consumption of some other circuitry powered by the self-supply circuit.

Table 3. Consumption of the self-supply circuit for different IC’s.

Device

L5991

10mA

(15 + 0.6)V

• (10 + 2)mA = 187 mW

L5991A

10mA

(9 + 0.6)V

• (10 + 2)mA = 115 mW

UC3842A/B

17mA

(15 + 0.6)V

• (17 + 5)mA = 343 mW

Table 3 summarizes a comparison concerning the power demanded to the self-supply circuit under light

load conditions by different IC’s. In addition to those considered in table 1 and 2, table 3 includes also

Ω resistor to turn

off the start-up MOS, but is definitely advantageous in terms of consumption. As a result, it is advisable

As to this concept, the L5991A is particularly advantageous. The undervoltage lockout hysteresis, how-

Power MOSFET

The incidence of the MOSFET on power losses at light load depends basically on the switching fre-

quency. Leaving out conduction losses, which can be neglected in this context, the power dissipation

due to the MOSFET under light load conditions consists of three contributions:

1 -Turn-on losses, due to the discharge of the total capacitance of the drain node inside the MOSFET. It

ternal capacitance of the MOSFET, modulated by the drain voltage (manufacturers specify the value

ter the secondary current has run dry in DCM operation (see fig. 9). In fact, when the transformer is

≈ 2π ⋅ √

(26).

Turn-on losses depend on the input voltage in a non-monotonic way. As shown in fig. 9, the value of the

voltage increase, despite raising the settling value of the oscillation, may lead to a lower value at turn-on

2 -Turn-off losses, due to the crossing of the active region that causes a voltage-current overlapping, as

bility (1.6A peak) and can be controlled with a series resistor placed between pin 10 and the gate of

the MOSFET.

The parasitic inductances (basically, the one located between source and ground) limit the maximum

3 - Gate drive losses, related to the charge to be delivered to the gate each time the MOSFET is turned on.

AN1049 APPLICATION NOTE

11/24