The switching power supply circuit of a frequency converter can be completely simplified to the circuit model shown above, which includes all the key elements of the circuit. Any complex switching power supply, after removing the extraneous details, will ultimately be reduced to a main structure like the one shown above. In fact, during troubleshooting, it's essential to have the ability to "simplify" complex circuits, to extract the main threads from seemingly chaotic circuit layouts. We should learn from the skilled butcher who skillfully dissects an ox, training ourselves to see not the overall switching power supply circuit, but only the individual components and their pathways—oscillation circuits, voltage regulation circuits, protection circuits, and load circuits, etc.
Take a look at how many pulses are in the circuit.
1. Oscillation Circuit: The drain-source terminals of the main windings N1 and Q1 of the switching transformer, and R4 form the path for the operating current of the power supply; R1 provides the starting current; the self-powered windings N2, D1, and C1 form the power supply voltage for the oscillation chip. The normal operation of these three components is a prerequisite for the power supply to oscillate.
Of course, the external timing components R2 and C2 connected to pin 4 of PC1, along with the PC1 chip itself, also constitute part of the oscillation circuit.
2. Voltage regulation circuit: The +5V power supply of N3, D3, C4, etc., and components such as R7-R10, PC3, R5, R6, etc. constitute the voltage regulation control circuit.
Of course, the PC1 chip and the peripheral components R3 and C3 at pins 1 and 2 are also part of the voltage regulation circuit.
3. Protection Circuit: The PC1 chip itself and the peripheral component R4 at pin 3 constitute an overcurrent protection circuit; the components D2, R6, and C4 connected in parallel on the N1 winding constitute the IGBT protection circuit; in fact, the voltage feedback signal of the voltage regulation circuit—the voltage regulation 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 activation of the protection circuit is often caused by abnormalities in the load circuit.
4. Load Circuit: The secondary windings of N3 and N4 and subsequent circuits are all load circuits. An abnormality in the load circuit will involve the protection circuit and the voltage regulation circuit, causing both circuits to take corresponding protection and adjustment actions.
The oscillator chip itself participates in and constitutes the first three circuits; if the chip fails, all three circuits will stop working simultaneously. Troubleshooting three or four circuits is only performed under the premise that the chip itself is functioning correctly. Furthermore, fault diagnosis should be approached with a holistic and systemic perspective, like playing chess, looking beyond the surface to the underlying cause. For example, an oscillation failure might not be caused by a damaged component in the oscillator circuit, but rather by a fault in the voltage regulator circuit or an abnormality in the load circuit, triggering the chip's internal protection circuit and stopping the PWM pulse output. It's crucial not to isolate each circuit completely during troubleshooting; the occurrence of a faulty component can have a domino effect.
Switching power supply circuits often exhibit the following three typical fault phenomena:
1. Secondary load supply voltage is 0V. The inverter has no response after power-on, the operation display panel shows no indication, and the 24V and 10V voltages at the control terminals are 0V. After checking that the main circuit charging resistor or pre-charge circuit is intact, the problem can be identified as a fault in the switching power supply. The troubleshooting steps are as follows:
1. First, use the resistance measurement method to check whether the switching transistor Q1 is short-circuited or short-circuited, and whether the current sampling resistor R4 is open-circuited. The switching transistor is the most easily damaged component in the circuit. When it is damaged, the resistance of R4 will increase or become open-circuited due to the impact. The series resistor at the gate of Q1 and the oscillator chip PC1 are often damaged by strong current surges and must be replaced at the same time; check the load circuit for short circuits and eliminate them.
2. Replace the damaged parts, or if a short-circuited component is not detected, perform a power-on inspection to further determine whether the fault lies in the oscillation circuit or the voltage regulation circuit.
Inspection method:
a. First, check if the starting resistor R1 is open-circuited. If it is normal, directly supply 18V DC power to pins 7 and 5 of the UC3844 to power the oscillation circuit separately. Measure that pin 8 should have a 5V output voltage; pin 6 should have a voltage output voltage of about 1V. This indicates that the oscillation circuit is basically normal, and the fault lies in the voltage regulation circuit.
If the voltage output at pin 8 is 5V, but the voltage at pin 6 is 0V, check the external R and C timing components connected to pins 8 and 4, and the external circuitry of pin 6.
If the voltage at pins 8 and 6 is 0V, the UC3844 oscillator chip is faulty and needs to be replaced.
b. Power on the UC3844 separately and short-circuit the PC2 input side. If the circuit starts oscillating, the fault is in the peripheral circuit of the PC2 input side; if the circuit still does not oscillate, check the PC2 output side circuit.
2. The switching power supply exhibits intermittent oscillations, accompanied by a "hiccup" or "squeak" sound, or sometimes no "hiccup" sound, but the operation display panel flickers on and off. This is a typical fault characteristic caused by an abnormal load circuit, leading to power supply overload and triggering the overcurrent protection circuit. An abnormal rise in load current causes a significant increase in the primary winding excitation current, generating a voltage signal above 1V on the current sampling resistor R4. This activates the internal current detection circuit of the UC3844, stopping the circuit oscillation. When the overcurrent signal on R4 disappears, the circuit restarts oscillation, and this cycle repeats, resulting in intermittent power supply oscillations.
Inspection method:
a. Measure the resistance values across C4 and C5 in the power supply circuit. If there is a short circuit, it may be due to a short circuit in rectifier diodes D3 and D4. Observe the appearance of C4 and C5 for bulging, liquid spraying, etc. If necessary, remove them for testing. If there is no abnormality in the power supply circuit, it may be due to a short circuit fault in the load circuit.
b. After checking the power supply circuit and finding no abnormalities, power on the circuit and use a process of elimination to troubleshoot each power supply line one by one. If disconnecting the fan power supply terminal allows the power supply to work normally and the operation display panel to show normally, then the 24V cooling fan is damaged. If disconnecting the +5V power supply connector or cutting the power supply copper foil allows the power supply to work normally, then there is a damaged component in the +5V load circuit.
3. The supply voltage of the load circuit is too high or too low. The oscillation circuit of the switching power supply is normal; the problem lies in the voltage regulation circuit.
The output voltage is too high, or the components in the voltage regulator circuit are damaged or inefficient, resulting in insufficient feedback voltage amplitude. Troubleshooting method:
a. Connecting a 10k resistor in parallel to the output of PC2 causes the output voltage to drop. This indicates that the voltage regulator circuit on the output side of PC2 is normal, and the fault lies in PC2 itself or the input side circuit.
b. Connecting a 500Ω resistor in parallel with R7 causes a significant drop in output voltage. This indicates that optocoupler PC2 is working properly, and the fault lies with inefficient PC3 or a change in the value of the external resistor connected to PC3. Conversely, if the voltage returns to normal, PC2 is faulty.
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 voltage regulator circuit components are faulty, resulting in an excessively large voltage feedback signal; 3. The switching transistor is inefficient, causing insufficient transduction in the circuit (switching transformer).
Inspection and repair methods:
a. Disconnect the load circuits of each power supply branch one by one (Note! Do not disconnect the load circuit by opening the rectifier tube of that power supply, especially the +5V power supply circuit with a voltage regulation feedback signal! The disappearance of the feedback voltage signal will cause the output voltage of each branch to rise abnormally, which will burn out a large part of the load circuit!) to determine whether the voltage drop is caused by excessive load; if the circuit returns to the normal value after disconnecting a certain power supply, it means that the switching power supply itself is normal, check the load circuit; if the output voltage is low, check the voltage regulation circuit.
b. Check the resistors R5-R10 in the voltage regulator circuit. If there is no change in value, replace PC2 and PC3 one by one. If it works normally, it means that the replaced components are inefficient and the conduction resistance has increased.
c. If replacing PC2 and PC3 does not resolve the issue, the fault may be due to inefficient switching transistors, problems with the switching and drive circuits, or inefficiency in the internal output circuit of the UC3844. Replace with high-quality switching transistors and the UC3844.
For general faults, the above troubleshooting methods are effective, but not always foolproof. If the oscillation circuit, voltage regulator circuit, and load circuit are all found to be normal, but the circuit still has a low output voltage, intermittent oscillation, or no response at all, these situations are all possible. Don't panic yet; let's analyze the causes of the circuit fault in more depth to help find the faulty component as quickly as possible. If the cause of intermittent oscillation or cessation of oscillation is not in the oscillation starting circuit or voltage regulator circuit, what other reasons could cause the circuit to fail to oscillate?
(1) The R, D, C circuit connected in parallel across the main winding N1 forms a peak voltage absorption network, providing a path for the release of magnetic field energy stored in the transformer during the switching transistor's off period (the reverse current path of the switching transistor), protecting the switching transistor from overvoltage breakdown. When D2 or C4 experiences severe leakage or short circuit, the power supply is equivalent to adding a very heavy load, causing the output voltage to drop significantly. U3844 experiences insufficient power supply, triggering the internal undervoltage protection circuit, which in turn causes the circuit to enter intermittent oscillation. Because the components are connected in parallel across the N1 winding, short circuits are difficult to detect and are often overlooked.
(2) Some switching power supplies have an overvoltage protection circuit for the input power supply voltage. If the circuit itself fails, the circuit will activate the overvoltage protection and stop oscillating.
(3) The current sampling resistor is faulty. For example, if the pin is oxidized, carbonized, or the resistance value increases, the voltage drop will increase, causing false overcurrent protection and the circuit to enter an intermittent oscillation state.
(4) The rectifier diode D1 of the self-powered winding is inefficient, and its forward conduction internal resistance increases, so the circuit cannot oscillate. Replace it for testing.
(5) If the quality factor of the switching transformer is reduced due to mold or moisture in the windings, replace it with the original model transformer for testing.
(6) The parameters of the R1 oscillation circuit vary, but no abnormality can be measured, or the switching transistor is inefficient. At this time, the circuit is checked and no abnormality is found, but it just does not oscillate.
Repair method:
Try changing the existing parameters and states of the circuit to expose the fault! Try reducing the resistance value of R1 (not lower than 200kΩ); the circuit should oscillate. This method can also be used as an emergency repair measure. If ineffective, replace the switching transistor, UC3844, and switching transformer for testing.
The output voltage is always slightly too high or too low, never reaching the normal value. No abnormalities can be found in the circuit or components; almost all components have been replaced, but the output voltage remains barely acceptable. Sometimes it seems to work normally, but it's unsettling, almost neurotic, as if it might malfunction at any moment. Don't give up! Adjust the circuit parameters to bring the output circuit to its normal value, achieving its normal operating state, so we can feel at ease. Variations in circuit parameters can be caused by the following:
1. Inefficient transistors, such as a decrease in the amplification factor of a transistor, an increase in its on-resistance, an increase in the forward resistance of a diode, and a decrease in its reverse resistance.
2. Capacitor-related dielectric losses, frequency losses, etc., that cannot be measured with a multimeter;
3. Aging and parameter drift of transistors and chip devices, such as reduced light transmission efficiency of optocouplers;
4. Inductive components, such as the reduced Q value of switching transformers;
5. The resistance value of the resistive element varies, but not significantly.
6. Several of the above five reasons are involved, forming a "combined effect".
The current state of the circuit, caused by various reasons, is a kind of "pathological condition." Perhaps we need to change our troubleshooting approach. Traditional Chinese medicine has a theory of "treatment based on syndrome differentiation," which we should also apply. The next prescription shouldn't target any particular component, but rather "regulate" the entire circuit, bringing it from a "pathological" state towards a "normal" state. In this way, "vaguely and unpredictably," the problem is cured.
Repair method (minor adjustment of component values):
1. Output voltage is too low:
a. Increase the value of R5 or decrease the value of R6; b. Decrease the values of R7 and R8 or increase the value of R9.
2. Output voltage is too high:
a. Decrease the value of R5 or increase the value of R6; b. Increase the value of R7 or R8 or decrease the value of R9.
The above adjustments were made after a thorough inspection of the circuit and replacement of inefficient components. The purpose was to adjust the relevant gain of the voltage regulation feedback circuit, thereby changing the duty cycle of the pulse output by the oscillator chip and the energy stored in the switching transformer, so that the output voltage of the secondary winding reaches a normal value, and the circuit enters a new "normal equilibrium" state.
Many seemingly unfixable faults were repaired smoothly by adjusting one or two resistor values.
Points to note during troubleshooting: 1. During the inspection and repair of the switching power supply, the power supply to the IGBT module of the three-phase output circuit should be disconnected to prevent abnormal drive power supply from damaging the IGBT module. 2. When repairing faults with excessively high output voltage, the +5V power supply to the CPU motherboard must be disconnected to prevent abnormal or high voltage from damaging the CPU and rendering the CPU motherboard unusable. 3. The voltage regulation circuit must not be interrupted, as this will cause the output voltage to rise abnormally! 4. The diodes in the switching power supply circuit, used for rectification and protection, are all high-speed diodes or Schottky diodes; ordinary IN4000 series rectifier diodes cannot be used as substitutes. 5. If the switching transistor is damaged, it is best to replace it with the original model. With the internet so developed now, sourcing goods is not a problem, and they can generally be purchased. Many things can be bought cheaply on Taobao, but pay attention to quality!
