Inverter switching power supply circuit diagram
The inverter switching power supply circuit can be simplified to the circuit model in the figure below, and the key elements in the circuit are included. For any complex switching power supply, after removing the branches and vines, the main trunk will be left as shown below. In fact, during maintenance, one must have the ability to “simplify” complex circuits, and find out these main threads from the seemingly chaotic circuit extension. You must learn from the chef who knows how to solve the problem, and train yourself that there is no overall switching power supply circuit in front of you, only the individual parts and the direction of the connections – oscillation circuit, voltage stabilizing circuit, protection circuit and load circuit, etc.
Look at how many connections there are in the circuit:
1. Oscillation circuit: The main winding N1 of the switching transformer, the drain-source of Q1, and R4 are the paths for the power supply operating current; R1 provides the starting current; the self-powered windings N2, D1, and C1 form the supply voltage of the oscillation chip. The normal operation of these three links is a prerequisite for the power supply to oscillate.
Of course, the 4-pin external timing components R2 and C2 of PC1 and the PC1 chip itself also form part of the oscillation circuit.
2. Voltage stabilization circuit: The +5V power supply of N3, D3, C4, etc., and R7-R10, PC3, R5, R6 and other components constitute the voltage stabilization control loop.
Of course, the PC1 chip and the peripheral components R3 and C3 on pins 1 and 2 are also part of the voltage stabilizing circuit.
3. Protection circuit:The PC1 chip itself and the 3-pin peripheral component R4 form an overcurrent protection circuit; the D2, R6, and C4 components connected in parallel on the N1 winding form the IGBT protection circuit; essentially the voltage feedback signal of the voltage stabilizing circuit – the voltage stabilizing signal, can also be regarded as a voltage protection signal. However, the content of the protection circuit is not limited to the protection circuit itself. The control of the protection circuit is often caused by abnormalities in the load circuit.
4. Load circuit: N3, N4 secondary windings and subsequent circuits are all load circuits. Abnormalities in the load circuit will involve the protection circuit and the voltage stabilizing circuit, causing the two circuits to take corresponding protection and adjustment actions.
The oscillation chip itself participates in and forms the first three loops. If the chip is damaged, all three loops will stop working together. rightThe maintenance of three or four circuits is carried out on the premise that the chip itself is normal. In addition, just like playing chess, we should use overall concepts and system ideas to judge faults, and see the essence through phenomena. If the oscillation failure occurs, it may not be caused by damage to the oscillation circuit components. It may be a voltage stabilizing circuit failure or an abnormal load circuit, which causes the chip’s internal protection circuit to take control and stop the output of PWM pulses. Each circuit cannot be completely isolated for maintenance. The occurrence of a certain faulty component is likely to have the effect of “impacting one thing and affecting the whole body”.
Switching power supply circuits often exhibit the following three typical fault phenomena
1. The supply voltage of the secondary load is 0V. There is no response after power-on, there is no indication on the operation display panel, and the 24V and 10V voltages measured at the control terminals are 0V.
Check that the main circuit charging resistor or pre-charging circuit is intact, and it can be judged that the switching power supply is faulty. The maintenance steps are as follows:
1. First use the resistance measurement method to measure whether the switching tube Q1 has a breakdown short circuit and whether the current sampling resistor R4 has an open circuit. The component that is easily damaged by electricity is the switching tube. When it is damaged, the resistance of R4 becomes larger or the circuit is broken due to the impact. The G pole series resistor of Q1 and the oscillation chip PC1 are often damaged by strong electric shocks and must be replaced at the same time; check whether there is a short circuit in the load circuit and eliminate it.
2. Replace damaged parts, or there are short-circuit components that are not detected. You can perform power-on inspection to further determine whether the fault is in the oscillation circuit or the voltage stabilizing circuit.
Inspection method:
a. First check whether the starting resistor R1 is open circuit. After normal operation, use 18V DC power supply directly to pins 7 and 5 of UC3844 to power up the oscillation circuit separately. Measure that pin 8 should have a voltage output of 5V; pin 6 should have a voltage output of about 1V. It means that the oscillation circuit is basically normal, and the fault is in the voltage stabilizing circuit;
If there is a 5V voltage output at pin 8, but the voltage at pin 6 is 0V, check that pins 8 and 4 are connected to external R and C timing components, and pin 6 is peripheral circuit; if the voltage measured at pin 8 and pin 6 is both 0V, the UC3844 oscillation chip is broken and needs to be replaced.
b. Power on UC3844 alone and short-circuit the input side of PC2. If the circuit vibrates, it means the fault is in the peripheral circuit on the input side of PC2; if the circuit still does not vibrate, check the circuit on the output side of PC2.
2. The switching power supply has intermittent oscillation.A “hiccup” or “squeak” sound can be heard, or no “hiccup” sound can be heard, but the display panel lights up and goes off when operating.
This is a typical fault characteristic caused by an abnormality in the load circuit, causing the power supply to overload and causing the overcurrent protection circuit to operate. The abnormal increase in the load current causes a substantial increase in the primary winding excitation current, forming a voltage signal above 1V in the current sampling resistor R4, causing the internal current detection circuit of UC3844 to start controlling and the circuit stops oscillating; the overcurrent signal on R4 disappears, and the circuit resumes operation. It starts to oscillate again, and the cycle repeats, and the power supply oscillates intermittently.
Inspection method:
a. Measure the resistance values at both ends of the power supply circuit C4 and C5. If there is a short-circuit through phenomenon, it may be the rectifier diodes D3 and D4. There is a short circuit; observe whether there are bulges, liquid spraying, etc. on the appearance of C4 and C5, and remove them for inspection if necessary; there is no abnormality in the power supply circuit, which may be a short-circuit fault component in the load circuit;
b. Check the power supply circuit If there is no abnormality, power on and use the troubleshooting method to troubleshoot each power supply one by one. If the fan power supply terminal is unplugged and the switching power supply works normally and the operation display panel displays normally, the 24V cooling fan is damaged; if the +5V power supply terminal is unplugged or the copper foil of the power supply is cut off and the switching power supply works normally, the +5V load circuit is There are damaged components.
3. The power supply voltage of the load circuit is too high or too low. The oscillation circuit of the switching power supply is normal, and the problem lies in the voltage stabilizing circuit. The output voltage is too high, and the components of the voltage stabilizing circuit are damaged or inefficient, resulting in insufficient feedback voltage amplitude.
Check method:
a. Connect a 10k resistor to the PC2 output end and the output voltage will drop. It shows that the output side voltage stabilizing circuit of PC2 is normal, and the fault lies in PC2 itself and the input side circuit;
b. Connect a 500Ω resistor in parallel to R7, and the output voltage drops significantly. It means that the photocoupler PC2 is good, and the fault is that PC3 is inefficient or the external resistance component of PC3 changes value. Otherwise, it means PC2 is defective.
If the load supply voltage is too low, there are three possible faults:
1. The load is too heavy, causing the output voltage to drop;
2. The components of the voltage stabilizing circuit are defective, causing the voltage feedback signal to be too large;
3. The switching tube is inefficient, resulting in insufficient energy conversion in the circuit (switching transformer). Inspection and repair methods:
a. Disconnect the load circuits of the power supply branch one by one (note! Do not disconnect the load circuit by opening the power supply rectifier of this circuit, especially if it is connected to a voltage stabilizing feedback signal +5V power supply circuit! The disappearance of the feedback voltage signal will cause the output voltage of each channel to rise abnormally and burn a large part of the load circuit!) Determine whether the voltage drops due to overload; if a certain channel’s power supply is cut off, the circuit rises to Normal value, indicating that the switching power supply itself is normal, check the load circuit; inputThe output voltage is low, check the voltage stabilizing circuit.
b. Check the resistance components R5-R10 of the voltage stabilizing circuit. There is no change in value; replace PC2 and PC3 one by one. If they are normal, it means that the replacement components are inefficient and the conduction internal resistance increases.
c. If the replacement of PC2 and PC3 is ineffective, the fault may be an inefficiency of the switch tube, or a problem with the switch and excitation circuit. It is not ruled out that the internal output circuit of UC3844 is inefficient. Replace with high quality switch tube, UC3844.
For general faults, the above troubleshooting methods are effective, but they are not necessarily 100% correct. If there is no abnormality in the oscillation circuit, voltage stabilizing circuit, and load circuit, but the circuit still has a low output voltage, or intermittent oscillation, or simply no response, this situation may occur. Don’t worry yet, let’s analyze the causes of circuit failures in depth to help identify faulty components as soon as possible. If the cause of the circuit’s intermittent oscillation or oscillation cessation is not the oscillation circuit and the voltage stabilizing circuit, what other reasons can cause the circuit to not oscillate?
(1) The R, D, and C circuits connected in parallel at both ends of the main winding N1 serve as a peak voltage absorption network and provide a discharge path for the magnetic field energy stored in the transformer during the turn-off period of the switching tube (the switching tube Reverse current channel), protecting the switch tube from overvoltage breakdown. When D2 or C4 has serious leakage or breakdown short circuit, the power supply is equivalent to adding a heavy load, causing the output voltage to drop seriously. U3844 has insufficient power supply and the internal undervoltage protection circuit is activated, causing the circuit to enter intermittent oscillation. Because the components are connected in parallel to the N1 winding, it is difficult to detect a short circuit and is often ignored;
(2) Some switching power supplies have input supply voltage (overvoltage) protection circuits. Once the circuit itself fails, The circuit will cause false overvoltage protection action and the circuit will stop vibrating;
(3) The current sampling resistor is defective, such as when the pin is oxidized, carbonized or the resistance becomes large, causing the voltage drop to rise and false overcurrent to occur. protection, causing the circuit to enter an intermittent oscillation state;
(4) The rectifier diode D1 of the self-powered winding is inefficient, the forward conduction internal resistance increases, and the circuit cannot oscillate, so replace it for testing;
(5) The quality factor of the switching transformer is reduced due to moldy windings, moisture, etc., so the original model transformer is used for the test;
(6) The parameters of the R1 oscillation circuit vary, but no abnormality is found in the measurement, or The switching tube is inefficient. At this time, there is no abnormality in the circuit, but it does not vibrate.
Repair method: Change the existing parameters and status of the circuit to expose the fault! Try reducing the resistance value of R1 (should not be less than 200kΩ), and the circuit can start to oscillate. This method can also be used as one of the emergency repair methods. If it is invalid, replace the switch tube, UC3844, and switch transformer for testing.
The output voltage is always a little higher or a little lower than the normal value. No abnormalities in the circuit or components were found, and almost all the components in the circuit were replaced. The output voltage of the circuit was still in a “reluctant and make-do” state. Sometimes it seemed to be “working normally”, but it made people feel uneasy and seemed to be neurotic. Like, I don’t knowWhen will there be an “abnormal behavior”? Don’t give up, adjust the circuit parameters so that the output circuit reaches normal values and reaches its working state, where we can feel “at ease”.
Variations in circuit parameters are due to the following reasons:
1. Inefficiency of the transistor, such as the amplification of the transistor is reduced, or the internal resistance of the transistor is changed. If the diode is large, the forward resistance of the diode becomes larger, the reverse resistance becomes smaller, etc.;
2. The related dielectric loss and frequency loss of the capacitor that cannot be measured with a multimeter;
3. Aging and parameter drift of transistors and chip devices, such as the reduction in light transmission efficiency of photocouplers; 4. The Q value of inductive components, such as switching transformers, etc.;
5. The resistance of resistive components Variation, but not significant.
6. Several of the above five reasons are involved, forming a “comprehensive effect”.
The “current” state of the circuit caused by various reasons is a kind of “sickness”. Maybe we have to change our thinking about maintenance. Traditional Chinese medicine has a theory of “syndrome differentiation and treatment”. We are going to use it too. The next recipe is not to target any one component, but to “condition” the entire circuit so that it changes from “pathological” to “normal”. Just like this “vague and confused”, the disease was cured.
Repair method (slight adjustment of component values):
1. The output voltage is low:
a. Increase R5 Or reduce the resistance value of R6;
b. Reduce the resistance value of R7 and R8 or increase the resistance value of R9.
2. The output voltage is too high:
a. Reduce the resistance value of R5 or increase the resistance value of R6;
b. Increase the resistance value of R7 and R8 or Reduce the R9 resistor value.
The above adjustments are made after a thorough inspection of the circuit and replacement of inefficient components. The purpose is to adjust the relevant gain of the voltage stabilizing feedback circuit, so that the pulse duty cycle output by the oscillation chip changes, and the energy storage of the switching transformer changes, so that the output voltage of the secondary winding reaches a normal value, and the circuit enters a new “normal balance” state. Many seemingly irreparable and difficult faults were repaired without incident after adjusting one or two resistor values.
Issues that need to be paid attention to during maintenance:
1. During the inspection and repair process of the switching power supply, the power supply to the IGBT module of the three-phase output circuit should be cut off. This is to prevent abnormal drive power supply from causing damage to the IGBT module.
2. When repairing a fault where the output voltage is too high, it is necessary to cut off the +5V power supply to the CPU motherboard to avoid abnormal or high voltage damage to the CPU and cause the CPU motherboard to be scrapped.
3. Do not interrupt the voltage stabilizing circuit, as this will cause the output voltage to rise abnormally!
4. The diodes of the switching power supply circuit, used for rectification and protection, are all high-speed diodes.Or Schottky diode, ordinary IN4000 series rectifier diodes cannot be used instead.
5. After the switch tube is damaged, it is best to replace it with the original model. Now that the Internet is so developed, the source of the goods is not a problem and can generally be purchased. You can buy many things at cheap prices online, so pay attention to the quality!
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