Notification 218 - Primary circular mils per amp (CMA) is below the recommended minimum

CATEGORY

ISSUE / RESULT

FIX / REFERENCE(S)

Warning

The primary circular mils per amp (CMA) calculation is below the recommended minimum for this power level (PO).

Table 1 contains minimum recommended current density limits that should be valid for most designs.  However, thermal performance will be affected by overall efficiency (N), the transformer construction scheme, system thermal factors and environmental constraints and specifications, such as heat sink design, airflow restrictions altitude of operation, etc.

The transformer may overheat, due to high resistive winding losses.  The worst-case conditions for this would be at low-line AC input and full load, at the highest rated ambient temperature.

• Use thicker wire (lower AWG, or larger cross-sectional area)

• Increase the number of parallel strands of wire (filars)

• Increase BWE by increasing the number of primary layers (L).

• Lower the number of primary (NP) and secondary (NS) turns, which will allow larger wire (lower AWG) to be used.

• Lower the primary RMS current (IRMS) by increasing the reflected output voltage (VOR).

• Decrease the current waveform parameter (KP) to lower the IRMS.

• Increase the minimum AC input voltage (VACMIN), which will increase the minimum input voltage (VMIN) and lower the IRMS.

• Increase the bulk input capacitance (CIN) to raise the VMIN and lower the IRMS.

• Increase the effective bobbin width (BWE) by increasing the core/bobbin size.

• If possible, lower the power supply output power requirement (PO).

• See Application Notes AN-32 (TOPSwitch-GX)  for details on primary CMA calculations.

Minimum recommended CMA limits

Power (PO)

CMA (min)

<20 W

200

20 W - 40 W

250

40 W – 80 W

300

80 W – 150 W

350

>150 W

400