I am fairly new to circuit design. I was wondering if I have correctly implemented the TVS diode (D8), flyback diode (D6) and the snubber circuit (R20 and C41) for this buck converter. I apologize in advance if the circuit design is messy/offensively incorrect! The only component downstream from this buck converter is a gate driver.
First, D8 isn't a TVS diode. and if it is, it's the wrong symbol. NBD, rookie mistake. Also, this isn't where you would typically put a TVS.
Flyback diode (D6) is in correct location and orientation. Snubber (C41 and R20) is also in the correct location.
D8 is redundant and doesn't really make sense.
Also, you are probobly going to want some more input capacitance and a small ceramic chip capacitor to decouple high-frequency power supply ripple that will cause the controller to behave erratically. Everything else seems fine as a first pass reviewing this highly offensive circuit.
Pro tip: If you have little experience drafting the schematic on your own, just start by copying the application circuit in datasheet; tweak from there. In first-order, it's going to be drafted perfectly because the app circuits are drafted by seasoned professionals. This makes everything from a communication standpoint clearer for all parties involved. With more experience, you will be able to draft the circuit from just the snippet circuit on page 1 of the datasheet. Not just for this chip, but all the chips. They all follow this paradigm, give or take.
Always start with the datasheet:
The TPS62136 and TPS621361 are high efficiency and easy to use synchronous step-down DC-DC converters [ed: emph]
The application diagrams show no catch diode, nor RC snubber; D6, and likely R20-C41, are superfluous.
A catch diode sometimes shows up in synchronous designs, when the low side switch isn't always driven synchronously, or to mitigate body diode recovery due to excessive LS-to-HS dead time; but mostly it's a hack, and dealing with the underlying problem is more effective (i.e. making it fully synchronous, or tightening the dead time). In a normal functioning synchronous design, the diode does nothing useful, and simply increases capacitance on the switch node, and therefore increases switching loss.
You could add the R+C to mitigate emissions somewhat; but mind this comes at the cost of efficiency, and is better solved in general by careful layout and appropriate component choice. It should be an unusual/exceptional case where both the snubber is needed, and provides enough value to be useful -- certain automotive EMC tests perhaps (near / E-field test?).
There may be notes about the capacitor choice as well, but their types will need to be given. To summarize: type 2 (X5R, X7R, Y5P, etc.) ceramic capacitors exhibit a reduction in value under DC (voltage) bias. The amount of reduction is independent of the voltage rating, so it is not enough to simply put in, say, 50 or even 100V rated capacitors. You must look for the characteristic data of the parts, and ascertain whether they are sufficient for the application. The main outcome will be: for 10 and 22uF parts, even at 3.3V, you will likely need 1206 chip size at least, and preferably 1210, and of a grade no worse than X5R to X8S.
No layout is given, but do pay close attention to the suggested layout in the application section. Layout is critical: every mm of trace length contributes a good 0.5nH or so of stray inductance. May not sound like much, but it adds up quickly -- pin and component body lengths count just as well as PCB traces do -- and at the edge rates in regulators such as these, it doesn't take much, maybe even 5 or 10 nH, to cause dubious behavior -- or outright malfunction or damage.
