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User Guide

SLUU087B - September 2001

10-Watt Flyback Converter Using the UCC3809

Power Supply Control Products

Contents

1 Introduction 2. . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . .

2 Description 5. . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . .

3 Theory of Operation 5. . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . .

4 UCC3809EVM–052 Operating Instructions 6. . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . .

5 Determine the Maximum on Time (D

MAX

) for the Primary Switch 7. . . . . . . . . . . . . . . . . . . . . . . .

6 Determine the Turns Ratio of the Flyback Inductor 7. . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . .

7 Primary Inductance 7. . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . .

8 Soft Start 8. . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . .

9 Self-Biasing Winding 8. . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . .

10 Selection of the Switching Element 8. . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . .

11 Current Sense 9. . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . .

12 Gate Drive 10. . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . .

13 Output Diode 10. . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . .

14 Output and Input Capacitors 11. . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . .

15 Loop Compensation 11. . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . .

16 Slope Compensation 13. . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . .

17 Silkscreen 13. . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . .

18 Conclusion 13. . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . .

19 References 13. . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . .

Summary of content (15 pages)

PAGE 1
User Guide SLUU087B - September 2001 10-Watt Flyback Converter Using the UCC3809 Power Supply Control Products Contents 1 Introduction . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 2 2 Description . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 5 3 Theory of Operation . . . . . . . . . . . . . . . . . . .
PAGE 2
SLUU087B 1 Introduction 1.1 Design Procedure The UCC3809EVM evaluation module is an isolated 10-W, 3.3-V converter designed for telecom input voltages, nominally –48 Vdc but ranging from –32 Vdc to –75 Vdc. The UCC3809 economy primary-side controller is used as the pulse-width modulator (PWM), combined with a TLV431 low-voltage adjustable-precision shunt regulator on the secondary side. The nominal switching frequency for this converter is approximately 400 kHz.
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SLUU087B Reference Diode Optoisolator QTY Description Manufacturer Part Number D1 1 Fast Switching, 70 V, 10 mA, SOT–23 Vishay/LiteOn Power Semiconductor (Diodes Inc.) BAV70DI D2 1 Ultra-Fast, 200 V, 1 A, SMA/DO–214AC Vishay/LiteOn Power Semiconductor (Diodes Inc.) ES1DT D3 1 SCHOTTKY, 30 V, 12 A, TO–252AA International Rectifier 12CWQ03FN D4 1 SCHOTTKY, 100 V, 1 A, SMA/DO–214AC Diodes, Inc.
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SLUU087B – C18 1 µF C2225C105K1RAC VIN = 32 – 75 V C6 5 4.7 nF R10 20 kΩ 1W – J2 . HIGH VOLTAGE – SEE EVM WARNINGS AND RESTRICTIONS R18 R8 3.3Ω 82.5 kΩ R5 10 Ω 3,4 D1 BAV70DICT J3 + C14 10 µF 6 1 D2 ES1D 2 C7 120 pF C10, C11, C12 Kemet 470µF 6.3 V T510X477(K)006AS D3 12CWQ03FN 1,3 2 T1 PB0028 J1 R11 150 W 1/4 W C8 1.5 nF R12 5.1Ω 1/4 W + VOUT= 3.3 V – 1,2 J4 D4 B1100–13 3 2 R24 22.1Ω 8 C13 47 pF 7 C19 1 µF C5 10µF 25 V Q3 IRFR220 D–Pak TO–252AA R6 1.1 kΩ R7 0.
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SLUU087B 2 Description A brief description of the circuit elements follows: D Transformer (a.k.a. flyback inductor) T1, transistor Q3, Schottky diode D3, input capacitor C18, and output capacitors C10 through C12 form the power stage of the converter. Power resistor R7 senses the primary-switch current and converts this current into a voltage to be sensed by the primary-side controller feed-back comparator. D Capacitor C14 filters out high-frequency noise on the output bus directly at the output diode.
PAGE 6
SLUU087B Peak current mode control responds immediately to line voltage changes and also provides over current protection to the switching device. A discontinuous current-mode controlled flyback converter does not have a right half plane zero and does not have the subharmonic-loop instability problems usually seen in the large duty–ratio continuous–inductor current topologies.
PAGE 7
SLUU087B 5 Determine the Maximum on Time (DMAX) for the Primary Switch Maximum duty cycle occurs at minimum input voltage and maximum load. Programming DMAX clamps the maximum on–time of the switch. This maximum on–time should incorporate enough of a margin so that control is well maintained at minimum input voltage. The UCC3809 has a programmable maximum duty-cycle clamp up to 70%.
PAGE 8
SLUU087B GAP + mO mR NP 2 AE 10 *2 LP where µo is the permeability of free space equal to 4π×10–7 H/m and µR is the relative permeability of free space, which is air, equal to 1. AE is the effective core area measured in cm2 . 8 Soft Start Soft start is used to reduce transformer and output capacitor stress and to reduce the surge on the input circuits when the converter action starts.
PAGE 9
SLUU087B The switch must be able to conduct the repetitive peak-primary current as determined by: I PEAK(primary) + ǒVIN(min) * VDSǓ t ON(max) LP The primary-current waveform of a discontinuous mode flyback converter is triangular in shape, therefore, its root mean square (rms) current is calculated by: I PRMS + Ǹ t ON(max) I PEAK(primary) Ǹ3 T The chosen device should also have a low RDS(on) value because the conduction losses of the device are proportional to the square of the primary rms curr
PAGE 10
SLUU087B 12 Gate Drive At 15 volts gate to source voltage the IRFR220 has a total gate charge of approximately 14 nC. The UCC3809 is capable of sourcing 400-mA of peak-drive current which would result in a turn-on time of 35 ns. To limit the peak current through the IC, an external resistor is placed between the totem-pole output of the IC and the gate of the MOSFET.
PAGE 11
SLUU087B 14 Output and Input Capacitors Output capacitors are selected based upon their capacitance value, equivalent series resistance (ESR), equivalent series inductance (ESL), and capacitor ripple current rating. The capacitance value controls the peak to peak output ripple voltage at the switching frequency.
PAGE 12
SLUU087B where COUT is the total capacitance of the output capacitor bank and ESR is the parallel combination of the output capacitor’s equivalent series resistance. The power stage contributes a pole located at: F PWRstg(pole) + 2 2 R OUT p C OUT Note that this pole is load dependent.
PAGE 13
SLUU087B 16 Slope Compensation All peak current-mode converters benefit from slope compensation, which is added to the current-sense signal, to cancel the error introduced by sensing peak current instead of average current. The amount of inductor down slope added to the FB pin was designed to be approximately 0.5 and the emitter follower configuration was used. The step-by-step calculations are beyond the scope of this design note.
PAGE 14
SLUU087B DYNAMIC WARNINGS AND RESTRICTIONS It is important to operate this EVM within the input voltage range of 32.0 V to 75.0 V, and the output voltage range of 3.218 V to 3.383 V, and the load current range of 0.0 A to 3.0 A. Exceeding the specified input range may cause unexpected operation and/or irreversible damage to the EVM. If there are questions concerning the input range, please contact a TI field representative prior to connecting the input power.
PAGE 15
IMPORTANT NOTICE Texas Instruments Incorporated and its subsidiaries (TI) reserve the right to make corrections, modifications, enhancements, improvements, and other changes to its products and services at any time and to discontinue any product or service without notice. Customers should obtain the latest relevant information before placing orders and should verify that such information is current and complete.