DE8710023U1 - Primary switched mode power supply - Google Patents

Description

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See 3462/87 j

7* July 1987 1

Werner Schaffer Transformers GmbH & Co KG
8340 Parish Churches

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! The innovation concerns a primary clocked switching |

\ power supply of the flyback converter type. The essential I

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electronic components of such a network |

partly a bridge rectifier, a downstream |

switched smoothing capacitorj a power over- |

carrier with a primary winding, one end of which \

connected to the bridge rectifier and with |

at least one secondary winding, furthermore a switching |

transistor, which is the other end of the primary winding J

periodically to ground, as well as a control element <

to control this switching transistor. After the |

Switching power components constructed on the flyback converter principle |

are particularly suitable for the transmission of small \

} Output up to approx. 200 watts. Compared to

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concepts

- with other converter types, to simple circuits

When designing power supplies for office equipment,
VDE guideline 0806 must be observed. A special

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The problem here is the occurrence of high-frequency interference, which can be traced back to the steep switching armatures of the switching transistor, which switches on and off at high frequency. The rectangular pulses that occur in the secondary circuit of the power transmitter generate high current pulses, which make appropriate dimensioning of the electronic components at the output of the circuit necessary. Compliance with the VDE guideline mentioned

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decorated winding structure of the transformer.

The aim of this innovation is to propose a primary-switched power supply of the flyback converter type that is particularly easy and cost-effective to manufacture and in which the high-frequency interference is greatly reduced.

The solution to this problem is based on |

a primary-switched power supply according to the type mentioned at the beginning; according to the first protection claim, the problem is solved in that the power supply f

Transformer is a two-chamber transformer in whose |

first chamber the primary winding and in its second

Chamber the secondary winding is housed, and |

by connecting a capacitor between the collector and emitter of the switching transistor. The primary winding and the secondary winding are spatially separated from each other in the two-chamber transformer used here. This results in a deliberately poor magnetic coupling, which leads to a large magnetic stray field. The capacitor connected parallel to the switching transistor

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transistor additional capacitor forms together with the inductance of the primary winding
a kind of series resonant circuit. As soon as a current begins to flow on the secondary side, this capacitor is charged. After it has been fully charged, the capacitor again supplies
electrical current into the primary winding of the two-chamber transformer. In the proposed switching power supply, this results in a line

IQ power transmission with an almost sinusoidal voltage curve. The switching transistor is switched on exactly during the zero crossing of the sinusoidal half-wave. Due to the "soft" magnetic coupling, hard magnetic feedback is avoided.

avoided and thus high-frequency interference is drastically reduced. Compared to the conventionally constructed power transformers used up to now, the use of a simple two-chamber transformer enables a significant reduction in production

^q costs of the entire switching power supply. The requirements according to VDE guideline 0806 are easily met.

The use of a two-chamber coil former, on which the primary winding and the secondary winding are wound next to each other and which sits on a common ferrite core, has proven to be advantageous.

go &idigr;&eegr; appropriate further development of the innovative power supply unit, one or more additional secondary windings are housed in the second chamber of the two-chamber coil body, so that on the output side

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Different DC voltages can be tapped*

The switching transistor is conveniently controlled by means of an integrated circuit that contains a comparator and a zero-crossing indicator. This control IC monitors the magnetization in the transformer. The zero-crossing indicator detects the zero crossing of the sinusoidal half-wave charging or discharging current of the capacitor between the emitter and collector of the switching transistor. The comparator stage also contained in the integrated circuit serves to monitor the output voltage so that the duration of the magnetization current can be controlled accordingly.

The capacitor in parallel with the switching transistor must be precisely matched to the primary winding in order to achieve the desired soft switching close to the zero point of the sine half-wave. Particularly good results were achieved with switching power supplies in which the inductance of the primary winding was around 5.4 mH and the capacitance of the capacitor was in the range of 10 nF.

An embodiment of the innovation is explained in more detail below with reference to the attached drawings.

Show it:

Fig. 1 a switching power supply with three outputs

outputs on the secondary side, in a circuit diagram;

-&dgr;-Fig. 2 the power transformer of the

Switching power supply according to Fig. I ₁ in a simplified vertical section.

The primary-switched power supply shown in Figure 1, which is constructed according to the flyback converter principle, is designed for an output of around 30 W. The input alternating voltage UIN is first rectified by means of a bridge rectifier consisting of four rectifier diodes D 1, D 2, D 3 and D 4. A smoothing capacitor C 1 is connected downstream. The power transformer TR 1 is constructed as a two-chamber transformer. It has a primary winding N 2, a secondary winding N 4 and further secondary-side windings N 3 and N 5. One end of the primary winding N 2 is connected to the bridge rectifier (D 3, C 1). A switching transistor T 1 periodically connects the other end of the primary winding N 2 to ground. An integrated circuit IC 1 of type TDA 4601 serves as the control element for controlling the switching transistor T 1.

A capacitor C 4 with a capacitance of 10 nF is connected between the collector and emitter of the switching transistor T 1. The inductance of the primary winding N 2 is approximately 5.4 mH. Due to the spatial separation of the primary winding N 2 and the secondary winding N 4, a leakage inductance of approximately 2.2 mH results.

If the switching transistor T 1 now connects the primary winding N 2 to ground, the magnetization current increases

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-δ-in the two-chamber transformer TR 1. A target-actual value comparison takes place via the connection of the integrated circuit IC 1. IC 1 limits the duration of the magnetization current by switching off the switching transistor T 1 depending on the voltage at the output plus 5 V. Magnetic energy is stored in the ferrite core of the transformer TR 1. At the same time, the magnetization is monitored via connection 2 of IC 1. As soon as T 1 has gone into the blocking state, the magnetic energy stored in the ferrite core is released to the secondary side of TR 1. While current begins to flow through the diode D 103, the capacitor C 4 is charged. After it is fully charged, C 4 supplies current back to the primary winding N 2. The result is a voltage curve in the form of a sine half-wave. The zero crossing of this sine half-wave is monitored by IC 1. As soon as its connection 2 is positive again, the switching transistor T 1 is switched on again. The switch-on time can be set as close as possible to the zero point of the sine half-wave by precisely adjusting the inductance of the primary winding N 2 and the capacitance of the capacitor C 4.

Figure ? shows schematically the winding structure of the two-chamber transformer TR 1. The coil body SK has two separate chambers K 2 and K 3 av ^f . The primary winding N 2 is located in the chamber K 2 , while the secondary winding N 4 is housed in the adjacent chamber K 3 . In this second chamber &kgr; 3 there are also further

1 Secondary windings N 4 and N 5, which are used to tap additional output voltages (see Figure 1). The interior of the coil body SK is penetrated by a common ferrite core FK.

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Claims (5)

Sch 3462/87
July 7, 1987 S ection claims 1» Primary switched mode power supply of the flyback converter type, with - a bridge rectifier, - a downstream smoothing capacitor, - a power transformer with a primary winding, one end of which is connected to the bridge rectifier ir ¹ :, and with at least one secondary winding, ~ a switching transistor that periodically connects the other end of the primary winding to ground, - a control element for controlling the switching transistor, characterized in that - the power transformer is a two-chamber transformer (TR 1), in the first chamber (K 1) of which the primary winding (N 2) is housed and in the second chamber (K 2) of which the secondary winding (N 4) is housed, and 3Q - that a capacitor (C 4) is connected between the collector and emitter of the switching transistor (T 1). 2. Switching power supply according to claim 1, characterized in that the primary I · t I · I ■ · r * winding (N 2) and the secondary winding (N 4) are wound next to each other on a two-chamber coil former (TR 1) which sits on a common ferrite core (FK). 3. Switching power supply according to claim 1 or 2, characterized by further secondary windings (N 3, N 5), which are also accommodated in the second chamber (K 2). 10 4. Switching power supply according to one of claims 1 to 3, characterized in that the control element for controlling the switching transistor is an integrated circuit (IC 1) which contains a comparator and a zero-crossing indicator. 5. Switching power supply according to one of claims 1 to 4, characterized in that the inductance of the primary winding (NZ) is approximately 5.4 mH and the capacitance of the capacitor is approximately 10 nF. 25 30 f36