Series 1 : Teardown Analysis of the 20kW DC Charger Power Module

Amplifiers, Supporting 20kW Power Module Current Detection

The "Duet" of OPA2171 and AMC1200B

“Analog signals never lie—you just need to understand their language.”

In a DC charging module, accurate current sampling is not only crucial to control precision, but also directly impacts the safety and stability of the entire system. And the combination of OPA2171 and AMC1200B is what makes all this possible.

Background of the Power Supply Module: Current Detection is the First Step in Control

In a 20kW-class three-phase DC charging pile module such as the TGOOD MC750K20, the current sampling circuit is mainly responsible for two tasks:

Accurately detect three-phase PFC current to ensure power factor control and energy efficiency conversion;
Provide high-voltage isolation for the signal path to prevent high-voltage side interference from affecting the low-voltage main control unit (MCU).

From the teardown analysis, we can see that the inside of the module is equipped with:

The isolation chip used for current sampling is from Texas Instruments (TI), with the model number AMC1200B. It is a high-precision isolation amplifier and adopts the SOIC8 package.

The dual operational amplifier (dual op-amp) is from Texas Instruments (TI), with the model number OPA2171. It is a single-supply general-purpose amplifier, used for current signal amplification, and adopts the SOIC8 package.

4 units of OPA2171: Serve as pre-stage current amplifiers;
3 units of AMC1200B: Used for isolation detection and differential output of PFC in each phase.

👇 The sampling path is as follows:

Three-phase inductor → Shunt resistor → OPA2171 (amplification) → AMC1200B (isolation and amplification) → MCU ADC

OPA2171: The Foundation for Stable, Accurate, and Efficient Current Amplification

The OPA2171 is a 36V-powered, low-noise, general-purpose dual-channel operational amplifier launched by TI. It is widely used in industrial control circuits for current detection, amplification and filtering, and other scenarios.

📌 Why choose it? Parameter performance of OPA2171:

Power supply range: 2.7V ~ 36V wide range, compatible with 12V or 24V rail input
Input bias current: typical value is only 8pA, suitable for small signal sampling
Input offset voltage: typical 0.25mV, with low full-temperature drift down to ±0.3μV/°C
Gain Bandwidth Product (GBW): 3MHz, the bandwidth meets medium and high-speed control requirements
Common Mode Rejection Ratio (CMRR): up to 120dB, effectively suppressing power supply noise
Power consumption: 475μA per channel, low power consumption facilitates high-density layout

✅ Typical application values:

Amplification of pre-stage current signals in three-phase sampling
Post-stage op-amp matching with isolation amplifiers or ADC inputs
Suppression of common-mode interference and improvement of system anti-interference capability
Low power consumption meets the requirements of high efficiency and low thermal design

AMC1200B: The Perfect Integration of Precision and Isolation

The AMC1200B is a high-precision differential isolation amplifier introduced by Texas Instruments (TI), commonly used in shunt-based current detection scenarios. It integrates an 8x fixed gain, a silicon oxide isolation layer, and a 108dB Common-Mode Rejection Ratio (CMRR), achieving both signal fidelity and safe isolation.

📌 Why Choose It? Key Specifications of the AMC1200B:

Input Range: ±250mV differential, compatible with shunt resistors
Fixed Gain: 8x, with a ±0.5% gain error
Nonlinearity Error: ≤0.075% (@5V supply)
Isolation Voltage: 4250Vpeak, compliant with the UL1577 standard
Bandwidth: Minimum 60kHz, suitable for PFC frequency bands
Common-Mode Transient Immunity (CMTI): ≥10kV/μs, effectively shielding surge interference

✅ Application Highlights:

Isolated current sampling, ensuring safe decoupling between the MCU and the high-voltage side
Differential output directly connected to ADC, improving sampling stability
Bandwidth covering typical operating conditions from 50/60Hz to tens of kHz
Compatible with industrial temperature range: -40°C ~ +105°C

Engineering Practice: Precision and Protection Logic of the Sampling Circuit

Current detection not only serves power metering and power control, but also acts as the first line of defense for overcurrent protection and feedback closed-loop systems.

Link Position	Function
Shunt resistor	Converts current to voltage, serving as the signal source
OPA2171	Amplifies small signals to improve Signal-to-Noise Ratio (SNR)
AMC1200B	Provides high-voltage isolation + high-precision differential output
MCU ADC	Digitizes the signal and determines whether overcurrent or offset occurs

If front-end distortion, gain error, or isolation failure occurs, it may lead to the following consequences:

Control error: The PFC inductor cannot be adjusted correctly.
Protection delay: Overcurrent signals are not recognized, which may burn the MOSFET (Metal-Oxide-Semiconductor Field-Effect Transistor).
EMC issue: High common-mode interference couples into the main control ground.

Suggestions for Analog Chip Selection

Comparison Dimension	OPA2171	Alternative Recommendations
Performance Stability	✅ Low temperature drift	OPA192 (lower drift)
Noise Requirement	✅ Low-frequency stability	OPA2333 (lower noise)
Package Option	Multiple options (VSSOP/SOIC)	–

Comparison Dimension	AMC1200B	Alternative Recommendations
Isolation Level	✅ 4250V	AMC3302 (higher CMTI)
Bandwidth	✅ PFC-compatible	AMC1301 (wider bandwidth)

Written in the End

“For a 20kW power module system, power performance depends on MOSFETs, while stability relies on the analog front-end.”

Behind every current sampling path, the performance and stability of analog chips determine the system’s precision, response speed, and safety threshold. The OPA2171 and AMC1200B form a “golden combination” of precision + isolation in the PFC detection circuit, serving as an irreplaceable component in the design of high-reliability DC EV charger.