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TEA152X Fiches technique(PDF) 5 Page - NXP Semiconductors |
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TEA152X Fiches technique(HTML) 5 Page - NXP Semiconductors |
5 / 17 page TEA152X_3 © NXP B.V. 2009. All rights reserved. Product data sheet Rev. 03 — 23 March 2009 5 of 17 NXP Semiconductors TEA152x SMPS ICs for low-power systems 8. Functional description The TEA152x family is the heart of a compact flyback converter, with the IC placed at the primary side. The auxiliary winding of the transformer can be used for indirect feedback to control the isolated output. This additional winding also powers the IC. A more accurate control of the output voltage and/or current can be implemented with an additional secondary sensing circuit and opto coupler feedback. The TEA152x family uses voltage mode control. The frequency is determined by the maximum transformer demagnetizing time and the time of the oscillator. In the first case, the converter operates in the Self Oscillating Power Supply (SOPS) mode. In the latter case, it operates at a constant frequency, which can be adjusted with external components RRC and CRC. This mode is called Pulse Width Modulation (PWM). Furthermore, a primary stroke is started only in a valley of the secondary ringing. This valley switching principle minimizes capacitive switch-on losses. 8.1 Start-up and undervoltage lockout Initially, the IC is self supplying from the rectified mains voltage. The IC starts switching as soon as the voltage on pin VCC passes the VCC(start) level. The supply is taken over by the auxiliary winding of the transformer as soon as VCC is high enough and the supply from the line is stopped for high efficiency operation. When for some reason the auxiliary supply is not sufficient, the high-voltage supply also supplies the IC. As soon as the voltage on pin VCC drops below the VCC(stop) level, the IC stops switching and restarts from the rectified mains voltage. 8.2 Oscillator The frequency of the oscillator is set by the external resistor and capacitor on pin RC. The external capacitor is charged rapidly to the VRC(max) level and, starting from a new primary stroke, it discharges to the VRC(min) level. Because the discharge is exponential, the relative sensitivity of the duty factor to the regulation voltage at low duty factor is almost equal to the sensitivity at high duty factors. This results in a more constant gain over the duty factor range compared to PWM systems with a linear sawtooth oscillator. Stable operation at low duty factors is easily realized. For high efficiency, the frequency is reduced as soon as the duty factor drops below a certain value. This is accomplished by increasing the oscillator charge time. To ensure that the capacitor can be charged within the charge time, the value of the oscillator capacitor should be limited to approximately 1 nF. 8.3 Duty factor control The duty factor is controlled by the internal regulation voltage and the oscillator signal on pin RC. The internal regulation voltage is equal to the external regulation voltage (minus 2.5 V) multiplied by the gain of the error amplifier (typically 20 dB which is 10 ×). |
Numéro de pièce similaire - TEA152X |
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Description similaire - TEA152X |
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