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IW1691-00 Datasheet(PDF) 8 Page - Dialog Semiconductor |
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IW1691-00 Datasheet(HTML) 8 Page - Dialog Semiconductor |
8 / 24 page iW1691 Digital PWM Current-Mode Controller for Quasi-Resonant Operation Rev. 2.0 iW1691 Page 8 FebRuaRy 3, 2012 _ () () g ON g peak ON M vt t i t L × = (9.2) This current represents a stored energy of: 2 _ () 2 M g g peak ON L E i t = × (9.3) When Q 1 turns off, ig(t) in LM forces a reversal of polarities on all windings. Ignoring the communication-time caused by the leakage inductance L K at the instant of turn-off, the primary current transfers to the secondary at a peak amplitude of: _ () ( ) P d g peak ON S N i t i t N = × (9.4) Assuming the secondary winding is master and the auxiliary winding is slave. VAUX 0V VAUX = -VIN x NAUX NP VAUX = VO x NAUX NS Figure 9.3 : Auxiliary Voltage Waveforms The auxiliary voltage is given by: () AUX AUX O S N V VV N = +∆ (9.5) and reflects the output voltage as shown in Figure 9.3. The voltage at the load differs from the secondary voltage by a diode drop and IR losses. The diode drop is a function of current, as are IR losses. Thus, if the secondary voltage is always read at a constant secondary current, the difference between the output voltage and the secondary voltage will be a fixed ΔV. Furthermore, if the voltage can be read when the secondary current is small; for example, at the knee of the auxiliary waveform (see Figure 9.3), then ΔV will also be small. With the iW1691, ΔV can be ignored. The real-time waveform analyzer in the iW1691 reads the auxiliary waveform information cycle by cycle. The part then generates a feedback voltage V FB. The VFB signal precisely represents the output voltage and is used to regulate the output voltage. 9.4 Constant Voltage Operation After soft-start has been completed, the digital control block measures the output conditions. It determines output power levels and adjusts the control system according to a light load or a heavy load. If this is in the normal range, the device operates in the Constant Voltage (CV) mode, and changes the pulse width (T ON), and off time (TOFF) in order to meet the output voltage regulation requirements. During this mode the PWM switching frequency is between 30 kHz and 130 kHz, depending on the line and load conditions. If less than 0.2 V is detected on V SENSE it is assumed that the auxiliary winding of the transformer is either open or shorted and the iW1691 shuts down. 9.5 Valley Mode Switching In order to reduce switching losses in the MOSFET and EMI, the iW1691 employs valley mode switching when I OUT is above 50%. In valley mode switching, the MOSFET switch is turned on at the point where the resonant voltage across the drain and source of the MOSFET is at its lowest point (see Figure 9.4). By switching at the lowest V DS, the switching loss will be minimized. Gate VDS Figure 9.4 : Valley Mode Switching Turning on at the lowest V DS generates lowest dV/dt, thus valley mode switching can also reduce EMI. To limit the switching frequency range, the iW1691 can skips valleys (seen in the first cycle in Figure 9.4) when the switching frequency becomes too high. iW1691 provides valley mode switching during constant output current operation. So, the EMI and switching losses are still minimized during CC mode. This feature is superior to other quasi-resonant technologies which only support valley mode switching during constant voltage operation. This is beneficial to applications, such as chargers, where the power supply mainly operates in CC mode. |
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