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ML4802CS Datenblatt(PDF) 6 Page - Micro Linear Corporation |
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ML4802CS Datenblatt(HTML) 6 Page - Micro Linear Corporation |
6 / 15 page ML4802 6 Datasheet August 2000 The ML4802 consists of a combined average-current- controlled, continuous boost Power Factor Corrector (PFC) front end and a synchronized Pulse Width Modulator (PWM) back end. It is distinguished from earlier combo controllers by its unique Green Mode operation and dramatically reduced start-up and operating currents. The PWM section has been optimized for use in current mode topologies. The PWM stage uses conventional trailing- edge duty cycle modulation, while the PFC uses leading- edge modulation. This patented leading/trailing edge modulation technique results in a higher useable PFC error amplifier bandwidth, and can significantly reduce the size of the PFC DC bus capacitor. The synchronization of the PWM with the PFC simplifies the PWM compensation due to the reduced ripple on the PFC output capacitor (the PWM input capacitor). The PWM section of the ML4802 runs at twice the frequency of the PFC, which allows the use of smaller PWM output magnetics and filter capacitors while holding down the losses in the PFC stage power components. In addition to power factor correction, a number of protection features have been built into the ML4802. These include soft-start, PFC over-voltage protection, peak current limiting, brown-out protection, duty cycle limit, and under-voltage lockout. GREEN MODE OPERATION Green Mode automatically improves efficiency by up to 20% or more during low power operation. This feature is particularly helpful in meeting the demands of Energy Star™ programs. When the PWM’s output falls to nominally 17% of its design maximum power, Green Mode operation is initiated. The upper Green Mode threshold corresponds roughly to 1/3 of rated full power level. In Green Mode, the PWM operates in a pulse- skipping mode. This significantly reduces the frequency of operation, and therefore the dissipation in the PWM output driver and switch. Since the pulse-skipping is synchronous to the PWM’s master clock, the noise spectrum of the PWM retains a strong relationship to its spectrum during continuous-mode operation, which eases input and output filter design. PWM pulse frequency reductions in excess of 10:1 are common, with no increase in peak-to-peak output ripple. During Green Mode, the PFC also cycles on and off, running only as often as necessary to maintain its feedback voltage (VFB) between 2.25V and 2.5V (corresponding typical values of VBUSS are 382V and 425V). The PFC uses a built-in soft- start to minimize line current peaks and component stress when turning on. See Figure 1 for a flow chart detailing Green Mode and Normal Mode operation. FUNCTIONAL DESCRIPTION VFB ≥ 2.50V? YES NO YES NO NO NO TURN PFC OFF VDC < 1.30V? YES YES TURN PFC ON VFB < 2.25V? TURN PWM ON VDC ≥ 2.25V? ISSUE PWM POWER PULSE NORMAL MODE OPERATION START Figure 2. Green Mode Section Block Diagram Figure 1. ML4802 Operational Flow Chart – + + – – + S R Q Q 2.25V VFB VDC VTH1 VTH2 2.5V PFC ON PWM OFF Entering Green Mode The Green Mode Controller is detailed in Figure 2. Key to the ML4802’s operation in Green-Mode is the fact that the PWM’s output power is related to the voltage on the VDC (PWM Duty Cycle Control Voltage) pin by a known transfer characteristic. Therefore, the output power POUT drawn from an ML4802 supply can be inferred by monitoring the value of VDC fed back to the ML4802 from the (external) reference/error amplifier combination. When the output power taken from the PWM is reduced, the voltage on VDC will decrease. When VDC falls below VTH1 (1.30V typical), the part enters Green Mode operation. Once this happens, the threshold to which VDC is compared for further PWM operation is set to a higher value VTH2 (1.58V typical). This causes the PWM to enter a pulse-skipping mode while maintaining the desired output voltage. Pulse-skipping occurs because VTH2 is a higher voltage than VTH1, and because the PWM drive (PWMOUT) is disabled until VDC ³ VTH2. Since the primary current of the PWM output transformer is determined by VTH2 in Green Mode, and VTH2 > VTH1, each PWM output pulse will carry slightly more energy during Green Mode operation than during all but the highest duty cycle regimes of continuous-mode operation. In Green Mode, the power in each PWM output pulse is: PPULSE µ IPRIMARY(PWM) x VBUSS PPULSE µ (VGMT/RSENSE[PWM]) x 380V |
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Ähnliche Beschreibung - ML4802CS |
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