Notification 205 - Zener Clamp loss (PZ) is greater than 3 W and may overheat

CATEGORY

ISSUE / RESULT

FIX / REFERENCE(S)

Warning

The estimated Zener Clamp loss (PZ) calculation indicates dissipation in excess of 3 watts:

Leakage inductance (LL) depends on transformer construction, including the number of primary turns (NP), the number of primary winding layers (L), primary winding wire size (ODA), stack-up and the Safety insulation system.  Zener clamp voltage (VCLO) is set at about 1.5 times the reflected voltage (VOR).  Transformer and PCB layout parasitic interaction should be verified by bench testing.

If a single 5-watt Zener diode is used, it may overheat.  High power applications may require additional clamp circuitry, including parallel RC and / or RCD components.

With TinySwitch-II devices, the calculations use effective switching frequency, which assumes that cycle skipping will occur.

• If the Zener Clamp is used, as is, thermal performance must be verified under worst-case conditions: full load, low-line AC, high ambient temperature and minimum airflow.

• Split primary windings reduce transformer leakage inductance (LL) and clamp dissipation (PZ), see Figure 1.  Above 40 watts, a split primary is recommended.

• Consider adding RC and/or D components to reduce Zener dissipation.  See parallel RC circuit, Figure 2 (RC component values will vary slightly, per application).  Contact PI for more RCD Clamp design tips.

• Low-voltage, high-current outputs may need secondary trace inductance (LSEC) and/or turns ratio (VOR) reduction.  Multi-layer PCBs may offer further LSEC reduction.  See Appendix B for more layout tips.

• Increasing Zener clamp voltage (VCLO) will reduce clamp dissipation, but, safe DRAIN-SOURCE voltages (VDRAIN) must be verified.  Note: VCLO is not user accessible.

• Increase overall efficiency (N), which will reduce peak primary current (IP).  Application Note AN-19  provides high-efficiency techniques.