Adapter features

AN2432

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The self supply circuit (Q2, R33, C23, L3, D6 and C6) ensures:

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enough energy during no-load periods

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a poor (under UVLO) supply voltage during short-circuit failures

A separate rectifying circuit (D11, R19, R28 and C13) derives a voltage level that best

matches the output voltage for accurate overvoltage protection.

As seen, the primary side is quite standard. The most interesting part of this demo board

lies in the secondary side. Here we can find the STSR30, a smart driver for flyback

synchronous rectification (SR). The flyback output diode is substituted with a power

MOSFET (a 75V - 10m

Ω) that dramatically reduces the conduction losses. A small Schottky

diode (D1) is mounted in parallel to the MOSFET body diode to keep low the voltage drop

during dead times (while the SR MOS is off and current is circulating in the secondary).

The STSR30 can work in both Continuous and Discontinuous conduction mode and uses 2

pins to synchronize the SR MOSFET with the flyback. The SR MOSFET drain provides the

synchronization information; when the primary side MOSFET is turned off, the drain voltage

down to zero. This falling edge is sensed by the CK pin and the IC turns on the SR

MOSFET. Behavior varies according to the flyback transformer operating mode:

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Continuous conduction mode (CCM): the STSR30 uses an internal digital counter to

predict when it has to turn off the SR MOSFET.

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Discontinuous conduction mode (DCM): the STSR30 senses the voltage on the

INHIBIT pin (that is, Rdson x Isec) and turns off the SR MOSFET when it reaches the

-25mV threshold (i.e. the current is approaching zero).

During CCM operation, a certain amount of anticipation is used to prevent cross-conduction

of Q3 and Q1. This anticipation can be selected among three values by biasing the SETANT

pin. In the demo board, the SETANT voltage is 2.5V so the anticipation is 225ns.

The STSR30 works at 5V so it is necessary to obtain such voltage from the output. A low

cost linear regulator (L78L05) is used. For the same reason the gate drive of the IC has a

high value of 5V so a low threshold (logic level) MOSFET has to be used.

Another interesting feature of the STSR30 is its disable input. This is useful at low loads to

turn off the IC and reduce its power consumption. In this condition, the Schottky diode D1

works like in a standard flyback. The information on the load level is obtained by averaging

the voltage on the CK pin using R6, R15 and C4. The CK pin is low (~ 0V) only when the

current in the secondary winding is flowing (SR MOSFET on). Otherwise, the pin is pulled

up at 5V. As the load decreases, the average voltage on CK pin becomes higher and higher.

This voltage level is monitored by the last IC used, the TSM1015, a CV/CC controller that

includes a voltage reference and two op-amps. The reference and the CV op-amp are used

for the voltage control loop of the converter. The CC op-amp is not used for the current

control loop but it acts as a comparator to sense the average voltage of the CK pin. At light

STSR30. By adding a little hysteresis (using R40), the DISABLE pin of the STSR30 is driven

digitally with a good noise rejection.

The next two pictures show some waveforms during normal operation at full load. It is