AND8272/D Implementing a Simple Brown−out Solution on the NCP101X Series Prepared by: David Sabatie ON Semiconductor http://onsemi.com APPLICATION NOTE Introduction Some AC−DC applications require a clean start−up sequence when the mains reaches a certain level to avoid stressing the power MOSFET at low−line, especially when the bulk capacitor starts to charge up. Also, at power−off, the power supply shall not go into hiccup as the bulk capacitor depletes and can no longer deliver the required energy. To provide such a behavior, the present application note describes a low−cost brown−out circuit successfully tested on the NCP101X switcher series. The NCP101X series integrates a fixed−frequency current−mode controller and a 700 V MOSFET. Housed in a PDIP−7, PDIP−7 Gull Wing, or SOT−223 package, the NCP101X offers everything needed to build a rugged and low−cost power supply, including soft−start, frequency jittering, short−circuit protection, skip−cycle and a Dynamic Self−Supply (no need for an auxiliary winding). Unlike other monolithic solutions, the NCP101X is quiet by nature: during nominal load operation, the part switches at one of the available frequencies (65 − 100 − 130 kHz). When the current setpoint falls below a given value, e.g. the output power demand diminishes; the IC automatically enters the so−called skip−cycle mode and provides excellent efficiency at light loads. Because this occurs at typically 1/4 of the maximum peak value, no acoustic noise takes place. As a result, standby power is reduced to the minimum without acoustic noise generation. Skip cycle is implemented by monitoring the feedback voltage. As it drops below a certain level, a dedicated comparator blanks the switching events. If a circuit maintains the pin below the skip level, we have a mean to stop the circuit, e.g. for a brown−out implementation… Figure 1 shows a possible implementation where no hysteresis is necessary. Vbulk 2 3 R1 R4 To opto−coupler Q2 Q1 BC547 R2 0 0 0 : Components to add Figure 1. In This Application, There is No Need for Hysteresis We set the startup Vbulk voltage by pulling the feedback pin low until enough voltage builds−up on the bulk capacitor. When Vbulk is too low, Q1 is blocked and Q2 base is pulled−up by R4 to VCC (≈ 8 V). Q2 being saturated, the feedback pin goes directly to ground. As we explained, the product does not start thanks to the presence of the internal skip−cycle comparator. When Vbulk reaches the selected value (determined by R1 and R2), Q1 saturates and Q2 base goes to ground, naturally blocking the transistor. The feedback is now solely driven by the opto−coupler and the system can freely operate. To implement this brown−out function, highlighted components are used, two standard NPN transistors and three resistors. The remaining components in the design are not modified. The design equations are as follows: 1. select the bridge current flowing in R1 and R2: 50ĂmA to avoid wasting power at high line. As explained in the introduction, some particular applications impose a minimum input voltage below which the power supply shall not work. To start the SMPS, a circuit constantly monitors the rectified mains voltage and pulls the feedback pin down to ground until the mains voltage reaches the desired value. When this happens, we need to introduce a certain amount of hysteresis to avoid going into a hiccup mode as the ripple on the bulk increases. August, 2006 − Rev. 0 8 VCC GND GND GND 7 GND DRV 4 FB 5 NCP1014 BC547 Brown−out © Semiconductor Components Industries, LLC, 2006 U1 1 1 Publication Order Number: AND8272/D AND8272/D 2. calculate R2 assuming a Vbe around 650 mV: R2 + 0.65 + 13 kW 50 m 4. R4 is fixed to 10 kW, a choice limiting the Dynamic Self Supply (DSS) loading current to a reasonable value. (eq. 1) 3. If we want a startup around 100 Vdc, then the upper resistor is found to be: R1 + 100 * 0.65 + 1.98 MW 50 m Adding Hysteresis to the Brown−out Detection, Auxiliary Winding Solution Now, let us see how we can add some hysteresis via the usage of the auxiliary winding. Figure 2 portrays the idea: (eq. 2) Vbulk R3 U1 D1 1 R1 8 VCC GND GND 2 GND 7 3 GND DRV 5 4 FB NCP1014 R4 R5 From auxillary winding 1N4148 + + C1 C2 0 0 To opto−coupler Q2 BC547 Q1 : Components to add BC547 R2 0 0 0 Figure 2. A Small Hysteresis is Added via R3. The main circuit actually does not differ compared to that of Figure 1. However, the addition of a small voltage offset changes the behavior at turn−off. During the startup phase, there is no auxiliary voltage. Hence, R3 right terminal is pulled down to ground. Thanks to D1 which isolates the device from the NCP1014 DSS level, there is no voltage. When the system starts to work, the auxiliary branch goes up and brings its level around 23 V (in our particular example). Via R3, it slightly raises Q1 Vbe voltage, now forcing Vbulk to reduce to a further down level, in comparison with the startup level. To implement this brown−out function, Figure 2 shows the additional components. the highlighted components are used. The other components in the design are not modified. The design equations now include the presence of R3, coming in parallel with R2 during the start−up sequence. However, we can neglect its contribution as its value is much higher than R2 as we will see: 1. select the bridge current flowing in R1 and R2: 50ĂmA to avoid wasting power at high line. 2. calculate R2 assuming a Vbe around 650 mV: R2 + 0.65 + 13 kW 50 m 4. Once the auxiliary voltage comes into play, the voltage over R2 including R3 is given by (applying superposition): Vbe + Vbulk solving for R3 gives: R3 + V R1R2(Vaux * Vbe) be(R1 ) R2) * VbulkR2 (eq. 5) Keeping similar values as in the above example, if we want a cutoff voltage of 70 Vdc with a 23 V auxiliary level (in this particular example), then R3 = 1.5 MW. 5. R4 is fixed to 10 kW, a choice limiting the Dynamic Self Supply (DSS) loading current to a reasonable value Adding Hysteresis to the Brown−out Detection, DSS Solution When using the NCP101X series in a non−auxiliary supply solution, the controller receives its operating supply from the DSS, bringing VCC between 7.5 and 8.5 V. Rather than connecting the hysteresis resistor to the auxiliary voltage, why not wiring it to the DSS supply. The arrangement is then slightly different as the DSS is already present at startup. The trick consists in shunting a resistor in series with the lower side element once the circuit has started: it artificially raises the level on the bottom transistor Q1 base. (eq. 3) 3. If we want a startup around 100 Vdc, then the upper resistor is found to be: R1 + 100 * 0.65 + 1.98 MW 50 m R2 ø R3 R1 ø R2 ) Vaux , R2 ø R3 ) R1 R1 ø R2 ) R3 (eq. 4) http://onsemi.com 2 AND8272/D Vbulk U1 8 VCC GND 2 GND GND 7 3 GND DRV FB 4 5 NCP1014 1 R1 R4 Q2 BC547 Q1 R2 To opto−coupler BC547 Q3 : Components to add BC547 0 R3 0 0 0 Figure 3. Altering the Bottom Resistor once the Circuit has Started Helps Creating the Necessary Hysteresis Designs equations are not complicated, compared to the previous calculations. At power−up, Q3 is biased by R4 and R3 goes off the picture (we neglect Q3 Vce,sat): R3 + Again, if we stick to our 70 Vdc cutoff level, we have R3 = 5.6 kW. R2 Vbe + V R2 ) R1 bulk Conclusion The method to evaluate R2 and R1 is similar to bullets 1 to 3 above. When the circuits starts to operate, in other words Q1 collector being low, Q3 is blocked and R3 appears in series with R2. The above equation becomes: R2 ) R3 Vbe + V R2 ) R3 ) R1 bulk Vbe(R1 ) R2) * VbulkR2 Vbulk * Vbe As we have shown here, various possibilities exist to implement brownout protection on NCP101X boards. Despite the natural Vbe variations with temperature, the obtained results are good for low−cost adapters. For more precise trip points, designer should consider a more efficient solution built around the NCP1027 which features a real programmable brown−out protection. (eq. 6) Solving this equation gives the series resistor value where Vbulk represents the turn−off value: ON Semiconductor and are registered trademarks of Semiconductor Components Industries, LLC (SCILLC). SCILLC reserves the right to make changes without further notice to any products herein. SCILLC makes no warranty, representation or guarantee regarding the suitability of its products for any particular purpose, nor does SCILLC assume any liability arising out of the application or use of any product or circuit, and specifically disclaims any and all liability, including without limitation special, consequential or incidental damages. “Typical” parameters which may be provided in SCILLC data sheets and/or specifications can and do vary in different applications and actual performance may vary over time. All operating parameters, including “Typicals” must be validated for each customer application by customer’s technical experts. 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