Dex0000 mains filter

DEX_MRHW0000 - basic line filter

A simple line filter circuit implements individual clearances for phase-to-neutral, mains-to-earth, and earth to PCB edge clearances, as well as small self-clearances for nets that share the same DC potential, but may be split to account for AC behavior. The latter is applied hier to the case of a common-mode choke.

Schematic

Net Classes

The Net Classes panel is used to

• declare net class names,
• set basic net class values and
• assign each net in the schematic a class.

Clearance values given here can be superseded by rules written in Design Rules > Rules.

Clearance Rules

When specifying only a basic clearance value in the Net Classes table, this value becomes the distance any net item (track segment, pad, ...) must keep form items in other nets, no matter the net class. Between two items of net classes with differing clearance requirements, the larger clearance is applied.

An equivalent rule for class_x with 0.2mm clearance has the form

(rule "class_X"
    (condition "A.NetClass == 'class_X'")
    (constraint clearance (min 0.2mm)))

Such a rule written in Design Rules > Rules supersedes the implicit definition through the Net Classes table, but is functionally equivalent until the boolean expression it contains is extended. Meanwhile, the Net Classes table values cover all cases not otherwise described in the design rules script.

The distinction between same-class net and other-class net clearances is made by forming two complementary rules. A and B are the corresponding board items between which the clearance resolution is determined.

(rule "N_mutual_clearance"
    (condition "A.NetClass == 'class_N' && B.NetClass != A.NetClass")
    (constraint clearance (min 3.0mm)))

(rule "N_self_clearance"
    (condition "A.NetClass == 'class_N' && B.NetClass == A.NetClass")
    (constraint clearance (min 0.75mm)))

Finally, order-of-evaluation needs to be kept in mind. In the example below, clearance rules named r1-r6 for short are thought to be written in consecutive order, which leads to a chevron stack pattern in the associated clearance matrix. r1, r3, r5 are mutual clearance rules, r2, r4, r6 are same-class clearance rules:

step 1:					step 2:					step 3:
	X	Y	Z			X	Y	Z			X	Y	Z
X		r1	r1		X	r2	r1	r1		X	r2	r3	r1
Y	r1				Y	r1				Y	r3		r3	
Z	r1				Z	r1				Z	r1	r3	

step 4:					step 5:					step 6:
	X	Y	Z			X	Y	Z			X	Y	Z
X	r2	r3	r1		X	r2	r3	r5		X	r2	r3	r5
Y	r3	r4	r3		Y	r3	r4	r5		Y	r3	r4	r5	
Z	r1	r3			Z	r5	r5			Z	r5	r5	r6

The roles of A and B in the rule description can be swapped, which makes the clearance matrix symmetric, so it's usually only given in triangle form.

In its most general form, clearance matrix elements can also be specified individually by writing out all (N²+N)/2 combinations of N net classes occuring in the design, including the diagonal ones:

(rule "clearance_<class_i>_<class_j>"
    (condition "A.NetClass == '<class_i>' && A.NetClass == '<class_j>'")
    (constraint clearance (min 0.2mm)))

where <class_i>, <class_j> are placeholders.

Rules validation

Design Rules can also describe constraints with respect to net classes not (yet) contained in the design. This may at a later stage be pointed out in the form of syntax check info messages. Always validate the correctness of the resulting clearances, for example by selecting two layout items and using Inspect > Clearance Resolution to see which rule and what clearance value was found to apply.

Clearance Rules For This Design

Please see the project files for the full list of rules. An excerpt is given below.

# Project Rules
#
# Phase is segmented to insert a fuse holder and one winding of a common-mode filter choke.
# A spark gap is inserted across the filter winding to prevent insulation breakdown in 
# the event of a transient overvoltage condition. The same concept is applied to neutral.
#
# Clearance Matrix equivalent:
#
#			class_L		class_N		class_PE
#	class_L		0.75mm		3.0mm		4.5mm
#	class_N				0.75mm		4.5mm
#	class_PE					0.2mm*
#
# * Note there is only PE_mutual_clearance for clearance to other nets, so class_PE self-clearance
#   defaults to 0.2mm from Project > Net Classes.

Net Class Inspection

All members of a net class can be highlighted by using the right-click option "Hightlight nets in " of a class listed in Appearance > Nets > Net Classes:

This project is part of a set of design examples. See https://github.com/MisterHW/KiCad-Design-Examples/ for more.