MC34063AP switching power supply principle and maintenance

When I was repairing the Jieenxi electronic display, I found that the power supply used the switching power supply composed of MC34063AP. For the convenience of inspection, I mapped the circuit, as shown in Figure 1.

Figure 1 Jie Enxi electronic display switching power supply circuit diagram

First, MC34063AP chip overview

MC34063AP is a cheap and practical first-generation switching power supply chip. It was introduced in the 1980s. Its switching power supply circuit is simple and flexible. It is a general-purpose integrated circuit. DIP-8 (dual in-line) and SO-8 (small outline package for surface mount) are common in external package types, and their pin functions are shown in Table 1.

Table 1 MC34063AP pin function

There has been a TDA9383 foot field loss or failure analysis: This fault belongs to the "field loss protection" automatic shutdown. According to the characteristics of Konka "K" series special protection circuit, the field output LA7845N internal and external circuit or field drop protection detection circuit TDA9383 pin (beam current, field circuit loss protection end) peripheral circuit such as: field output circuit LA7845N7 foot to TDA983 pin connection When any component of the component between the components of the T4027 pin (ABL) and TDA9383 is abnormal, the CPU (TDA9383) detects the "field loss" message and sends a protection shutdown via the internal bus (standby). The instruction causes the CPU1 pin (standby control terminal) and the interface circuit to perform a work ban on the line scan circuit and the decoding circuit, that is, the line excitation pulse HD and the 8V power supply voltage output are cut off, causing the related circuit to stop working, so that the whole machine is in the protection standby state. . The beam current drop protects the automatic shutdown fault.

Second, the rectification, filtering part

It is mainly composed of transformer T (220V/12V, 10W), bridge rectifier circuit (D5~D8) and filter capacitor C26 (470μF/35V). Its function is: 12V AC is generated by the step-down of the transformer T, and after rectification and filtering, an unstable DC voltage of about +12~16V is generated, and the voltage varies with the display content.

Third, MC34063AP switching power supply works

The function of the switching power supply is to generate a stable +5V voltage. The internal structure block diagram of the integrated circuit MC34063AP is shown in Figure 2. It consists primarily of a 1.25V voltage reference, voltage comparator, oscillator, latch, driver stage, and output stage.

Figure 2 MC34063AP structure block diagram

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