Power Distribution Unit (PDU)

The Power Distribution Unit (PDU) is an independent control unit that controls the power supply to all electrical consumers in the vehicle's low-voltage system. It consists of various high and low-side switches controlled by a microcontroller. The microcontroller receives instructions from the main control unit (MCU) via the CAN bus. Pulse width modulation (PWM) can be used to control the power output to individual loads either directly or by generating a control signal for a secondary controller. In addition, the PDU provides diagnostic functions that can be used to detect faults in the vehicle. ## Summary of current version The current version of the PDU uses four MC33984 dual switches capable of switching up to 30A and suitable for loads with high inrush current, as well as motors and all types of resistive and inductive loads [^1]. Additionally, there are three MC06XS3517 switches with a total of five channels [^2]. The MC06XS3517 is designed for automotive lighting applications, so special care must be taken when switching inductive loads. Both chips feature an SPI interface and programmable PWM generators. - 8 channel @ 8mOhm - 9 channel @ 6mOhm - 6 channel @ 17mohm - 6x PWM out (3v3) - 2x Low-Side (BSS123) / Input - 2x +5V (BSS123) / Input ### List of current / expected consumers #### Default ECU´s | Count | Consumer | I_max | P_max | |:-----:|------------------|:-----:|:-----:| | 1 | MCU | 1A | 12W | | 1 | IU | 1A | 12W | | 1 | steering wheel | 0.2A | 2W | | 4 | data acquisition | 0.2A | 10W | | 1 | brakelight | 2A | 24W | | 1 | RTDS | 2A | 24W | | 1 | CAN-logger | 1A | 12W | | 1 | router | 1A | 12W | | 1 | switch | 1A | 12W | | 1 | kistler | 3A | 36W | #### Autonomous System | Count | Consumer | I_max | P_max | |:-----:|------------------|:-----:|:-----:| | 1 | EBS | 1A | 12W | | 1 | RES | 1A | 12W | | 3 | ASSI | 2A | 72W | | 1 | emergency sound | 1A | 12W | | 1 | steering actuator| 12A | 144W | | 1 | car pc | 20A | 240W | | 1 | LiDAR | 3A | 36W | #### Electric drivetrain | Count | Consumer | I_max | P_max | |:-----:|------------------|:-----:|:-----:| | 1 | TSAL | 2A | 48W | | 1 | LE | 5A | 60W | | 1 | BMS | 3A | 36W | | 1 | shutdown circuit | 2A | 24W | | 1 | LE pump | 4A | 48W | | 1 | LE cooler | 4A | 48W | | 1 | motor pump | 7.1A | 86W | | 1 | motor cooler | 4A | 48W | | 4 | battery fan | 3.1A | 150W | The maximum current per consumer and the maximum power draw per type of consumer are roughly estimated. The purpose of this list is to estimate the number of channels required and their current capacity. | Current Range | Number of channels | |:-------------:|--------------------| | 0.0-1.9 A | 20x | | 2.0-4.9 A | 7x | | 5.0-9.9 A | 3x | | > 10 A | 2x | ### Problems / Shortcomings - Number of channels not sufficient for Driverless - Potentially susceptible to demag pulses (KIT19c) - Quickly switching large inductive loads on and off causes the PDU to shutdown - There is no circuitry on the PDU to protect it from voltage spikes on the supply rail. Inductive loads should be connected in parallel to a freewheeling diode, but even this neglects the parasitic inductance of the wiring harness (TODO: find out what order of magnitude we are talking about) - More signal inputs would be useful + protection / buffer - Probe shutdown circuit at different point for diagnostics - had to improvise before (e.g. FSG 2019 driverless signal for TS-active LED IIRC) - Current sensing not implemented or to inaccurate - Voltage measurement not accurate and low frequency - BSS123 probably not strong enough to drive bigger Relays - could be used to switch on the Car-PC / steering actuator ## Design Goals **Input voltage :** 8 - 24 V **Input current :** 40 A (60 A for 10s) **Protection features :** overcurrent, overtemperature, overvoltage (transient), reverse polarity, load disconnect, short to ground, short to vcc **Output channels :** ? ## Design ### Chip selection **Resources:** - [Infineon](https://www.infineon.com/cms/en/product/power/smart-low-side-high-side-switches/high-side-switches/classic-profet-24v-automotive-smart-high-side-switch/) - [STMicroelectronics](https://www.st.com/en/automotive-analog-and-power/high-side-switches.html) - [Texas Instruments](https://www.ti.com/de-de/power-management/power-switches/high-side-switches/products.html) - [NXP](https://www.ti.com/de-de/power-management/power-switches/high-side-switches/products.html) - [Dialog semiconductors](https://www.dialog-semiconductor.com/hfet-1-high-voltage-high-current-ultra-low-rdson-integrated-power-control-switches) - [Rohm semiconductors](https://www.rohm.de/products/power-management/power-management-switch-ics?SupplyVoltageMax_num=28.0|32.0) **Selection Criteria:** - $R_{DS(on),max}$ - maximum operating voltage - package size - protection / convenience feature --> easy integration - cost / availability The PCB area required to dissipate the power losses of a switch is proportional to its $R_{\text{DS(on)}}$. The increase in temperature at the junction per Watt is given by the thermal resistance of the switch $R_{\theta\text{JA}}$. This parameter is measured according to JEDEC JESD51-2 on a 2s2p test board. The junction temperature for a given current is therefore: $T_{J} = T_{A} + R_{\theta\text{JA}} \cdot R_{\text{DS(on)}} I^2$ Consequently, the product of thermal resistance and drain-source resistance is proportional to the junction temperature. To asses the space requirements of a switch in relation to its current capability the parameter $\phi$ is introduced: $\phi = \frac{A}{N} \cdot R_{\theta\text{JA}} \cdot R_{\text{DS(on),150}} \cdot 10^{-3}$ #### High current switches (10A) - **MC33984**: Dual Intelligent High-current Self-protected Silicon High Side Switch (4.0 mOhm) $\color{green}{\Leftarrow}$ - $R_{\text{DS(on),max}} = 6.8\text{m}\Omega \quad \Leftrightarrow \quad T_J = 150 \text{°C}$ <img src=https://www.nxp.com/assets/images/en/chip-images/MC33984_IMG.jpg width=200px align=right> - $\phi = 14.688$ - operating voltage: 6-27V - size: 12mm x 12mm (144 mm2) - channels: 2 - cost: 10,19 € - same as current switch, but lower resistance - **MC33988** Dual Intelligent High-current Self-protected Silicon High Side Switch (8.0 mOhm) $\color{red}{\Leftarrow}$ - $R_{\text{DS(on),max}} = 13.6\text{m}\Omega \quad \Leftrightarrow \quad T_J = 150 \text{°C}$ - $\phi = 29.376$ <img src=https://www.mouser.de/images/freescale/images/PQFN-16.JPG width=200px align=right> - operating voltage: 6-27V - size: 12mm x 12mm (144 mm2) - channels: 2 - cost: 6,88 € - no problems in the past - **MC06XS3517** Dual 24V high-side switch (6.0 mOhm) - $R_{\text{DS(on),max}} = 28.9\text{m}\Omega \quad \Leftrightarrow \quad T_J = 150 \text{°C}$ - $\phi \Rightarrow$ n.a. (thermal resistance must be guaranteed by design) <img src=https://www.mouser.de/images/freescale/lrg/MC06XS3517.jpg width=200px align=right> - operating voltage: 7-20V / 6-28V (extended range) - size: 12mm x 12mm (144 mm2) - channels: 2 - cost: 7,97 € - **BTS50085-1TMA** (9 mOhm) - $R_{\text{DS(on),max}} = 17\text{m}\Omega \quad \Leftrightarrow \quad T_J = 150 \text{°C}$ - $\phi = 41.134$ <img src=https://assets.lcsc.com/images/szlcsc/900x900/20180914_Infineon-Technologies-BTS50085-1TMA_C82655_front.jpg width=200px align=right> - size: 13.85mm x 10mm (138.5 mm2) - channels: 1 - cost: 5,33€ - **BTS7008-2EPZ** (8 mOhm) - $R_{\text{DS(on),max}} = 18\text{m}\Omega \quad \Leftrightarrow \quad T_J = 175 \text{°C}$ - $\phi=8.098$ - operating voltage: 6-28V - size: 6.3mm x 5.2mm (32.76 mm2) - channels: 2 - cost: 3,07 € - continuous current at 85°C 7.5A - **BTS7004-1EPP** (4.4 mOhm) - $R_{\text{DS(on),max}} = 8\text{m}\Omega \quad \Leftrightarrow \quad T_J = 150 \text{°C}$ - $\phi=8.334$ <img src=https://www.infineon.com/export/sites/default/media/products/Power_Semiconductors/PG-TSDSO-14-22.PNG_1708265422.png width=200px align=right> - operating voltage: 3.1-28V - size: 6.3mm x 5.2mm (32.76 mm2) - channels: 1 - cost: 2,40 € - continuous current at 85°C 15A - **BTS7002-1EPP** (2.6 mOhm) - $R_{\text{DS(on),max}} = 4.8\text{m}\Omega \quad \Leftrightarrow \quad T_J = 150 \text{°C}$ - $\phi=4.827$ - operating voltage: 3.1-28V - size: 6.3mm x 5.2mm (32.76 mm2) - channels: 1 - cost: 2,90 € - continuous current at 85°C 21A #### Medium current switches (5A) - **BTS5215L** Smart high-side switch, two channels (90 mOhm) - $R_{\text{DS(on),max}} = 180\text{m}\Omega \quad \Leftrightarrow \quad T_J = 150 \text{°C}$ <img src=https://www.infineon.com/export/sites/default/media/products/Power_Semiconductors/PG-DSO-12-2.jpg_1544565615.jpg width=200px align=right> - $\phi = 325.377$ - operating voltage: 5.5-40V - size: 10.3mm x 7.8mm (80.34mm2) - cost: 2,66€ - **SLG59H1019V** 24 V, 5 A Integrated Power Switch (13 mΩ) - $R_{\text{DS(on),max}} = 19.2\text{m}\Omega \quad \Leftrightarrow \quad T_J = 85\text{°C}$ - $\phi = 3.686$ <img src=https://www.mouser.de/images/dialogsemi/lrg/STQFN-18_DSL.jpg width=200px align=right> - operating voltage: 4.5-25.2V - size: 3.0mm x 1.6mm (4.8 mm2) - cost: 0,436€ (min. 3000) - channels: 1 - **BV1HJ045EFJ-C** (45 mOhm) - $R_{\text{DS(on),max}} = 90\text{m}\Omega \quad \Leftrightarrow \quad T_J = 150 \text{°C}$ - $\phi = 68.531$ - operating voltage: 8-28V - size: 4.9mm x 6.0mm (29.4 mm2) - cost: not available - channels: 1 #### Low current switches (1A) - **BTS71040-4ESA** (22.5 mOhm) - $R_{\text{DS(on),max}} = 38\text{m}\Omega \quad \Leftrightarrow \quad T_J = 150 \text{°C}$ - operating voltage: 3.1-28V - size: 8.65mm x 6.0mm (51.9 mm2) - cost: 3,24€ - channels: 4 ### MC33984 design - [AN3274, eXtreme Switch Protection Guidelines](https://www.nxp.com/docs/en/application-note/AN3274.pdf) - add 4.7uF and 10nF capacitor to $V_{\text{PWR}}$ (chapter 6.1) - add 22nF and 470nF capacitor to each output (chapter 6.4) - (optionally) add 6.2V Zener diode and 100R series resistance to $V_{\text{DD}}$ to protect against short to battery (chapter 7.2) - Clamping transient voltages caused by inductive loads - add TVS Diode on outputs - P6SMB24A / P6SMB27A for 12-16 V - P6SMB30A for 24V, but Vc is slightly to high (41.4 vs 41.0V) - external current sensing, because internal has +-25% deviation - $I_{\text{min}} = 3A$; $I_{\text{max}}=20A$; - $(15A)^2 \cdot 1.5m\Omega = 0.3375 W$ - Isabellenhütte BVF-V-R0015-1.0 (2W) ### Connector #### Wire size selection | cross section | AWG (approx.) | current rating (max.) | current rating (nom.) | |:-----------------:|:---:|:--------------:|:---------:| | 0.25$\text{mm}^2$ | 23 | 5A | 3.05A | | 0.35$\text{mm}^2$ | 22 | 8A | 4.87A | | 0.50$\text{mm}^2$ | 20 | 12A | 7.31A | | 0.75$\text{mm}^2$ | 19 | 15A | 9.14A | | 1.00$\text{mm}^2$ | 17 | 19A | 11.57A | | 1.50$\text{mm}^2$ | 15 | 24A | 14.62A | | 2.50$\text{mm}^2$ | 13 | 32A | 19.49A | Calculations are based on Lapp LiY 1x series. According to [^3] there is a derating to the current capability of the cable based on the ambient temperature and the number of current carrying cable in the same cable duct. Assuming an ambient temperature of 40°C and an average of 5 current carrying wires in the same part of the wiring harness, we have to apply the factors $\alpha_T = 0.87$ and $\alpha_N = 0.70$ to the current capability at 30°C. Note that this is still an optimistic approximation; Temperatures can rise higher than 40°C in the car and there will be parts with more conductors at the same spot. #### Connector Selection - **Molex CMC 36638** <img src=https://www.mouser.de/images/molex/lrg/36638_series_iso.jpg width=200px align=right> - positions: 48 - configuration: 8x12A; 40x2.5A - voltage rating: 14VDC - size (w x l x h): 58.6x56.5x30.9mm - wire size: 0.35mm2 - 1.0mm2; - - **TE Ampseal 1-776163-1** <img src=https://www.te.com/content/dam/te-com/catalog/part/017/761/631/1-776163-1-t1.jpg/jcr:content/renditions/product-details.png width=200px align=right> - positions: 35 - configuration: 35x17A - voltage rating: 48DC / 250AC - size (w x l x h): 76.9x37.9x32.1mm - wire size: 16-20AWG (1.5 - 0.5mm2) - **TE Superseal 1.0** <img src=https://www.te.com/content/dam/te-com/catalog/part/CAT/SU7/63H/CAT-SU763-H342-t2.jpg/jcr:content/renditions/product-details.png width=200px align=right> - positions: 34 - configuration: 34x15A - voltage rating: 250V AC/DC - size (w x l x h): 45.5x36.5x31.4mm - wire size: 1.25 - 0.5mm2 [^1]: [Datasheet MC33988](https://www.nxp.com/docs/en/data-sheet/MC33988.pdf) [^2]: [Datasheet MC06XS3517](https://www.nxp.com/docs/en/data-sheet/MC06XS3517.pdf) [^3]:[Strombelastbarkeit Grundtabellen](https://www.lappkabel.de/fileadmin/catalog/2010_de_pdf/T12_Belastbarkeit_Grundtabelle_Reduktionstabellen.pdf)