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PIIPM25P12B008 Datasheet(PDF) 14 Page - International Rectifier |
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PIIPM25P12B008 Datasheet(HTML) 14 Page - International Rectifier |
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14 / 28 page  www.irf.com 14 PIIPM25P12B008 I27147 02 - Oct reference voltages. In fact if the 5V linear regulator drifts in temperature or time, then all references (even the 3.30V being this a simple partitioning) follow in track and still keep the overall chain precision. The trimming is then done only once, in a single point of the measuring chain, that is the conditioning op-amp collecting the current sensing ICs signal as will then be described in the following chapter. 3. Current sensing interfaces, over-current protections and signal conditioning. This block is the real critical point of the system. Current measuring performances directly impact on motor control performances in a servo application: errors in current evaluation, delay in its measuring chain or poor overall precision of the system, such as scarce references or lower number of significant A/D bits, inevitably results in unwanted trembling and unnatural noise coming from the motor while running at lower speed or at blocked shaft conditions. In the PI-IPM25P12B008 the current sensing function is done through three sensing resistors dropout measurement, one on each output phase, with the benefit of a lower area and somewhat a lower cost compared to the well-known Hall effect devices. This solution has the added value of having the shunts element embedded in the power module with all Kelvin connections available, avoiding any noise due to long routing of power paths. As the block schematic on page 2 shows, the voltage across each sensing resistor is applied, through an anti-aliasing 400kHz filter, at the input of a current sense IC and then to a signal conditioning circuit. Though the block schematic here shows an Op- Amp plus an external passive filter this is simply realized implementing a VCVS cell (i.e. a Constant Gain or Sallen – Key cell) configured so that the offset and gain is easily trimmed by three on board resistors. The filter implemented is a second order Bessel with 5.5kHz pole frequency, the reason for this is that this type of polynomials are calculated with the aim of having a constant group delay within the pass-band frequencies, thus giving the minimum waveform distortion to the output signal up to almost twice the filter pole. In other words we could also say that the group delay of the signal chain from the sensing resistor up to the ADC input of the DSP is constant from 0 to 5.5kHz. Signal outputted from the overall chain has a 0 to +3.30V dynamic, with a sensing resistor of 4mohms the input measured current range is +/- 50A then we have a situation as follows: V A V A V A 30 . 3 50 65 . 1 00 . 0 0 . 0 50 = + = = − Summing up our current measurements performances are shown in the following table: The “2406A” DSP has a 10bit ADC, consequently the PI-IPM25P12B008 has a minimum appreciable current step of approximately: Α = = 0976 . 0 2 50 * 2 10 LSB that is: mA LSB 98 1 ≅ The over current protection is provided also through the current sensing ICs, the related fault signal is activated when a 250mV voltage across sensing pins is detected, this means an over- current detection level of approximately 25%. The delay of this line is around 3 µs, fast enough to let the DSP react within the 10 µs IGBTs short circuit rating, thus providing full device protection for any phase-to-ground and phase-to- phase short circuits. The only failure not covered in this way is the shoot-through, where high current levels cannot be detected from outside the module rather internally between two IGBTs of PI-IPM Current sensing chain typical performances Value Units current range +/- 50 A Gain and Offset precision +/- 1.8 % bandwidth 5.5 kHz latency time 10 µs |
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