design idea DI-122 tinyswitch-iii 13 w (17.2 w peak) non-isolated power supply for white goods: <150 mw no-load power consumption figure 1. tinyswitch-iii 13/17.2 w white good power supply. DI-122 january 2007 application device power output input voltage output voltage topology white goods tny279p 13 w / 17.2 w pk 85-265 vac -5 v, 2.0 a / -12 v, 0.25 a / 0.6 a pk flyback design highlights ? low cost, low component count solution ? impedance free path from neutral to output rtn, ideal for applications driving triacs ? <150 mw no-load consumption at 230 vac input ? both outputs exhibit good cross regulation ? maximum ambient temperature of up to 70 c ? meets cispr-22 class b/en55022 b emi limits without an input common mode choke operation the non-isolated fyback converter shown in figure 1 uses a tny279 to provide two output voltages: -5 v at 2 a and -12 v at 250 ma continuous and 600 ma peak. the mosfet integrated within u1 switches the primary of transformer t1. each time it turns on, the primary current ramps until it reaches an internal current limit and the mosfet is turned off. during the off time, the energy stored in t1 transfers to the secondary where it is rectifed and fltered by d4 and c11 and d5 and c9. the primary side rcd clamp (d2, c6, r1 and r2) limits the peak drain voltage spike caused by transformer leakage inductance. feedback is taken directly from the -5 v output. once the output exceeds the voltage defned by the v be of q1 and rating of vr1, q1 is biased on. this pulls current out of the en/uv pin of u1 and disables the next switching cycle. by adjusting the number of enabled to disabled cycles, output regulation is maintained. the value of r7 sets the bias current through vr1, operating the zener closer to its specifed test current. resistor r6 limits the base current of q1, r5 and c8 roll-off of high frequency gain and r3 limits the current out of the en/uv pin. diode d3 ensures the en/uv pin is clamped to > -0.3 v with respect to the source pin of u1. power integrations e-shield ? transformer construction techniques, the rcd clamp, a simple flter (c4, c5 and l1), small x-type (c1) and y-type (c2) capacitors and the frequency jitter function of the tinyswitch-iii family, provide good emi margin (see figure 3). using a one square inch metal shield, located underneath u1 and t1 (connected to the source pin of u1) can further reduce emi noise generation. ? c9 470 f 25 v c11 2200 f 10 v c12 47 f 16 v 1 1 0 8 7 3 rtn line safety earth ground neutral -5 v, 2 a -12 v, 250 ma (600 ma pk) c10 47 f 25 v c8 100 nf 50 v c7 100 nf 50 v c6 1 nf 1 kv d1 1n4007 d4 sb530 d5 sb3100 d3 1n5817 vr 1 bzx79-b4v7 4.7 v, 2% q1 pn2222a d2 1n4007gp c1 47 nf 275 vac c4 33 f 400 v c5 33 f 400 v c2 1 nf 250 vac f1 3.15 a pi-4498-112906 d s en/uv bp/m u1 tny279p tinyswitch-iii l1 1000 h l3 3.3 h t1 ef20 r3 4.7 k ? r6 5.1 k ? r4 47 ? r7 620 ? *one square inch metal shield (connected to the s pin of u1) under u1 and t1 eliminates the need for c2. **for high-line only operation (180 vac input, minimum), 10 f (400 v) capacitors can be used for c4 and c5. ** * ** r5 1 k ? r1 200 k ? 0.5 w r2 200 ?
power integrations reserves the right to make changes to its products at any time to improve reliability or manufacturability. power integrations does not assume any liability arising from the use of any device or circuit described herein. power integrations makes no warranty herein and specifically disclaims all warranties including, without limitation, the implied warranties of merchantability, fitness for a particular purpose, and non-infringement of third party rights. the products and applications illustrated herein (transformer construction and circuits external to the products) may be covered by one or more u.s. and foreign patents or potentially by pending u.s. and foreign patent applications assigned to power integrations. a complete list of power integrations' patents may be found at www.powerint.com . power integrations grants its customers a license under certain patent rights as set forth at http://www.powerint.com/ip.htm. the pi logo, topswitch , tinyswitch , linkswitch , dpa-switch , peakswitch , ecosmart , clampless , e-shield , filterfuse , stackfet , pi expert and pi facts are trademarks of power integrations, inc. other trademarks are property of their respective companies. ?copyright 2007, power integrations, for the latest updates, visit www.powerint.com di-89 DI-122 rev. a 1/07 power integrations 5245 hellyer avenue san jose, ca 95138 phone: 1-408-414-9200 apps: 1-408-414-9660 apps fax: 1-408-414-9760 for a complete listing of worldwide sales offces, please visit www.powerint.com table 2. transformer construction information. key design points ? the secondary side bias current, set by r7, should be kept low to minimize light load and no-load power consumption. ? the output voltage that is refected across the transformer (v or ) should be kept low to minimize the losses in the primary side rcd clamp. ? optimize the turns ratio of the two outputs for voltage centering. ? for good output cross regulation, design for continuous conduction mode (minimize the value of k p in the pi xls spreadsheet). ? use a schottky diode for d3 to limit en/uv pin > -0.3 v. ? as the forward drop of d3 is a function of temperature, diode type and current, use a schottky diode. do not reduce the value of r3, and verify en/uv pin absolute maximum ratings are observed at low temperatures (~ < -10 c). transformer parameters core material ef20 tdk pc40, or equivalent a lg of 142 nh/t 2 bobbin ef20, 10 pin (5 + 5) winding details shield: 23t, primary: 95t shield: 9t (triflar), C5 v: 5t, C12 v: 7t primary inductance 1.28 mh 10% figure 3. conducted emi at 230 vac, full load. figure 2. full load effciency vs. input voltage. table 1. worst case cross regulation at 85 vac. 85 115 175 145 235 205 265 ac input v oltage (v ) efficiency (%) 82 80 78 76 74 72 70 pi-4499-090606 nominal loa d pi-4500-090706 1.0 en55022b limits 0.15 10.0 70.0 -20 -10 0 10 20 30 40 50 60 80 70 mhz db v qp av qp av ? the value of r4 can be used to make small output voltage adjustments. ? the regulation point is the sum of the voltage rating of vr1, v be(q1) and the voltage across r4 ((v be(q1) /r7) r4). ? biasing the zener below its test current lowers its voltage drop, in this case to ~4.3 v. ? select transformer wire gauge sizes so that each winding layer occupies the entire bobbin width (lowers leakage inductance and improves output cross regulation). ? use option to add e-shield windings in pi transformer designer software to reduce conducted emi noise generation. ? no-load power consumption can be lowered by adding a bias winding to feed the supply current of u1 into the bp pin.
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