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Can a switching power supply replace a low-frequency transformer?
Switch-mode power supplies and transformers have a wide range of applications, yet they also differ significantly—both in terms of function and purpose. Simply put, a transformer is an electronic component integrated into a single circuit, while a switch-mode power supply is a more complex circuit composed of multiple electronic components, designed to efficiently convert electrical energy. These distinctions become clear when examining their roles and intended uses.
2020-08-03
Can a switching power supply replace a low-frequency transformer?
1. Low-frequency transformers will not be replaced;
2. Low-frequency transformers do not have the pulse interference typical of switch-mode power supplies;
3. Low-frequency transformers provide isolation against various power grid pulse disturbances;
4. Low-frequency transformers possess surge resistance capabilities that switching power supplies simply cannot match—especially when it comes to lightning strikes.
5. Linear power supplies made with low-frequency transformers offer the best ripple performance.
6. The output of the low-frequency transformer can use a low-voltage switching chip, which provides isolation while also delivering high efficiency.
In industry, low-frequency transformers effectively isolate interference caused by high-power frequency converters, servos, and thyristor-based phase-control systems.
The difference between switching power supplies and transformers
Switching power supplies and transformers have a wide range of applications, yet they also differ significantly—both in terms of functionality and intended use. Simply put, a transformer is an electronic component integrated into a single circuit, while a switching power supply is a more complex circuit composed of multiple electronic components, designed to efficiently convert electrical energy. These distinctions become clear when examining their roles and practical applications. Refer to the diagram below for a visual comparison:
Can a switching power supply replace a low-frequency transformer? A brief discussion on transformers in switching power supplies
I. The Functional Differences Between Switching Power Supplies and Transformers
1. Switch-mode power supplies convert DC to DC, while transformers handle AC to AC;
2. Switching power supplies operate at relatively high frequencies—typically in the tens of kilohertz—while transformers can handle both high and low frequencies. The pulse transformers used in switching power supplies are specifically high-frequency types, featuring magnetic cores made of ferrite or similar materials. These transformers are compact in size yet highly efficient, with efficiency increasing as the operating frequency rises.
When people think of transformers, they usually picture power-frequency transformers, which operate at a standard 50 Hz—making them low-frequency devices. For these low-frequency transformers, the core must be constructed from laminated sheets of silicon steel, a soft magnetic material, to prevent eddy currents that could generate unwanted Joule heating.
3. The two have slightly different functional scopes: a switching power supply includes a pulse transformer as well as rectification components at both the input and output stages, whereas transformers—regardless of their frequency range—serve solely to step up or step down voltage, without any rectifying function.
4. Switching Power Supplies: Compared to traditional power supplies, switching power supplies boast advantages such as low power consumption, high efficiency, compact size, lightweight design, and a wide voltage regulation range. As a result, they are widely used in fields like computers and peripherals, telecommunications, industrial automation, household appliances, and more. However, a significant drawback of switching power supplies is their tendency to generate strong electromagnetic interference. These interference signals not only span a broad frequency spectrum but also carry substantial amplitude levels. Through both conducted and radiated pathways, they can contaminate the electromagnetic environment, disrupting communication devices and electronic products. If not properly managed, the switching power supply itself may even become a major source of interference.
Traditional transformer power supplies suffer from drawbacks such as bulky size and low efficiency. However, manufacturers on the market today often cut corners—using inferior selenium steel sheets or even counterfeit materials—in pursuit of higher profits. This leads to overheating in transformers and further reduces their already low efficiency.
2. What is the difference in use between switching power supplies and transformers?
Switching power supply: A switching power supply can stably convert voltages within a specified range into highly precise low or high voltages—for example, with an input of 110V–250V, the output voltage can be accurately maintained at the desired level, with deviations no greater than ±0.5V.
Transformer: The output voltage of the transformer continuously varies with the input voltage—specifically, as the input voltage increases, the output voltage also rises, and when the input voltage decreases, the output voltage drops accordingly.
In summary, it can be concluded that switch-mode power supplies first convert AC electricity into DC, and then use a power switching transistor to transform this DC into higher-frequency AC. This high-frequency AC is subsequently stepped up or down in voltage via a high-frequency transformer. Not only does this process significantly improve efficiency, but the increased frequency also allows for a much smaller overall design, reducing both copper and iron losses.
Moreover, since the output voltage is controlled by the power switching transistor, the transistor remains on for shorter durations during low-current conditions—sufficient to maintain the desired output voltage. Conversely, when the load increases, the transistor operates continuously to ensure stable output voltage.
As a result, switch-mode power supplies deliver highly stable output voltages, making them an ideal choice for precision instruments such as LED displays.
In summary, switch-mode power supplies are now being used in an even wider range of applications and better meet the demands of the market. After understanding the distinctions between switch-mode power supplies and transformers as outlined above, we also hope that manufacturers of switch-mode power supplies will prioritize quality control—ensuring that profit-driven decisions never come at the expense of customer satisfaction or trust.
Causes of Switching Power Supplies and Transformers Squealing
The main reasons for noise in switch-mode power supply transformers fall into four key areas: transformer manufacturing issues, loop-related problems within the transformer, core-related issues, and load problems specific to the switch-mode power supply. These will be analyzed one by one below.
(1) Transformer Manufacturing Issues
① Improper impregnation and drying lead to an unstable magnetic core, causing mechanical vibrations that result in audible noise.
② The air gap is improperly sized, causing the transformer to operate unstably and produce noise.
③ If the wire harness isn’t wrapped tightly, the anchor point can still cause a noise.
④ When the magnetic core assembly has an air gap, it causes air vibrations at high frequencies, resulting in audible noise (though transformers treated with full vacuum impregnation typically do not produce any sound).
(2) Transformer Loop Issues
The transformer's loop issue refers to oscillations occurring in the transformer's loop, which cause the transformer to produce a buzzing noise.
① Improper PCB wiring causes interference, leading to oscillations and resulting in audible noise.
② Improperly set feedback loop parameters can lead to loop instability, causing oscillations that result in audible noise.
③ Quality issues with components in the circuit—such as insufficient capacitance in the input filter capacitor, poor-quality fast-recovery rectifier diodes at the output, substandard power MOSFETs, or low-quality high-voltage capacitors or diodes in the RCD snubber circuit—can all potentially lead to oscillations that cause audible noise.
(3) Transformer Core Issues
When the transformer core becomes saturated, the current in the coil increases, causing the transformer to heat up and triggering self-excited oscillations. These oscillations in the coil then set the surrounding air into vibration, producing an audible sound.
(4) Load Issues with Switching Power Supplies
① In switch-mode power supplies, oscillation can occur under no-load or light-load conditions at certain operating points, manifesting as transformer humming and unstable output.
This phenomenon occurs because, during no-load or light-load conditions, the switch’s instantaneous turn-on time becomes excessively long, leading to an overabundance of output energy. As a result, the voltage overshoot also becomes significant, taking a relatively long time to return to normal levels. Consequently, the switch must pause its operation for a brief period, causing it to enter an intermittent operating mode. This, in turn, induces lower-frequency vibrations in the transformer—characterized by either regular, intermittent full-cut-off cycles or rapid fluctuations in the duty cycle.
② The transformer is operating under severe overload conditions, with a constant risk of burning out—this is precisely why many power supplies "scream" right before they fail.
Switching Power Supply Transformer Testing Methods
(1) Visual Inspection. Switching power supply transformers operate at a relatively high frequency—15,625 Hz—and typically use magnetic materials for efficient magnetic flux conduction. Common faults include leakage current or short circuits between windings. During inspection, first examine the exterior for any signs of burning or scorching, check if the surface is excessively dirty, and look for contaminants between the pins (especially in some switching power supply transformers where pin spacing is very narrow, such as the Sanyo 83P chassis transformer). Since these transformers are mounted directly against the printed circuit board, they tend to accumulate dust, which can lead to arcing and sparking when exposed to humid weather or environmental conditions.
(2) Multimeter Testing:
Inspect the switching power supply transformer visually for any obvious issues. If it appears intact, use a multimeter to measure its resistance and check for open-circuit faults in the coils. Additionally, even severely shorted transformers can often be detected using this method—simply select an appropriate resistance range on the multimeter so that the measured resistance falls near the mid-scale value. By referring to the winding’s number of turns and the wire gauge used, you can determine the ohmic value per meter of the enameled wire. Then, calculate the total resistance of the winding and compare it with the measured resistance. If they match closely, it suggests no short circuit; however, if there’s a discrepancy, it may indicate a shorted condition. Note, though, that this is only a rough diagnostic tool. In some cases, especially when inter-turn or inter-layer insulation has broken down, the resistance values might remain nearly identical, making it difficult—or even impossible—to detect the fault using this method.
The insulation resistance between each winding of the switching power supply transformer should be infinite, and the insulation resistance between each winding and the magnetic core (iron core) should also be infinitely high.
(3) Substitute Measurement. If you have a switch-mode power supply transformer of the same specifications on hand, you can use a substitution test, which is straightforward and saves time. Hope this helps!
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Power-frequency transformers, also known as low-frequency transformers, are distinguished from high-frequency transformers used in switch-mode power supplies. Historically, power-frequency transformers were widely employed in conventional power supplies, which relied on linear regulation to maintain stability—hence the term "linear power supplies." The term "power frequency" typically refers to the frequency of mains electricity, which in China is 50 Hz, though other countries use 60 Hz. A transformer capable of altering the voltage of AC power while maintaining this specific frequency is precisely what we call a power-frequency transformer.
The Role of High-Frequency Transformers _ Where Are High-Frequency Transformers Used?
Currently, switch-mode power supplies typically use a half-bridge power conversion circuit, which includes components like a high-frequency transformer and transistors. When the circuit operates, the transistors alternate between conducting states, generating high-frequency pulses at a rate of 100 kHz. These pulses are then stepped down via the high-frequency transformer, ultimately producing a lower-voltage AC output. The exact output voltage level is determined by the turns ratio of the individual windings in the high-frequency transformer. In most designs, three separate transformers are employed: the main transformer, the driver transformer, and the auxiliary transformer. Each of these transformers has its own specific design specifications and critical functions, making them all indispensable to the overall operation of the system.
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