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AN2432 Datasheet(PDF) 7 Page - STMicroelectronics |
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AN2432 Datasheet(HTML) 7 Page - STMicroelectronics |
7 / 27 page Adapter features AN2432 7/27 The self supply circuit (Q2, R33, C23, L3, D6 and C6) ensures: ● a constant VCC voltage with respect to load variations ● enough energy during no-load periods ● 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 of the SR MOSFET falls from VOUT + VIN/n (where n is the transformer turns ratio n1/n2) 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: ● Continuous conduction mode (CCM): the STSR30 uses an internal digital counter to predict when it has to turn off the SR MOSFET. ● 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 loads, the CK voltage exceeds the threshold (VREF) and the TSM1015 turns off the 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 possible to see that the converter operates in CCM at 115 VRMS and in DCM at 230 VRMS. |
Similar Part No. - AN2432 |
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Similar Description - AN2432 |
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