MX881 12 7/30/07
MX881
IXYS
Layout Notes:
• Parasitic capacitance on node VSEC or VPRI will cause loss of efficiency. Keep wires on this
node as short as possible.
• Parasitic inductance on the transformer connections can cause overshoot during switching that
could damage the part. Keep connections from the capacitors and MX881 to the transformer
short.
• Also keep the connection from pin 2 of D1 to C3 as short as possible.
• The VFB node is sensitive to coupling from T1 and the VSEC node. The VSEC node ramps to
over 300 volts in just a few nano-seconds. Keep R1, R2, and R3 as far from T1 as possible. If
possible, shield resistors by placing on the other side of the board from T1.
• VCC should be well bypassed to average charge current to the battery and reduce noise to
analog circuits within MX881.
• VFB is a very fast and short pulse. Keep VFB connection to R7 short to minimize capacitance on
this node. If VFB has too much capacitance, the feedback voltage may be filtered so that the
output voltage does not stop at 330V but continues to a much higher voltage.
• A good ground plane is extremely important. Large pulse currents of 0.7 amps will be flowing in
the ground. The large bottom pad of the MX881 is the only ground pad and requires a low
impedance return to C1, C2, and R7.
• The ground plane must have adequate clearance from high voltage nodes VHV and VSEC to
avoid arcing. Also, the node connecting R1 to R2 will be at approximately 170V.
• R1 and R2 must be at least 1206 size surface mount to withstand 200V each.
Transformer
For 330V charger, the transformer should have a turns ratio of approximately 10 to insure that the fly-
back voltage on the primary side of the transformer does not damage the power switch internal to the
MX881. The fly-back voltage should be equal to the output voltage divided by the turns ratio plus the
VCC supply voltage. The fly-back voltage will be greater than this due to the energy stored in the
transformer leakage inductance. The actual voltage will depend on the transformer leakage inductance
and the parasitic capacitance of the circuits and layout.
The MX881 is designed to work with a transformer having a primary inductance of approximately 10µH.
Larger values are acceptable as long as leakage inductance is small. Smaller primary inductance values
may cause feedback pulses that are too short and lead to higher than expected output voltage.
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publication and reserves the right to make changes to specifications and product descriptions at any time without notice. Neither
circuit patent licenses nor indemnity are expressed or implied. Except as set forth in IXYS’ Standard Terms and Conditions of Sale,
IXYS Corporation assumes no liability whatsoever, and disclaims any expressed or implied warranty, relating to its products
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Drawing No. 088109 www.claremicronix.com
