P1012AP datasheet(16/25 Pages) ONSEMI

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Design Procedure

The design of an SMPS around a monolithic device does

not differ from that of a standard circuit using a controller

and a MOSFET. However, one needs to be aware of certain

characteristics specific of monolithic devices:

Figure 28. The Drain−Source Wave Shall Always be Positive . . .

1.004M

1.011M

1.018M

1.025M

1.032M

−50.0

50.0

150

250

350

> 0 !!

1. In any case, the lateral MOSFET body−diode shall

never be forward biased, either during startup

(because of a large leakage inductance) or in

normal operation as shown by Figure 28.

As a result, the Flyback voltage which is reflected on the

drain at the switch opening cannot be larger than the input

voltage. When selecting components, you thus must adopt

a turn ratio which adheres to the following equation:

N · (Vout

) Vf) t Vin min (eq. 14) . For instance, if

operating from a 120 V DC rail, with a delivery of 12 V, we

can select a reflected voltage of 100 Vdc maximum:

120–100 > 0. Therefore, the turn ratio Np:Ns must be

smaller than 100/(12 + 1) = 7.7 or Np:Ns < 7.7. We will see

later on how it affects the calculation.

2. A current−mode architecture is, by definition,

sensitive to subharmonic oscillations.

Subharmonic oscillations only occur when the

SMPS is operating in Continuous Conduction

Mode (CCM) together with a duty−cycle greater

than 50%. As a result, we recommend to operate

the device in DCM only, whatever duty−cycle it

implies (max = 65%). However, CCM operation

with duty−cycles below 40% is possible.

3. Lateral MOSFETs have a poorly dopped

body−diode which naturally limits their ability to

sustain the avalanche. A traditional RCD clamping

network shall thus be installed to protect the

MOSFET. In some low power applications,

a simple capacitor can also be used since

Vdrain max

+ Vin ) N · (Vout ) Vf) ) Ip ·

Ctot

(eq. 15)

, where Lf is the leakage inductance,

Ctot is the total capacitance at the drain node

(which is increased by the capacitor wired between

drain and source), N the Np:Ns turn ratio, Vout the

output voltage, Vf the secondary diode forward

drop and finally, Ip the maximum peak current.

Worse case occurs when the SMPS is very close to

regulation, e.g. the Vout target is almost reached

and Ip is still pushed to the maximum.

Taking into account all previous remarks, it becomes

possible to calculate the maximum power that can be

transferred at low line.

When the switch closes, Vin is applied across the primary

inductance Lp until the current reaches the level imposed by

the feedback loop. The duration of this event is called the ON

time and can be defined by:

ton

Lp · Ip

Vin

(eq. 16)

At the switch opening, the primary energy is transferred

to the secondary and the flyback voltage appears across

Lp, resetting the transformer core with a slope of

N · (Vout

) Vf)

. toff, the OFF time is thus:

toff

Lp · Ip

N · (Vout

) Vf)

(eq. 17)

If one wants to keep DCM only, but still need to pass the

maximum power, we will not allow a dead−time after the

core is reset, but rather immediately restart. The switching

time can be expressed by:

Tsw

+ toff ) ton + Lp · Ip ·

Vin

)

N · (Vout

) Vf)

(eq. 18)

P1012AP Datasheet(PDF) 16 Page - ON Semiconductor

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