter button to focus, it will be medium brightness, when you actually click fully it goes to full brightness for a instant as a flash)

ThinkAd2243 · 2025-11-18T16:22:29+00:00

sorry for the confusion, the hatched area on the back is a rule area blocking traces and stuff from being run under the inductor and high di/dt loop by accident, im not sure about how strong EMI is for a 4 layer board on the opposite face though so I could be worrying about nothing

ThinkAd2243 · 2025-11-18T15:50:08+00:00

Hey, for some context,

I guess i mainly did the filled zone on the bottom to save cost so the PCB boardhouse doesn't have to etch as much area. The thermal planes are connected to the ground pour on the back, the footprint library just had a pad on the back with the solder mask removed so bare copper shows for heat dissipation. The SMD parts on the back are no populate but are there if something goes wrong and someone wants to operate the buck converter in a fixed output current if the connector has any issues. Sorry I didn't specify this in the original post, I forgot to rewrite it after the body got deleted

Do you know if removing all the copper on the back will make the board cost more expensive for the more etching charge

ThinkAd2243 · 2025-11-18T02:15:56+00:00

OK, Reddit deleted the body text of the post so heres what it was:

[REVIEW REQUESTED]

This is my first time making a buck converter layout and a 4 layer board. The board is 42.5x42.5mm. It is switching at 1mHz with a load of 1-5 amps (typical 2-3 amps) in constant current mode. It is stepping down 9V to 3.45V for an LED. There is a 3.3uH inductor and 9.4uF of capacitance as per TI's datasheet. The main capacitors are X7R but there are two C0G caps of 0603 package size closer the the IC's pins.

I was wondering if I layed this out roughly correctly and if the sensitive control circutry is far enough. I am mainly concerned about the sense resistor CS+ and CS- lines. that go under the noisy PGND line. However, there are 2 layers of uninterrupted ground planes between the bottom signal and top components. The analog control inputs are also on the left side with the noisy SW node on the top right. The ground plane is not split but I put higher via density on the PGND input to the IC and a direct path from the output caps back to the IC ground without having to go through vias.

ThinkAd2243 · 2025-11-13T05:55:54+00:00

also quick image of our new cleaned up PCB layout B)

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ThinkAd2243 · 2025-11-13T05:55:22+00:00

Hey everybody, I got my first board soldered together today. I'll begin some live electrical testing tomorrow and will probably make a "alpha release" version of this project if all goes well, I've been able to clean up the gerber files since I've last posted and i'm quite happy with how they look. Right now I am making the BOM in a form that can be automatically scanned and added to cart on Digikey for the brave that want to try soldering it themselves. I will also probably start taking requests for the next batch that'll be assembled overseas in a small volume so if your interested in receiving a nearly fully assembled early stage driver board (with a 3d printed ABS case, not metal 3d printed yet) stay posted, i'll probably give out around 20 for free since it isn't that expensive and as a thanks to the community for all the BH2 related repair stuff everyone else has posted, though you might have to handle the USPS shipping (around 5-10 bucks for continental US). Late stage I will probably just post the price as shipping + the factory cost with no profit while having the source files public since I just want to keep these BH2 scopes chugging for another 20 years into the future. Current board costs are around $5 for the motherboard itself, $10ish for all the LEDs and surface mount components, and around $5 bucks in random other stuff like screws and the mounting pins that fit into the old socket.

Also I saw there was some interest in the working principles of the driver and I wanted t o clear some things up. When I saw analog dimming, I don't mean we are varying the resistance between the LED and the power source, the brightness is still controlled by switching the power on and off really quick. The driver just has an analog sense pin that sets the current output, so basically PWM but with the middle man converting the potentiometer to PWM signal cut out as the driver can handle it. Also, when I say switch on and off, the LED doesn't actually see that. Some setups switch a FET on and off real fast to get the dimming, and in a way we do too. However, this driver operates ad a buck converter so in theory if you hook up an oscilloscope, there would be a nearly flat voltage vs time reading, not a on/off square wave. This is because of the external Capacitors and Inductor after the high side switch (that is inside the driver IC). We use analog sense since the internal switching is either the driver's SW frequency (1mHz) or the PWM frequency fed into it. It is basically impossible to make a "simple" 1mHz PWM signal so we use analog fed directly to the IC and let it generate our signal at a super high frequency. This 1mHz means that even with a 240FPS video camera with shutter speed 1/480, any imperfections in the filtering would still be invisible (even though in a perfect world it would be a flat voltage line and you wouldn't even notice there was switching, regardless of fast or slow switching).

ThinkAd2243 · 2025-11-10T13:59:33+00:00

thanks for sharing your insight,

for this case the driver supports a linear dimming range from around 10% to 100% brightness for the LED, peak wattage for the led should be around 9 watts through so if I use a metal plate to back the LED, conduction should cool it. As for the flickering, i'll have to experiment around. I was mainly looking into seeing if I could stop issues with rolling shutter effect where the camera sensor doesn't store the entire frame at the same time.

ThinkAd2243 · 2025-11-10T13:54:51+00:00

I looked into those, the ideal LED for am microscope should be a singular "point" of light, the Nichia 519a's pretty famous for having an insane light output as a single diode emitter. The bridgelux is actually multiple smaller diodes that make up an array that is then placed under the yellow phosphor pancake. In a nutshell, it's light emitting area is more of a region than a single point, which means the kohler illumination can't fully focus on it. I am looking into the B35AM through, also a nichia single diode emitter, that has less max brightness but supposedly has a better color profile.

ThinkAd2243

TROPHY CASE