MOSFETs of such low voltage classes are available from ST in TO-220 package for through-hole as-

semblies and in DPAK / D2PAK packages for surface mount assemblies, as summarized in Table 1.

The table shows the devices suggested for typical output current ratings mentioned in Intel’s Instantly

Available PC (Power Supply ’98) specification.

Table 1. Suggested MOSFETs for typical output current ratings (3.3V output).

Part Number (TO-220)

Part Number (DPAK)

Part Number (D2PAK)

6

STP30NE06

55mohm

STD20NE06

40mohm

-

-

8

STP36NE06

40mohm

STD20NE06

40mohm

-

-

10

STP55NE06

22mohm

-

-

STB55NE06

22mohm

13

STP80NE06-10

10mohm

-

-

STB80NE06-10

10mohm

Q1 is driven with a gate-drive transformer (T2), with

a coupling capacitor to eliminate the dependence of

the gate-drive voltage on the duty cycle.

The control circuit is based on the L6561, a 8-pin

Transition Mode PFC controller that turns out to be

extremely useful for this application. Please refer to

[1] for a detailed description of the device.

The supply voltage of the L6561 can be got in dif-

ferent ways, depending on the architecture of the

entire power supply. A couple of examples are

shown in fig. 5.

The operation of the circuit of fig. 4 will be now de-

scribed, with the aid of the time diagrams shown in

fig. 6.

Q1 is turned on on the negative edge of the secon-

dary voltage, detected through the ZCD pin, that is

when the primary switch is turned off. The rectifier

D1 withstands the reverse secondary voltage.

The key point is: when Q1 is switched on, the cur-

rent through D1 and Q1 is zero, thus Q1 does not

experience any switching loss and the reverse re-

covery of D1 is here of no concern.

When the primary switch is turned on, the secon-

dary voltage becomes positive and, being Q1 al-

ready on, D1 conducts. The secondary voltage (re-

duced by D1 forward drop) minus the output voltage

is applied to the output inductor Lo, causing the in-

ductor current to ramp up.

A current-sense transformer (T3) picks up the in-

ductor current (flowing through Q1 as well) and de-

rent. This voltage, applied to CS pin, is internally

compared to the output of the multiplier of the

L6561 and when the two quantities are equal, Q1 is

switched off. The system performs therefore a trail-

ing edge modulation.

The multiplier output voltage is proportional to the

product of the error amplifier output voltage (at pin

COMP) times the multiplier input voltage (at pin

MULT). COMP voltage is in turn a function of the

amplified difference between a portion of the output

voltage (set by R1 and R2) and the internal 2.5V

12V / 4A

L4955

(13 < V < 18)

5V

TO FEEDBACK

MAIN

OUTPUT

Figure 5. Circuits for the generation of the

L6561 supply voltage

PRIMARY

SWITCH

ON

OFF

SECONDARY

VOLTAGE

Q1 GATE

VOLTAGE

Q1 & D1

CURRENT

D2

CURRENT

Figure 6. L6561-based switcher time diagrams.

AN1007 APPLICATION NOTE

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