AN1262 Datasheet, PDF (32/42 Pages) STMicroelectronics – APPLICATION NOTE

AN1262 APPLICATION NOTE
Some fundamental rules will be given to enable the designer to successfully produce a good layout.
All of traces carrying high currents, especially if pulsed (the bold ones in figures 18 and 19), should be as short
and fat as possible. This will keep both resistive and inductive effects to a minimum, in favor of efficiency as well
as radiated RFI. If a two layer PCB is used, some of these traces could be routed parallel on both sides.
Noise coupling and radiation will also be reduced by minimizing the area circumscribed by current loops where
high pulsed currents flow, that is the bolded ones in figures 18 and 19. The most critical loop is that including
the input bulk capacitor, the transformer and the L6590, thus these components should be next to one other. In
figure 20 an example of possible component placement is given.
Figure 19. Suggested ground routing for converters with primary feedback
Vin
Vout
L6590D only
Vac
DRAIN
Secondary GND
BOK
L6590
Vcc
C
Y1
L6590D
VFB
GND
Primary Signal GND
One-point GND
Primary Power GND
Current returns (or ground) routing is also very important. All of them (signal ground, power ground, shielding,
etc.) should be routed separately and should be connected only at a single ground point, as suggested in figures
18 and 19.
Generally, traces carrying signal currents should run far from others carrying pulsed currents or with quickly
swinging voltages like the bolded ones of figures 18 and 19. From this viewpoint, particular care should be taken
of the feedback path. In case of two layer PCB, it is a good practice to route signal traces on one PCB side and
power traces on the other side.
Some crucial points of the circuit need or may need filtering, such as the V_CC pin or the BOK pin. In case, high-
frequency filter capacitors (with plastic film or ceramic dielectric) should be placed between these pins and the
"signal ground" route, as close to the IC as possible.
Reduction of common mode emissions requires a Y1 class capacitor (or two series connected Y2 class ones)
connected between the primary and secondary ground. This decoupling capacitor should be connected as close
to the transformer as possible.
Another important point is related to creepage distance: this must be observed between primary and secondary
ground (8mm), between the phases of the input voltage (4 mm) and the opposite ends of the primary winding
of the transformer (4mm). Concerning the primary-to-secondary ground separation, no component or traces
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