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MC33067 Datasheet(PDF) 11 Page - ON Semiconductor |
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MC33067 Datasheet(HTML) 11 Page - ON Semiconductor |
11 / 16 page MC34067, MC33067 http://onsemi.com 11 Fault Latch S R Q Figure 20. Fault Detector and Soft−Start CSoft−Start Fault Input 10 9.0 mA 1.0 V 11 6 Ground Soft−Start Buffer Error Amp Clamp UVLO + Fault UVLO Fault Comparator Fault Soft−Start Circuit The Soft−Start circuit shown in Figure 20 forces the variable frequency Oscillator to start at the maximum frequency and ramp downward until regulated by the feedback control loop. The external capacitor at the CSoft−Start terminal is initially discharged by the UVLO+Fault signal. The low voltage on the capacitor passes through the Soft−Start Buffer to hold the Error Amplifier output low. After UVLO+Fault switches to a logic zero, the soft−start capacitor is charged by a 9.0 mA current source. The buffer allows the Error Amplifier output to follow the soft−start capacitor until it is regulated by the Error Amplifier inputs. The soft−start function is generally applicable to controllers operating below resonance and can be disabled by simply opening the CSoft−Start terminal. APPLICATIONS INFORMATION The MC34067 is specifically designed for zero voltage switching (ZVS) quasi−resonant converter (QRC) applications. The IC is optimized for double−ended push−pull or bridge type converters operating in continuous conduction mode. Operation of this type of ZVS with resonant properties is similar to standard push−pull or bridge circuits in that the energy is transferred during the transistor on−time. The difference is that a series resonant tank is usually introduced to shape the voltage across the power transistor prior to turn−on. The resonant tank in this topology is not used to deliver energy to the output as is the case with zero current switch topologies. When the power transistor is enabled the voltage across it should already be zero, yielding minimal switching loss. Figure 21 shows a timing diagram for a half−bridge ZVS QRC. An application circuit is shown in Figure 22. The circuit built is a dc to dc half−bridge converter delivering 75 W to the output from a 48 V source. When building a zero voltage switch (ZVS) circuit, the objective is to waveshape the power transistor’s voltage waveform so that the voltage across the transistor is zero when the device is turned on. The purpose of the control IC is to allow a resonant tank to waveshape the voltage across the power transistor while still maintaining regulation. This is accomplished by maintaining a fixed deadtime and by varying the frequency; thus the effective duty cycle is changed. Primary side resonance can be used with ZVS circuits. In the application circuit, the elements that make the resonant tank are the primary leakage inductance of the transformer (LL) and the average output capacitance (COSS) of a power MOSFET (CR). The desired resonant frequency for the application circuit is calculated by Equation 6: L L 2CR 1 = π 2 ƒr (eq. 6) In the application circuit, the operating voltage is low and the value of COSS versus Drain Voltage is known. Because the COSS of a MOSFET changes with drain voltage, the value of the CR is approximated as the average COSS of the MOSFET. For the application circuit the average COSS can be calculated by Equation 7: measured at CR 1 2 2 * COSS = in V (eq. 7) The MOSFET chosen fixes CR and that LL is adjusted to achieve the desired resonant frequency. However, the desired resonant frequency is less critical than the leakage inductance. Figure 21 shows the primary current ramping toward its peak value during the resonant transition. During this time, there is circulating current flowing through the secondary inductance, which effectively makes the primary inductance appear shorted. Therefore, the current through the primary will ramp to its peak value at a rate controlled by the leakage inductance and the applied voltage. Energy is not transferred to the secondary during this stage, because the primary current has not overcome the circulating current in the secondary. The larger the leakage inductance, the longer it takes for the primary current to slew. The practical effect of this is to lower the duty cycle, thus reducing the operating range. |
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Descripción similar - MC33067 |
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