AND9455/D How to Use Auxiliary Winding Voltage for Biasing the NCP1602 CSZCD Pin NCP1602 combines the current sense (CS) and the core demagnetization detection (ZCD, zero crossing detection) signals into a single pin named CSZCD. In some noisy environment, a non-optimum choice of sensing resistors combined with a dense PCB layout can lead to a unstable operation of 1602. In this particular situation, the adoption of a more conventional way of sensing via a dedicated winding is an advantageous solution bringing ease of implementation and stronger noise immunity. This application note describes how the NCP1602, primarily designed to work without the need of an auxiliary voltage for ZCD detection, can use the auxiliary voltage for a better CSZCD pin immunity to noise. www.onsemi.com APPLICATION NOTE Auxiliary Winding Circuit for CSZCD Pin (to Avoid Sensitivity to PCB Parasitics) To avoid pin CSZCD sensitivity, the circuitry shown in Figure 2 can replace the original circuitry of Figure 1. There is no VAUX voltage reflected by the auxiliary winding when the part does not switch and, consequently, no current is flowing through the RCS resistors so the standby current is not affected by RCS resistors. The total resistance value of the RCS bridge can then be set under the 1-MW limit originally set to avoid PCB parasitic capacitances and distortion of the CSZCD voltage signal. When an auxiliary winding voltage is available, the circuitry of Figure 2 is then preferred especially in applications where very low standby consumption is important. The only drawback of Figure 2 circuitry is that product options including brown-out detection cannot be used because the controller cannot start switching in this configuration. The reason of this non-functionality is that the brown-out high level can never be reached for allowing the controller to start switching because switching activity is needed for the brown-out level to be sensed through the CSZCD pin. Drain Connection for CSZCD Pin (Sensitivity to PCB Parasitics) When trying to reduce the no-switching standby consumption current by increasing (above 1 MW) the impedance (RCS1 + RCS2 ) of the CSZCD bridge connected between the drain and the source of the power MOSFET (see Figure 1), the sensitivity of the CSZCD pin to PCB parasitic capacitors is increased, leading in some cases to non-functionality caused for example by a constant false triggering of OCP (Over Current Protection) or OVP (Over Voltage Protection). This is due to the fact that the CSZCD voltage is distorted and internal circuitry cannot work as intended. Recommendations for avoiding such CSZCD pin sensitivity are given in [1]. CAUX VDRAIN VDRAIN RAUX VX DAUX1 RCS1 RCS0 DRV VAUX N AUX N PRIM RCS1 CSZCD RCS0 DRV CSZCD VM RCS2 RCS2 VSOURCE VSOURCE RSENSE RSENSE Figure 1. CSZCD Connection without Auxiliary Winding © Semiconductor Components Industries, LLC, 2016 June, 2016 − Rev. 0 Figure 2. CSZCD Connection with Auxiliary Winding 1 Publication Order Number: AND9455/D AND9455/D Calculations of Component Values Used in CSZCD Circuitry During the on-time, capacitor CAUX is quickly charged (see Figure 3) through the low value resistor RAUX to a voltage value VCAUX given by: The CSZCD pin has originally been designed to work with the information contained in the instantaneous drain-source voltage (case of RCS1 connected to power MOSFET drain as shown in Figure 1). For the CSZCD pin to work when RCS1 is not connected to the power MOSFET drain but to VX node (see Figure 2), the VX node (which name also represents the voltage between this node and GND) must be proportional to the instantaneous power MOSFET drain voltage VDRAIN . Let’s analyze the waveforms corresponding to Figure 2 schematic. During the on-time, VAUX voltage is given by: V AUX + * V IN @ N AUX V CAUX + V IN @ N AUX N PRIM V AUX + V DRAIN,AC @ N PRIM V X + V AUX ) V CAUX + V OUT @ (eq. 2) + V DRAIN,INST @ + (eq. 3) + V DRAIN,INST @ RAUX DAUX1 N PRIM N AUX N AUX N PRIM + (eq. 7) N PRIM N AUX N PRIM (eq. 8) So vX (t) is a scaled down (by NAUX / NPRIM which is auxiliary to primary turns ratio) version of vDRAIN (t). vX (t) acts as a scaled down instantaneous drain voltage and is then divided by the resistor bridge made with RCS1 and RCS2 . In order to have the same voltage waveform on CSZCD pin as when RCS divider was directly connected to the power MOSFET drain, the resistor divider ratio must be lowered. With RCS1 connected to drain voltage, the design equation to get RCS1 and RCS2 is: CSZCD VM RCS2 VSOURCE (eq. 6) N AUX v X(t) + v DRAIN(t) @ N AUX N PRIM RCS1 CAUX Discharge CAUX Charge RCS0 DRV + We just have demonstrated that, during on-time, demagnetization time and dead-time, which means whatever time we have: VAUX VX N PRIM V X + V AUX ) V CAUX + ǒV IN ) V DRAIN,ACǓ @ − CAUX VDRAIN N AUX During the dead-time, by combining Equation 3 and Equation 4 we get: Where VDRAIN,AC is equal to VDRAIN − VIN (during dead-time VDRAIN is ringing around its mean value which is equal to VIN ). VCAUX (eq. 5) N PRIM Because vDS (t) during the on-time is equal to zero. During the demagnetization time, by combining Equation 2 and Equation 4 we get: During the dead-time, VAUX voltage is given by: N AUX N AUX V X + V AUX ) V CAUX + 0 + V DRAIN,INST @ Where VIN is the rectified mains voltage, right after the diode rectifier bridge, NAUX & NPRIM are respectively the number of turns of auxiliary and primary windings of the transformer which primary inductor serves as the PFC boost inductor. During the demagnetization time, VAUX voltage is given by: V AUX + ǒV OUT * V INǓ @ (eq. 4) N PRIM The capacitor can only charge up to a greater value given by Equation 4 so it means that the end of one switching cycle, the voltage across CAUX must be such as VX voltage is slightly negative so during the following on-time CAUX can be charged at the value given by Equation 4. We can also mention that the voltage drop across DAUX1 has been neglected to keep simple equations. During the on-time, by combining Equation 1 and Equation 4 we get: (eq. 1) N PRIM N AUX RSENSE R CS1 ) R CS2 R CS2 Figure 3. Charge and Discharge of CAUX Capacitor www.onsemi.com 2 + K CS + 138 (eq. 9) AND9455/D Let’s Explain CAUX Capacitor Calculation Now with RCS1 connected to VX node, the design equation to get RCS1 and RCS2 is: ǒ N AUX Ǔ N PRIM *1 @ R CS1 ) R CS2 R CS2 + K CS + 138 The CAUX capacitor charges up during on-time to ((NAUX / NPRIM ) V VIN ) minus a DAUX diode Vf and while VIN is rising. It is recommended for DAUX to use a signal diode like the 1N4148. CAUX is charging very fast cycle by cycle because RAUX and CAUX values are chosen so that their time constant equals 100 ns. As can be seen on Figure 4 when VIN is decreasing and the CAUX discharge is not fast enough, the voltage across CAUX cannot track ((NAUX / NPRIM ) V VIN ) versus time. The absolute value of CAUX voltage discharge slope when vIN (t) is decreasing must be greater than the slope of ((NAUX / NPRIM ) V vIN (t)) and while the equations are too complex to be shown here, the following design Equation 12 for determining CAUX value can be used. (eq. 10) For both cases and dictated by internal circuitry, KCS must be as close as possible to 138 target value, within ±10% and RCS2 not being allowed to be under 20 kW, it is advised to set it to the normalized value of 22 kW. It is also advised to use 1% tolerance resistors for RCS1 and RCS2 as they are, with internal voltage references, setting the line level detection, the OVP2 (Second Over-Voltage protection), RCS0 value is set using the following design equation: ƪǒRCS1 ńń RCS2Ǔ ) RCS0ƫ @ 10 pF + 500 ns (eq. 11) ǒR CS1 ) R CS2Ǔ @ C AUX + 640 ms " 10% Where 10 pF is the parasitic input capacitance of the CSZCD pin and 500 ns the time constant of an internal zero. This zero is there to cancel the un-wanted pole made by associating the RCS resistors with the parasitic input capacitance of the CSZCD pin (10 pF).The internal zero ensures a non-distorted CSZCD voltage signal. (eq. 12) Once CAUX value is calculated, RAUX is calculated using the following equation which allows the CAUX capacitor to be fully charged during on-time: R AUX @ C AUX + 100 ns CAUXVoltage (V) (NAUX /Np)*VIN Diode Vf CAUX = 47 nF CAUX = 15 nF CAUX = 5 nF CAUX = 2.2 nF time (ms) Figure 4. Voltage Across CAUX Capacitor vs. Time and Different CAUX Values www.onsemi.com 3 (eq. 13) AND9455/D Practical Example of Component Values Calculation The closest standard value is: CAUX = 2.2 nF. Let’s start with: N AUX N PRIM + 0.1 To calculate the RAUX value we will use the design Equation 13 which gives: (eq. 14) R AUX + RCS2 must not be less than 20 kW so let’s adopt the standard value RCS = 22 kW. Solving the following design Equation 10 for RCS1 gives: ǒ R CS1 + R CS2 @ K CS @ N AUX N PRIM Ǔ *1 + 22 k @ VDRAIN 1 + 132.7 0.1 + R CS1 @ R CS2 R CS1 ) R CS2 270 k @ 22 k 270 k ) 22 k 640 m + RCS2 22 kW 640 m (eq. 20) RAUX 2.2 nF 47 W 0.1 N AUX N PRIM CSZCD 20 kW VSOURCE RSENSE + (eq. 17) Figure 5. CSZCD Schematic Circuitry with Component Values Previously Calculated + 20.34 kW 270 k ) 22 k + 45.45 W CAUX RCS0 DRV (eq. 16) ǒR CS1 ) R CS2Ǔ @ C AUX + 640 ms " 10% R CS1 ) R CS2 2.2 n RCS1 270 kW So we will take the standard value of RCS0 = 20 kW (we could have taken also 22 kW because 10% error is acceptable for matching a time constant) Now we have to calculate CAUX capacitance value using Equation 12 which gives: C AUX + 100 n DAUX1 1N4148 Which is acceptable because 138−10% = 124.2 and KCS = 132.7. This value is above 138−10%. Now that we have RCS1 and RCS2 values, let’s solve RCS0 using the design Equation 11 which gives: R CS0 + 50 kW * + All the calculated component values which have been calculated are now reported on Figure 5 schematic. (eq. 15) Let’s select the closest standard value which is: RCS1 = 270 kW. Now let’s recalculate KCS to see if the new value is within 138 ±10% The newly calculated KCS value is: 270 k ) 22 k C AUX The closest standard value is: RAUX = 47 W. + 22 k @ (138 @ 0.1 * 1) + 281.6 kW K CS + 100 n References [1] Application Note AND9218/D “5 Key Steps to Designing a Compact, High-Efficiency PFC Stage Using the NCP1602” which can be downloaded at: http://www.onsemi.com/pub_link/Collateral/ AND9218−D.PDF (eq. 18) + 2.19 nF (eq. 19) ON Semiconductor and are trademarks of Semiconductor Components Industries, LLC dba ON Semiconductor or its subsidiaries in the United States and/or other countries. ON Semiconductor owns the rights to a number of patents, trademarks, copyrights, trade secrets, and other intellectual property. A listing of ON Semiconductor’s product/patent coverage may be accessed at www.onsemi.com/site/pdf/Patent−Marking.pdf. ON Semiconductor reserves the right to make changes without further notice to any products herein. ON Semiconductor makes no warranty, representation or guarantee regarding the suitability of its products for any particular purpose, nor does ON Semiconductor 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. Buyer is responsible for its products and applications using ON Semiconductor products, including compliance with all laws, regulations and safety requirements or standards, regardless of any support or applications information provided by ON Semiconductor. “Typical” parameters which may be provided in ON Semiconductor 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. ON Semiconductor does not convey any license under its patent rights nor the rights of others. ON Semiconductor products are not designed, intended, or authorized for use as a critical component in life support systems or any FDA Class 3 medical devices or medical devices with a same or similar classification in a foreign jurisdiction or any devices intended for implantation in the human body. Should Buyer purchase or use ON Semiconductor products for any such unintended or unauthorized application, Buyer shall indemnify and hold ON Semiconductor and its officers, employees, subsidiaries, affiliates, and distributors harmless against all claims, costs, damages, and expenses, and reasonable attorney fees arising out of, directly or indirectly, any claim of personal injury or death associated with such unintended or unauthorized use, even if such claim alleges that ON Semiconductor was negligent regarding the design or manufacture of the part. ON Semiconductor is an Equal Opportunity/Affirmative Action Employer. This literature is subject to all applicable copyright laws and is not for resale in any manner. PUBLICATION ORDERING INFORMATION LITERATURE FULFILLMENT: Literature Distribution Center for ON Semiconductor 19521 E. 32nd Pkwy, Aurora, Colorado 80011 USA Phone: 303−675−2175 or 800−344−3860 Toll Free USA/Canada Fax: 303−675−2176 or 800−344−3867 Toll Free USA/Canada Email: [email protected] N. American Technical Support: 800−282−9855 Toll Free USA/Canada Europe, Middle East and Africa Technical Support: Phone: 421 33 790 2910 Japan Customer Focus Center Phone: 81−3−5817−1050 www.onsemi.com 4 ON Semiconductor Website: www.onsemi.com Order Literature: http://www.onsemi.com/orderlit For additional information, please contact your local Sales Representative AND9455/D