4.3 Battery Current Sense

For battery charger applications, it is essential to keep the charging current permanently under control. In the automotive space, the preference is for high-side sense, as if low-side sense is used. Then, if the cable gets shorted to the chassis, the current cannot be detected.

The requirements for the current sensor are shown in Table 4-1.

Isolated current sensors using different technologies are available that fit these criteria. Hall effect sensors are quite popular, but for certain applications, Anisotropic Magneto Resistive (AMR) sensors may be more suited.

All isolated current sensors are inherently noisy at all current levels due to internal amplifier and calibration circuitry.

Hall effect sensors can have up to 50mVpp noise in a very broad frequency range (including a low kHz range). A typical DC/DC current loop compensator could have 10dB+ of gain up to a few kHz, so this level of noise will essentially turn the current compensator into a random noise power amplifier (50mVpp x 10dB equates to swings of 3A to 5A on the controlled current, which is unacceptable). This noise is too powerful to filter at the power stage. Instead, it must be filtered at the source, either by placing a pole at the current sensor output or by reducing the gain of the current compensator. Both options degrade system performance.

AMR-based sensors have lower noise in the bandwidth of interest and, hence, were found to be a better option. A simple RC filter with a pole at 7.2kHz was used at the current sensor output to filter higher-frequency noise and the switching ripple before the signal reaches the ADC.