Na osnovu PCB raspored napajanja, ovaj rad predstavlja najbolju metodu postavljanja PCB -a, primjere i tehnike za optimizaciju performansi jednostavnog sklopnog modula napajanja.

Prilikom planiranja rasporeda napajanja, prvo se razmatra fizičko područje petlje dvije sklopne strujne petlje. Although these loop regions are largely invisible in the power module, it is important to understand the respective current paths of the two loops because they extend beyond the module. In loop 1 shown in Figure 1, the current self-conducting input bypass capacitor (Cin1) passes through the MOSFET to the internal inductor and output bypass capacitor (CO1) during the continuous conduction time of the high-end MOSFET, and finally returns to the input bypass capacitor.

Schematic diagram of loop in the power module www.elecfans.com

Figure 1 Schematic diagram of loop in power module

Loop 2 is formed during the turn-off time of the internal high-end MOSFEts and the turn-on time of the low-end MOSFEts. Energija pohranjena u unutarnjem induktoru teče kroz kondenzator izlaznog premosnika i niske MOSFE -ove prije nego se vrati u GND (vidi sliku 1). The region where two loops do not overlap each other (including the boundary between loops) is the region with high DI/DT current. The input bypass capacitor (Cin1) plays a key role in supplying the high frequency current to the converter and returning the high frequency current to its source path.

Izlazni zaobilazni kondenzator (Co1) ne nosi veliku izmjeničnu struju, ali djeluje kao visokofrekventni filter za prebacivanje šuma. Iz gore navedenih razloga, ulazni i izlazni kondenzatori trebaju biti postavljeni što je moguće bliže odgovarajućim VIN i VOUT pinovima na modulu. As shown in Figure 2, the inductance generated by these connections can be minimized by making the wiring between the bypass capacitors and their respective VIN and VOUT pins as short and wide as possible.

Slika 2 Jednostavna preklopna petlja

Minimiziranje induktiviteta u rasporedu PCB -a ima dvije velike prednosti. Prvo, poboljšajte performanse komponenti promicanjem prijenosa energije između Cin1 i CO1. Ovo osigurava da modul ima dobru hf premosnicu, minimizirajući vršne vrijednosti induktivnog napona zbog visoke DI/DT struje. Također smanjuje buku uređaja i naponski napon kako bi se osigurao normalan rad. Drugo, minimizirajte EMI.

Kondenzatori povezani s manje parazitske induktivnosti pokazuju niske impedancijske karakteristike prema visokim frekvencijama, čime se smanjuje provedeno zračenje. Ceramic capacitors (X7R or X5R) or other low ESR type capacitors are recommended. Dodatni ulazni kondenzatori mogu se aktivirati samo ako su dodatni kondenzatori postavljeni blizu krajeva GND i VIN. The Power module of the SIMPLE SWITCHER is uniquely designed to have low radiation and conducted EMI. However, follow the PCB layout guidelines described in this article to achieve higher performance.

Circuit current path planning is often neglected, but it plays a key role in optimizing power supply design. In addition, ground wires to Cin1 and CO1 should be shortened and widened as much as possible, and bare pads should be directly connected, which is especially important for input capacitor (Cin1) ground connections with large AC currents.

Uzemljeni pinovi (uključujući gole jastučiće), ulazni i izlazni kondenzatori, kondenzatori za meki start i povratni otpornici u modulu trebaju biti povezani na sloj petlje na PCB-u. Ovaj sloj petlje može se koristiti kao povratni put s izuzetno niskom strujom induktivnosti i kao uređaj za odvođenje topline koji će biti opisan u nastavku.

Sl. 3 Shematski dijagram modula i PCB -a kao toplinska impedancija

Povratni otpornik također treba postaviti što je moguće bliže FB (povratnom) pinu modula. To minimize the potential noise extraction value at this high impedance node, it is critical to keep the line between the FB pin and the feedback resistor’s middle tap as short as possible. Available compensation components or feedforward capacitors should be placed as close to the upper feedback resistor as possible. For an example, see the PCB layout diagram in the relevant module data table.

For AN example layout of LMZ14203, see the application guide document AN-2024 provided at www.naTIonal.com.

Heat Dissipation Design Suggestions

Kompaktan raspored modula, iako pruža električne prednosti, ima negativan utjecaj na dizajn rasipanja topline, gdje se ekvivalentna snaga rasipa iz manjih prostora. To address this problem, a single large bare pad is designed on the back of the Power module package of the SIMPLE SWITCHER and is electrically grounded. The pad helps to provide extremely low thermal impedance from the internal MOSFEts, which typically generate most of the heat, to the PCB.

Toplinska impedancija (θJC) od spoja poluvodiča do vanjskog paketa ovih uređaja je 1.9 ℃/W. While achieving an industry-leading θJC value is ideal, a low θJC value makes no sense when the thermal impedance (θCA) of the outer package to the air is too great! If no low-impedance heat dissipation path is provided to the surrounding air, the heat will accumulate on the bare pad and cannot be dissipated. So what determines θCA? The thermal resistance from bare pad to air is completely controlled by the PCB design and associated heat sink.

Now for a quick look at how to design a simple PCB without fins, figure 3 illustrates the module and PCB as thermal impedance. Because the thermal impedance between the junction and the top of the outer package is relatively high compared to the thermal impedance from the junction to the bare pad, we can ignore the θJA heat dissipation path during the first estimate of the thermal resistance from the junction to the surrounding air (θJT).

Prvi korak u dizajnu rasipanja topline je određivanje količine energije koja će se rasipati. The power consumed by the module (PD) can be easily calculated using the efficiency graph (η) published in the data table.

We then use the temperature constraints of the maximum temperature in the design, TAmbient, and the rated junction temperature, TJuncTIon(125 ° C), to determine the thermal resistance required for the packaged modules on the PCB.

Finally, we used a simplified approximation of the maximum convective heat transfer on the PCB surface (with undamaged 1-ounce copper fins and numerous heat sink holes on both the top and bottom floors) to determine the plate area required for heat dissipation.

Aproksimacija potrebne površine PCB -a ne uzima u obzir ulogu koju imaju rupe za rasipanje topline koje prenose toplinu iz gornjeg metalnog sloja (paket je spojen na PCB) u donji metalni sloj. Donji sloj služi kao drugi površinski sloj kroz koji konvekcija može prenositi toplinu s ploče. Da bi aproksimacija površine ploče bila važeća, potrebno je koristiti najmanje 8 do 10 rupa za hlađenje. Toplinski otpor hladnjaka se aproksimira sljedećom jednadžbom.

Ova se aproksimacija primjenjuje na tipičnu prolaznu rupu promjera 12 milja sa bakrenom bočnom stijenkom od 0.5 oz. Što je moguće više rupa za hladnjak treba biti projektirano u cijelom području ispod gole podloge, a ti otvori za hladnjak trebaju tvoriti niz s razmakom od 1 do 1.5 mm.

zaključak

Modul napajanja SIMPLE SWITCHER nudi alternativu složenim projektima napajanja i tipičnim rasporedima PCB -a koji su povezani s DC/DC pretvaračima. Iako su izazovi oko rasporeda eliminirani, potrebno je još obaviti neke inženjerske radove kako bi se optimizirale performanse modula s dobrim dizajnom premosnice i rasipanja topline.