Buck Converter PCB Layout
Step 1: Spot the Key Power Parts Within the Whipper Circuit
The first factor is to spot the key power parts within the whipper circuit. These area unit (reference the schematic in Figure 1):
Filter capacitors: Cin and Cout quickly supply and sink giant levels of AC current. Power switch: U1, the series pass component, is sometimes MOSFET. It's going to be one or additional distinct devices, or engineered into the controller, if present. Inductor: L1, the magnetic component, provides storage for energy that may be recovered whereas the switch is off. Diode: D1, the output rectifier, is sometimes a Schottky diode, however in ultra-efficient (e.g., synchronous rectifier) whipper styles, this operate are going to be performed by a MOSFET. The inductance associated Cout electrical device kind an LC filter, providing high-frequency voltage-ripple filtering. Sometimes, there's a high-frequency bypass condenser (Cbypass or Chf) connected in parallel with Cin. This electrical device has to be placed terribly near the switch input. Usually the ability parts for a whipper layout area unit “on-chip”, that is, they reside on the controller. The PCB routing can follow an equivalent rules as once these parts area unit “off-chip.”
Understanding however the oppressor works needs us to spot the multiple essential current loops, DC (continuous) and AC (alternating).
Step 2: Buck Converter Current Path
The DC current loops square measure 1) the Input Loop, from the input supply, through the electrical device Cin, and returning to the supply, and 2) the Output Loop, from the electrical device Cout, through the output load, and returning back to Cout. Figure 2 shows the locations of the Input and Output loops. These loops need to be connected both directly at their respective filter capacitor’s terminals, and with short, wide traces for low impedance.
These two separate DC loops can be thought of as the current flow paths of the unregulated source, and the regulated load voltages. The AC current loops are the Power Switch Loop and the Output Rectifier Loop. The power switch loop is formed while the power switch is ON. During this time, the forward current flows from Cin, through the switch, the inductor, through Cout and returns back to Cin. The output rectifier loop is formed while the switch is OFF. Now energy is recovered from the inductor (magnetic storage). During this cycle, the forward current flows from the inductance to Cout, and returns from Cout through the rectifier and back to the inductance. It may facilitate to consider a switcher’s operate as changing DC at its input into AC so back to DC at its output, for the aim of power potency.
Step 3: Buck Converter Layout
The AC current loops square measure the foremost essential connections in any oppressor layout. These ways take priority over all others. Their placement and routing have to be compelled to be planned initial and that they have to be compelled to be routed with short, low inductance ways (see Figure 3).
The AC current return paths should be matched to the respective forward current paths as much as possible. The best way to do this is to use a full ground plane in close proximity on the next adjacent PCB layer. By minimizing the loop area and making the return path closely follow the forward current path, the opposing magnetic fields will tend to cancel each other out. This reduces unwanted EMI. The return path should not be occupied with too many non-ground vias, which could undermine the effective copper for this path by creating openings or slots in this plane. It is also best to line up these vias, leaving wide alleyways of copper in the direction of the return path. The difference of the two AC current return paths (from the anode of the rectifier to the negative terminal of Cin) should be a short, low impedance, common point ground connection that includes the negative terminal of Cout and, if applicable, the controller thermal pad and any PGND connections.
All power components should be located on the same side of the board, and forward current path connections should be made without thermal relief and without the use of vias. The ground vias should also be connected to the plane without thermal relief. The output of the switch is called the SW node, and is part of the forward AC current path. It carries the fast-switching, high-amplitude voltage swings (high dV/dT), along with high peak currents. This connection in particular needs to be as short as possible. It is important at the controller level to make this connection very low inductance, and it should be wide enough for the current flowing through it. Widening the connection to compensate for a longer distance is not recommended. This is because the likelihood of this connection becoming an antenna and radiating EMI is directly related to its length. The switcher circuitry should be placed in such a way that this connection is kept away from other circuitry, including other switchers on the same board. The SW node connection should not be part of the copper flooding that is used to help dissipate heat, even if it is the best mechanical way to extract heat from the switcher (see Figure 4). Copper flooding for thermal management should utilize the low impedance and quiet DC connections (GND, VOUT, and VIN). Air flow direction may also need to be examined when placing tall components, like the inductor and filter capacitors, around discrete power switches. The compact size of a switcher layout, which is necessary to reduce EMI, can also make effective heat extraction a challenge.