JPS6111456B2 - - Google Patents

Description

DETAILED DESCRIPTION OF THE INVENTION The present invention relates to a pulse transformer for a switching power supply for transforming the rectified voltage of a pulsed commercial power supply.

Figure 1 shows a circuit example of a switching power supply using this type of pulse transformer. Commercial power is supplied to a double-wave rectifier circuit D1 through line filters C1, C2, L1, L2, and C3, and this rectified voltage is is smoothed by an electrolytic capacitor C4, converted into a high-frequency pulse by a switching circuit SW1, and transformed by a transformer T1. This transformer T1
The transformed secondary output is rectified by a diode D2 and smoothed by a choke L3 and an electrolytic capacitor C5 to generate a predetermined DC voltage. In addition, D3 is the flywheel diode of Chiyoke L3, and C6
and C7 is a line filter. This circuit configuration makes it possible to make the power supply smaller and lighter, but on the other hand, in order to rectify and switch the commercial power supply voltage such as 100V, there is always a power supply on the primary side of the transformer T1.
High-frequency pulses of 100V to several 100V are generated, and there is a drawback that common mode noise sources occur on the primary and secondary sides. As shown in Fig. 2, these noise sources are divided into voltages and fed together by the capacitance between the primary and secondary windings of the transformer T1 to become a common noise source _eN to the ground, and the input side filters L1 and L
2, C1, C2 and output side filters C6, C7, and are sent out to the outside, that is, to the power supply line and load, as primary common mode noise and secondary common mode noise, respectively. Therefore, in order to suppress the transmission of these noises to the outside, a filter capacitor C1 connected to the frame ground FG,
It was necessary to increase the capacitance of C2, C6, and C7, and as a result, the leakage current of the commercial power supply through C1 and C2 increased, making it particularly difficult to adopt it for ME equipment from the viewpoint of safety.

SUMMARY OF THE INVENTION Therefore, an object of the present invention is to provide a pulse transformer for a switching power supply that can greatly suppress the transmission of noise to the outside, regardless of the capacitance of the line filter.

Next, the present invention will be explained based on the illustrated embodiments.

A secondary winding 3 is first wound around the inner leg of the EI type core 1 via an insulating paper 2, and a primary winding 4 is wound on the outside of the secondary winding 3.
are wound concentrically, and these windings 3 and 4
A secondary electrostatic shielding foil 8 and a primary electrostatic shielding foil 9 with an insulator 5 interposed therebetween are inserted through insulating papers 6 and 7, respectively, and are wound.
Outside the primary winding 4, there is another secondary winding 3'.
are wound, and insulating paper 7', primary electrostatic shielding foil 9', insulator 5', secondary electrostatic shielding foil 8' and insulating paper are similarly placed between these windings 4 and 3'. 6'
The secondary windings 3 and 3' are wound in sequence, and the starting and ending ends of the respective windings are connected to each other to form a parallel circuit.

When a pulse transformer for a switching power supply having such a configuration is used as the transformer T1 in FIG. The secondary electrostatic shielding foils 8, 8' are likewise connected to the negative terminal and to a point where no pulses occur, for example to the negative terminal of the electrolytic capacitor C5. As a result, common mode noise on the primary side returns to the negative terminal of the electrolytic capacitor C4 through the primary winding 4 and the primary electrostatic shield foils 9, 9', and is confined inside. Similarly, common mode noise on the secondary side is transmitted through the electrostatic capacitor C5 through the secondary electrostatic shielding foils 8 and 8'.
returns to the negative terminal of the terminal and is trapped inside. That is, the level of the noise source e _N relative to the ground of the primary and secondary circuits explained in FIG. 2 is greatly reduced. In this case, since the electrostatic shielding foils 8, 9 and the plate-shaped insulator 5 are interposed between the secondary winding 3 and the primary winding 4, the mutual spacing increases, and such a winding structure itself causes leakage magnetic flux. Although the pulse transmission loss increases and the waveform characteristics deteriorate due to the increase in Due to the presence of the secondary windings 3', which are windings connected in parallel with each other, the direction of leakage magnetic flux is reversed between the two secondary windings 3, 3', so that the leakage inductance is conversely reduced. In other words, even if the insulators 5 and 5' that insulate between the two shield foils are made thicker to achieve high withstand voltage and high insulation, common mode noise can be confined inside without deteriorating the efficiency or pulse waveform. .

In the case of parallel winding, the secondary winding can be sandwiched between the primary winding in a sandwich-like manner, contrary to the above embodiment. The core is limited to the EI type, and can also be shaped like a cup.

As described above, by applying mutually insulated electrostatic shields to the primary and secondary windings of the present invention, the transmission of pulse noise from the switching power supply to the power supply line and load is significantly reduced, and therefore the capacitor of the line filter is The capacitance can be reduced, and this also reduces leakage in the power supply line, and it is also possible to reduce the application of power supply voltage to the load through the line filter on the power supply side when the frame ground is floating from the ground. In addition, by using two windings for either the primary or secondary windings and using sandwich winding, the leakage current and withstand voltage can be reduced to increase the thickness of the insulator between both electrostatic shielding foils without deteriorating the pulse transformation characteristics. Switching power supplies can be easily adopted even for ME equipment that has strict standards regarding power supply.

[Brief explanation of the drawing]

FIG. 1 shows an example of a circuit for a switching power supply, FIG. 2 is an explanatory diagram of a path for sending common mode noise to the outside in the circuit of FIG. 1, and FIG. 3 is a sectional view of a pulse transformer for a switching power supply according to the present invention. 3, 3': Secondary winding, 4: Primary winding, 5, 5':
Insulator, 2, 6, 6', 7, 7': Insulating paper, 8,
8': Secondary electrostatic shielding foil, 9, 9': Primary electrostatic shielding foil.

Claims (1)

[Claims] 1 A primary winding and a secondary winding are wound concentrically around a common core, the primary winding is connected to a primary circuit that generates pulses using a rectified commercial power source, and the secondary winding is a transformer. In a pulse transformer for a switching power supply, which is connected to a secondary circuit that rectifies and smoothes the pulses, the primary and secondary windings are connected in parallel in the same winding direction and number of turns as the inner winding. A winding is wound outside the other winding, and two electrostatic shielding foils are interposed between each winding through an insulator, and these electrostatic shielding foils are connected to the circuit on the winding side to which they belong. A pulse transformer for a switching power supply characterized by being connected to each reference potential point.