Application Principles and Scenarios of Alloy Resistors in On-Board Chargers (OBC)

author: Milliohm Electronics

2025-11-21

I. What is an OBC?

An on-board charger (OBC) is a device that converts AC power to DC power. It converts the AC voltage from a charging station into the DC voltage required by the battery. OBCs are installed in electric vehicles to charge them using AC power from residential or public charging stations. The output power of an OBC typically ranges from 3.6 kW to 22 kW. Another advantage of OBCs is that they can be used to charge electric vehicles from a household power outlet.

II. Application Principles of Alloy Resistors in OBC

① Current Sampling

Alloy resistors, characterized by low resistance, high precision, and low temperature coefficient, are commonly used for high-current sampling in OBC circuits. In the AC/DC and DC/DC conversion circuits of the OBC, accurate current detection is required to achieve precise control and protection of the charging process.

The alloy resistor is connected in series in the current loop. According to Ohm's law I=V/R, by measuring the voltage difference V across the resistor and combining it with the known resistance value R, the current value I in the loop can be calculated. This current signal is then converted into a voltage signal and fed back to the control circuit for real-time monitoring and control, ensuring a safe and stable charging process.

② Power Factor Correction (PFC)

In the OBC, to improve the power factor and limit harmonic currents, a power factor correction circuit is needed. Boost-type APFC technology is one of the commonly used PFC topologies. In Boost-type APFC circuits, an average current control method is typically used. This method requires detecting the inductor current to form a dual-loop control system with an outer voltage loop and an inner current loop.

Alloy resistors can be used to sample the inductor current, specifically by connecting an alloy resistor to ground in the main circuit. Since the APFC circuit generates inrush current when the filter capacitor charges upon power-up, the alloy resistor must have high inrush current withstand capability. Simultaneously, to ensure current sampling accuracy, its parasitic inductance must be minimized.

III. Application Scenarios of Alloy Resistors in OBC

① Input Current Sampling in AC/DC Rectifier Circuits

The OBC first rectifies the AC power from the grid (e.g., single-phase 220V or three-phase 380V) into DC power. The input current at this stage is a fundamental parameter of the charging system and needs to be monitored in real time to avoid grid overload or OBC input anomalies.An alloy resistor is connected in series in the AC input circuit at the front end of the rectifier bridge to detect the instantaneous current and RMS value of the input AC power.

Function: By sampling the current and feeding it back to the control chip, it enables input overcurrent protection (e.g., cutting off the circuit when the rated current is exceeded), grid harmonic suppression (optimized in conjunction with PFC circuit), and provides data support for power calculation (P = UI).

② Input Current Sampling of DC/DC Rectifier Circuit

The DC/DC stage of the OBC needs to convert the rectified high-voltage DC power (e.g., 300-400V) to a voltage suitable for the power battery (e.g., 200-800V, depending on the vehicle model). This stage involves high DC current and requires strict monitoring to ensure battery safety.

Bus Current Sampling: An alloy resistor is connected in series on the high-voltage bus of the DC/DC converter (e.g., between the primary-side switch and the transformer) to detect the DC current on the primary side. This is used to control the on/off sequence of the switch to prevent overcurrent on the bus from burning out the switch.

Output Current Sampling: Connected in series on the secondary side of the DC/DC converter (close to the battery interface), this directly detects the charging current flowing to the power battery and is one of the most critical parameters during the charging process.

Functions: Real-time feedback to ensure the charging current remains within the set range (e.g., current accuracy needs to be controlled within ±2% during constant current phase), enabling output overcurrent protection and current limiting control (e.g., reducing current when the battery is near full charge), and providing a basis for calculating charging capacity (charge = current × time).

③ Inductor current sampling in Power Factor Correction (PFC) circuit

The core of the PFC circuit in an OBC (mostly a Boost topology) is to correct the input current waveform to a sine wave in phase with the input voltage to improve the power factor. This process requires accurate detection of the inductor current to achieve closed-loop control.Alloy resistors are typically connected in series between the PFC inductor and ground (or in the main power circuit) to collect the inductor's instantaneous current (including ramp current in continuous mode or pulse current in discontinuous mode).

Functions: Provides feedback signals for the average current control or peak current control of PFC, ensuring the inductor current tracks the voltage command, stabilizing the output voltage, and withstanding the capacitor charging surge current during PFC startup (at which point the current may be several times the rated value).

④ Auxiliary Power Supply Current Monitoring

The OBC requires an auxiliary power supply (typically 12V or 5V) to power low-voltage components such as the control chip, drive circuit, and sensors. Although the current is small (generally a few amperes), it needs to be stable and reliable to ensure the normal operation of the control system. Alloy resistors can be used in the output circuit of the auxiliary power supply to detect the operating current on the low-voltage side.

Function: To achieve overcurrent protection for the auxiliary power supply (e.g., quickly cutting off power when the control chip is short-circuited), ensuring a stable power supply to the internal control system of the OBC.

⑤ Relay/Contactor Pull-in Current Detection

Relays or contactors are used in the OBC to control the switching of high-voltage circuits (e.g., the connection control between the charging gun and the battery). A large inrush current is generated at the moment of pull-in, which needs to be monitored to avoid damage to the relay contacts.Alloy resistors are connected in series in the relay coil circuit or main contact circuit to detect the peak current and stable operating current at the moment of pull-in.

Function: To determine whether the relay is properly pulled in (abnormal current may indicate contact sticking or coil failure), and to limit the inrush current through control logic, extending the relay's lifespan.

IV. Summary

Compared to other resistors, alloy resistors offer four core advantages: low resistance and high precision, high pulse withstand capability, low parasitic inductance, and strong environmental adaptability. These advantages perfectly match the requirements of OBCs for high-current sampling, high-frequency interference immunity, and reliability in harsh environments, making them key components for current detection and protection in on-board chargers.

Milliohm Electronics is committed to providing the electric vehicle industry with the latest component solutions to meet the ever-evolving technological demands, bringing higher performance and sustainable development to customer products. This provides greater design flexibility for products.

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