Dear TI engineers,
The INA 253 seemed very promising when testing it without any common mode voltage at the input. However, when applying it for phase current measurements in a full-bridge inverter with a switching frequency of 800kHz, its output voltage seems to behave weirdly. In the following example, the three inverter legs receive the same gate signal with a duty cycle of 50% and the current sensors are connected after a LC filtering stage. The sensors are supplied with 5V (with decoupling capacitor of 100nF) and connected to an external reference voltage of 2.5V. Since no current can flow through the sensors, their output is expected to be flat at 2.5V.
In a first attempt, the star-point of the filter is isolated from GND. Without any gate signal at the switches, the output looks nice at 2.5V:
But as soon as the gate signal is activated, the output (channel C1 yellow) gets an offset (0.48V -> 1.2A) and oscillates arbitrarily (the INA253 variant used is the one with 400mV/A, a division represents then 0.5 A). Might there be a problem when the voltage at C3 (blue curve in figure below) goes below -4 V?
If the star-point of the filter is connected to ground, the offset disappears, but an unwanted ripple still exists:
Questions:
- Can then the INA253 be used in such a configuration (even preferably without connection from star to GND)?
- And what could be the reason for this strong deviations?
- In the data sheet PWM rejection is mentioned as special feature.
- How das the PWM rejection works?
- Might there be a problem when the PWM frequency is at 800 kHz?
Thank you for your consideration
Kind regards
Timon Achtnich