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UCC1857 Datasheet(PDF) 6 Page - Texas Instruments |
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UCC1857 Datasheet(HTML) 6 Page - Texas Instruments |
6 / 11 page 6 UCC1857 UCC2857 UCC3857 4 2 1 12 VIN IAC UVLO 13.75V / 10V CRMS RMS DETECT AND CONDITIONING X ÷ X MULT 10 VA– 3.0V 0.5V ZERO POWER 9 SS 10 µA 11 VAO 7 CA– ENBL 1.0V ALWAYS ON SD 7.5V REF VREF 5 REF GOOD 3 ENBL VOLTAGE AMP 13 PKLMT CURRENT AMP PEAK LIMIT COMP 8 MOUT CAO 20 CT 19 RT OSCILLATOR TOGGLE F/F Q Q PWM LATCH Q R R S R PWM COMP SD SD TRAILING EDGE DELAY DELAY 15 VD 14 MOSDRV 16 IGDRV1 18 IGDRV2 17 PGND 6 AGND DRIVER DRIVER DRIVER VD VD APPLICATION INFORMATION (continued) BLOCK DIAGRAM through the transformer and the output rectifier. It can be seen that the¸ ÷ operation on the primary side of the circuit is that of a boost converter and UCC3857 pro- vides input current programming using average current mode control to achieve unity power factor. The trans- former turns ratio can be used to get the required level of output voltage (higher or lower than the peak line volt- age). The transformer also provides galvanic isolation for the output voltage. Power stage optimization involves design and selection of components to meet the performance and cost objec- tives. These include the power switches, transformer and inductor design. The choice of IGBTs is based on their advantage over MOSFETs at higher voltages. For universal line opera- tion, the voltage stress on the push-pull switches can approach 1000V. However, the slow turn-off of IGBTs can contribute high switching losses and the use of MOSFET (QA) helps turn the IGBTs off with zero voltage across them (ZCS turn-off). This is accomplished by keeping QA on (beyond the turn-off of Q1 or Q2 – see Fig. 1 for waveforms) to allow the inductor current to di- vert from IGBT to MOSFET while the IGBT is turning off and still maintain zero volts. The MOSFET delay time (TD1) effectively adds to the boost inductor charge pe- riod. The voltage stress of the MOSFET is half the stress of the IGBTs under normal operating conditions. How- ever, QA can see much higher voltage stress under start-up and short circuit conditions as the converter oper- ates in a flyback mode then. For different operating re- quirements or constraints (e.g. single North American line operation), the choice of switching components may be different (e.g. MOSFETs for Q1 and Q2 and no QA) as the voltage stress is different. In that case, UCC3857 can still be used without using the MOSDRV output. Transformer design is very critical in this topology. The push-pull transformer must have minimal leakage induc- tance between the primary and secondary windings. Simi- larly, the leakage between the two primary windings must be minimized. In practice, it is hard to achieve both tar- gets without using sophisticated construction techniques such as interleaving, use of foils etc. In many cases, it may be beneficial to use a planar transformer to achieve these objectives. The effects of higher leakage induc- tance include higher voltage stresses, ringing, power losses and loss of available duty cycle. The high voltage levels make it difficult to design effective snubber circuits for this leakage induced ringing. UDG-98218 |
Similar Part No. - UCC1857_08 |
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Similar Description - UCC1857_08 |
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