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FSFR1700 Datasheet(PDF) 10 Page - Fairchild Semiconductor |
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FSFR1700 Datasheet(HTML) 10 Page - Fairchild Semiconductor |
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10 / 18 page  © 2007 Fairchild Semiconductor Corporation www.fairchildsemi.com FSFR series Rev.1.0.9 10 Functional Description 1. Basic Operation: FSFR-series is designed to drive high-side and low-side MOSFETs complementarily with 50% duty cycle. A fixed dead time of 350ns is introduced between consecutive transitions, as shown in Figure 16. High side MOSFET gate drive Low side MOSFET gate drive Dead time time Figure 16. MOSFETs Gate Drive Signal 2. Internal Oscillator: FSFR-series employs a current- controlled oscillator, as shown in Figure 17. Internally, the voltage of RT pin is regulated at 2V and the charging/discharging current for the oscillator capacitor, CT, is obtained by copying the current flowing out of RT pin (ICTC) using a current mirror. Therefore, the switching frequency increases as ICTC increases. Figure 17. Current Controlled Oscillator 3. Frequency Setting: Figure 18 shows the typical voltage gain curve of a resonant converter, where the gain is inversely proportional to the switching frequency in the ZVS region. The output voltage can be regulated by modulating the switching frequency. Figure 19 shows the typical circuit configuration for RT pin, where the opto- coupler transistor is connected to the RT pin to modulate the switching frequency. 0.6 0.8 1.0 1.2 1.4 1.6 1.8 Gain 140 150 60 70 80 90 100 110 120 130 freq (kHz) f min f normal f max f ISS Soft-start Figure 18. Resonant Converter Typical Gain Curve Control IC VDL LVcc RT CON SG PG R min R max C ss R ss Figure 19. Frequency Control Circuit The minimum switching frequency is determined as: min min 5.2 100( ) k fkHz R Ω =× (1) Assuming the saturation voltage of opto-coupler transistor is 0.2V, the maximum switching frequency is determined as: max min max 5.2 4.68 ( ) 100( ) kk fkHz RR Ω Ω =+ × (2) To prevent excessive inrush current and overshoot of output voltage during startup, increase the voltage gain of the resonant converter progressively. Since the voltage gain of the resonant converter is inversely proportional to the switching frequency, the soft-start is implemented by sweeping down the switching frequency from an initial high frequency (f IS S ) until the output voltage is established. The soft-start circuit is made by connecting R-C series network on the RT pin, as shown |
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