ÓÉ Foxit PDF Editor ±à¼°æȨËùÓÐ (c) by Foxit Software Company, 2004 ½öÓÃÓÚÆÀ¹À¡£ PR8275 Green-Mode PWM Controller with High Voltage Start-Up 汪工 ＴＥＬ：１３８２８７１９４１０ ＱＱ：１９２９７９４２３８ Features High-Voltage(500V) start up Circuit Current Mode Control Programmable Switching Frequency Internal Slope Compensation Under Voltage Lockout(UVLO) Over Voltage Protection (OVP) Over Load Protection(OLP) Leading _Edge Blanking LCD Monitor/TV Power Applications AC/DC Adapter Open Frame Switching Power Supply General Description The PR8275 is a current-mode PWM controller with excellent power-saving operation. It provides a high voltage current source to directly supply the startup current from bulk capacitor and further to provide a lossless startup circuit. The integrated functions such as the leading-edge blanking of the current sensing, internal slope compensation, and the small package provide the users a high efficiency, minimum external component counts, and low cost solution for AC/DC power applications. Furthermore, the embedded over voltage protection, over load protection and the special green-mode control provide the solution for users to design a high performance power circuit easily. The PR8275 is offered in both SOP-8 and DIP-8 package Typical Application Fig.1 V1.0 1/10 High Voltage Start-Up PWM Controller PR8275 Pin Descriptions PIN NAME 1 RT 2 COMP 3 4 5 6 7 CS GND OUT VCC NC 8 HV FUNCTION This pin is to program the switching frequency. By connecting a resistor to ground to set the switching frequency. Voltage feedback pin (same as the COMP pin in UC384X), By connecting a photo-coupler to close the control loop and achieve the regulation. Current sense pin, connect to sense the MOSFET current Ground Gate drive output to drive the external MOSFET Supply voltage pin Unconnected Pin Connect this pin to positive terminal of bulk capacitor to provide the startup current for the controller. When Vcc voltage trips the UVLO(on), this HV loop will be off to save the power loss on the startup circuit. Fig.2 V1.0 2/10 High Voltage Start-Up PWM Controller PR8275 Absolute Maximum Ratings Supply Voltage VCC ------------------------------------------------------------------30V High-Voltage Pin, HV ---------------------------------------------------------------- -0.3V~500V COMP, RT, CS ----------------------------------------------------------------------- -0.3 ~7V Junction Temperature ----------------------------------------------------------------150°C Operating Ambient Temperature ------------------------------------------------- -40°C to 85°C Storage Temperature Range------------------------------------------------------ -65°C to 150°C Package Thermal Resistance (SOP-8) ------------------------------------------160°C/W Package Thermal Resistance (DIP-8) -------------------------------------------100°C/W Power Dissipation (SOP-8, at Ambient Temperature = 85°C)-------------- 400mW Power Dissipation (DIP-8, at Ambient Temperature = 85°C) --------------650mW Lead temperature (Soldering, 10sec) --------------------------------------------260°C Gate Output Current -----------------------------------------------------------------500mA Caution: Stresses beyond the ratings specified in “Absolute Maximum Ratings” may cause permanent damage to the device. This is a stress only rating and operation of the device at these or any other conditions above those indicated in the operational sections of this specification is not implied. Recommended Operating Conditions Item Supply Voltage Vcc Vcc Capacitor Switching Frequency V1.0 Min. 11 10 50 Max. 25 47 130 Unit V μF KHz 3/10 High Voltage Start-Up PWM Controller PR8275 Electrical Characteristics (TA = +25°C unless otherwise stated, Vcc =15.0V) PARAMETER PARAMETER CONDITIONS High-Voltage Supply (HV Pin) High-Voltage Current Source Vcc< UVLO(on), HV=500V Off-State Leakage Current Vcc> UVLO(off), HV=500V Supply Voltage (Vcc Pin) Startup Current Operating Current VCOMP=0V (with 1nF load on OUT pin) VCOMP=3V Protection tripped (OLP, OVP) UVLO (off) UVLO (on) OVP Level Voltage Feedback (Comp Pin) Short Circuit Current VCOMP=0V Open Loop Voltage COMP pin open Green Mode Threshold VCOMP Current Sensing (CS Pin) Maximum Input Voltage Leading Edge Blanking Time Input impedance Delay to Output Oscillator (RT pin) Frequency Green Mode Frequency Temp. Stability Voltage Stability Gate Drive Output (OUT Pin) Output Low Level Output High Level Rising Time Falling Time OLP (Over Load Protection) OLP Trip Level OLP Delay Time (note) RT=100KΩ Fs=65.0KHz (-40°C ~105°C) (Vcc =11V-25V) Vcc =15V, Io=20mA Vcc =15V, Io=20mA Load Capacitance=1000pF Load Capacitance=1000pF Fs=65KHz MIN TYP MAX UNITS 0.5 1.0 1.5 35 mA μA 100 3.0 4.0 9.0 15.0 25.0 2.0 2.5 0.5 10.0 16.0 27.5 μA mA mA mA V V V 1.5 6.0 11.0 17.0 30.0 2.2 mA V V 0.90 V nS MΩ nS 70.0 KHz KHz % % 2.35 0.80 0.85 350 1 100 60.0 65.0 20 3 1 1 9 50 30 5.0 30 160 60 V V nS nS V mS Note: The OLP delay time is proportional to the period of switching cycle. So that, the lower RT value will set the higher switching frequency and the shorter OLP delay time. V1.0 4/10 PR8275 High Voltage Start-Up PWM Controller Application Information Operation Overview As long as the green power requirement becomes a trend and the power saving is getting more and more important for the switching power supplies and switching adaptors, the traditional PWM controllers. are not able to support such new requirements. Furthermore, the cost and size limitation force the PWM controllers need to be powerful to integrate more functions to reduce the external part counts. Internal High Voltage start up and UVLO Fig.3 Traditional circuit powers up the PWM controller through a startup resistor to provide the startup current. However, the startup resistor consumes significant power which is more and more critical whenever the power saving requirement is coming tight. Theoretically, this startup resistor can be very high resistance value. However, higher resistor value will cause longer startup time. To achieve an optimized topology, as shown in figure 3, PR8275 implements a high-voltage startup circuit for such requirement. During the startup, a high-voltage current source sinks current from the bulk capacitor to provide the startup current as well as charge the Vcc capacitor C1. During the startup transient, the Vcc is lower than the UVLO threshold thus the current source is on to supply a V1.0 current with 1mA. Meanwhile, the Vcc supply current is as low as100µA thus most of the HV current is utilized to charge the Vcc capacitor. By using such configuration, the turn-on delay time will be almost same no matter under low-line or high-line conditions. Whenever the Vcc voltage is higher than UVLO(on) to power on the PR8275 and further to deliver the gate drive signal, the high-voltage current source is off and the supply current is provided from the auxiliary winding of the transformer. Therefore, the power losses on the startup circuit can be eliminated and the power saving can be easily achieved. Fig.4 An UVLO comparator is included to detect the voltage on the Vcc pin to ensure the 5/10 High Voltage Start-Up PWM Controller PR8275 supply voltage enough to power on the PR8275 PWM controller and in addition to drive the power MOSFET. As shown in Fig. 4, a hysistersis is provided to prevent the shutdown from the voltage dip during startup. The turn-on and turn-off threshold level are set at 16V and 10.0V, respectively. Current Sensing, Leading-edge Blanking and the Negative Spike on CS Pin The typical current mode PWM controller feedbacks both current signal and voltage signal to close the control loop and achieve regulation. The PR8275 detects the primary MOSFET current from the CS pin, which is not only for the peak current mode control but also for the pulse-by-pulse current limit. The maximum voltage threshold of the current sensing pin is set as 0.85V. Thus the MOSFET peak current can be calculated as: I peak (max .) = 0.85 Rs A 350nS leading-edge blanking (LEB) time is included in the input of CS pin to prevent the false-trigger caused by the current spike. In the low power application, if the total pulse width of the turn-on spikes is less than 350nS and the negative spike on the CS pin is not exceed -0.3V, the R-C filter (as shown in figure 5) can be eliminated. However, the total pulse width of the turn-on spike is related to the output power, circuit design and PCB layout. It is strongly recommended to add the small R-C filter (as shown in figure 6) for higher power application to avoid the CS pin damaged by the negative turn-on spike. Fig.5 Fig.6 Output Stage and Maximum Duty-Cycle An output stage of a CMOS buffer, with typical 500mA driving capability, is incorporated to drive a power MOSFET directly. And the maximum duty-cycle of PR8275 is limited to 75% to avoid the transformer saturation. Voltage Feedback Loop The voltage feedback signal is provided from the TL431 in the secondary side through the photo-coupler to the COMP pin of PR8275. The input stage of PR8275 with 2 diodes voltage offset then feeding into the voltage divider with 1/3 ratio, that V1.0 6/10 High Voltage Start-Up PWM Controller PR8275 is, 1 V+ = (Vcomp − 2V f ) 3 thus can be eliminated on the external circuit. A pull-high resistor is embedded internally Oscillator and Switching Frequency Connecting a resistor from RT pin to GND according to the equation can program the normal switching frequency: f sw = 65 ⋅100( KHz ) RT ( K Ω) Internal Slope Compensation A fundamental issue of current mode control is the stability problem when its duty-cycle is operated more than 50%. To stabilize the control loop, the slope compensation is needed in the traditional UC384X design by injecting the ramp On/Off Control The PR8275 can be controlled to turn off by pulling COMP pin to lower than 1.2V. The gate output pin of PR8275 will be Dual-Oscillator Green-Mode Operation There are many difference topologies has been implemented in different chips for the green-mode or power saving requirements such as “burst-mode control”, “skipping-cycle Mode”, “variable off-time control “…etc. The basic operation theory Over Load Protection (OLP) To protect the circuit from the damage during over load condition or short condition, a smart OLP function is implemented in the PR8275. Figure 7 shows the waveforms of the OLP operation. Under such fault condition, the feedback system will force the voltage loop toward the saturation and thus pull the voltage on COMP pin (VCOMP) to high. Whenever the VCOMP trips the OLP threshold 5.0V and keeps longer than V1.0 The suggested operating frequency range of PR8275 is within 50KHz to 130KHz. signal from the RT/CT pin through a coupling capacitor. In PR8275, the internal slope compensation circuit has been implemented to simplify the external circuit design. disabled immediately under such condition. The off mode can be released when the pull-low signal is removed. of all these approaches intended to reduce the switching cycles under light-load or no-load condition either by skipping some switching pulses or reduce the switching frequency. 30mS (when switching frequency is 65KHz), the protection is activated and then turns off the gate output to stop the switching of power circuit. The 30mS delay time is to prevent the false trigger from the power-on and turn-off transient. A divide-2 counter is implemented to reduce the average power under OLP behavior. Whenever OLP is activated, the output is latched off and the divide-2 counter starts to count the number of UVLO(off). The 7/10 High Voltage Start-Up PWM Controller PR8275 latch is released if the 2nd UVLO(off) point is counted then the output is recovery to switching again. By using such protection mechanism, the average input power can be reduced to very low level so that the component temperature and stress can be controlled within the safe operating area. Fig.7 OVP (Over Voltage Protection) on Vcc The Vgs ratings of the nowadays power MOSFETs are most with maximum 30V. To prevent the Vgs from the fault condition, PR8275 is implemented an OVP function on Vcc. Whenever the Vcc voltage is higher than the OVP threshold voltage, the output gate drive circuit will be shutdown simultaneous thus to stop the switching of the power MOSFET until the next UVLO(on). The Vcc OVP function in PR8275 is an auto-recovery type protection. If the OVP condition, usually caused by the feedback loop opened, is not released, the Vcc will tripped the OVP level again and re-shutdown the output. The Vcc is Fault Protection A lot of protection features have been implemented in the PR8275 to prevent the power supply or adapter from being damaged caused by single fault condition on the open or short condition on the pin of PR8275. Under the conditions listed below, working as a hiccup mode. Figure 8 shows its operation. On the other hand, if the OVP condition is removed, the Vcc level will get back to normal level and the output is automatically returned to the normal operation. Fig.8 the gate output will be off immediately to protect the power circuit --1. RT pin short to ground 2. RT pin floating 3. CS pin floating Pull-Low Resistor on the Gate Pin of MOSFET In PR8275, an anti-floating resistor is implemented on the OUT pin to prevent V1.0 8/10 PR8275 the output from any uncertain state which may causes the MOSFET working abnormally or false triggered-on. However, such design won’t cover the condition of disconnection of gate resistor Rg thus it is still strongly recommended to have a resistor connected on the MOSFET gate terminal (as shown in figure 9) to provide extra protection for fault condition. This external pull-low resistor is to prevent the MOSFET from damage during power-on under the gate resistor is disconnected. In such single-fault condition, as show in figure 21, the resistor R8 can provide a discharge path to avoid the MOSFET from being false-triggered by the current through the gate-to-drain capacitor Cgd. Therefore, the MOSFET is always pull-low and kept in the off-state Protection Resistor on the Hi-V Path In some other Hi-V process and design, there may cause a parasitic SCR between HV pin, Vcc and GND. a small negative spike on the HV pin may trigger this parasitic SCR and causes the latch up between Vcc and GND. And such latch up is easy to damage the chip because of the equivalent short-circuit which is induced by V1.0 High Voltage Start-Up PWM Controller whenever the gate resistor is disconnected or opened in any case. Fig.9 such latch up behavior.So that PR8275 is with higher capability to sustain negative voltage than similar products. However, a 10K Ω resistor is recommended to implement on the Hi-V path to be played the role as a current limit resistor whenever a negative voltage is applied in any case 9/10 High Voltage Start-Up PWM Controller PR8275 SOP-8L Symbol Min. A 1.346 A1 0.101 b Min. 1.752 0.053 0.254 0.004 E 3.810 e 1.016 Max. 0.069 0.010 0.008 4.978 1.270 Inch Typ. 0.016 0.203 4.648 0.183 3.987 0.150 1.524 0.040 0.381X45° F V1.0 Max. 0.406 c D Millimeter Typ. 0.196 0.157 0.050 0.060 0.015X45° H 5.791 6.197 0.228 0.244 L 0.406 1.270 0.016 0.050 θ˚ 0° 8° 0° 8° 10/10