Home /Related News /Low-frequency transformer /Solve the problem of voltage imbalance in multiple output channels /
Solve the problem of voltage imbalance in multiple output channels
2026-07-07
Multiple sets of schemes for distributing the low-frequency transformer load reasonably, solving the problem of voltage imbalance in multiple outputs
The multi-winding output is the most common design form of power frequency transformers. One transformer can simultaneously output two, three or even more sets of different voltages, providing power for the main controller, relays, signal modules and actuators on the circuit board. During the debugging and mass production of actual prototypes, a difficult problem is often encountered: the loads of each path are not the same, resulting in severe voltage imbalance. The voltage of the light-load winding rises significantly, far exceeding the specified value; the voltage of the heavy-load winding drops severely, failing to meet the design standards. Many engineers simply attribute the fault to the unreasonable design of the transformer's turns. They repeatedly modify the mold and adjust the turns, but the situation gets more and more chaotic. In fact, the voltage deviation of multiple windings is mostly not caused by incorrect turn calculation, but by the uneven distribution of loads, unreasonable winding arrangement, inconsistent internal resistance drop, and unbalanced magnetic coupling. If the load distribution plan is not planned in advance, even if the coil turns are repeatedly modified, the voltage drift cannot be completely solved. This article starts from the underlying principle of voltage imbalance, sorts out four rectification measures including winding layout, internal resistance control, load ratio, and structural optimization, and provides standardized load distribution principles. It controls the voltage drop of each output path from the design source, ensuring that the voltages of each group remain within a reasonable range under light-load, half-load and full-load conditions, avoiding repeated mold modification and shortening the project development cycle.
- The Core Causes of Voltage Imbalance in Multiple Output Voltages
1. Inconsistent DC Resistance of Windings, With Severe Voltage Drop Differentiation
Each secondary coil has a conductor resistance. The larger the load current, the greater the IR voltage drop, and the more significant the decrease in output voltage. If the wire diameters of multiple windings are different and the lengths of the wires vary greatly, the internal resistances of each path will be significantly different. When one path is fully loaded and the other is almost unloaded, the voltage of the heavily loaded circuit is pulled down, while the unloaded circuit has almost no voltage drop. The voltage rises sharply, and the voltage difference between the two paths is rapidly widened. Many custom transformers only ensure that the voltage meets the standard under rated full-load conditions, completely ignoring the extreme situation where the loads of each path are unbalanced. Once one path is lightly loaded, the voltage exceeding the standard problem immediately becomes apparent.
Each secondary coil has a conductor resistance. The larger the load current, the greater the IR voltage drop, and the more significant the decrease in output voltage. If the wire diameters of multiple windings are different and the lengths of the wires vary greatly, the internal resistances of each path will be significantly different. When one path is fully loaded and the other is almost unloaded, the voltage of the heavily loaded circuit is pulled down, while the unloaded circuit has almost no voltage drop. The voltage rises sharply, and the voltage difference between the two paths is rapidly widened. Many custom transformers only ensure that the voltage meets the standard under rated full-load conditions, completely ignoring the extreme situation where the loads of each path are unbalanced. Once one path is lightly loaded, the voltage exceeding the standard problem immediately becomes apparent.
2. The tightness of primary and secondary magnetic coupling is different
Due to the different winding positions, there are differences in the coupling coefficients between the windings and the primary. The secondary windings that are closely attached to the inner layer of the core and close to the primary have a tight coupling and a small leakage inductance, resulting in a small voltage drop under load; while the secondary windings placed on the outer layer are far from the primary, with a large leakage inductance, and the voltage drop becomes more significant when the load increases. The arrangement of multiple windings is disordered, with some close to the primary and some on the outer layer, and the coupling degree varies unevenly. This is a key structural factor causing voltage imbalance in multi-winding transformers.
3. Extremely unbalanced load conditions (the main human-induced factor)
During equipment operation, the outputs of each path will not simultaneously reach the rated full load. The common situation is that one path operates at full load for a long time, while the other paths only provide weak standby current. A heavy load on a single path will cause a significant impedance voltage drop on the secondary winding, while the no-load winding does not have a voltage drop. The voltage difference between the two paths is further amplified. During the transformer design stage, if no correction coefficient for unbalanced loads is reserved, and the number of turns is calculated based on the ideal condition of simultaneous full load for each path, once a single path is heavily loaded, voltage imbalance will become an inevitable phenomenon.
4. Mutual disturbance of magnetic flux in common iron core
Multiple secondary windings share the same iron core. When a large current load is connected to one of the windings, it will alter the magnetic flux distribution inside the iron core, causing an induced electromotive force disturbance and indirectly affecting the induced voltages of the other windings. Especially for EI-type transformers, the windings are arranged in a dispersed manner, and the magnetic flux disturbance is more obvious, further exacerbating the differentiation of voltages for each path.
- Core Design Principles for Multi-winding Load Distribution
To control voltage deviation, the first step is to plan the load ratio during the design stage and establish an equal distribution rule. It is not advisable to make corrections after problems arise during prototype testing.
Principle 1: Try to balance the power distribution evenly across all paths to avoid any single path dominating.
Optimal design solution: Distribute the total power of the entire machine evenly among each group of secondary windings, and try to ensure that the loads on all paths rise and fall simultaneously, maintaining synchronization between full load and light load conditions. Engineering practice standard: The rated power of any group of secondary windings should not exceed 60% of the total secondary power. Avoid extreme layouts where one path accounts for more than 80% of the power, while the remaining paths only provide weak standby power supply. If the power distribution cannot be evenly divided by the circuit function, and there is a large power main circuit and a small power auxiliary circuit, the main power winding must be strengthened, reducing its own internal resistance drop, and minimizing magnetic flux interference with other windings.
Optimal design solution: Distribute the total power of the entire machine evenly among each group of secondary windings, and try to ensure that the loads on all paths rise and fall simultaneously, maintaining synchronization between full load and light load conditions. Engineering practice standard: The rated power of any group of secondary windings should not exceed 60% of the total secondary power. Avoid extreme layouts where one path accounts for more than 80% of the power, while the remaining paths only provide weak standby power supply. If the power distribution cannot be evenly divided by the circuit function, and there is a large power main circuit and a small power auxiliary circuit, the main power winding must be strengthened, reducing its own internal resistance drop, and minimizing magnetic flux interference with other windings.
Principle 2: High-power windings are placed as close as possible to the primary winding to enhance coupling capability.
In the winding arrangement, the main output winding with the highest power and current is wound closely to the primary winding to ensure the tightest magnetic coupling and minimize leakage inductance. The voltage drop in the high-power circuit is controlled, so that large current disturbances will not cause voltage drift in other windings. The small current auxiliary windings are arranged on the outer layer. Because their current is small, the IR voltage drop is already very low. Even if the leakage inductance is large, the voltage drop amplitude is very limited, and it has little impact on the overall balance. This is the simplest and most effective means of balancing multiple voltages in the winding process.
Principle 3: Use the same wire diameter for coils of the same specification to ensure consistent internal resistance
One of the major causes of voltage imbalance is the inconsistency of internal resistances. For multiple coils of the same voltage level, the same wire diameter and the same length of enameled wire must be used to ensure that the deviation of DC resistance is within 5%. For low-voltage high-current coils, the wire diameter should be appropriately increased to reduce the resistance of the conductor and the voltage drop caused by the load; for small current coils, do not blindly choose thin wires to avoid the voltage of the unloaded circuit rising. All secondary coils should be independently wound separately, and shared lead-out wires should not be used to prevent one current from affecting the potential of another circuit.
Principle 4: Differentiate between continuous loads and instantaneous impact loads, and allocate separate independent windings.
Relay, solenoid valve, motor and other instantaneous impact loads have high starting currents and can cause a sudden drop in voltage. Such impact loads must occupy a separate group of independent windings and should not share the same set of coils with the main control chip and signal circuits. If the impact load shares the winding with the weak current circuit, the sudden voltage drop caused by the large current will directly lead to the reset of the single-chip microcontroller and the interruption of communication. For strong and weak current loads, the windings are independent of each other and do not interfere with each other, so the stability of each voltage can be guaranteed.
Principle 5: Reserve compensating turns for uneven load voltage drop
The number of turns in a transformer should not be calculated solely based on the simultaneous full-load condition of each path. For the extreme condition of "one path fully loaded while the others are empty", a 3% to 6% voltage drop compensation turns must be reserved. The main power winding should be wound with a slightly larger number of turns to compensate for the internal resistance voltage drop under high current; the number of turns in the light-load auxiliary winding should be slightly reduced to suppress excessively high no-load voltage, and keep the voltage fluctuations under ±5% for both light and heavy loads.
- Optimization of winding structure, further reducing voltage deviations of each path
1. Adopting the sandwich winding structure to balance the coupling coefficient
Traditional winding method: All the primaries are wound first, then multiple secondary windings are wound successively. The coupling difference and voltage drop of the outer layer winding are large. Optimization solution: Adopting the "primary - secondary - primary" sandwich winding, placing the secondary windings between the two layers of primaries, making the magnetic coupling degree of all secondary windings consistent, and the leakage inductance of each path becomes more uniform, significantly reducing the load voltage drop difference. For multi-output transformers with three or more paths, the sandwich structure can reduce the dispersion of voltages of each path by more than half.
Traditional winding method: All the primaries are wound first, then multiple secondary windings are wound successively. The coupling difference and voltage drop of the outer layer winding are large. Optimization solution: Adopting the "primary - secondary - primary" sandwich winding, placing the secondary windings between the two layers of primaries, making the magnetic coupling degree of all secondary windings consistent, and the leakage inductance of each path becomes more uniform, significantly reducing the load voltage drop difference. For multi-output transformers with three or more paths, the sandwich structure can reduce the dispersion of voltages of each path by more than half.
2. Each secondary winding is independently isolated to prevent common copper busbar voltage drop.
It is strictly prohibited to share the same lead copper sheet among multiple secondary coils. Each winding has its own terminals and tails, and is separately led out to pins. The common lead will cause shared voltage drop, resulting in fluctuations in current, and the voltages of all windings will change accordingly. By independently connecting the wires, the voltage drops of each path are not interrelated, significantly improving the voltage stability.
3. Select the core structure reasonably
For EI transformers, the windings are distributed on two core columns, resulting in relatively prominent magnetic flux imbalance issues; for ring-type transformers, the windings are uniformly wound around the entire ring, which leads to better coupling consistency among multiple secondary windings and better voltage balance across multiple paths compared to the EI structure. If the project has strict requirements for the consistency of multiple voltages, it is recommended to choose the ring-type core; when using EI cores due to cost constraints, try to distribute multiple secondary windings evenly on the left and right core columns to balance the magnetic flux distribution.
- On-site Debugging: Rectification Steps for Voltage Imbalance Caused by Unbalanced Load
When a voltage spike and a voltage drop occur simultaneously on the sample machine, strictly follow the sequence to troubleshoot and rectify, avoiding random modification of the number of turns.
Step 1: Measure the DC resistance of each winding group. The winding with significantly larger resistance is the root cause of severe voltage drop during heavy load. Prioritize increasing the wire diameter, reducing the internal resistance, and minimizing IR voltage drop.
Step 2: Simulate extreme conditions, fully load the main circuit while keeping the other circuits unloaded. Record the voltage deviation. This is the most stringent testing condition for evaluating the balance of multiple windings. Measuring only at full load synchronously cannot detect potential problems.
Step 3: Adjust the winding arrangement, moving the high-power coils to the inner layer, closer to the primary, to reduce leakage inductance. In many cases, merely adjusting the winding sequence does not require changing the number of turns, and can control the voltage deviation within the acceptable range.
Step 4: Fine-tune the number of turns for voltage drop compensation. For heavy-load windings, appropriately increase the number of turns to compensate for the load-induced voltage drop; for auxiliary windings with excessively high voltage during no-load, slightly reduce the number of turns. Each fine-tuning of the number of turns should not be excessive, with 1 to 3 turns each time. Repeatedly measure under light and heavy load conditions.
Step 5: Add a voltage stabilizing auxiliary at the circuit end. For weak electrical circuits with extremely high voltage accuracy requirements, a simple voltage stabilizing circuit can be added in the subsequent stage to absorb the small voltage fluctuations caused by the windings, without repeatedly modifying the transformer structure and saving the cost of mold modification.
Step 1: Measure the DC resistance of each winding group. The winding with significantly larger resistance is the root cause of severe voltage drop during heavy load. Prioritize increasing the wire diameter, reducing the internal resistance, and minimizing IR voltage drop.
Step 2: Simulate extreme conditions, fully load the main circuit while keeping the other circuits unloaded. Record the voltage deviation. This is the most stringent testing condition for evaluating the balance of multiple windings. Measuring only at full load synchronously cannot detect potential problems.
Step 3: Adjust the winding arrangement, moving the high-power coils to the inner layer, closer to the primary, to reduce leakage inductance. In many cases, merely adjusting the winding sequence does not require changing the number of turns, and can control the voltage deviation within the acceptable range.
Step 4: Fine-tune the number of turns for voltage drop compensation. For heavy-load windings, appropriately increase the number of turns to compensate for the load-induced voltage drop; for auxiliary windings with excessively high voltage during no-load, slightly reduce the number of turns. Each fine-tuning of the number of turns should not be excessive, with 1 to 3 turns each time. Repeatedly measure under light and heavy load conditions.
Step 5: Add a voltage stabilizing auxiliary at the circuit end. For weak electrical circuits with extremely high voltage accuracy requirements, a simple voltage stabilizing circuit can be added in the subsequent stage to absorb the small voltage fluctuations caused by the windings, without repeatedly modifying the transformer structure and saving the cost of mold modification.
- Common Mistakes Avoidance
Mistake 1: Only measure the synchronous full-load voltage, without conducting single-path heavy-load tests. The vast majority of potential problems occur when the loads of each path are not synchronized. Even the most perfect synchronous full-load parameters will still result in voltage imbalance when a single path is under heavy load.
Mistake 2: Multiple windings share a single lead wire and use a common copper resistor to cause a cumulative voltage drop. One path fluctuates, and the others follow and drift.
Mistake 3: High-power windings are placed on the outermost layer of the coil, resulting in excessive leakage inductance. Once a load is applied, the voltage drops sharply. Even after repeatedly adjusting the number of turns, it still fails to meet the standard.
Mistake 4: Both strong and weak electrical loads share a single secondary winding. The impact current causes severe voltage fluctuations, and the weak electrical circuit operates abnormally as a result.
Mistake 2: Multiple windings share a single lead wire and use a common copper resistor to cause a cumulative voltage drop. One path fluctuates, and the others follow and drift.
Mistake 3: High-power windings are placed on the outermost layer of the coil, resulting in excessive leakage inductance. Once a load is applied, the voltage drops sharply. Even after repeatedly adjusting the number of turns, it still fails to meet the standard.
Mistake 4: Both strong and weak electrical loads share a single secondary winding. The impact current causes severe voltage fluctuations, and the weak electrical circuit operates abnormally as a result.
Multiple sets of output transformers have experienced voltage imbalance. The surface cause is mismatched turns, while the underlying reason is the unreasonable load distribution, inconsistent winding coupling, and excessive difference in winding resistance. To solve the problem at its root, relying solely on modifying the turns after the fact is not sufficient. During the scheme design phase, load planning should be done well: evenly distribute the power of each path, separate the main power winding independently, and allocate the impact load and weak electrical load to different windings; in terms of process, large power coils should be wound close to the primary coil, unify the wire diameter of each set of windings, reduce the resistance difference, and use sandwich winding to balance the magnetic coupling; finally, reserve turns compensation for the extreme working condition of "full load in one path and no load in the others". Strictly follow the load distribution rules + winding structure optimization. Even if the loads of each path change dynamically, the output voltages of each group can still remain stable, completely solving the problem of voltage being high in light load and low in heavy load, significantly reducing the number of mold modifications, ensuring the success of the first sample production, and smoothly achieving mass production.
Continuous operation: How to extend the service life of low-frequency transformers
Eliminating the ring-shaped transformers noise at the power frequency level
Related Article
High-temperature faults in transformers are not simply caused by the heat resistance of materials; more than 80% of power supply abnormalities stem from heat accumulation resulting from defective heat dissipation structural designs.
Abnormalities and potential hazards that may arise from the overheating transformer
To achieve precise voltage output throughout the entire life cycle of the transformer and prevent long-term parameter drift, the key does not lie in merely optimizing the winding resistance, but rather in reducing the iron core's magnetic hysteresis loss from all dimensions including materials, structure, design, and process. This is to stabilize the magnetic circuit operation state and lock in the long-term output accuracy.
Hysteresis loss in transformer directly affects the output accuracy.
The power frequency low-frequency transformers, with their electromagnetic coupling buffering characteristics, magnetic circuit energy storage voltage stabilization capabilities, and electrical isolation protection advantages, inherently possess the ability to resist voltage fluctuations.
The anti-voltage fluctuation capability of the power frequency transformer ensures stable power supply
In the design system of power frequency transformers, the number of turns in the windings is not merely a basic parameter for meeting the voltage ratio requirements. Instead, it is the sole core variable that can be adjusted in both directions to control copper loss and iron loss, thereby achieving the minimization of total loss.
The design of the number of windings directly affects the balance between copper and iron loss in the transformer