AN1262 APPLICATION NOTE

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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

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.

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

Vin

Vout

L6590

L6590D

DRAIN

GND

Vcc

Vac

VFB

Primary Power GND

Primary Signal GND

One-point GND

BOK

Secondary GND

L6590D only

C

Y1