DN05060/D Design Note – DN05060/D Input Over Voltage Protection Circuit Device NCL300001: NCP431 and NCS2220 or LM2903 Application Input Voltage Output Power Topology Offline Power Conversion, LED lighting 90 – 315 Vac Various Various Typical Shutdown Voltage Typical Restart Voltage Bias Current (LM2903) Bias Current (NCS2220A) Circuit Description Overvoltage events on the power grid can be thought of in broad terms as impulse-like or “long term”. “Impulse” events are well understood and there are a range of surge suppression techniques (ex: TVS, MOV etc.) utilized by power supply designers. Longer term excess voltage events are typically due to poor regulation or faults in the power grid and can exceed 300 Vac and last for hours or be less than 50 msec in duration. Some form of mitigation is required in order to provide a reliable system with a long service life. This is especially true in applications like outdoor LED lighting. Essentially these long term events have limitless power capability and the first line of defense is to select components which are rated to withstand the anticipated stress. Depending on the magnitude of excess voltage, it may not be practical to use parts with suitable rating. This design note describes an input voltage monitoring solution and is suitable for systems which can be disabled for the duration of the excess voltage event circumventing unnecessary stress on some components. Hysteresis provides more stable operation by lowering the input voltage threshold necessary to restore normal operation. Typically, power converter input voltage is specified in terms of RMS voltage. The detection method used in this example circuit is based on peak voltage providing a simpler implementation. Throughout the remainder of this design note, conversion between RMS and peak voltages will be done assuming sinusoidal waveforms. February 2014, Rev. 0 266 Vac 253 Vac 1.25 mA 0.17 mA Even though the power converter is not operational during the overvoltage event, the monitoring circuit remains active to detect when the input voltage returns to normal levels so bias power must be supplied directly from the rectified ac source. The simplest method is via a linear regulator, and in this case a shunt configuration is appropriate and cost effective. Any linear regulator will generate heat and the amount is proportional to the required current and the difference between input and output voltage. To minimize dissipation the circuit must draw minimal current. Since the comparator draws the majority of the current, careful consideration should be given to the tradeoff of cost and power consumption. High resistances are selected for dividers and pull-up devices to meet this end. Note that an appropriate number of resistors are needed in this shunt regulator to dissipate the power without exceeding component ratings. The bias in this circuit performs two functions. Not only does this voltage provide the current to operate the circuit, it is also used as the main reference to establish the input over voltage threshold. A reference with tight tolerance is desirable to control accuracy of the shutdown threshold; however, once again there is a tradeoff between a low cost solution such as a zener diode and the more accurate NCP431 shunt regulator. Details on utilizing the more accurate NCP431 are shown in a later section. Moreover compared to an industry standard TL431, the NCP431 requires only 8% of the minimum operating current offering a more power efficient solution. Circuit Operation Figure 1 shows a solution based on the industry standard LM2903 comparator and a low current zener diode for the bias voltage. Two resistors, R1 and R2, are used to www.onsemi.com 1 DN05060/D form the upper portion of the input divider due to maximum voltage rating of 200 volts for a 1206 surface mount resistor. Two resistors also provide finer resolution on setting the shutdown threshold. C1 is a noise filter for this initial divider. The peak detecting comparator U1A accepts the input from the first divider and compares it to a reference derived from the bias voltage. R6 provides hysteresis and noise immunity for the peak level sensing. R8 is a pull up for the LM2903 open collector comparator output. D2 couples the peak detecting comparator to a simple resistor-capacitor timer formed by R9, R10 and C2. This R-C timer integrates over each half cycle of the ac line providing a more constant level indicative of the average ac input voltage. R10 provides quick response to high peak voltages on the input. R9 provides a slow response filling the gaps between half cycles of the applied voltage. When U1A detects a peak voltage above the set threshold or trip level, C2 is pulled low. When C2 voltage falls below the threshold of the second comparator U1B, the output of this comparator switches to a high state turning on Q2. FET Q2 is connected to the appropriate control signal of the target power converter stopping all switching and reducing voltage stress in response to the excessive input voltage. The second comparator also turns on Q1 forming a conducting path for R7. This resistor increases the hysteresis for the first comparator lowering the detection threshold. The value of R7 is adjusted to provide the desired input line voltage level below which the power converter will be allowed to restart. As long as the peak detecting comparator U1A senses input voltage above the reset threshold, D2 will keep C2 discharged, and the second comparator will maintain Q2 in a conducting state forcing the power converter off. When the first comparator no longer detects excessive peak voltages corresponding to normal RMS voltage levels, C2 will begin to charge through R9. After a delay, the second comparator will switch to a low state providing hysteresis through R13 for stable detection. Subsequently, Q2 will switch off allowing the power converter to restart. Q1 will also switch off raising the first comparator threshold back to the higher trip level corresponding to the RMS input voltage threshold to shut down the power converter. Bias Setup Energy to power the detection circuit must be derived from the rectified ac input. By its nature, bias power is supplied when the circuit is operating at high input voltage. A series connection of resistors is used to deliver the required current dividing the voltage and power stress amongst multiple devices. Maintaining each device within ratings enhances reliability. Collectively, this series connection of resistors is referred to as R15. February 2014, Rev. 0 The design example of Figure 1 is based on the MMSZ4689 5.1 volt zener with 5% tolerance. Note this low current zener diode is specified at 50 µA bias current which avoids significant dissipation compared to a MMSZ5231 which is guaranteed 5% at 20 mA bias current. The required bias current is largely dependent on the selected comparator. The LM2903 dual comparator draws about 1 mA bias current. The remaining circuitry draws about 0.25 mA depending on resistor values plus 50 µA for the zener diode, totaling 1.25 mA for this circuit. A lower power solution based on the NCS2220 comparator is discussed in a later section. The current supplied by R15 is dependent on the input voltage. As the ac input voltage is reduced, the available bias current will reduce. By the nature of the application, full performance of this circuit is required only at elevated input voltage. However, the circuit must not interfere with power supply operation down to the minimum operating input voltage of the system. Empirical testing shows full bias current is required at approximately 250 volts to avoid false operation. In this case, R15 = (250 – 5.1) / 1.25 mA = 196 k ohms is the maximum resistance value. During an excessive input event of say 310 Vac less 2 volts in the bridge rectifier, the peak voltage will be 308 * 1.414 = 436 Vdc. Subtracting the 5.1 volt bias leaves 434.9 Vdc applied across R15. Dissipation in R15 follows 434.9 squared divided by 196 k = 0.96 watts. Using the typical 125 mW allowable stress per 1206 resistor, this means 8 resistors are required to handle the dissipation. 196 k divided by 8 equals 24.5 k ohm per resistor. Therefore, R15 is comprised of 8 resistors of 24.5 k ohm each. Optionally, a two watt through-hole resistor could be used for R15. Specific types are available which are rated for higher operating voltage. Improved Accuracy The bias voltage is used as the reference to establish shutdown or trip voltage. Any deviation in the bias voltage will directly reflect in accuracy of the trip voltage. The MMSZ4689 low current zener provides a simple bias regulator, but carries with it a 5% tolerance as well as variation due to current through the device. Changing to the NCP431 shunt regulator reduces the error to 1% or even 0.5% depending on the version selected. The NCP431 requires 100 µA bias current. This is a relatively small increase in current and dissipation. Capacitor C3 must be less than 200 pF to ensure stability. www.onsemi.com 2 DN05060/D Figure 2 shows the implementation of the NCP431 in place of the zener diode. Each particular application should evaluate these tradeoffs between accuracy, dissipation, and cost. In addition, the NCS2220 will sink and source current which eliminates two resistors, R8 and R14. The solution requires less board space, fewer components and reduced bias current. Design Example A design example is presented based on the NCL30001 High Efficiency, Single Stage, High Power Factor LED driver. A similar approach could be used for any off-line power converter. If the NCS2220 comparator is used with the NCP431 higher accuracy reference, the typical bias current is reduced to about 14% of the circuit shown in Figure 1. Dissipation is reduced from 0.94 to 0.13 watts. A typical implementation requires only 2 bias resistors compared to 8 of the 1206 size surface mount resistors. The circuit of Figure 1 is used in this example, The goal is to disable the converter when the input voltage exceeds 266 V ac. Normal operation shall be restored when the input voltage reaches 253 V ac. A design example based on a 60 watt output power supply will help put these numbers in perspective. If this power supply had an efficiency of 85%, then the input power would be 60 / 0.85 = 70.59 watts. The NCL30001 can be disabled through the ‘Vff’ function of pin 5. Pulling this pin below 0.45 volts activates the brown out function. The drain of Q2 in Figure 1 will be connected to NCL30001 pin 5. When an over voltage event is detected, Q2 will pull pin 5 below the threshold causing the controller to shut off all switching. Figure 3 shows a schematic detailing connection. Adding the Input Over Voltage Protection circuit of Figure 1 increases the input power by 0.94 watts. As a consequence, the efficiency of this power supply will drop from 85% to 83.9%. Figure 4 shows the applied input voltage in yellow transitioning from 260 V ac to 270 V ac. This step was selected for clarity, noting that the circuit actually responds at 268 V ac. The blue trace shows the gate voltage of Q2 rising which initiates a shutdown of the NCL30001 controller. The input voltage must be reduced to below 253 V ac to restore operation. A delay of approximately 40 ms is used to ensure the input voltage has returned to the proper level and the Over Voltage monitor is not responding to normal zero crossing events. Figure 5 shows the response of the circuit to a reduction of input voltage from 268 V ac to 253 V ac. The blue trace shows the gate of Q2 dropping after a delay. The circuit provides clean transitions from excessive input voltage to shut down state and back to normal operation when required. The hysteresis and delay of the circuit provide robust protection. Reduced Current/Dissipation Dissipation and effect on system efficiency may be a concern in some applications. While the LM2903 dual comparator is a cost effective solution, the bias current is about 1 mA. This introduces significant power loss given the circuit must be powered by a linear regulator from a high voltage source. If the Input Over Voltage Protection circuit of Figure 2 was incorporated, the input power increase is only 0.13 watts. The power supply efficiency would be 84.8%. The circuit of Figure 2 represents an improvement of about 1% for this example power supply compared to the circuit of Figure 1. A Bill of Materials is presented in Figure 6 below showing all parts including options for lower power comparator and higher accuracy operation. Design Tool A design tool is available at the ON Semiconductor website. This Excel® spreadsheet helps establish shut off and start up voltages. In addition, the designer can select which type of comparator and reference to use and guidance is given on selecting the bias resistors. Conclusion Many applications be it lighting, communications, or computing which are exposed to poor power quality may benefit from a protection solution which does not involve more expensive or lossy semiconductor components. Since this solution does disable the power converter during the over voltage event it may not be suitable for critical applications which cannot be interrupted. The monitor circuit will automatically restore normal operation when the input voltage returns acceptable levels. This design does not use any electrolytic capacitors which enhances reliability and reduces PCB space. ON Semiconductor offers another dual comparator which draws less than 3.5% of the LM2903 bias current. The NCS2220 comparator draws only 34 µA. Details for this circuit are shown in Figure 2. February 2014, Rev. 0 www.onsemi.com 3 DN05060/D *R15 is a collection of resistors based on dissipation and voltage derating R15 Rectified AC 196k R1 499k R4 100k R8 R9 R11 R14 100k 470k 220k 47k Shutdown Low R2 U1B U1A LM2903 8 499k 8 LM2903 3 + R10 5 1k 1 6 + Q2 7 2N7002 - - 4 2 D1 MMSD4148 4 R6 2meg R7 C1 Vcc bias 5.1 Volt R13 1.5meg 220k 10nF R3 R5 10k 430k C2 R12 C3 100nF 220k 100nF D2 MMSZ4689 Q1 2N7002 Primary Return Figure 1. LM2903 Schematic R15 *R15 is a collection of resistors based on dissipation and voltage derating Rectified AC 1.36 meg R1 511k R4 100k R9 R11 470k 220k Shutdown Low R2 U1A NCS2220A 511k 3 D1 8 + - 2 U1B NCS2220A R10 1k Q2 5 + - 1 7 2N7002 6 1 R6 MMSD4148 2meg Vcc bias R13 5.0 Volt C1 R7 1.5meg 220k R16 10nF R3 R5 10k 430k C2 R12 C3 100nF 220k 100pF U2 NCP431 220k R17 Q1 220k 2N7002 Primary Return Figure 2. NCS2220A Schematic February 2014, Rev. 0 www.onsemi.com 4 DN05060/D Figure 3. NCL30001 schematic showing connection to Input OVP circuit February 2014, Rev. 0 www.onsemi.com 5 DN05060/D Figure 4. Circuit responding to excessive input voltage Figure 5. Restoring normal operation for normal input voltage February 2014, Rev. 0 www.onsemi.com 6 DN05060/D 1 Value Description Tol (+/-) Footprint Manufacturer Cap 10nF 50V Ceramic X7R 10% 0603 Cap 100nF 50V Ceramic X7R 10% 0603 1 Cap 100nF 50V Ceramic X7R 10% D1 1 Diode MMSD4148 100V, 200mA D2 1 Diode MMSZ4689 Low current, 5.1V Q1 Q2 2 2N7002 NFET, 60V, 7.5Ω R1 R2 2 Res 499k 1/4W R3 1 Res 10k 1/10W R4 R8 2 Res 100k R5 1 Res 430k R6 1 Res R7 1 R9 Ref Qty Type C1 1 C2 1 C3 Manufacturer Part Number Sub Allowed TDK C1608X7R1H103K080AA Yes TDK C1608X7R1H104K080AA Yes 0603 TDK C1608X7R1H104K080AA Yes - SOD-123 ON Semiconductor MMSD4148T1G No 5% SOD-123 ON Semiconductor MMSZ4689T1G No - SOT-23 ON Semiconductor 2N7002LT1G 1% 1206 Yes 1% 0603 Yes 1/10W 1% 0603 Yes 1/10W 1% 0603 Yes 2 meg 1/10W 1% 0603 Yes Res 1.5 meg 1/10W 1% 0603 Yes 1 Res 470k 1/10W 1% 0603 Yes R10 1 Res 1k 1/10W 1% 0603 Yes R11 R12 R13 3 Res 220k 1/10W 1% 0603 Yes R14 1 Res 47k 1/10W 1% 0603 Yes R15 8 Res 24k 1/4W 5% 1206 U1 1 Comp LM2903 - SOIC8 Tran Dual Comparator No Yes ON Semiconductor LM2903DG No Optional Components when implementing low power comparator and higher accuracy reference of Figure 2 C3* R1 R2* 1 Cap 100pF 50V Ceramic X7R 10% 0603 TDK C1608C0G2A101K080AA Yes 2 Res 511k 1/4W 1% 1206 Yes R15* R16 R17* 2 Res 680k 1/4W 5% 1206 Yes 2 Res 220k 1/10W 1% 0603 U1* 1 Comp NCS2220A Dual Comparator - UDFN8 ON Semiconductor NCS2220AMUT1G No U2* 1 Reg NCP431 Low Current Ref - SOT-23 ON Semiconductor NCP431AVSNT1G No Yes Figure 6 Bill of Materials 1 © 2014 ON Semiconductor. Disclaimer: ON Semiconductor is providing this design note “AS IS” and does not assume any liability arising from its use; nor does ON Semiconductor convey any license to its or any third party’s intellectual property rights. This document is provided only to assist customers in evaluation of the referenced circuit implementation and the recipient assumes all liability and risk associated with its use, including, but not limited to, compliance with all regulatory standards. ON Semiconductor may change any of its products at any time, without notice. Design note created by Jim Young, e-mail:[email protected] February 2014, Rev. 0 www.onsemi.com 7