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DPA423GN Datasheet(PDF) 11 Page - Power Integrations, Inc. |
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DPA423GN Datasheet(HTML) 11 Page - Power Integrations, Inc. |
11 / 20 page AN-31 C 7/04 11 U1 U2 D S C OUTPUT OUTPUT RETURN R6 L2 R12 R10 C10 R11 C14 R9 R4 C5 U3 TL431 C6 C11 C16 PI-2876-062204 CONTROL CONTROL DPA-Switch Input Return + Figure 9. Essential Components of the Feedback Circuit. The Schematic Does Not Show ESR of the Output Capacitors (Component Designators are the Same as in the EP-21 Prototype). The phase margin is specified at the crossover frequency. It is the difference between the phase of the loop gain and 180 degrees. A stringent specification will call for a phase margin of at least 60 degrees under worst case conditions. In no case should the phase margin be less than 45 degrees.This means the phase would have to decrease by that amount for the system to become unstable. Phase margin is also related to the dynamic characteristics of the system. A low phase margin suggests an oscillatory response to a load step or other disturbance. It is also important that the loop gain decrease in magnitude beyond the crossover frequency. This requirement is generally specified as gain margin. Gain margin is the difference between 0 dB and the magnitude of the loop gain at the frequency where the phase is 180 degrees. An acceptable gain margin is greater than10dB.Thismeansthemagnitudewouldhavetoincreaseby thatamountforthesystemtobecomeunstable.Loopgainshould be measured at worst case conditions (generally maximum input voltage with maximum load) and at the extremes of the specified ambient temperature, since important component parameters (especially capacitor ESR) can change greatly with temperature. StabilizingahighfrequencyforwardDC-DCconverterpresents some challenges due to the inherently high bandwidth of this topology. Many DC-DC converter designs use cycle-by-cycle current-mode control. The DPA-Switch uses classic voltage mode control to allow operation at duty ratios greater than 50% withouttheneedforthestabilizingramp(“slopecompensation”) required with current-mode control. The fundamental system characteristicsoftheforwardconverterincontinuousconduction mode with voltage mode control call for a compensation circuit with multiple poles and zeros to achieve the desired loop response. The crossover frequency for a control loop that uses DPA-Switch in a forward converter with an optocoupler should be limited to 10 kHz or less at maximum input voltage and room temperature. The DPA-Switch has one internal pole at approximately 30 kHz to filter switching noise. Other poles at higher frequencies contribute additional phase shift at 30 kHz. The optocoupler has two poles at approximately 100 kHz. The phase shift from these poles, combined with the phase shift introduced by the LC filter at the output of the converter, is difficult to compensate above 10 kHz. The objective of the feedback design is to reduce the magnitude of the loop gain to zero dB at a frequency of 10 kHz or less with a phase margin near 60 degrees.Although system requirements and the DPA-Switch fix some quantities that determine loop characteristics, the designer can manipulate many components in the feedback circuit to optimize loop stability. Figure 8 shows the essential components of a feedback circuit that uses an ordinary TL431 regulator to achieve the high loop gain |
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