Here is a typical WD bridge PCB:
Western Digital My Book Essential bridge PCB, 4061-705094:
http://www.alexsoft.org/viewtopic.php?f=112&t=1032&p=4805#p4805
This is a typical functional block diagram:
dual MOSFET switch
.---------. .--------.
| _/ | +12V| |
+12V o----------+--------------o/ o------------->| |
| | | | SATA |
.----'----. | _/ | | Power |
o | +5V reg |---+-----o/. o------------->| |
| '---------' | | | | +5V| |
=== | '----|----' | |
GND +-------+ |HDD Power | |
| | | On/Off | |
.---'--. .--'---. | control | |
switchmode |Vcore | | Vio | | | HDD |
regulators '---.--' '--.---' | | |
| | | | |
| | | | |
.--. USB 2.0 .---'-------'---. | | |
| |<------->| |---+ | |
| | | USB-SATA | SATA Tx | |
| |<--------| Bridge IC |------------------->| SATA |
| |-------->| | | data |
'--' USB 3.0 | |<-------------------| |
'-.-----.-----.-' SATA Rx | |
VCC | | | '--------'
+ | | | .----.
| | | | | |
.-. | | +-------| | EEPROM/flash
| | | | | |(bridge firmware)
R | | | .-. '----'
'-' | | | R
| | | | R
+---------+ '-' ___ LED
| | |----|___|------|<-----o VCC
| | | Error,
| o | |/ Activity
|=|> Pushbutton +------| Q
| o Switch |>
| |
| |
=== ===
GND GND
When an external drive is overvolted with a typical 19VDC laptop adapter, the bridge board usually survives. In those case where it does not, the typical failures are the dual MOSFET switch IC, and less often in the PWM controller IC for the 5V switchmode regulator.
Repairing Bridge PCBs in External HDDs:
http://www.alexsoft.org/viewtopic.php?f=100&t=155&p=311
