Mitsubishi FX1S Switching Power Supply Maintenance and STRG6551 Chip Circuit Analysis and Circuit Diagram

This is the circuit schematic diagram of the Mitsubishi PLC switch power supply board that I surveyed based on physical objects. There are two (isolated by L2, which can be called two) 24V outputs, and the 5V power supply of the motherboard is obtained through this voltage reduction. The Power circuit board is connected to the motherboard through copper needle shaped hard wires. The 5 and 6 pins of the socket are stabilized and processed into 5V after entering the motherboard, providing power for the motherboard. The 4 and 7 pins are used as input control power for the X terminal, and can also be used as an external control power supply for DC24V, for external measuring instruments such as encoders. The two sets of power supplies are isolated through L2 bidirectional filters, and the function of L2 also blocks high-frequency interference pulses from the internal radiation of the power supply to the outside and from the external radiation of the power supply to the inside, making the system operation more stable.
Below is a brief description of the working principle of the circuit: the power supply of 220V at power frequency enters the power board through the L and N terminals of the PLC. C1, C2, C4, and L1 form a bidirectional low-pass filtering network. The function of L1 and L2 is the same. The advantage of bidirectional filtering is that the high-frequency components in the circuit are magnetically coupled and cancelled out, greatly enhancing the filtering effect. The power supply enters the full wave rectification circuit through dual fuses F1 and TH1. F1 is an overload protection quick fuse, and TH1 is a temperature fuse. When the ambient temperature is too high or the component heats up and reaches the cutoff threshold of TH1, but the current value does not reach the level of F1 melting, TH1 will disconnect in advance to protect the PLC from burning out due to excessive temperature rise. After the temperature drops, TH1 can return to its on state on its own. The mains power is rectified to a DC voltage of about 280V, which is added to the oscillation and voltage stabilization circuit of the primary switch transformer TB1, with the power oscillation module STRG6551 as the core.
Even if there is no circuit information for STRG6551 at hand, it is easy to see its pin function from the circuit structure: pins 4 and 3 are used for power supply; 1. Two pins are internally connected to the source and drain of a power switch (MOS) transistor, and the two pins also provide negative feedback on the switch’s operating current; Pin 5 introduces feedback voltage. The function of STRG6551 is similar to the commonly used switch mode power supply oscillator U3844, except that it will be turned on
Guan Guan is just integrated into it. Once the functions of these pins are understood, it is not difficult to analyze the role of peripheral circuits.
It can be roughly divided into three circuits for analysis. (1) Power on vibration and power supply branch, abbreviated as oscillation circuit; (2) Voltage stabilizing circuit; (3) Protection circuit;
First, look at the (1) branch – oscillation circuit: When powered on, the DC voltage after 280V rectification is reduced and embedded on 30V through R1, R11, and D5. Then, R12 is sent to the power supply pin 4 of STRG6551 to provide the internal COMS switching transistor with the starting voltage and current, forming a current path from the initial winding of TB1 to the switching transistor source to the 280V negative pole. Then, the secondary winding 1 of TB1
The induced current is rectified and filtered by R8, D3, C8, etc., and used as the working power supply for STRG6551.
Next, let’s take a look at (2) branch one – voltage stabilizing circuit: TB1 winding 2 induces voltage (current), which is rectified and filtered by D4 and C10. It serves as the overall working power supply of the PLC, input terminal control power supply, and output relay power supply. The stability of this voltage determines the working performance of the PLC. Therefore, R9, IC2, and PC1 output voltage sampling circuits are used. IC2 is a 24V voltage stabilizing device, providing
The voltage reference for the sampling circuit forms the change in input current on the PC1 optocoupler device. This change is fed into the 5 pins of STRG6551 through R4 at the output end of the optocoupler. The internal comparison amplification circuit is used for processing, controlling the on/off time of the switching transistor, i.e. controlling the duty cycle of the oscillation frequency, in order to achieve stable output voltage.
Looking at branch (3) – protection circuit: The sampling resistance of the working current of the internal MOCS switching tube is R2. When the load abnormality causes a sharp increase in current, this current/voltage change is introduced through R3 to pin 5 of STRG6551, causing a decrease in output voltage to lower the load current. When the voltage drop on R2 reaches a certain threshold, the internal circuit of STRG6551 disconnects the driving circuit of the switch tube, causing the circuit
Stop vibration and implement power outage protection; This circuit is a current protection implemented in case of abnormal load, and there is also a voltage protection branch in case of abnormal input, composed of components such as R5, R6, R11, D1, etc. When the input voltage is abnormal (high), such as poor contact of the neutral line, causing 220V to rise to 260V or higher. After dividing the voltage by R5 and R6, it still reaches the breakdown value of D1. D1 breakdown feeds this voltage into pin 5 of STRG6551, causing the switching power supply to stop vibrating and protecting the subsequent circuit from overvoltage damage. It can be seen that pin 5 is the summary pin for various protection signals, and both voltage and current abnormal signals are converted into voltage signals, which are sent from this pin. In essence, the output voltage feedback signal from the optocoupler device – a voltage stabilizing signal, can also be seen as a voltage protection signal. When the voltage sampling circuit is abnormal, it can also cause the switching power supply to stop vibrating, playing a protective role. In addition, D6 is connected in parallel at both ends of the primary coil of the switch transformer, providing a discharge path for the magnetic field energy stored in the transformer during the cut-off period of the switch tube, protecting the switch tube from overvoltage breakdown, and is also considered a protective branch.
Let’s talk about the repair of switch mode power supply:
A common fault of a switching power supply is probably the lack of output voltage. Essentially, due to the involvement of its protection and control circuits, when the load is abnormal, the input is abnormal, or the circuit itself is abnormal, the internal circuit of STRG6551 will send a cut-off command to the switching transistor, causing the switching power supply to stop vibrating and resulting in no output. The phenomenon of no output from the power supply is caused by various reasons, and even a simple circuit may encounter difficulties in maintenance. Adopting reasonable maintenance steps and scientific maintenance methods is a major way to improve maintenance efficiency.
Simply put, the F1 and TH1 fuses are blown, which is a common fault. Check the switch tube and load circuit for any abnormal short circuit points. After replacing the fuse, it can generally be repaired; When replacing the fuse and the power supply still does not vibrate, or even after replacing STRG6551, it is necessary to first distinguish whether the fault is in the vibration circuit or the voltage stabilization circuit.

1. Disconnect the PLC motherboard and use a voltage regulator to set the input voltage below AC100V. It is best to use a 100 ohm 5-watt resistor indirectly on terminals 5 and 6 as a false load. Short circuit PC1 optocoupler
2. Two pins, namely the light-emitting diode side, make the voltage feedback signal zero (PC1 pins 3 and 4 should be measured first and not short circuited). When powered on, there are two situations: the power supply has output, but it is not a stable voltage value; The power supply still has no output. The former is a voltage stabilizing circuit fault, check IC2 and other components; The latter is a fault in the oscillation circuit, check the above oscillation circuit.
   • For general faults, the above troubleshooting methods are effective, but sometimes they may not be very effective. Let’s delve deeper into analyzing the cause of circuit stalling to help identify faulty components as soon as possible. What other reasons can cause the circuit to stop vibrating when it is not in the starting and stabilizing circuits?
     (1) Abnormal protection circuit, such as D1 breakdown, feeding the main circuit voltage directly into pin 5, causing the internal circuit to mistakenly believe that the input voltage is abnormal, and causing the switching tube to stop vibrating;
     (2) D6 breakdown or leakage is equivalent to the switching transformer being connected in parallel with an extremely heavy load, causing insufficient induced voltage amplitude and stopping vibration. Due to D4 being connected in parallel to the primary winding, it is difficult to detect a short circuit, so attention should be paid;
     (3) When the load side circuit is abnormal, it is manifested as difficulty in stopping or starting the circuit, that is, intermittent oscillation produces a “squeaking” sound. If D4 or C10 are faulty or there is a short circuit fault in the subsequent circuit, the voltage drop on the sampling resistor of the switching tube current will increase, causing the power supply to stop vibrating:
     (4) The parameters of the R1, R11, and D5 oscillation branch may vary, but no major abnormalities can be detected, or the switch tube in STRG6551 may age and the efficiency may decrease. At this time, after checking the circuit, there are no abnormalities, but the oscillation does not occur, leaving people feeling at a loss. What should we do? Change the existing parameters and status of the circuit to expose faults! Try to reduce the resistance value of R2, even if the output voltage is lowered, you can
     When the resistance test is short circuited and the R2 value is reduced to a certain value, the circuit vibrates. This method can also be used as one of the emergency repair methods. Replace D5, C8, and C12 with STRG6551, which has excellent performance.
     When the internal switch tube of STRG6551 is broken, R2 often burns out, so attention should be paid.
     The online measurement of key components is measured using a pointer multimeter. The online pin resistance values of STRG6551 and SE024 are attached in the circuit diagram for reference.