70824| Datasheet

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AN724
Vishay Siliconix
AN724

Designing A High-Frequency, Self-Resonant Reset Forward DC/DC For Telecom Using Si9118/9 PWM/PSM Controller
by Thong Huynh

FEATURES
Fixed Telecom Input Voltage Range: 30 V to 80 V Self-Resonant Reset 5-V Output Voltage, 5-A Max., Total 25-W Continuous Power Current Mode Control with Slope Compensation High Efficiency 500-kHz Switching Frequency Optional Pulse Skipping Mode for Light Load Efficiency Integrated Start-Up Circuitry

DESIGN CALCULATION
Circuit parameter definition: VI: Input Voltage Output Voltage VO: Output Current IO: LO: Output Inductor Power Transformer Primary Winding Number of Turns NP: Power Transformer Secondary Winding Number of Turns NS: FSW: Switching Frequency Switching Period TSW: D: Switching Duty Cycle in Percent Power Transformer Design clamp, also involves losses. The demo board for the Si9118/ 9119 features a self-resonant reset circuit which resets the magnetizing current and also recovers this magnetizing energy by charging it back to the input. The reset circuit consists of only parasitic elements and requires no additional external components. Detail operation of this circuit is beyond the scope of this application note. For this circuit to function properly, the primary inductance needs to satisfy the following equation:
1 LMp ------- CR ( V o + 0.5 ) N P 1 1 ------------------------------------- -----------------Fsw V lm N S
2

Duty Cycle Range
The Si9118/Si9119 is capable of operating at up to 80% duty cycles, leaving adequate margins for leakage inductance effects and load transient response. Its >50% duty cycle capability reduces rms current in the transformer windings and the primary circuitry, thus reducing circuit conduction losses. For most circuits, the maximum duty cycle should be fixed at around 65% at the minimum input voltage. The transformer turn ratio can be determined using the equation below:
( V O + 0.5 ) 1 NS ------ = -------------------------- --------DM V lm NP (1)

(2)

Where
NS 2 C R = C DS + C J ------ N P LMp is the primary winding magnetizing inductance VIm is the minimum input voltage CDS is the output capacitance of the main MOSFET switch CJ is the junction capacitance of the output forward rectifier CXFMR is the power transformer primary winding capacitance (3)

Where:
VIm is the minimum input voltage DM is the maximum duty cycle chosen

Example: For VO = 5 V. VIm = 30 V, DM = 0.65 NS/NP = 0.28. Choose NS = 7 and NP = 22 gives NS/NP = 0.31 which give DM = 60%.

Self-Resonant Reset
After each switching duty cycle, the magnetizing current in the power transformer has to be reset to prevent core saturation. Traditional reset circuits use either an RCD dissipative clamp or a tertiary reset winding on the transformer. This type of circuit uses extra components and, in the case of the RCD
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This equation ensures that the resonant reset circuit is fast enough to reset the core magnetizing current within the offtime of each period. The worst-case condition for reset is at low-line, light-load, and maximum switching frequency. For high-frequency operation, the power transformer core can be gapped to obtain the required LMp. Example: For CDS = 100 pF, CXFRM = 10 pF. CJ = 200 pF, NP = 22, NS = 7, VIm = 30 V, and FSW = 500 kHz, LMp must be equal or less than 559 uH.
11-Dec-98 1