P1012AP13 datasheet(18/25 Pages) ONSEMI

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NCP1010, NCP1011, NCP1012, NCP1013, NCP1014

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MOSFET Protection

As in any Flyback design, it is important to limit the

drain excursion to a safe value, e.g. below the MOSFET

BVDSS which is 700 V. Figure 29 presents possible

implementations:

Figure 29. Different Options to Clamp the Leakage Spike

NCP101X

CVcc

Rclamp

Cclamp

CVcc

NCP101X

Figure 29A: The simple capacitor limits the voltage

according to Equation 15. This option is only valid for low

power applications, e.g. below 5.0 W, otherwise chances

exist to destroy the MOSFET. After evaluating the leakage

inductance, you can compute C with Equation 15. Typical

values are between 100 pF and up to 470 pF. Large

capacitors increase capacitive losses.

Figure 29B: This diagram illustrates the most standard

circuitry called the RCD network. Rclamp and Cclamp are

calculated using the following formulas:

Rclamp

2 · Vclamp · (Vclamp

* (Vout ) Vf sec) · N)

Lleak · Ip2 ·Fsw

(eq. 27)

Cclamp

Vclamp

Vripple · Fsw · Rclamp

(eq. 28)

Vclamp is usually selected 50−80 V above the reflected

value N x (Vout + Vf). The diode needs to be a fast one and

a MUR160 represents a good choice. One major drawback

of the RCD network lies in its dependency upon the peak

current. Worse case occurs when Ip and Vin are maximum

and Vout is close to reach the steady−state value.

Figure 29C: This option is probably the most expensive of

all three but it offers the best protection degree. If you need

a very precise clamping level, you must implement a Zener

diode or a TVS. There are little technology differences

behind a standard Zener diode and a TVS. However, the die

area is far bigger for a transient suppressor than that of Zener.

A 5.0 W Zener diode like the 1N5388B will accept 180 W

peak power if it lasts less than 8.3 ms. If the peak current in

the worse case (e.g. when the PWM circuit maximum

current limit works) multiplied by the nominal Zener

voltage exceeds these 180 W, then the diode will be

destroyed when the supply experiences overloads. A

transient suppressor like the P6KE200 still dissipates 5.0 W

of continuous power but is able to accept surges up to 600 W

@ 1.0 ms. Select the Zener or TVS clamping level between

40 to 80 V above the reflected output voltage when the

supply is heavily loaded.

P1012AP13 Datasheet(PDF) 18 Page - ON Semiconductor

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