Analysis for MOSFET Parameter – EAS (single-pulse avalanche energy)

In the application of flyback topology power supply, the drain-source voltage of MOSFET may exceed the rated value under certain extreme conditions. Engineers prefer to choose MOSFET with higher voltage endurance to increase its margin. When the drain-source voltage of MOSFET exceeds the rated value, the MOSFET will have an avalanche breakdown; the avalanche current IAR and avalanche energy EAS will be the key parameters to measure the avalanche capability of MOSFET.

1. E_AS test circuit

E_AS= LI²()   Formula 1

V_AV: avalanche voltage V_{BUS :} Voltage of the voltage source for avalanche testing.

Because V_BUS is far lower than V_AV, the Formula 1 is approximately equal to below Formula 2:

E_AS= LI²           Formula 2

The temperature of MOSFET will rise because of avalanche breakdown. If the junction temperature caused by the avalanche exceed 150℃, MOSFET will fail because of overheat.

2. The EAS in the specification

The E_AS in the specification is tested with defined test condition, e.g.:

Table 1 **10A80E Specification Excerpt

E_AS value of **10A80E is consistent with the specification if calculated as Formula 2.

Table 2 ***60R580P Specification Excerpt

However, for the E_AS parameters of ***60R580P, the test condition is not mentioned on datasheet. E_AS has necessary connections with inductance as well as avalanche current. There is no comparision between E_AS of different MOSFETs based on different test conditions.

3. Table 3 CYG WAYON WMO11N60C2 specification Excerpt

The E_AS test condition of CYG WAYON WMO11N65C2 is clearly mentioned in the specification.

4. Test validation

Based on the same test condition on table 3, increasing the I_AS to 2.3A, the E_AS of WMO11N65C2 is 206mJ, which is higher than 179mJ calculated E_AS value of ***60R580P.

2. CYG WAYON WMO11N65C2 E_AS Limitation Test

10pcs test data of ultimate avalanche energy value:

2. E_AS Margin for power supply

In the practical applications of Flyback power supply, the leakage inductance is about 3% of the total inductance.

E.g.: output power: 75W; switching frequency:60KHZ; E36 magnetic core; the primary winding: N67.

Primary inductance: Lprimary = 557 mH;

Primary leakage inductance: Lleakage = 0,03 × Lprimary = 16.71 μH;

Primary input voltage: VBus = (265Vrms ×) – VDiode = 372V

Primary VReset: = 100 V

VDS(Flyback) = VBus + VReset = 472 V, referring to use 600V MOSFET, meet the requirement of 0.8 times margin;

The switching cycles of MOFET: T=16.667 μs; Duty Cycle: D=0.2

MOSFET t_ON=D*T=3.517 μs

I_D(Peak) =*t_ON=2.348 A

Considering some extreme input conditions, I_D(Peak)=5A; E_AS=LI²=0.5*0.557mH*25=6.96mJ;(above calculations are based on 75W power supply)

In 45W power supply, I_D is smaller, more E_AS margin is there.