AN1680 MOTOROLA | Alldatasheet

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2 Motorola Applications Data

Figure 2. A Simple FLYBACK Configuration current needs to be higher than theoretically calculated to deliver the full rated output power. MC3337X family, the maximum voltage shall stay within 700 V. Worse case arises when the mains is a its highest level, e.g.

4 Motorola Applications Data

, with F the switching frequency and Vz the nominal zener level. Figure 3. Clipping with a Zener is Possible Solution Figure 3b. Clipping Waveforms with a Zener Diode overshoot upon the drain. Measurements have to be carried upon the final board to confirm the safety of the final drain level.

6 Motorola Applications Data

that the amount of charge Q will equally split between Rclamp and Cclamp at turn–off we can write: Vripple . C clamp = IRclamp. T. 100 nF. With a 118 µH leakage inductance and a 13.8 turn ratio, the normal operating clamping voltage was measured at 238 V. Figure 5. Turn–on Sequence Showing Vclamp Running

/C0065/C0078/C0049/C0054/C0056/C0048

8 Motorola Applications Data

Depending on the dVDS /dt slope and maximum reverse voltage conditions, the following MOTOROLA references can be used: Reference VRRM Ton (typical) IF max MUR160 600 V 50 ns 3 A MUR100E 1000 V 25 ns 3 A 1N4937 600 V 200 ns 1 A MSR860* 600 V 100 ns 8 A MSRB860–1* 600 V 100 ns 8 A * soft recovery diodes Parameter Evolutions with Temperature Once protected, the equipment must survive in its operating environment. That is to say, despite the component’s key specs (e.g. Tfr for the series diode) variations with the ambient temperature, they must not jeopardize the SMPS operation. The below lines give an idea on the way these parameters move with temperature: MC3337X’s MOSFET BVDSS: positive temperature coefficient MC3337X’s maximum current limit: +1.03% at TJ = 125°C MC3337X’s oscillator frequency: 115 kHz at max TJ Zener breakdown voltage: positive temperature coefficient Diode reverse recovery time: positive temperature coefficient Conclusion In FLYBACK converters, leakage inductance always stresses the switching elements. An efficient active or passive protection network is therefore mandatory to ensure the SMPS will survive in any operating conditions. This paper details some possibilities on how to implement the adequate level of protection. Data–sheets can be downloaded at: http://mot–sps.com/books/current.html Spice models of diodes and MC3337X: http://mot–sps.com/models/html/models.html Motorola reserves the right to make changes without further notice to any products herein. Motorola makes no warranty, representation or guarantee regarding the suitability of its products for any particular purpose, nor does Motorola 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 consequential or incidental damages. “Typical” parameters which may be provided in Motorola 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. Motorola does not convey any license under its patent rights nor the rights of others. Motorola products are not designed, intended, or authorized for use as components in systems intended for surgical implant into the body, or other applications intended to support or sustain life, or for any other application in which the failure of the Motorola product could create a situation where personal injury or death may occur. Should Buyer purchase or use Motorola products for any such unintended or unauthorized application, Buyer shall indemnify and hold Motorola 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 Motorola was negligent regarding the design or manufacture of the part. Motorola and are registered trademarks of Motorola, Inc. Motorola, Inc. is an Equal Opportunity/Affirmative Action Employer. Mfax is a trademark of Motorola, Inc. How to reach us: USA / EUROPE / Locations Not Listed: Motorola Literature Distribution;JAPAN : Motorola Japan Ltd.; SPD, Strategic Planning Office, 141, P.O. Box 5405, Denver, Colorado 80217. 1–303–675–2140 or 1–800–441–2447 4–32–1 Nishi–Gotanda, Shinagawa–ku, Tokyo, Japan. 81–3–5487–8488 Customer Focus Center: 1–800–521–6274 Motorola Fax Back System – US & Canada ONLY 1–800–774–1848 2, Dai King Street, Tai Po Industrial Estate, Tai Po, N.T., Hong Kong. – http://sps.motorola.com/mfax/ 852–26629298 HOME PAGE : http://motorola.com/sps/ AN1680/D◊