Quick Teardown: High current DC motor controller - Automotive PAS - Page 1
Forum section: electronics / general

author: max_torque
author_rank: Super Contributor
timestamp: September 15, 2014, 01:50:45 pm
content: Found this SHOWA brushed DC motor H bridge controller cheap on ebay. They are used on the Mazda RX8 electric power steering that has a brushed low voltage (12v nominal) motor driving the rack to add power assistance to the system.However, it looks like these units would make a great base for any low voltage high current brushed DC motor control project, being cheap (around £20 in the UK) and capable of high current operation:The controller is a nice, quite heavy (for thermal mass i think) thick die cast ally enclosure, with an all-in-one proprietary molded connector on one end, and capped with a thin pressed steel top plate.Basic layout is a single control pcb mounted on top, with the power stage in a custom welded/molded assy in the bottom:Power stage (4 fets in classic H bridge layout, 2 isolation relays (in & out) filter caps, and an output current sensor:Fets are 60v 85A continuous TO-247s, with approx 4mO RDS(on), and are extremely well heat sinked to the casing. Construction is typical of high current, mass produced automotive power distribution, using multiple, large conduction area, stamped plated copper bus bars, held in place with an "overmolded" thermoplastic plastic structure. Components are joined to those bus bars by a mixture of ERW and ultrasonic welding depending on the size of the join. No obvious snubbing or filtered bar the two large filter caps on the DC bus (105deg rated, 35V 2700uF) and a single small ceramic cap.What is quite nice is that all the control signals for the FET gates, and the current sensor & isolation relay I/O are brought up to the PCB level with nice, easy to join to "through hole" vertical pins. Making a custom control pcb to go into this housing would be simple!The control pcb is 4 layer, double side loaded, rectangular, 96mm x 80mm, std FR4 1.6mm thick, with a light conformal coating.IO from the power stage and the units IO connectors is via hand soldered (during unit assy) through holes, rest of pcb is very neat, reflow soldered. Main connector IO is at front end of pcb via 19w connector, current sensor on RHS (5way) and gate drive at rear (9way)A couple of SOT-23 packaged logic level N fets are present and used to control the main power isolation relays (being "safety critical" as it's part of the cars steering system) the ecu doesn't just rely on turning off the H bridge FETs, but can isolate both the Vbatt supply to the H bridge and the output arms as well (prevents shorted FETs from "locking" motor and stopping it being turned, for un-impeded mechanical steering functionality)Also an unused 8way connector footprint labelled "CN1", could be programming or debugPCB is labelled "NESTECinc" on the silk screen (who were taken over by SHOWA) and IC date codes are around the middle of 2003. Layout is fairly neat, and components are well labelledMosfet Gate drive is achieved with a Toshiba TD7203F "3 phase" driver, of which only 2 phases are used.Main processor: Labelled 6432612A87FA 1ACJ60100200 BL01231 JAPANSecondary device: NEC C6718 0340A3003 (might be specific motor control IC or 'safety supervisor'?)There is also a small SO-28 IC, that i suspect is a sin wave resolver driver and feedback IC that is used to excite the resolver based steering column torque sensor. Looks to be SPI linked to main processorAlso std automotive CAN transciever, Philips A82C250 in SO-8 and usual common mode CAN choke next to itUnderside of PCB:Usual automotive power and load dump stuff. Nice big TVS diode (On-semi SM6A27) a smattering of descretes for IO and decoupling etc. Also a 16Kbit EEPROM in SO-8 format (not sure what this for, on my unit it is blank (all 0xFF) Could be for model coding and error code storage etc