C4520X7R3D102K

ISL6844 INTERSIL | Alldatasheet

Document overview

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Technical content

• A e = 4.3mm2
• A L = 35nH/n2
• l e = 13.1mm
• V e = 56.5mm3 This section provides general guideline to calculate the number of turn and wire size. For more details on designing transformer parameters, please contact a Pulse representative. The number of turns on the primary side, Np, can be determined from: Therefore, the primary side has 26 turns. With the turn ratio of 1, the secondary side and the auxiliary primary side also have 26 turns. Next the calculate the maximum flux density to make sure that it is below the saturation limit.Where: For the operating power level, the wire sizes of the primary, secondary, and auxiliary windings are selected such that the current density in each winding is about 0.25335 cm2/A (50 circular mil/A). To simplify transformer winding, AWG#37 is used for all primary, secondary and auxiliary windings. Primary MOSFET Selection The primary MOSFET needs to be able to handle the voltage stress, given by: As a good design practice, some margin is provided to this peak stress voltage to accommodate transient spikes and for a good reliable performance over time. Providing a 30% design margin as a rule of thumb, the minimum rating on the primary MOSFET needs to be 54.6V.

FIGURE 2. TYPICAL OPERATIONAL CURRENT WAVEFORMS

3 AN1612.1 November 28, 2011 The RMS current through the MOSFET can be calculated from: Selecting the conduction loss in the MOSFET to 1% of total output power, 0.03W. The required MOSFET’s rDS(ON) to achieve the required conduction loss is shown in Equation 10. Vishay’s SI4436DY is selected in this design. Output Diode Selection Schottky diodes are recommended for the output diode due to their low forward voltage drop. The voltage stress across the output diode can calculated by: Diodes Inc’s B180 are employed in this design. Output Filter The output capacitance needs to meet the ripple and noise requirements, and also be able to handle the ripple current. Assuming ceramic capacitors are used as the output filter, the voltage ripple from the capacitor’s ESR is negligible. The minimum capacitance required to meet specifications can be approximately calculated from Equation 12. 10µF ceramic capacitors are selected for each output. Design margin has been provided to account for noise spikes. Snubber Circuit When the MOSFET switches off, it interrupts the current that flows through the transformer leakage inductance. An RCD snubber circuit is typically used in flyback converters to clamp voltage spikes on the MOSFET. Assuming that the transformer leakage inductance is 2% of the magnitizing inductance, the energy stored in the leakage inductance during MOSFET’s on-time is: Average power transferred to the snubber circuit is: To limit peak voltage spikes across the MOSFET to 50V, the snubber voltage is set to: The average power transferred to the snubber circuit in Equation 14 is dissipated by the snuuber resistor, so RS is determined by: So RS = 10kΩ is selected. Cs is selected such that the RSCS time constant is substantially longer than the switching period to keep low ripple voltage on the snubber circuit. A time constant of 10 times the switching period is used for calculation: CS = 3.33nF is used in the design. Feedback Network The feedback is being tapped off of the primary auxiliary winding. This is one of the advantages of selecting the flyback topology, since the auxiliary winding voltage follows the output. This scheme was fully exploited, since the load fluctuation is minimal, and that load regulation does not suffer much at these power levels. For tighter regulation requirements, an opto-coupled solution would need to be used, which leads to additional cost. Referring to the schematic on page 8, the output voltage can be set by: R 23 = 1kΩ and R22 = 5.23kΩ are selected. The control-to-output transfer function of the DCM flyback converter is [1]: Where: RE = Equivalent load resistor reflected to the auxiliary output. Irms FET, Ipk d 3---⋅= (EQ. 9) 1.06 0.35 rDS ON() PFET cond loss–, IFET rms, 0.03 VDiode nV×= IN MAX, VOUT+ COUT ΔVPP 1d 2–() TSW⋅ IOUT (EQ. 12)0.42μF> 50 3–×10 0.1 300 3×10⋅ WL 2--- LL ILM 2⋅⋅= (EQ. 13)1 PL WL FSW⋅= (EQ. 14) VS peakV MOSFET VIN MIN,–= (EQ. 15) 50 21.6 – 28.4V== RS VS Pl (EQ. 16) 28.42 CS 10 TSW RS (EQ. 17) 10= 3.33 6–×10 10 3×10 R22 R23 VOUT VF+ Vref (EQ. 18)15 0.6+ Gvc K RE LM FSW⋅⋅

CE = Equivalent capacitor reflected to the auxiliary output. ESR = Equivalent series resistance of the output capacitor. auxiliary output delivers all of the output power. VC(MAX) has value of 1.1V, clamped by ISL6844’s internal circuit. Along with the result from Equation 22, K has a value of 0.97. 24 is selected to be 16.2kΩ. FIGURE 3. GAIN OF G VC

FIGURE 12. OVER CURRENT RESPONSE

8 AN1612.1 November 28, 2011 Schematic POWER INPUT 24V DNP VREF RS 10k Vin- 150pF 1 2 0.82ohm 1 2 C13 1nF/2kv 1 2 CS 3.3nF +15V B180 R22 10k 1 2 ISL6844 RTCT 4 CS 3 GND5 OUT6 COMP 1 FB 2VDD7 VREF8 10uF 1 2 1uF 1 2 B180 2 1 C18 0.1uF 1 2 C12 220pF R11 5.1 SI4436DY 100 1 2 C24 10uF 1 2 R14 6.49k R24 16.2k 12 -15V C11 10uF1 2 Vin+ BAT46W 2 1 R25 4.02k1 2 RTN C10 68pF BAT46W 10k 1 2 R15 C23 10uF 1 2 BAT54 1uF 1 2 12.1k1 2 BZT52C18 R10 5.1 2.7nF RTN C17 0.1uF 1 2 C16 0.1uF 1 2 1 2 R23 1 2 R26 4.02k1 2 C14 0.1uF 1 2 C15 2.2uF1 2

9 AN1612.1 November 28, 2011 Bill of Materials REF DES QTY PART NUMBER DESCRIPTION PACKAGE VENDOR U1 1 ISL6844IUZ IC, PWM Controller MSOP-8 Intersil Q2 1 SI4436DY MOSFET, N-channel, 60V SOP-8 Vishay D1 1 BAT54WS Schottky Diode, 30V SOD323F Diodes Inc. D2, D3 2 BAT46W Schottky Diode, 100V SOD123 Diodes Inc. D4, D5 2 B180 Schottky Diode, 80V, 1A SMA Diodes Inc. D7 DNP BZT52C18 Zener Diode 18V SMA Diodes Inc. T1 1 PA3374NL Transformer, Custom Pulse C3 1 C5750X7R1H106K Capacitor, ceramic, X7R, 10µF, 20%, 50V SM_2210 Generic C4 1 Capacitor, ceramic, X7R, 1.0µF, 20%, 50V SM_0805 Generic C6 1 Capacitor, ceramic, X5R, 150pF, 20%, 50V SM_0603 Generic C8 1 Capacitor, ceramic, X5R, 1.0µF, 20%, 25V SM_0805 Generic C9 1 Capacitor, ceramic, X5R, 2.7nF, 20%, 50V SM_0603 Generic C10 1 Capacitor, ceramic, X5R, 68pF, 20%, 50V SM_0603 Generic C11 1 Capacitor, ceramic, X5R, 10µF, 20%, 25V SM_1206 Generic C12 1 Capacitor, ceramic, X7R, 220pF, 20%, 50V SM_0603 Generic C13 1 C4520X7R3D102K Capacitor, ceramic, X7R, 1000pF, 10%, 2kV SM_1808 TDK C14, C16, C17, C18

4 Capacitor, ceramic, X7R, 100nF, 10%,

SM_0603 Generic CS 1 Capacitor, ceramic, X7R, 3.3nF, 20%, 50V SM_0805 Generic C15 1 Capacitor, ceramic, X7R, 2.2µF, 20%, 10V SM_0603 Generic C23, C24 2 Capacitor, ceramic, X5R, 10µF, 10%, 25V SM_1812 Generic R1 1 Resistor, 0.82 Ω, 1%, 1/4W SM_1206 Generic R3 1 Resistor, 12.1k Ω, 1%, 1/16W SM_0603 Generic R4, R22 2 Resistor, 10k Ω, 5%, 1/16W SM_0603 Generic R5 1 Resistor, 1k Ω, 1%, 1/16W SM_0603 Generic R6 1 Resistor, 100 Ω, 1%, 1/16W SM_0603 Generic R10, R11 2 Resistor, 5.1 Ω, 1%, 1/16W SM_0603 Generic R14 1 Resistor, 6.49k Ω, 1%, 1/16W SM_0603 Generic R15 1 Resistor, 10 Ω, 1%, 1/16W SM_0603 Generic RS 1 Resistor, 10k Ω, 5%, 1/4W SM_1206 Generic R23 1 Resistor, 2k Ω, 1%, 1/16W SM_0603 Generic R24 1 Resistor, 16.2k Ω, 5%, 1/16W SM_0603 Generic R25, R26 2 Resistor, 4.02k Ω, 5%, 1/16W SM_0603 Generic