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CM6807XIRTR Datasheet(PDF) 11 Page - Champion Microelectronic Corp. |
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CM6807XIRTR Datasheet(HTML) 11 Page - Champion Microelectronic Corp. |
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11 / 16 page  CM6807 10-PIN Green-Mode PFC/PWM Combo CONTROLLER for High Density AC Adapter 2010/04/20 Rev1.0 Champion Microelectronic Corporation Page 11 The ISENSE Filter is a RC filter. The resistor value of the ISENSE Filter is between 100 ohm and 50 ohm. By selecting RFILTER equal to 50 ohm will keep the offset of the IEAO less than 5mV. Usually, we design the pole of ISENSE Filter at fpfc/6, one sixth of the PFC switching frequency. Therefore, the boost inductor can be reduced 6 times without disturbing the stability. Therefore, the capacitor of the ISENSE Filter, CFILTER, will be around 283nF. IAC, RAC, Automatic Slope Compensation, DCM at high line and light load, and Startup current There are 4 purposes for IAC pin: 1.) For the leading edge modulation, when the duty cycle is less than 50%, it requires the similar slope compensation, as the duty cycle of the trailing edge modulation is greater than 50%. In the CM6807, it is a relatively easy thing to design. Use an more than 2KK ohm resistor, RAC to connect IAC pin and the rectified line voltage. It will do the automatic slope compensation. If the input boost inductor is too small, the RAC may need to be reduced more. 2.) During the startup period, Rac also provides the initial startup current, 100uA;therefore, the bleed resistor is not needed. 3.) Since IAC pin with RAC behaves as a feedforward signal, it also enhances the signal to noise ratio and the THD of the input current. 4.) It also will try to keep the maximum input power to be constant. However, the maximum input power will still go up when the input line voltage goes up. Start Up of the system, UVLO, VREFOK and Soft Start During the Start-up period, RAC resistor will provide the start up current~100uA from the rectified line voltage to IAC pin. Inside of CM6807 during the start-up period, IAC is connected to VCC until the VCC reaches UVLO voltage which is 13V (UVB) and internal reference voltage is stable, it will disconnect itself from VCC. During the Start up, the soft start function is triggered and the duration of the soft start will last around 10mS. PFC section wakes up after Start up period After Start up period, PFC section will softly start since VEAO is zero before the start-up period. Since VEAO is a slew rate enhanced transconductance amplifier (see figure 3), VEAO has a high impedance output like a current source and it will slowly charge the compensation net work which needs to be designed by using the voltage loop gain equation. Before PFC boost output reaches its design voltage, it is around 380V and VFB reaches 2.5V, PWM section is off. PWM section wakes up after PFC reaches steady state PWM section is off all the time before PFC VFB reaches 2.45V. Then internal 10mS digital PWM soft start circuit slowly ramps up the soft-start voltage. PFC OVP Comparator PFC OVP Comparator sense VFB pin which is the same the voltage loop input. The good thing is the compensation network is connected to VEAO. The PFC OVP function is a relative fast OVP. It is not like the conventional error amplifier which is an operational amplifier and it requires a local feedback and it make the OVP action becomes very slow. The threshold of the PFC OVP is 2.5V+10% =2.75V with 250mV hysteresis. PFC Tri-Fault Detect Comparator To improve power supply reliability, reduce system component count, and simplify compliance to UL1950 safety standards, the CM6807 includes PFC Tri-Fault Detect. This feature monitors VFB (Pin 5) for certain PFC fault conditions. In case of a feedback path failure, the output of the PFC could go out of safe operating limits. With such a failure, VFB will go outside of its normal operating area. Should VFB go too low, too high, or open, PFC Tri-Fault Detect senses the error and terminates the PFC output drive. PFC Tri-Fault detect is an entirely internal circuit. It requires no external components to serve its protective function. VCC OVP and generate VCC For the CM6807 system, if VCC is generated from a source that is proportional to the PFC output voltage and once that source reaches 17.9V, PFCOUT, PFC driver will be off. The VCC OVP resets once the VCC discharges below 16.4V, PFC output driver is enabled. It serves as redundant PFC OVP function. Typically, there is a bootstrap winding off the boost inductor. The VCC OVP comparator senses when this voltage exceeds 17.9V, and terminates the PFC output drive. Once the VCC rail has decreased to below 16.4V the PFC output drive be enabled. Given that 16V on VCC corresponds to 380V on the PFC output, 17.9V on VCC corresponds to an OVP level of 460V. It is a necessary to put RC filter between bootstrap winding and VCC. For VCC=15V, it is sufficient to drive either a power MOSFET or a IGBT. |
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