開關電源設計外文翻譯-其他專業(yè)(編輯修改稿)
2025-02-24 11:19 本頁面
【文章內(nèi)容簡介】 two current pulses of width , the input power is = (Vdc/2)( Ipft)(),where Ipft is the peak equivalent flattopped primary current pulse. Then Ipft (half bridge) = () Primary Wire Size Selection Primary wire size must be much larger in a half bridge than in a pushpull circuit of the same output power. However, there are two half primaries in the pushpull, each of which has to support twice the voltage of the halfbridge primary when operated from the same supply voltage. Consequently, coil sizes for the two topologies are notmuch different. Halfbridge primary RMS current is Irms = Ipft and from Eq. Irms = () At 500 circular mils per RMS ampere, the required number of circular mils is Circular mils needed = 500 () Secondary Turns and Wire Size Selection In the following treatment the number of secondary turns will be selected using Eqs. to for Ton = , and the term Vdc – 1 will be replaced by theminimumprimary voltage, which is (Vdc/2)?1. The secondary RMS currents and wire sizes are calculated from Eqs. and , exactly as for the fullwave secondaries of a pushpull circuit. Output Filter Calculations The output inductor and capacitor are selected using Eqs. and as in a pushpull circuit for the same inductor current ramp amplitude and desired output ripple voltage. Blocking Capacitor to Avoid Flux Imbalance To avoid the fluximbalance problem discussed in connection with the pushpull circuit (Section ), a small capacitor Cb is fitted in series with the primary as in Figure . Recall that flux imbalance occurs if the voltsecond product across the primary while the core is set (moves in one direction along the hysteresis loop) differs from the voltsecond product after it moves in the opposite direction. Thus, if the junction of C1 and C2 is not at exactly half the supply voltage, the voltage across the primary when Q1 is “on” will differ from the voltage across it when Q2 is “on” and the core will walk up or down the hysteresis loop, eventually causing saturation and destroying the transistors. This saturating effect es about because there is an effective DC current bias in the primary. To avoid this DC bias, the blocking capacitor is placed in series in the primary. The capacitor value is selected FIGURE The small blocking capacitor Cb in series with the halfbridge primary (Figure ) is needed to prevent flux imbalance if the junction of the filter capacitors is not at exactly the midpoint of the supply voltage. Primary current charges the capacitor, causing a droop in the primary voltage waveform. This droop should be kept to no more than 10%. (The droop in primary voltage, due to the offset charging of the blocking capacitor, is shown as dV.) as follows. The capacitor charges up as the primary current Ipft flows into it, robbing voltage from the flattopped primary pulse shown in Figure . This DC offset robs voltseconds from all secondary windings and forces a longer “on” time to achieve the desired output voltage. In general, it is desirable to keep the primary voltage pulse
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