Notification 222 - Primary current density (A/mm^2) is above the recommended maximum

CATEGORY

ISSUE / RESULT

FIX / REFERENCE(S)

Warning

The primary current density (A/mm^2) calculation is above the maximum recommended for this power level (PO).

Table 1 contains recommended current density limits that are 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.

• Use thicker wire (increase cross-sectional area)

• Use more parallel strands of wire (filars)

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

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

• Lower the primary rms current (IRMS) and the A/mm^2 by raising 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.

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

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

• See Application Note AN-26 (-FX)  or AN-32 (-GX)  for details on primary CMA calculations.

Maximum recommended A/mm² limits

Power (PO)

A/mm² (max)

< 20 W

9.8

20 W – 40 W

7.8

40 W – 80 W

6.5

80 W – 150 W

5.6

> 150 W

4.9