What is DC Bias of Magnetic Cores?

DC bias refers to a phenomenon in high-frequency switching power supplies where the magnetic core of a transformer or inductor, under the influence of an alternating magnetic field, is superimposed with a DC component. This causes the magnetization state of the magnetic core to deviate from its symmetric equilibrium position, ultimately leading to magnetic core saturation. Though seemingly abstract, this phenomenon affects power supply performance through a series of chain reactions and requires in-depth understanding from three aspects: mechanism, impacts, and countermeasures.

01 Mechanism of DC Bias Generation: The “Unbalanced” State of Magnetic Cores

The core components of high-frequency switching power supplies—transformers and inductors—rely on the electromagnetic induction principle of their magnetic cores (e.g., ferrite, nanocrystalline alloy) to achieve energy conversion or energy storage through alternating magnetic fields generated by alternating currents. Under normal circumstances, the magnetization process of the magnetic core should proceed symmetrically along the hysteresis loop: the core is magnetized in the positive direction during the positive half-cycle of the current and in the reverse direction during the negative half-cycle. The magnetic flux changes in the positive and negative half-cycles are equal, maintaining an overall balanced state.

The occurrence of DC bias essentially stems from the disruption of this balance. When a DC component (which may come from current or voltage) exists in the magnetic core, the hysteresis loop shifts toward one direction, resulting in asymmetric magnetic flux changes between the positive and negative half-cycles. For instance, if the volt-second product (product of voltage and time) in the positive half-cycle is greater than that in the negative half-cycle, the magnetization of the magnetic core will gradually accumulate in the positive direction, eventually exceeding the core’s saturation magnetic flux density and entering a saturated state.

Common sources of DC components include:

• Circuit topology asymmetry: In topologies such as full-bridge and half-bridge, differences in the on-state voltage drop and switching speed of power switches (e.g., MOSFETs, IGBTs) lead to unbalanced on-time (duty cycle) between the positive and negative half-cycles.

• Control signal deviation: Inconsistent pulse widths of the drive circuit, different signal transmission delays, or failure of the PWM control algorithm to compensate for DC components in real time.

• Component parameter discreteness: Parameter errors of components like resistors and capacitors further amplify circuit asymmetry.

• Input voltage fluctuation: Instantaneous deviations in the DC bus voltage may introduce short-term DC components.

02 Hazards of DC Bias: From Performance Degradation to Device Damage

Once the magnetic core enters a saturated state due to DC bias, a series of chain reactions will occur, seriously affecting the operation of the high-frequency switching power supply:

Sudden Drop in Inductance and Surge in Current

After the magnetic core is saturated, its permeability decreases significantly (even dropping to less than 10% of the initial value), leading to a sharp reduction in the inductance of the transformer or inductor. According to the volt-ampere characteristic of inductors (V = L×di/dt), the decrease in inductance (L) causes an instantaneous increase in the current change rate (di/dt). The magnetizing current may surge from the normal several amperes to dozens of amperes, exceeding the rated current of power devices and eventually burning out components such as switches and diodes.

Increased Losses and Reduced Efficiency

In the saturated state, the hysteresis loss and eddy current loss of the magnetic core increase significantly. At the same time, large currents raise the conduction losses of line resistors and switches. In high-frequency scenarios above 100kHz, such losses can reduce power supply efficiency by 5%-10%, not only wasting energy but also causing excessive temperature rise of the equipment.

Aggravated Electromagnetic Interference (EMI)

Distortion of the magnetizing current generates a large number of high-frequency harmonics, which interfere with surrounding electronic equipment through radiation or conduction, causing problems such as communication interruption and instrument measurement errors. Additionally, magnetic core saturation triggers mechanical vibration, producing harsh noise that affects the acoustic performance of the equipment.

Deteriorated Dynamic Response

The sudden change in inductance damages the closed-loop control stability of the power supply, resulting in increased output voltage ripple and decreased regulation rate. When the load changes abruptly, the power supply may fail to respond quickly, leading to overvoltage or undervoltage and affecting the normal operation of downstream equipment.

03 Identification and Countermeasures of DC Bias: From Detection to Suppression

The key to identifying DC bias lies in monitoring the magnetization state of the magnetic core. Common methods include:

• A: Real-time detection of the DC component of the transformer’s primary current (via Hall sensors or sampling resistors).

• B: Monitoring changes in inductance (via impedance analyzers or on-line inductance measurement circuits).

• C: Observing changes in power supply temperature rise and noise (saturation is usually accompanied by a sudden temperature increase and abnormal noise).

To suppress DC bias, comprehensive measures should be taken from three aspects: circuit design, control algorithms, and magnetic core selection:

• Circuit topology optimization: Connect a DC-blocking capacitor in series to block DC components, or adopt symmetric topologies such as active clamping and dual active bridge (DAB) to reduce volt-second product imbalance.

• Control algorithm improvement: Real-time compensation of DC components through digital PI control, or adoption of average current mode to ensure current symmetry between positive and negative half-cycles.

• Magnetic core parameter adjustment: Select magnetic core materials with high saturation magnetic flux density (e.g., nanocrystalline alloy), or open an air gap in the middle column of the magnetic core to enhance anti-bias capability.

In summary, DC bias in high-frequency switching power supplies is essentially a phenomenon where the magnetization balance of the magnetic core is disrupted by DC components. Its hazards range from performance degradation to device damage, making it a key challenge in power supply design. Understanding its generation mechanism, identification methods, and suppression measures not only improves the reliability and efficiency of power supplies but also lays a foundation for the development of high-frequency and miniaturized power supplies. With the continuous advancement of power electronics technology, precise control of DC bias has become an important indicator to measure the design level of high-frequency switching power supplies.