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an10580 greenchip tea1761 synchronous rectifi cation and feedback controller rev. 01 ? 19 march 2008 application note document information info content keywords greenchip sr, tea1761, synchronous rectification, high efficiency, flyback, voltage regulation abstract the tea1761 is a member of the new generation of synchronous rectifier controller ics for switched mode power supplies. its high level of integration allows the design of a co st effective power supply with a very low number of external components. the tea1761 is a controller ic dedicated for synchronous rectification on the secondary side of discontinuous conduction mode and quasi-resonant flyback converters. besides electronics for synchronous rectification, circuitry for output voltage and output current regulation is integrated. the tea1761 is fabricated in a silicon on insulator (soi) process. this nxp soi process makes a wide voltage range possible.
an10580_1 ? nxp b.v. 2008. all rights reserved. application note rev. 01 ? 19 march 2008 2 of 11 contact information for more information, please visit: http://www.nxp.com for sales office addresses, please send an email to: salesaddresses@nxp.com nxp semiconductors an10580 greenchip tea1761 sr and feedback controller revision history rev date description rev. 01 20080319 first edition
an10580_1 ? nxp b.v. 2008. all rights reserved. application note rev. 01 ? 19 march 2008 3 of 11 nxp semiconductors an10580 greenchip tea1761 sr and feedback controller 1. introduction the tea1761 is a controller for synchronous re ctification, to be us ed in quasi-resonant and discontinuous conduction mode flyback converters. besides control of the sr mosfet, the tea1761 contains the voltage reference and amplifiers to regulate and control the output voltage and output current of the power supply. 2. application schematic tea1761 figure 1 shows a typical synchronous rectification application using the tea1761. see ta b l e 1 in section 6.2 for the component values, which are relevant to the application?s behavior. 3. functional descript ion and application 3.1 sr control the tea1761 uses the srsense pin as an input to control the mosfets. there is no adjustment necessary for the sr control. fig 1. example of the tea1761 in a 90 w adapter 014aaa051 opto srsense driver gnd isense n.c. vsense v cc r34 r35 r36 r37 q3 q4 r38 c33 c34 c35 r40 r39 r42 r33 c38 r41 1 2 c36 c37 r32 r30 u2 ? 2 c39 r31 c31 1 2 r18 r19 c8 d3 c9 q2 r15 t1 9, 10 7, 8 1 2 4 v out gnd u3 5 6 3 7 2 4 1 8
an10580_1 ? nxp b.v. 2008. all rights reserved. application note rev. 01 ? 19 march 2008 4 of 11 nxp semiconductors an10580 greenchip tea1761 sr and feedback controller the sr mosfet is switched on when the voltage at the srsense pin is lower than ? 310 mv. when the voltage at the srsense pin reaches ?55 mv (i d r dson ), the driver decreases and regulates the gate voltage of the mosfet in order to maintain the ?55 mv at the srsense pin. when the voltage at the srsense pin rises above ? 12 mv (typical), the sr mosfet is switched off. the synchronous rectification remains active in standby-mode, as long as the secondary stroke is less than 2 s (typical). the driver of the tea1761 has been designed such that there is no additional power consumption in standby with the mosfet active. for the best performance it is advisabl e to connect the srsense pin as close as possible to the drain of the mosfets. also see section 6.1 . it is not necessary to place a resistor between the driver output and the mosfet gate. if such a series resistor is required, e.g. for reasons of reducing switching noise, then it must be checked if the sr mosfet is kept off under all circumstances, especially at high temperature of the sr mosfet. at switch-on of the primary side mosfet, the voltage at the drain of the sr mosfet goes up with a high v/ t. the steep v/ t causes a current flow through the c dg capacitor, from gate to drain. this current increases the gate voltage of the mosfet. if this rises above the threshold voltage, v th(en) , the sr mosfet is switched on. this should be prevented. 3.2 function of resistors in series with pin srsense in the tea1761 there is an esd protection at every pin for handling during production. because this esd protection can still be trigge red by an esd event or test during normal operation, additional protection by a resistive path is recommended. if the esd protection circuit is activated by an external esd event in the application, then there will be a short circuit betw een the srsense pin and gnd pin. in this event the ic could be damaged. the function of the resistors (r34 and r35 in figure 1 ) is to limit the current in the srsense pin if the esd protection is trigge red. a total resistance value of 1 k is sufficient to protect pin sr sense. because of the peak power rating, two smd 1206 resistors are used. 3.3 output voltage regulation the application of the voltage feedback circuit is similar to well known circuits using a tl431 or tsm103. the internal reference voltage is 2.5 v, accuracy within 1 %. a voltage divider (r32 and r33 in figure 1 ) is used to set the output voltage of the application. the output voltage can be calculated with the equation: or when v o and r32 are known, for example v o = 19.5 v and r32 = 35.7 k , then v o 2.5v r 32 r 33 + r 33 ---------------------- - =
an10580_1 ? nxp b.v. 2008. all rights reserved. application note rev. 01 ? 19 march 2008 5 of 11 nxp semiconductors an10580 greenchip tea1761 sr and feedback controller the phase and gain margin of the system can be set with a feedback network between the opto output and the vsense pin (r31 and c31 in figure 1 ). 3.4 output current limit the output current of the application can be limited by sensing the voltage across a current-sense resi stor (r42 in figure 2 ). the internal reference voltage of the current-sense circuit is 50 mv. therefore the voltage drop across the current-sense resistor must be more than 50 mv. the resistor divider (r39 and r40 in figure 1 and figure 2 ) is used to adjust the actual output-current limit and to act as an rc filter in combination with c35. with the tea1761 both signals for the voltage feedback and the current feedback are transferred through one opto coupler to the primary side of the application. when an output overcurrent occurs, the flyback controlle r at the primary side should limit the output power. a commonly used method is to do this by triggering the undervoltage lock out (uvlo) of the flyback controller. to trigger the uvlo, three conditions must be met: 1. the tracking of the supply voltage (v cc ) of the flyback controller must be coupled very closely to the output voltage. this requires a well designed transformer with a low leakage inductance and a well designed peak clamp. 2. the output power must be decreased gradually to enable tracking of the flyback v cc with the output voltage. to achieve this, th e time constant of r40 x c35 should be approximately 100 ms. 3. the number of turns on the primary side of the auxiliary winding must be kept as low as possible. this is necessary to trigger the uvlo of the flyback controller before the uvlo of the tea1761 is reached. r 33 r 32 v ref v o v ref ? () -------------------------- 35.7k 2.5v 19.5v 2.5v ? () ------------------------------------ 5.25k 5.23k 1 % () ? == = fig 2. output current limit 014aaa052 c35 r40 r39 r42 c34 r30 opto isense 50 mv gnd v out + v out ? tea1761
an10580_1 ? nxp b.v. 2008. all rights reserved. application note rev. 01 ? 19 march 2008 6 of 11 nxp semiconductors an10580 greenchip tea1761 sr and feedback controller the value of c34 should be chosen such that the output current limit operates correctly at different overload levels between the tea1761 setting point and a real short circuit. bouncing of the tea1761 opto output should be prevented, as long as the flyback is switching. in practice, values between 470 pf and 22 nf have proven to give a good result in different applications. for setting the required current limit, the following procedures can be used. 3.4.1 adjusting the current limit in combination with the greenchip ii and other brands 1. disable the isense circuit of the tea17 61, by connecting pin isense to ground. 2. adjust the flyback current-sense resistor such that the adapter can deliver 130 % to 140 % of the nominal output current at a low mains voltage. 3. adjust the opp resistor (t ea1552, tea1654 or tea1533), or external opp network (tea1532) such that the adapter can deliver 130 % to 140 % of the nominal output current at a high mains voltage. 4. enable the tea1761 isense circuit and adjust the voltage divider (r39 and r40 in figure 1 ) such that the output current is li mited to 120 % of the nominal output current. some adjustment of the value of c35 may be necessary. 5. adjust c34 such that there is no bouncing of the tea1761 opto output during switching of the flyback. 3.4.2 adjusting the current limit in combination with the greenchip iii tea1750 1. adjust the time-out circuit at the fbctrl pin. 2. disable the isense circuit of the tea17 61, by connecting pin isense to ground. 3. adjust the flyback current-sense resistor such that the adapter can deliver 130 % to 140 % of the nominal output current at a low mains voltage. 4. enable the tea1761 isense circuit and adjust the voltage divider (r39 and r40 in figure 1 ) such that the output current is li mited to 120 % of the nominal output current. some tuning of the value of c35 could be necessary. 5. adjust c34 such that there is no bouncing of the tea1761 opto output during switching of the flyback. 3.5 opto output the functions of the voltage feedback and the current feedback are combined to one open-drain opto output. it is po ssible to make two separate feedback circuits from this single output, one to pi n vsense (r31 and c31 in figure 1 ) and one to pin isense (c34 in figure 1 ). 3.6 v cc v startup is typically 8.6 v and v cc(uvlo) is typically 8.1 v. if necessary, a capacitor c39 can be placed between pin v cc and pin gnd to stabilize the supply voltage and to limit the noise at the ic ground track.
an10580_1 ? nxp b.v. 2008. all rights reserved. application note rev. 01 ? 19 march 2008 7 of 11 nxp semiconductors an10580 greenchip tea1761 sr and feedback controller 3.7 driver output between 0 v and uvlo, an active current sink circuit keeps the external mosfet(s) in an off-state. 3.8 internal overtemper ature protection (otp) the tea1761 has an internal overtemperature protection at 150 c. the ic will maximize the opto sink current at the mome nt the otp level has been reached. this will limit (or shut down) the output pow er of the primary side controller. 4. application examples 4.1 using the tea1761 with a 3.3 v or 5 v output the tea1761 has a start level of 8.6 v (typical). an extra secondary winding must be used to supply the ic when using the tea1761 for a 3.3 v or 5 v output. figure 3 gives the configuration of such a circuit. in some cases, where the nominal output voltage is close to v startup , the extra supply winding could be necessary for proper output current limiting behavior. 5. meeting emc requirements in some applications, it can be more difficul t to meet the cispr 22 requirements with the synchronous rectification compared to a solution with schottky diodes. this is caused by the placing of the sr mosfets in the secondary ground path instead of the ?normal? placing of the schottky diode in the positive vo ltage path. this problem is typical for this topology. there are two ways to solve this problem without increasing the common mode filtering. 5.1 the powered shield figure 4 shows the powered shield. in most transformer designs, the shielding between the primary and secondary windings is already present. all the present shields must be connected to an extra winding at the primary side. this winding injects a current into the fig 3. v cc supply with a 3.3 v or 5 v output tea1761 15 v 3.3 v v cc secondary ground v dc prim primary ground sec 014aaa05 3 10 f
an10580_1 ? nxp b.v. 2008. all rights reserved. application note rev. 01 ? 19 march 2008 8 of 11 nxp semiconductors an10580 greenchip tea1761 sr and feedback controller shielding. the phase of the injected current is opposite to the phase of the common mode current through the fl yback transformer and will reduce the common mode current through the input and output cables by compensation. the exact number of turns of the shield winding should be determined experimentally. with the powered shield a significant reduction of the y-cap value and thus leakage current is possible. in some cases the y-capacitor even can be omitted. 5.2 the powered y-cap figure 5 shows the powered y-cap which is an alte rnative to the powered shield solution. this solution needs an additional winding on the transformer and sometimes it also requires an additional y-cap, to meet the esd surge and fast transient tests. the extra winding at the primary side, injects current through a y-cap into the secondary ground. the phase of the injected current is opposite to the phase of the common mode current through the flyback tr ansformer and this will, by compen sation, reduce the common mode current through the input and output cables. the exact required number of turns in the extra winding and the y-cap value should be determined experimentally. eight winding turns and a y-cap with a value of 100 pf would be recommended as a good starting point. fig 4. powered shield tea1761 secondary ground v dc prim primary ground sec 014aaa05 4 y-cap shield v out fig 5. powered y cap tea1761 secondary ground v dc prim primary ground sec 014aaa091 y-cap 2 y-cap 1 v out
an10580_1 ? nxp b.v. 2008. all rights reserved. application note rev. 01 ? 19 march 2008 9 of 11 nxp semiconductors an10580 greenchip tea1761 sr and feedback controller 6. appendix 6.1 layout considerations the ic ground copper track should be as wide and as low ohmic as possible. the ic ground is used as a reference for the circuits , but also has to conduct the high current of the driver and the currents through the mosfet c dg . the ic ground is used as a reference for the voltage and current regulation and for the control of the sr mosfet. therefore a compromise has to be made with respect to the connection of the ic ground to the surrounding circuits. it is advisable to connect the ic ground to the output electrolytic ca pacitor closest to the mosfet source. the srsense pin must be connected as close as possible to the mosfet drain pin to guarantee a proper detection of the mosfet v ds and thus control of the sr mosfets. it is advisable to reserve a separate copper tr ack in the pcb layout for this function. 6.2 bill of materials table 1. bill of materials relevant component values for a 19.5 v / 4.62 a application with the tea1761. description position resistor, smd 0603 thin film chip, 1 k , 5 % r30 resistor, smd 0603 thin film chip, 10 k , 5 % r31, r39 resistor, smd 0603 thin film chip, 35.7 k , 1 % r32 resistor, smd 0603 thin film chip, 5.23 k , 1 % r33 resistor, smd 1206 thin film chip, 560 , 5 % r34, r35 resistor, smd 0805 thin film chip, 47 , 5 % r36, r37 resistor, smd 0603 thin film chip, 51 k , 5 % r40 resistor, mu-cu wire, 10 m , 1 % r42 mlcc, smd 0805, 1 f/50 v, y5v c39 mlcc, smd 0805, 0.01 f/50 v, x7r c31 mlcc, smd 0805, 470 pf/50 v, x7r c34 mlcc, smd 0805, 2.2 f/10 v, x7r c35
an10580_1 ? nxp b.v. 2008. all rights reserved. application note rev. 01 ? 19 march 2008 10 of 11 nxp semiconductors an10580 greenchip tea1761 sr and feedback controller 7. legal information 7.1 disclaimers general ? information in this document is believed to be accurate and reliable. however, nxp semiconductors d oes not give any representations or warranties, expressed or implied, as to the accuracy or completeness of such information and shall have no liability for the consequences of use of such information. right to make changes ? nxp semiconductors reserves the right to make changes to information published in this document, including without limitation specifications and product descriptions, at any time and without notice. this document supersedes and replaces all information supplied prior to the publication hereof. suitability for use ? nxp semiconductors products are not designed, authorized or warranted to be suitable for use in medical, military, aircraft, space or life support equipment, nor in applications where failure or malfunction of a nxp semiconductors product can reasonably be expected to result in personal injury, death or severe property or environmental damage. nxp semiconductors accepts no liability for inclusion and/or use of nxp semiconductors products in such equi pment or applications and therefore such inclusion and/or use is at the customer?s own risk. applications ? applications that are described herein for any of these products are for illustrative purpos es only. nxp semiconductors makes no representation or warranty that such applications will be suitable for the specified use without further testing or modification. 7.2 trademarks notice: all referenced brands, produc t names, service names and trademarks are the property of their respective owners. greenchip ? is a trademark of nxp b.v.
nxp semiconductors an10580 greenchip tea1761 sr and feedback controller ? nxp b.v. 2008. all rights reserved. for more information, please visit: http://www.nxp.com for sales office addresses, please se nd an email to: salesaddresses@nxp.com date of release: 19 march 2008 document identifier: an10580_1 please be aware that important notices concerning this document and the product(s) described herein, have been included in section ?legal information?. 8. contents 1 introduction . . . . . . . . . . . . . . . . . . . . . . . . . . . . 3 2 application schematic tea1761. . . . . . . . . . . . 3 3 functional description and application . . . . . . 3 3.1 sr control . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 3 3.2 function of resistors in series with pin srsense . . . . . . . . . . . . . . . . . . . . . . . . . . . . 4 3.3 output voltage regulation . . . . . . . . . . . . . . . . . 4 3.4 output current limit . . . . . . . . . . . . . . . . . . . . . . 5 3.4.1 adjusting the current lim it in combination with the greenchip ii and other brands . . . . . . . . . . . . . 6 3.4.2 adjusting the current lim it in combination with the greenchip iii tea1750 . . . . . . . . . . . . . . . . . . 6 3.5 opto output. . . . . . . . . . . . . . . . . . . . . . . . . . . 6 3.6 v cc . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 6 3.7 driver output . . . . . . . . . . . . . . . . . . . . . . . . . . . 7 3.8 internal overtemperature protection (otp). . . 7 4 application examples . . . . . . . . . . . . . . . . . . . . 7 4.1 using the tea1761 with a 3.3 v or 5 v output . 7 5 meeting emc requirements. . . . . . . . . . . . . . . . 7 5.1 the powered shield . . . . . . . . . . . . . . . . . . . . . 7 5.2 the powered y-cap. . . . . . . . . . . . . . . . . . . . . . 8 6 appendix . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 9 6.1 layout considerations. . . . . . . . . . . . . . . . . . . . 9 6.2 bill of materials . . . . . . . . . . . . . . . . . . . . . . . . . 9 7 legal information. . . . . . . . . . . . . . . . . . . . . . . 10 7.1 disclaimers . . . . . . . . . . . . . . . . . . . . . . . . . . . 10 7.2 trademarks. . . . . . . . . . . . . . . . . . . . . . . . . . . 10 8 contents . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 11

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