Abstract
System integration is one of the market driving issues in power electronics. In this paper the integra-
tion of precise shunts into IGBT modules, like it is done in Infineons MIPAQTM family, is compared to
on-IGBT-chip current sense functionality. Temperature measurement via NTC-resistor on DBC-level or
on-IGBT-chip integrated temp-sense-diodes will be treated as well. Further processing of the low volt-
age sense signals is done by sigma-delta converter including a functional isolation barrier by Core-
lessTransformerTechnology (CLT).
1. Introduction
Modern industrial inverter applications like pre-cise high speed servo drives require accurately dosed electrical energy at high power levels pro-vided by modern semiconductor inverter sys-tems. Forspeed/torque control and protective functions of the drive system, several measure-ment points like instantaneous inverter output current, dc-link voltage, high power semiconduc-tor junction temperature, etc. have to be cap-tured during converter operation. Recent re-search, development effort and improvements in power electronics allow the integration of sense functionality into high power modules to reduce space, to increase the reliability and to save costs of the converter system. In this paper, two ways of integrating current and temperature
measurement for control and protection functions will be shown, explained and discussed.
2. Integrating Current and Temperature Sense Solu-tions
The first approach is the integration of high power precise shunts into next generation IGBT modules. Temperature measurement is done by using common NTC-resistors. The second ap-proach is the integration of temperature and cur-rent sense into the IGBT chip itself, realizing the Temperature-Current-Sense (TCS) IGBT.
The galvanic isolated transfer of the measure-ment signals to the digital control side is done by sigma-delta-conversion. The galvanic isolation barrier is formed by coreless-transformer-technology (CLT) [2, 3], which is integrated on-chip with the sigma-delta converter.
2.1. NTC & Shunt integrated IGBT Module
In Figure 1, the block diagram of the IGBT in-verter full bridge module with integrated NTC, output current shunts and sigma-delta converters including galvanic isolation of digital signals is shown. This configuration is encapsulated by the proven Econo3 module case and belongs to the MIPAQ™ sense family [1].
Shunt measurement configurations for speed and torque control belongs to the least expen-sive solutions and can be easily soldered on the DBC inside the module. Doing this, the shunts can be easily isolated and cooled in an optimized way. The disadvantage of shunt measurement is the additional power dissipation, which is quite negligible as compared to the power semicon-ductor losses, however. A further drawback can be the reduced area on DBC level available for IGBT and diode dies.
Inside the module a NTC is integrated on the DBC. This configuration is proven and offers po-tential free temperature measurement. However, it does not provide a direct chip (IGBT/diode) re-lated signal. For most drive applications it is suf-ficient to have a “low-pass-filtered” temperature information signal for observing IGBT tempera-ture limits.
The voltage drop above the shunt is captured and digitalized via sigma-delta converter. Further features of small signal sigma-delta-conversion are treated below.
Fig. 1. Block diagram of the IGBT inverter full bridge module MIPAQ™ sense with integrated
NTC, output current shunts and sigma-delta-converter including galvanic isolation for all digi-
tal signals
2.2. TCS-IGBT
The on-chip current sense functionality isachieved by separation of a defined number ofIGBT cells from the power emitter metallization.Doing this, an additional sense emitter appearswhich should be connected to a low ohmic senseresistor (Figure 2).
Fig. 2. Current-Sense-IGBT device (inside das-hed line) with low ohmic sense resistor RSense Thereby the sense current iSense is a fraction of the load current iL of the whole device. Ideally iSense is only determined by the load current mul-tiplied with the number of sense cells divided by the total cell number of the IGBT. So the meas-ured sense current needs just to be multiplied by the cell number ratio in order to get the whole device current. The application of the sense re-sistor into the sense path causes a current den-sity mismatch between the emitter-isolated sense cells and the main power cells. The higher the sense resistor value is, the higher the devia-tion between scaled sense voltage and meas-ured load current becomes. On the other hand, for a appropriate voltage drop, a certain amount of sense resistance is required.
Figure 3 shows a 600V/200A sense IGBT ramp-ing current in double pulse test. Choosing RSense to 0.5Ω, the nominal scaling is 50mV/div. Apply-ing the nominal scaling for the vSense curve, the
sense signal is something higher than the con-ventionally measured current curve iL.
Fig. 3. 600V/200A TCS-IGBT: comparison of conventionally measured current iL (red, 100A/div) and voltage drop vSense across a 0.5Ω sense resistor (green, 50mV/div)
Adjusting the deviation by a scaling factor of 0.9for vSense, the situation like given in Fig. 4 ap-pears. For smaller currents, the vSense curve fitsthe conventionally measured current curve. Within creasing current, the sense voltage deviatesmore and more from the curve iL. The reason behind this is, that the higher the voltage drop across the sense resistor becomes, the more the vCE of the IGBTs sense cells and the power cells vary. This causes an increasing current density mismatch inside these cell parts leading to a
lowered sense resistor voltage drop.
Fig. 4. data from Fig. 5, assuming a scaling fac-tor of 0.9 for vSense
Fig. 5. Dynamic responses of the sense resistor voltage drop @ 2xIN: comparison of conven-tionally measured current iL (100A/div) and voltage drop vSense across a 0.5Ω sense re-sistor (50mV/div)
The dynamic responses of sense resistor voltag drop at switching 2 times the nominal current cabe seen in Fig. 5. Turning-off the TCS-IGBT, thsense voltage reacts with ringing, but after 1µ the signal is stable. The turn-on process doenot show any ringing. It reproduces the revers recovery current peak of the diode in a qualitative manner followed by a transient 5µs undershoot (also visible in Fig. 3 and 4).
Summarizing, the current sense feature of thTCS-IGBT does not provide most precise curren information like it is mandatory for electric machine control. But beside this, its performance iterms of accuracy and dynamic behaviour cabe used for over-current and short-circuitdetection. Doing so, no additional high voltagdiode is necessary like it is custom for conventional vCE(sat) detection. The sense signal is re
lated to the emitter potential of each IGBT, allowing a simple approach of the current sense functionality. But one should keep in mind, that versmall signals need to be handled for quite important protection functionality.
For temperature sense feature, on the top side othe TCS-IGBT there were pn-diodes integrated In general, they are isolated from any potentiaappearing on the IGBT die, but their pn-junctiovoltage drop should be referred to emitter potential.
For constant current, the pn-junction voltage drop is a good indicator for the chip temperature. For investigating the diodes voltage step re-sponse to a thermal event in the chip, a 8µs short circuit was performed with the TCS-IGBT (Fig. 6).
Fig. 6. Short circuit of the TCS-IGBT: gate emit-ter voltage (blue, 10V/div) and load current (red, 500A/div)
Figure 7 shows the changing in temp sense di-ode voltage drop as an answer of the short cir-cuit event shown in Fig. 6. The internal heat peak reaches the diode after a delay of more than 100µs.
Fig. 7. Temp-Sense-Diode step response: changing in pn-junction voltage drop (green,
15mV/div) and load current (red, 500A/div)
Because of the thermal time constant inside the IGBT die, the temp sense feature seems not suit-able to sense a short circuit. But apart from this, the temp sense feature provides a chip related and more or less integrated temperature infor-mation.
2.3. Sigma-Delta Converter with functional isolation capability
The sigma-delta conversion of the measurement signals generated by the above mentioned sense functions offer a big advantage compared to conventional A/D converter solutions. It only needs two channels for data streaming and clocking [4]. Both information lines easily can be galvanic isolated via micro transformer technol-ogy or so called CoreLessTransformer technol-ogy [2, 3]. Figure 8 shows the clock (fCLK = 10 MHz) and the serial data waveforms of the Σ/∆ bit stream generated by the Infineon´s 2nd
order sigma-delta converter IC type 1EC010I12-F. Latter includes a micro transformer isolation suitable for 1200V IGBT applications. The falling edge of the clock (Ch3, green) indicates the va-lidity/”point of time” of the data stream.
Fig. 8. Captured waveforms of the Σ/∆ converter 1EC010I12-F. Ch3: Clock (CLK) / Ch4: Serial Data Output (SD)
The so modulated and isolation-transmitted ana-logue measurement signal has to be digitally fil-tered before further processing in µC or in DSP for IGBT converter control. This sigma-delta modulated signal consists of its own origin fre-quency spectra plus quantization noise in the higher frequency region (see Figure 9b).
Fig. 9. Measurement signal depicted in time and frequency domain
a) before sigma-delta conversion
b) after sigma-delta conversion
c) after digital decimation filter process
Note, a narrow-band signal extraction from a wideband noised source has to be done. There-fore a digital filter with low-pass properties (red line in Figure 9b) has to be grind-in the transmis-sion path between modulator output and con-verter control input. This filter also has to convert the modulated signal, toggling between “1” and “0” into parallel multi-bit words with length w and to divide down the high modulator clock speed (here: 10 MHz) for signal processing in µC/DSP.This normally require very fast multipliers, com-
plex shift registers and long filter block chains which is often the largest bottleneck in a DSP system. Therefore here a very simple but effi-cient filter algorithm so called Cascaded-Integration-Comb (CIC) filter [5] is shown here, which can easily implemented into a FPGA. The CIC structure is depicted in Fig. 10.
Fig. 10. Principle of a k=3rdorder Cascaded-Integrator-Comb (CIC) filter
This filter structure consists of delay elements and adders which are working as integrators and differentiators and can be managed without mul-tipliers. Between the integrators I and the combs C the clock fCLK is divided by the decimation ratio OSR. At the output multi-bit word appears whose width w depends on the order k and OSR, which is described in [4]. OSR is equivalent to an over-sampling of the captured sigma-delta input sig-nal. Therefore only a simple 1st order RC low-pass filter has to be added to its input. The time-discrete transfer function H(z) in the z-domain can be described as follow:
Here is k the order, OSR the oversam-pling/decimation ratio. The filter frequency trans-fer function |H(f)| is a so called Sinck-function (si-nus cardinalis):
The sinc3 filter response for an OSR = 16 is shown in Fig. 11.
Fig. 11. sinc3 filter response for an OSR = 16
Here, the -3dB point is located at f-3dB = 164 kHz. This fulfills the required low-pass function de-picted in Figure 9b). Further discussion about the Sinck-function transfer behavior is done in [5].
For testing and investigating the integrated Σ/∆ converter 1EC010I12-F transfer characteristics, a 1200V IGBT halfbridge output current was measured via a PEARSON-probe and the sigma-delta converter clock and bit stream sig-nals were captured simultaneously. These wave-forms are shown in Fig. 12. The captured wave-forms were read from a personal computer spreadsheet calculation program, where the
clock and bit stream signals were 1-bit-digitalized. With this spreadsheet program a 3rdorder CIC filter structure, the Sinc3 filter, was modeled to calculate line-by-line and simulative the status of the integrators and combs shown in Figure 10. This time-discrete filter processing represents an implementation into a FPGA. Here, different oversampling rates
Fig. 12. Ch2: IGBT halfbridge output current (red, depicted in V instead of A) Ch1 + Ch3: sigma-delta converter output clock + bit stream (“noise” in the bottom) Ch4: internally filtered analogue output signal from 1EC010I12-F
(OSR) were chosen to clarify the different ef-fects. In Fig. 13 the result after simulative digital filtering of the bit stream is shown. It clearly shows the nearly exact reconstruction of the sigma-delta modulated output current (the di-rectly captured output current “I_pearson” wave-form is covered by the filter output signals).
Fig. 13. Sinc3-filtered sigma-delta modulation signal with different oversampling/decimation
rates
The higher the oversampling/decimation rate OSR is, the more exact is the reconstructed sig-nal and the longer the time response becomes. There is a linear trade-off between conversion speed tconv and OSR with the assumption, the registers implemented in the FPGA are triggered by the sigma-delta clock of fCLK = 10MHz (inte-grators I) or fCLK/OSR (combs C) and the step response of the above mentioned sinc3-filter was investigated:
A trade-off between needed accuracy and con-version speed has to chosen for every applica-tion. Now the so decimated and filtered sigma-delta modulation signal can be used by the µC/DSP for power electronics application control. Further investigations and characterization of the integrated shunt/sigma-delta converter configu-ration are ongoing.
3. Conclusion
In this paper the future trend of integrating cur-rent and temperature measurement sense fea-tures inside IGBT modules, like it is realized with Infineon´s MIPAQ™ family, and even into IGBT chip devices is shown. In this study, two possible integration solutions were presented and ex-plained. Their pros and cons were enumerated and discussed. In summary, shunt integration into IGBT modules is suitable for speed and
torque control while the on-chip current sense seems to fit only for protection functions. Compared with the NTC approach, the temp sense with on-chip-integrated diodes does not provide the provenlow-passed temp signal but a chip related information. However, the NTC measurement signal is potential free, while the diode signal needs to be transferred by sigma-delta converter to the inverter control side.
Except for the NTC temp sense approach, the treated integration solutions small signals are digitalized and transmitted to the µC-/DSP-port via Σ/∆ converter including micro transformer technology for galvanic isolation. Compared to a classical converter with discrete sense equip-ment and digitalization units they are less expen-sive and more reliable.
4. Literature
[1] Schulz, M.; Kanschat, P.; Zhang, X.: „Higher Functionalities in Power Electronic Modules“, Bodo’s Power, September 2008, pp. 26 -28.
[2] Strzalkowski, B. et al.: “High performance IGBT-driver in microtransformer technology providing outstanding insulation capability”, Proceedings of PCIM 2003, Nürnberg, 2003.
[3] Strzalkowski, B.; Schwarzer, U.; Jansen, U.: “High performance IGBT-driver in micro-transformer technology providing out-standing insulation capability”, Proceedings of PCIM2007, Nürnberg, 2003.
[4] Park, S.: “Principles of Sigma-Delta-Modulation for Analog-to-Digital Converters”, MOTOROLA Application Notes.
[5] Hogenauer, E. B.: “An Economial Class of Digital Filters for Decimation and Interpola-tion”, IEEE Transactions on Acoustics, Speech and Signal Processing, Volume 29, Issue 2, Apr 1981 pp. 155 - 162
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