Maxim Integrated 78M6610+PSU Hardware Design Guidelines - Manual (Page 6)

78M6610+PSU Hardware Design Guidelines AN_6610_107
6 Rev 0
2.2 Safety Precautions
With V
3P3
directly connected to NEUTRAL, the 78M6610+PSU’s Ground signal can be -3.3 V
below earth ground. Any external test equipment attached to the 78M6610+PSU will be
subject to this -3.3 V ground reference disparity. External test equipment must
be
disconnected from earth ground to avoid equipment damage due to this ground reference
disparity.
A more serious safety issue may arise due to mis-wiring of the AC circuit. If the LINE and NEUTRAL wire
connections at the AC plug are reversed between the 78M6610+PSU and the external test equipment,
the 78M6610+PSU and external test equipment will see a 120/240 VAC voltage difference rather than
-3.3 V. This scenario can occur when the 78M6610+PSU and external test equipment are powered from
different wall outlets or power strips, one of which is mis-wired. With proper earth grounding, the external
equipment is always referenced to earth ground via their enclosures.
2.3 Powering the 78M6610+PSU
The 78M6610+PSU requires a single 3.3VDC supply. The supply circuit should provide about 10 mA to
power the 78M6610+PSU. Additionally, power is required for the surrounding circuitry (i.e. opto-couplers).
In the following diagrams, circuit protections such as Metal-Oxide Varistors (MOV), chokes, etc. are
omitted.
Commonly power supplies have a main (SMPS) channel that powers the DC loads (servers, storage,
communication equipment etc.) and a stand-by (SMPS) channel that powers the controllers and
supervisory circuits. The most common way to power the 78M6610+PSU in a power supply is to use an
auxiliary winding (tap) of the transformer used in the stand-by channel. The advantage is that the
winding is isolated and the circuitry requires a minimal number of components.
STAND-BY POWER
SUPPLY
Voltage
Regulator
V3P3
GND