As we all know, the power supply voltages we encounter are mostly 220V or 380V, but some applications require even lower voltages, such as control boards and DC contactors. Using a transformer to step down the voltage can lead to instability because the transformer's output voltage constantly changes with the input voltage. To obtain a more stable voltage, a switching power supply is needed. A switching power supply is a voltage conversion circuit that converts rectified DC voltage into any desired voltage through the switching of transistors and a transformer. Essentially, a switching power supply is an oscillation circuit.
Switching power supplies can be classified into forward converters and flyback converters based on energy conversion. A flyback converter operates in a state where the transformer secondary is cut off when the switching transistor is saturated and conducting, storing energy; when the switching transistor is cut off, the transformer secondary is turned on, releasing energy to the load. A forward converter, on the other hand, induces a corresponding voltage on the transformer secondary while the switching transistor is saturated and conducting, outputting it to the load—the energy conversion is exactly the opposite of a forward converter.
Flyback power supplies are widely used in small to medium power supplies due to their simple structure, eliminating the need for an inductor and using a dedicated power oscillator chip. Some data shows that flyback power supplies can reach a maximum power of 300 watts, which can meet the needs of most applications. Forward power supplies commonly use two transistors to alternately enter the saturation and cutoff regions to form an oscillation cycle, eliminating the need for a power supply chip and optimizing the use of components. Due to the simplicity of the circuit, it will not be described in detail here.
The most common flyback switching power supply is the one composed of UC3844. Its main components are the power oscillator chip UC3844, the switching transistor, and the transformer.
1. Power oscillator chip UC3844
Because the UC3844 integrates a divide-by-two trigger, its output frequency is only half the oscillation frequency of pin 4, resulting in a maximum output duty cycle of 50%. Compared to general voltage-controlled pulse-width modulation switching power supplies, switching power supplies constructed with this integrated circuit have advantages such as simpler external circuitry, better voltage regulation, better frequency response, larger stability amplitude, and overcurrent limiting, overvoltage protection, and undervoltage lockout.
Pin Description: Pin 1 is the output of the error amplifier, and pin 2 is the inverting input of the error amplifier, used to adjust the output voltage of the switching power supply. An external circuit connects pins 1 and 2 to a series-parallel negative feedback network of resistors and capacitors to determine the amplifier's bandwidth, frequency response, and amplification factor. Since the non-inverting input of the error amplifier internally provides 2.5V, when a voltage higher than 2.5V is input to pin 2, the error amplifier outputs a lower voltage, maintaining the load voltage below its normal value, thereby reducing the output voltage and keeping the voltage at pin 2 at 2.5V. The reverse is also true.
I encountered this situation once: the measured output voltages on the transformer secondary side were all higher than normal, except for one 18V output voltage which was the same as normal. After connecting the control board, the switching power supply oscillated intermittently, and the control panel did not display anything. This situation was caused by an abnormality in the voltage regulation circuit. Since the voltage at pin 2 is taken from this 18V, during normal operation, when the transformer secondary side output voltages are all at their normal values, the 18V output voltage is only around 16V. At this time, a low output voltage signal is fed back to pin 2, forcing the load voltage to rise, raising the 16V voltage to a stable 18V. All other voltages rise to varying degrees. After connecting the control board, the load capacity is insufficient, and the switching power supply stops working. Upon inspection, it was found that one of the voltage sampling resistors was abnormal.
Pin 3 is the current sensing and comparison terminal. When the voltage at pin 3 exceeds 1V, the PWM latch will block the output pulse and activate the overcurrent protection function of the circuit. After the switching transistor is turned on, the current flows through the sampling resistor R12 connected to the source S, is filtered by a capacitor, and then fed to pin 3. Since the maximum current of the switching transistor IRFPF50 is 6.7A and the maximum allowable operating current is 2A, the voltage after passing through the 0.5Ω resistor is 1V, which is then fed to pin 3. A more common transistor, such as the K2225, has a maximum current of 2A and a maximum operating current of 0.6A. Calculations show that a sampling resistor of 1.5 ohms is needed to achieve the required 1V voltage at pin 3. Under normal conditions, the voltage at pin 3 is below 1V.
Because the current sampling resistor has a small resistance value, when the switching transistor is broken down, the sampling resistor is often damaged as well, so it needs to be carefully measured and checked.
Pin 4 is the input terminal for the oscillator timing component, similar to a crystal oscillator in a CPU. Pin 8 is the reference power output terminal, providing a stable +5V reference voltage. The power supply for the RC oscillation circuit composed of pin 4 is often taken from pin 8. The oscillation frequency can be calculated using the formula F=1.72/(R·C). In the diagram above, R=1K ohms and C=4700pF, calculating the oscillation frequency to be 366kHz. The oscillator's maximum oscillation frequency can reach 500kHz. The oscillation frequency in typical switching power supplies is around 50kHz.
Sometimes, when a machine is powered on, a piercing sound can be heard from the switching power supply. This is very likely due to a damaged oscillation circuit. Since the highest frequency that the human ear can hear is 20kHz, this situation occurs when capacitor C ages and its capacitance increases, causing the oscillation frequency to drop below 20kHz.
Pins 5 and 7 are the power supply terminals GND and VCC. The UC3844 has a 16V start-up voltage and a 10V undervoltage lockout voltage, unlike the UC3843 which has an 8.5V start-up voltage and a 7.6V undervoltage lockout voltage. The UC3844 operates normally between 10V and 17V.
Pin 6 is the PWM waveform output pin, which drives the switching transistor to open and close, with a maximum output current of 1A.
2 switching transistors
K1317 and K2225 field-effect transistors are commonly used. Taking K1317 as an example, it has a withstand voltage of 1200V and a maximum current of 2.5A. Since a reverse diode is connected between the drain and source, measuring the forward and reverse diode characteristics of the drain and source can be used as a basis for judgment. There is also a more accurate method: apply an 8V forward power supply between G and S. At this time, the switching transistor is forward-biased, and the resistance between D and S is very small. When the 8V power supply is reversed, with S connected to positive and G connected to negative, the switching transistor is reverse-biased and cut off, and the resistance between D and S is high, indicating that the switching transistor is normal.
Some power supply circuits use bipolar transistors such as BU508A. Since there is no reverse diode connected in parallel between the emitter and collector, the polarity can only be determined by applying forward and reverse voltages.
3 transformers
Transformer faults are both the easiest and the most difficult to diagnose. Transformer damage falls into two categories: inter-turn short circuits, where two windings are directly connected; and open circuits within windings. Because transformers have numerous primary and secondary windings, it's crucial to understand the relationships between them for accurate diagnosis.
