High-frequency transformer for microwave oven

A high-frequency transformer for a microwave oven comprises: an E-shaped first core of ferrite of a low-voltage side; a primary winding wound around the first core over a coil bobbin; an E-shaped second core of ferrite of a high-voltage side disposed in opposition with the first core with a specific gap therebetween; a gap-filling insulation material interposed between the first and second cores; a secondary winding wound around the second core; and a grounding plate connected electroconductively between the first core and a bottom plate of the microwave oven, wherein the distance between the primary winding and the first core is greater than the gap between the two cores.

BACKGROUND OF THE INVENTION 
This invention relates generally to voltage transformers and to microwave 
devices and more particularly to high-frequency transformers for microwave 
cookers or ovens. More specifically the invention relates to a 
high-frequency transformer for a microwave oven in which transformer 
short-circuiting between the windings of the primary coil and the 
secondary coil is positively prevented. 
In general, a microwave cooker or oven for cooking by heating materials to 
be cooked is provided with a built-in magnetron, to which high-frequency 
power from a step-up transformer is supplied through a rectifier. A 
step-up transformer of this type has a primary winding connected to a 
commercial power supply and a secondary winding connected to the magnetron 
side, which windings are wound on a common coil bobbin (for example, as 
described in Japanese Patent Publication No. 3636/1981 and Utility Model 
Publication No. 32733/1976). Although ample electrical insulation is 
originally provided between these primary and secondary windings at the 
time of manufacture, the electrical insulation layer ruptures and causes 
short-circuiting in some cases after use over a long period at a high 
potential difference. 
SUMMARY OF THE INVENTION 
Accordingly, it is a general object of this invention to provide a 
high-frequency transformer for a microwave oven in which the above 
described problem accompanying known high-frequency transformers is 
solved, and the insulation between the primary winding of the low-voltage 
part and the secondary winding of the high-voltage part is improved, 
whereby short-circuiting therebetween is prevented and the degree of 
safety is elevated. 
According to this invention there is provided a high-frequency transformer 
for a microwave oven comprising: an E-shaped first core of ferrite of a 
low-voltage side; a primary winding wound around the first core over a 
coil bobbin; an E-shaped second core of ferrite of a high-voltage side 
disposed in opposition with the first core with a specific gap 
therebetween; a gap-filling insulation material interposed between the 
first and second cores; a secondary winding wound around the second core; 
and a grounding plate connected electroconductively between the first core 
and a bottom plate of the microwave oven, wherein the distance between the 
primary winding and the first core is greater than the gap between the two 
cores. 
According to this invention, the distance between the primary winding and 
the E-shaped ferrite core on the low-voltage side is made large, and at 
the same time, this ferrite core is grounded to the chassis of the 
microwave oven. Therefore, short-circuiting between the primary and 
secondary windings is prevented, and safety at the time of use is assured. 
The nature, utility, and further features of this invention will be more 
clearly apparent from the following detailed description with respect to 
preferred embodiments of the invention when read in conjunction with the 
accompanying drawing, briefly described below.

DETAILED DESCRIPTION OF THE INVENTION 
Referring first to FIGS. 1, 2, and 3, the example of the high-frequency 
transformer according to this invention shown therein has a ferrite core 
assembly comprising an E-shaped lower ferrite core 1 and an E-shaped upper 
ferrite core 6 assembled in mutually opposed relationship with a specific 
gap therebetween. On the lower core 1 is fitted a primary coil bobbin 3, 
around which is wound a primary coil 2 having lead wires 2a. Above the 
primary coil bobbin 3 is disposed a secondary coil bobbin 5, around which 
is wound a secondary coil 4 of enameled wire (solid wire). On this 
secondary coil bobbin 5 is fitted the upper core 6. 
At the lower part of the secondary coil bobbin 5 are heater coil leads 7. 
Terminals 8a and 8b supply filament voltage to, for instance, a magnetron. 
Furthermore, a gap filling insulation material 9 made of insulative paper 
is interposed between the upper surface of the lower core 1 and the lower 
surface 6a of the upper core 6. This gap filling insulation material 9 
forms the insulation distance T according to the creepage distance. A 
feature of this insulation distance T is that it positively maintains the 
insulation distance between the high-voltage part 4a of the secondary coil 
4 and the upper and lower cores 6 and 1 and constitutes an insulation part 
in a manner to improve the magnetic saturation of the upper and lower 
cores 6 and 1. 
Furthermore, as shown in FIG. 4, a portion of the secondary coil 4 extends 
downward into the lower core 1. The transformer is mounted on the bottom 
plate 10 of a microwave oven (not shown), the lower core 1 being fixed to 
the bottom plate 10 by mounting screws 14 with a copper plate 11 for 
grounding interposed between the lower surface of the lower core 1 and the 
bottom plate 10 for connection therebetween. The copper plate 11 for 
grounding is fixed by an outer peripheral tape 12 for fixing. Around the 
outer periphery of the secondary coil bobbin 5 and the heater coil 7 is 
provided an insulation 13 made of insulative paper. The outer peripheral 
tape 12, which is fixing the copper plate 11 for grounding, is wrapped 
around the outer surfaces of the lower and upper cores 1 and 6. 
An important feature of the construction of this transformer is that, 
between the outer periphery of the primary coil 2 and the lower core 1, a 
creepage space distance A (FIG. 1) is provided, and that this distance A 
is formed to be amply greater than the insulation distance T based on the 
creepage distance and formed by the gap filling insulation material 9. The 
significance of this creepage space distance will be described 
hereinafter. 
The principal parts of the high-frequency transformer of the above 
described construction according to this invention are assembled and 
mounted in the following manner. 
The primary coil 2 is wound around the primary coil bobbin 3, which is then 
fitted on the lower core 1. The secondary coil 4 is wound around the 
secondary coil bobbin 5, which is then positioned above the primary coil 
bobbin 3, and on which the upper core 6 is fitted. The gap filling 
insulation material 9 is interposed between the upper core 6 and the lower 
core 1, which are thus assembled. Then, in the case where the transformer 
is to be mounted in a microwave oven, the copper plate 11 for grounding is 
inserted between the lower core 1 and the bottom plate 10 of the oven, and 
the primary coil bobbin 3 is then fixed to the bottom plate 10 by means of 
the mounting screws 14. 
The construction as described above of the transformer of this invention 
affords the following meritorious effects and advantages. 
As mentioned briefly hereinbefore, the creepage space distance A between 
the primary coil 2 and the lower core 1 is made amply greater than the 
insulation distance T defined by the gap filling insulation material 9. 
Because of this provision, even if the insulation material 9 should 
rupture for some unlikely reason, an electric potential of high voltage 
will be impressed between the upper core 6 and the lower core 1 across the 
insulation distance (gap) T. Furthermore, since the lower core 1 is 
grounded by way of the copper plate 1 for grounding to the bottom plate 10 
of the oven the high-voltage side and the ground side assume the same 
potential, whereby a closed circuit is formed. 
In this manner, an ample insulation distance is secured between the 
low-voltage part of the primary coil 2 and the high voltage part of the 
secondary coil 4 and the heater coil 7. Furthermore, since the lower core 
1 of low voltage is grounded by way of the grounding copper plate 11 to 
the bottom plate 10, not only can short-circuiting between the primary 
coil 2 of high voltage and the secondary coil 4 be prevented, but 
improvement in safety can be attained. In addition, since a portion of the 
secondary coil 4 is lapped over the lower core 1, even if the high-voltage 
insulation should rupture, the high voltage will be grounded through the 
grounding copper plate. 11 to the bottom plate 10 without being impressed 
on the primary coil 2, whereby safety is assured.