AN-6846

www.fairchildsemi.com AN-6846 Applying SG6846 to Control a Flyback Power Supply with Surge Current Output Summary This application note describes a detailed design strategy for a high-efficiency, compact flyback converter. Design considerations, mathematical equations, and guidelines for printed circuit board layout are presented. Applications General-purpose switch-mode power supplies and flyback power converters, including: Power Adapters Open-frame SMPS SMPS with Surge-current Output, such as for Printers, Scanners, Motor Drivers Figure 1. Pin Configuration RI 4 0.9V/0.7V VDD VIN 3 Over-Power Compensation 1.18-0.08 X VIN Soft Driver 8 GATE VD D -TH-G VDD 7 UVLO Internal BIAS Q S R OSC Green Mode 6V 3R 16.5V/10.5V + 1R R 2 FB Latch Slope Compensation Blanking Circuit 6 SENSE 23.6V 1R Debounce OCP Delay IRT RT 5 1.05V 1R /2R 1.15V 1 GND Figure 2. Block Diagram © 2008 Fairchild Semiconductor Corporation Rev. 1.3.2 • 9/26/08 www.fairchildsemi.com AN-6846 APPLICATION NOTE Table 1. Comparison Symbol Feedback Input Section tD-OLP Parameter SG6846A SG6846B SG6846C SG6846G Delay Time for Open-Loop Protection (RI = 26kΩ) 1700ms NA 110ms Over-Temperature Protection (OTP) Section tDOTP-LATCH Over-Temperature Latch-off Debounce (RI = 26kΩ) 100ms 100µs Current Sense Section tD-SCP tD-OCP VSTH (VIN=1V) VSTH (VIN=3V) VSTH – 2/3 (1/2) Delay Time for output short circuit protection (RI = 26kΩ) Delay Time for Over-Current Protection (RI = 26kΩ) Threshold Voltage for Current Limit Threshold Voltage for Current Limit Threshold Voltage for Over Current Protection 100ms 1700ms 0.83V 0.7V VSTH × 2/3 7ms 200ms NA 110ms 1.1V 0.94V VSTH × 1/2 Symbol Oscillator Section VFB-N VFB-G VFB-ZDC Parameter SG6846A/B/C SG6846G FB Threshold Voltage For Frequency Reduction (RI = 26kΩ) FB Voltage at Green-mode Minimum Frequency (RI = 26kΩ) FB Threshold Voltage for Zero-duty 2.1 1.6 VFB-G 2.8V 2.3V 2V Figure 3. SG6846A/B/C PWM Frequency Figure 4. SG6846G PWM Frequency © 2008 Fairchild Semiconductor Corporation Rev. 1.3.2 • 9/26/08 www.fairchildsemi.com 2 AN-6846 APPLICATION NOTE Description The SG6846 is a highly integrated PWM controller IC that provides special features satisfying the needs for low standby power consumption. It also incorporates multiple protection functions. With low start-up current and low operating current, high-efficiency power conversion is achieved. The typical start-up current is only 8µA and the operating current is around 3.7mA. In nominal loading conditions, the SG6846 operates at a fixed PWM frequency. To minimize standby power consumption, a proprietary green-mode function provides off-time modulation to continuously decrease the switching frequency under lightload conditions. Under no-load conditions, the power supply enters burst-mode. Green mode dramatically cuts the power loss at no-load and light-load conditions, enabling the power supply to meet stringent power conservation requirements. The SG6846 is specially designed for SMPS with surgecurrent output, incorporated with a two-level OCP function. Besides the cycle-by-cycle current limiting; if the switching current is higher than OCP threshold for a delay time, overcurrent protection is activated such that the SG6846 is totally shutdown. Other protection functions include AC input brownout protection with hysteresis and VDD overvoltage protection. For over-temperature protection, an external NTC thermistor can be applied to sense the ambient temperature. When OCP, VDD OVP, or OTP are activated, an internal latch circuit is used to latch-off the controller. If the latch circuit is triggered by over-temperature conditions, it resets when the temperature cools off sufficiently (SG6846CX) or when the AC supply is disconnected (SG6846LX). When the latch is triggered by VDD overvoltage conditions, the latch resets only when the AC supply is disconnected. Other features of this controller include built-in synchronized slope compensation and proprietary internal compensation for constant output power limit over universal AC input range. The gate output is clamped at 18V to protect the external MOSFET from over-voltage damage. practical. As the voltage of VDD reaches the startup threshold voltage VTH-ON, the SG6846 activates and drives the entire power supply. The IC then operates based on the auxiliary winding energy. Using one startup resistor (510KΩ) with a 10µF/50V holdup capacitor C1 can satisfy the three-second maximum power-on delay requirement in most applications. After startup, the energy is supplied from C2. The capacitance at VDD must be large enough to hold VDD above the off threshold voltage, VTH-OFF, at a step change of load. When an output short circuit occurs, the duration before VDD drops to below the off threshold voltage must be longer than the OCP delay time or it fails to trigger the internal latch circuit. A 100µF/50V capacitor is suggested for C2. Figure 6. Circuit Providing Power The maximum power dissipation of RIN is: PRIN,max ≅ 2 V AC,max 2 × RIN (1) where VAC,max is the maximum rectified input voltage. Taking a wide-range input (90VAC-264VAC) as an example, VAC,max =264V: PRIN,max = 264 2 2 × 510 × 10 3 ≅ 68mW RI (2) RI SG6846 In addition to the low startup current, SG6846 consumes less operating current than a traditional UC384x. GND Oscillator and Green-Mode Operation Resistor RI programs the frequency of the internal oscillator. A 26Ω resistor RI determines PWM frequency as 65kHz: fPWM (kHz ) = 1690 RI (kΩ ) Figure 5. Setting PWM Frequency (3) Startup Circuitry When the power is turned on, the input voltage charges the hold-up capacitor C1 via the startup resistors. As shown in Figure 2, one or two resistors from the AC utility are both © 2008 Fairchild Semiconductor Corporation Rev. 1.3.2 • 9/26/08 3 A PWM frequency range of between 47kHz ~ 109kHz is recommended. www.fairchildsemi.com AN-6846 APPLICATION NOTE The proprietary green mode provides off-time modulation to reduce the PWM frequency at light-load and no-load conditions. The feedback voltage of the FB pin is taken as a reference. As shown in Figure 7, when the feedback voltage is lower than about 2.1V, the PWM frequency decreases. Because most losses in a switching-mode power supply are proportional to the PWM frequency, the off-time modulation reduces the power consumption of the power supply at lightload and no-load conditions. For a typical case of RI=26KΩ, the PWM frequency is 65kHz at nominal load and decreases to 22.5kHz at light loads, about one third of the nominal PWM frequency. The power supply enters burst mode at zero-load conditions. Built-in Slope Compensation A flyback converter can be operated in discontinuous current mode (DCM) or continuous current mode (CCM). With the same output power, a converter in CCM exhibits smaller peak inductor current than in DCM. Therefore, a small-sized transformer and a low-rating MOSFET can be applied. On the secondary side of the transformer, the rms output current of DCM can be up to twice that of CCM. Larger wire gauge and output capacitors with larger ripple current ratings are required. DCM operation also results in higher output voltage spikes. A large LC filter has to be added. Therefore, a flyback converter in CCM achieves better performance with lower component cost. Despite the above advantages of CCM operation, there is one concern – stability. In CCM operation, the output power is proportional to the average inductor current, while the peak current is controlled. This causes the well-known subharmonic oscillation when the PWM duty cycle exceeds 50%. Adding slope compensation (reducing the current-loop gain) is an effective way to prevent oscillation. The SG6846 introduces a synchronized positive-going ramp (VSLOPE) in every switching cycle to stabilize the current loop. Therefore, the SG6846 enables design of a cost-effective, highly efficient, compact flyback power supply operating in CCM without adding external components. The positive ramp added is: VSLOPE = VSL ⋅ D Figure 7. PWM Frequency vs. FB Voltage FB Input The SG6846 is designed for peak-current-mode control. A current-to-voltage conversion is accomplished externally with current-sense resistor RS. Under normal operation, the FB level controls the peak inductor current: I pk = VFB − 1.2 4 .2 ⋅ R S (6) (4) where VFB is the voltage of FB pin. When VFB is less than 1.6V, the SG6846 terminates the output pulses. 0 is a typical feedback circuit mainly consisting of a shunt regulator and an opto-coupler. R1 and R2 form a voltage divider for output voltage regulation. R3 and C1 are adjusted for control-loop compensation. A small-value RC filter (e.g. RFB= 47Ω, CFB= 1nF) placed from the FB pin to GND can increase stability substantially. The maximum source current of the FB pin is about 2mA. The phototransistor must be capable of sinking this current to pull the FB level down at no load. The value of the biasing resistor, Rb, is determined as: Vo − VD − VZ ⋅ K ≥ 2mA Rb Figure 8. Feedback Circuit where VSL = 0.33V and D = duty cycle. (5) where: VD is the drop voltage of photodiode, about 1.2V; VZ is the minimum operating voltage, 2.5V of the shunt regulator; and K is the current transfer rate (CTR) of the opto-coupler. For an output voltage VO=5V, with CTR=100%, the maximum value of Rb is 650Ω. © 2008 Fairchild Semiconductor Corporation Rev. 1.3.2 • 9/26/08 4 Figure 9. Synchronized Slope Compensation www.fairchildsemi.com AN-6846 APPLICATION NOTE Leading-Edge Blanking (LEB) A voltage signal proportional to the MOSFET current develops on the current-sense resistor RS. Each time the MOSFET is turned on, a spike induced by the diode reverse recovery and by the output capacitances of the MOSFET and diode, appears on the sensed signal. A leading-edge blanking time (about 360ns) is introduced to avoid premature termination of the MOSFET by the spike. Therefore, only a small-value RC filter (e.g. 100Ω + 470pF) is required between the SENSE pin and RS. A non-inductive resistor for RS is recommended. As the current-sense signal of the SENSE pin exceeds the internal limit VSENSE, 0.9V typically, the SG6846 stops the PWM pulses immediately. The output power of a flyback power supply in DCM is calculated as: POUT = 1 2 ⋅ L ⋅ I pk ⋅ fS ⋅ η 2 (7) where: L is the inductance; Ipk is the peak inductor current; fs is the PWM frequency; and η is the conversion efficiency. If the conversion efficiency remains unchanged for a wide input voltage range, the maximum output power would be the same for a fixed Ipk, which is limited by the internal current limiting threshold voltage VTH and RS. However, due to the time delay from the comparator to output stage inside the SG6846, the maximum output power with high line input is always higher than with low line. A 30% error is common for the universal input voltage range if the converter is operated in DCM. In CCM operation, the deviation becomes even worse. For the purpose of constant output power limit, the peak current limit VTH must be adjustable according to the input voltage. In the SG6846, the peak-current threshold is adjusted by the voltage of the VIN pin for constant output power limit over universal input-voltage range. Since the VIN pin is connected to the rectified AC input line voltage through the resistive divider, a higher line voltage generates a higher VIN voltage. The threshold voltage decreases as the VIN voltage increases, making the maximum output power at high line input voltage equal to that at low line input. RS Figure 10. Turn-on Spike Output Driver / Soft Driving The output stage is a fast totem-pole gate driver capable of directly driving external MOSFETs. An internal Zener diode clamps the driver voltage under 18V to protect MOSFETs against over voltage. By integrating circuits to control the slew rate of switch-on rising time, the external resistor RG may not be necessary to reduce switching noise, thereby improving EMI performance. Figure 12. Universal Line Voltage Compensation for Constant Output Power Limit Brownout Protection Since the VIN pin is connected through a resistive divider to the rectified AC input line voltage, it can also be used for brownout protection. If the VIN voltage is less than 0.7V, the PWM output shuts off. As the VIN voltage reaches 0.9V, the PWM output is turned on again. The hysteresis window for on/off is around 0.2V. The recommended values for R1, R2, and C1 are 2M (1M+1M), 16.2K, and 4.7µF. Using these values in the evaluation board, the power supply is turned off at 75V (max. load) / 64V (min. load) and recovered at 82V. www.fairchildsemi.com 5 Figure 11. Gate Driver Constant Output Power Limit The maximum output power of a flyback converter can be designed by the current-sense resistor RS. When the load increases, the peak inductor current increases accordingly. © 2008 Fairchild Semiconductor Corporation Rev. 1.3.2 • 9/26/08 AN-6846 APPLICATION NOTE Thermal Protection A constant current, IRT, is provided from the pin RT. The resistor connected to pin RI determines its magnitude: IRT = 1.8V / RI For example, IRT = 70µA if RI = 26KΩ. For over-temperature protection (OTP), an NTC thermistor RT in series with a resistor Ra can be connected between the RT pin and ground. When the voltage of the RT pin drops below 1.065V, PWM output is latched off. A debounce time around 100µs is added to prevent false triggering. After the latch is reset and cleared (SG6846CX: VRT>1.165V; SG6846LX: AC unplugged), PWM turns on again. If the RT pin is not used, connect a 100kΩ resistor between the RT pin and ground to disable this thermal protection function. Beside NTC and Ra, a capacitor should be connected to RT pin to eliminate switching noise. This capacitor’s value is less than 1nF (shown as Figure 14). (8) Figure 13. Universal Line Voltage Compensation for Constant Output Power Limit Two-level Over-Current Protection (OCP) The cycle-by-cycle current limiting shuts down the PWM immediately when the switching current is over the peakcurrent threshold. In addition, an over-current protection circuit is built-in. When the switching current is higher than the OCP threshold, the internal counter starts counting up. When the switching current is lower than the OCP threshold, the internal counter counts down. When the total accumulated counting time is more than OCP delay time (SG6846A: 1600ms; SG6846C: 110ms), the controller is latched off. By adjusting the RS resistance to let the peak-current at maximum load under OCP threshold and over this threshold at peak load condition, the two-level OCP protection is enabled. These functions are especially designed for an SMPS with surge current output, such as for printers, scanners, and motor drivers. Figure 15. T C1 (