SP6850 Green-Mode PWM Controller APPLICATION INFORMATION Operation Overview The SP6850 integrate more functions into one controller , to reduce the external components counts and the size , SP6850 is for low-power switching mode power supplies. SP6850 meets the green power requirement and is intended for use in those modern switching power suppliers and switching adaptors, demanding higher power efficiency and power saving. It integrated. Under Voltage Lockout (UVLO) An UVLO comparator is implemented to detect the voltage on the VCC pin to ensure the supply voltage is enough to power on the SP6850 PWM controller and further to drive the power MOSFET. As shown in Fig. 1, a hysteretic is implemented 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. Therefore, the current through R1 will provide the startup current as well as charge the capacitor C1.Whenever the Vcc voltage is higher enough to power on the SP6850 and further to deliver the gate drive signal, the supply current is provided from the auxiliary winding of the transformer. The lower startup current requirement on the PWM controller will help to increase the R1 value and then reduce the power consumption on R1. By using BiCMOS process and the special circuit design, the typical startup current of SP6850 is only 8μA. If a higher resistance value of the R1 is picked, it will usually take more time to startup. To carefully select the value for R1 and C1 will optimize the power consumption and startup time. Fig.2 Fig. 1 Startup Current and Startup Circuit The typical startup circuit to power up the SP6850 is shown in Fig. 2. During the startup transient, the Vcc is lower than the UVLO threshold thus there is no gate pulse generated from SP6850 to drive power MOSFET. 2007/02/05 Ver.2 Current Sensing and Leading-edge Blanking The typical current mode PWM controller feedbacks both current signal and voltage signal to close the control loop and achieve regulation. Each time the MOSFET is turned on, a spike, which is induced by the diode reverse recovery and by the output capacitances of the MOSFET and diode, inevitably appears on the sensed signal. As shown in Fig. 3., the SP6850 detects the primary MOSFET current from the CS pin, which is not only Page 1 SP6850 Green-Mode PWM Controller 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 VCS . From above, the MOSFET peak current can be calculated as: IPEAK(MAX) = VCS / RCS Output Stage and Maximum Duty-Cycle An output stage of CMOS buffer, with typical 300mA driving capability, is incorporated to drive a power MOSFET directly. And the maximum duty-cycle of SP6850 is limited to 75% to avoid the transformer saturation. Oscillator and Switching Frequency Connecting a resistor from RT pin to GND according to the equation can program the normal switching frequency: f SW(KHz) = ( 65. 0×100 )/ RT(KΩ) The suggested operating frequency range of SP6850 is within 45KHz to 120KHz. Fig. 3 A leading-edge blanking time is included in the input of CS pin to prevent the false-trigger caused by the current spike and further to eliminate the need of R-C filter which is usually needed in the typical PWM controller application (Fig. 4). Voltage Feedback Loop The voltage feedback signal is provided from the voltage Regulator in the secondary side through the photo-coupler to the COMP pin of SP6850. The input stage of SP6850 , like as typical PWM controller , is with 2 diodes voltage offset then to feed the voltage divider with 1/3 ratio, that is, V+ (PWMCOMPARATOR) = (1/3) * (VCOMP − 2×VF) A pull-high resistor is embedded internally, it can therefore be eliminated from the external circuit. Internal Slope Compensation In conventional application , the problem of stability control is a critical issue for current mode controlling , when it operates more than 50% of the duty-cycle. As typical PWM controller , It takes slope compensation from injecting the ramp signal from the RT/CT pin through a coupling capacitor. Well, there is no excess design needed for the SP6850 since it has integrated this function with it already. Fig. 4 2007/02/05 Ver.2 On/Off Control The SP6850 can be turned off by pulling COMP pin lower than 1.3V. The gate output pin of the SP6850 will be disabled immediately under such condition. The off-mode can be released when the pull-low signal is removed. Page 2 SP6850 Green-Mode PWM Controller Green-Mode Operation In difference topologies , that has been implemented in different chips for the green-mode or power saving requirements such as “ burst-mode ”, “ skipping-cycle mode ”, “ variable off-time “…etc. The basic operation theory of all these approaches intended to reduce the switching cycles under light-load or no-load condition either by skip some switching pulses or reduce the switching frequency. SP6850 has the Dual-Oscillator function to the green mode control. The first oscillator is to take care the normal switching frequency, which can be set by the RT pin through an external resistor. Under this operation mode, the second oscillation (green-mode oscillator) is not activated. Therefore, the rising-time and the falling-time of the internal ramp will be constant to achieve good stability over all temperature range. Under the normal operation, this oscillator will dominate the switching frequency. The green-mode oscillator detects the signal of COMP pin to determine if it is within the green-mode operation. When the detected signal V+ is lower than the green-mode threshold VGREEN, the green-mode oscillator will be on. The green-mode oscillator, implemented by a VCO (voltage controlled oscillator), is a variable frequency oscillator. By using this dual-oscillator control, the green-mode frequency can be well controlled and further to avoid the generation of audible noise. Fault Protection Several curtail protection features have been integrated in the SP6850 to protect the power supply or adapter from being damaged. Those damage usually come from open or short condition on the pins of SP6850. Under the conditions listed below, the gate output will active off immediately to protect the power circuit : * RT pin short to ground * RT pin floating * CS pin floating OVP (Over Voltage Protection) on Vcc Most of the VGS ratings of the modern power MOSFETs are 20V~30V maximum. To prevent the VGS from the fault condition , SP6850 also integrates an OVP function on Vcc. Whenever the Vcc voltage is higher than the threshold voltage, the output gate drive circuit will be shut down simultaneously to stop the switching of the power MOSFET. Whenever the Vcc level gets back to lower level , the output will automatically return to the normal operation. 2007/02/05 Ver.2 Page 3 SP6850 Green-Mode PWM Controller 20W Converter Circuit --- 20W (12V/1.66A) (A) Circuit Diagram (B) Layout Diagram 2007/02/05 Ver.2 Page 4 SP6850 Green-Mode PWM Controller 20W Converter Characteristic (TA=25℃) 2007/02/05 Ver.2 Page 5 SP6850 Green-Mode PWM Controller Information provided is alleged to be exact and consistent. SYNC Power Corporation presumes no responsibility for the penalties of use of such information or for any violation of patents or other rights of third parties, which may result from its use. No license is granted by allegation or otherwise under any patent or patent rights of SYNC Power Corporation. Conditions mentioned in this publication are subject to change without notice. This publication surpasses and replaces all information previously supplied. SYNC Power Corporation products are not authorized for use as critical components in life support devices or systems without express written approval of SYNC Power Corporation. ©The SYNC Power logo is a registered trademark of SYNC Power Corporation ©2004 SYNC Power Corporation – Printed in Taiwan – All Rights Reserved SYNC Power Corporation 9F-5, No.3-2, Park Street NanKang District (NKSP), Taipei, Taiwan, 115, R.O.C Phone: 886-2-2655-8178 Fax: 886-2-2655-8468 http://www.syncpower.com 2007/02/05 Ver.2 Page 6