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Very Wide Input Voltage Range, Off-Line Flyback Switching Power Supply Prepared by: Marty Brown http://onsemi.com APPLICATION

AN1327/D AN1327/D Very Wide Input Voltage Range, Off-Line Flyback Switching Power Supply Prepared by: Marty Brown http://onsemi.com APPLICATION NOTE One of the many problems besetting the power supply designer today is being able to design a switching power supply that is able to operate in all the power systems within their international marketplaces. Forward-mode switching power supplies typically operate over a single power system's range of voltage, that is, 90 to 130 VAC or 200 to 270 VAC. Boost-mode converters can just make the range of 90 to 270 VAC. Any higher input voltages would then require a different design. This leads companies to create products targeted at specific marketplaces, which can be costly, or to have their customers arrange jumpers to accommodate their power system which can be annoying or lead to costly errors. Added to this are those industrial companies which may not only have their products reside on residential power systems but also have the varied international industrial power systems. This means that a single product family might have to operate from an input voltage of 90 to 600 VAC, well beyond the residential limits of 90 to 270 VAC. This paper reviews one method of enabling a discontinuous-mode flyback converter to operate beyond its traditional range of input voltage of 3:1 to a range of more than 6.6:1 without affecting the reliability of its operation. This is done by changing its mode of operation and the use of recently available power MOSFETs with breakdown voltage ratings of 1,200 V. Figure 1. The Wide Input Range Flyback Power Supply Demonstration Board This document may contain references to devices which are no longer offered. Please contact your ON Semiconductor representative for information on possible replacement devices. © Semiconductor Components Industries, LLC, 2002 November, 2002 - Rev. 1 1 Publication Order Number: AN1327/D AN1327/D AN1327/D AN1327/D A Summary of the Operation of Fixed Frequency Flyback Converters The most common topology for those applications less than 150 W has been the fixed frequency, current-mode controlled, flyback converter. Its block diagram can be seen in Figure 2. +Vin(DC) FLYBACK TRANSFORMER STARTUP CKT OUTPUT RECTIFIER AND FILTER STAGE OUTPUT VOLTAGE OSCILLATOR VOLTAGE REF CLOCK + PEAK CURRENT COMPARATOR - S POWER SWITCH Q R VOLTAGE FEEDBACK CIRCUIT OUTPUT DRIVER ERROR AMP LEADING EDGE SPIKE SUPPRESSOR CURRENT SENSE ELEMENT FEEDBACK VOLTAGE INPUT RETURN Figure 2. Block Diagram of a Fixed Frequency Current-Mode, Flyback Converter Here a fixed frequency oscillator initiates a power switch conduction period which is terminated by either the current within the power switch reaching a predetermined limit as set by the error amplifier or the oscillator terminating the period and initiating the next power switch conduction period. A representative flyback converter can be seen in Figure 3. T V ·T ipk + in on Lpri Vout Vin (eq. 1) The slope of the current ramp is Vin/Lpri. The flyback topology, as with all boost-mode converters, operate under the principle of storing energy within the core material of the transformer. The energy stored during each conduction period is given by: D + - The power switch essentially places the primary inductance of the flyback transformer across the input voltage source when it is turned on. The secondary is disconnected because the output rectifier (D) is reverse biased. The primary winding's current takes the form of a linear ramp starting from zero amps and whose peak value is given by: Q CONTROL Esto + Ipri 2 Lpri · i pk 2 (eq. 2) To meet the short-term steady-state power demands of the load(s), the following relationship must be met: Figure 3. A Simplified Schematic of a Flyback Converter Pout v http://onsemi.com 2 2 fop · Lpri · i pk 2 (eq. 3) AN1327/D AN1327/D power MOSFET is used as the power switch in most modern flyback power supplies. At high input voltages, the on-time of the power switch becomes so short (300 - 600 nS) that the output driver cannot source enough instantaneous current to drive the MOSFET into a saturated condition before turning it back off. The effect is the power switch operates in the linear conduction mode during these short "on'' pulses. This causes a drastic drop in power switch operating efficiency and jeopardizes the power supply's reliability. In reality, for any one output power, the current-mode controller strives to maintain a constant value for Ipk over the entire range of input voltages as visualized in Figure 4. PRIMARY CURRENT HI INPUT VOLTAGE LOW INPUT VOLTAGE Ipk The New Method of Control By a very simple modification to the traditional fixed frequency current-mode controlled flyback converter design, one can greatly extend its operational input voltage range. The modifications make the control method one of variable on-time, and variable frequency. Figure 5 illustrates the newly added and redefined functional blocks of this new method of control. TIME Figure 4. Peak Currents at Differing Input Voltages The shortcoming arises in the output drivers of the typical current-mode control IC and the power switch. Typically a +Vin(DC) FLYBACK TRANSFORMER STARTUP CKT VOLTAGE TRANSLATOR VOLTAGE REF - OUTPUT VOLTAGE CONTROL PEAK CURRENT DETECTOR + OUTPUT RECTIFIER AND FILTER STAGE VCO CLOCK S POWER SWITCH Q R VOLTAGE FEEDBACK CIRCUIT OUTPUT DRIVER ERROR AMP CURRENT RAMP TIME DELAY CURRENT SENSE ELEMENT FEEDBACK VOLTAGE INPUT RETURN Figure 5. Block Diagram of the Wide Input Range, Flyback Converter A VCO (voltage-controlled oscillator) is created by removing the timing capacitor's charging circuit from a fixed voltage or current source and placing it under the control of the error voltage. In the design example shown later, it means simply removing the timing resistor from the voltage reference and wiring it to a variable voltage created by the output of the error amplifier. A voltage translator is placed between the output of the error amplifier and the control input to the VCO. It consists of a simple biased 3.3 V zener diode so that the error amplifier may make use of its entire output voltage swing. The other new block is really a redefinition of an old familiar function - the leading edge spike filter from the current sensing element. Here, the formerly annoying parasitic of time lag serves an important function within the control algorithm. It now delays the actual current ramp prior to being sensed by the control IC. It allows the actual peak current to increase with increasing input voltages while the controller sees a lowering peak current needed by this control strategy. This will be discussed later. http://onsemi.com 3 AN1327/D AN1327/D fixed-frequency flyback converter. This will allow us to determine the appropriate value for the primary inductance. In the sample design, the frequency of operation at the highest input voltage will drop to one-half from that at the lowest input voltage. Equation 4 then dictates: The ultimate goal of the new control methodology is to force the on-time of the power switch to be greater than this minimum effective on-time over the power supply's entire line/load operating range. Its operation can be best understood by examining equation 3. The error amplifier/VCO section of the circuit lowers the operating frequency as the input voltage is increased. This requires the energy stored per conduction period to increase to meet the short-term power requirement of the output. This is done by extending the on-time of the power switch. If the key component parameters such as maximum operating flux density (Bmax) of the transformer, the avalanche ratings of the diodes and power switch, and the current ratings of the output rectifiers are adequate, then no degradation in the reliable operation of the supply is experienced. Its operation can be better defined by rearranging equation 3 and neglecting any power loss due to the inefficiency of the supply one gets: ipk + out Lpri2·Pf(f(Ve) ipk(hi) [ 2 ipk(lo) If the desired maximum operating flux density (Bmax) is one-half the core material's saturation flux density at 100°C and at the high input line, then the operating flux density at the low input voltage should be: Bmax(lo) [ 2 Pout · Lpri f(f(Ve) Bsat(min) 2 2 (eq. 7) For most common ferrite materials such as 3C8, N27, or F, the operating flux density at low line will be at approximately 1,300 gauss. The Bmax at the high input voltage will be no more than one-half the saturation flux density at 100°C. The inductance can now be calculated by using equation 1 at the low input voltage, and an air gap calculated using any one of the common methods. It is important to determine the secondary inductance such that the core's energy can be emptied as close to 50 percent duty-cycle (1/fop(hi) as possible. This will minimize the RMS currents to their lowest possible point over the entire operating range. The output peak current at any operating point is described as: (eq. 4) where f(f(Ve) is the controlled frequency of the power supply. As one can see, the peak current is inversely proportional to the square root of the frequency of operation, since all the other terms are fixed in the short-term operation and by the circuit design. By substituting equation 1 into equation 4 one further gets: ton + 1 · Vin (eq. 6) ipk(out) [ 2 · Iout(av) · Tdisch · fop (eq. 5) (eq. 8) This would describe both the peak currents flowing through the output rectifiers and the peak ripple currents flowing into and out of the output filter capacitors. Within the sample design with one ampere rated outputs, at low line the peak-to-peak rectifier currents would be four times the average output current. At the high line, the peak-to-peak currents would be eight times the average output current for the rated output current. There are more unknowns than there are independent equations, but at the low input line voltage and at the rated output load, one can solve equation 5. The input voltage is known to be 125 VDC (90 VAC), the frequency will be at its highest point as designated by the designer, the on-time will be one-half of the entire operating period and the peak current will be calculated as it is in a common http://onsemi.com 4 AN1327/D AN1327/D The Wide-Range Flyback Converter Demonstration Board The wide-input range off-line flyback converter described has the following maximum and performance ratings. Output Power: 17 Watts Outputs: +5.0 Vdc @ 1.0 Amp Max +12 Vdc @ 1.0 Amp Max Input Voltage Range: 90 VRMS - 600 VRMS Maximum Input Voltage: 675 VRMS J1 D1 C2 L1 R1 R2 R9 D3 + H2 C1 + D4 C3 C4 C5 R5 R16 D5 U1 R12 C7 C8 D10 R6 + T1 C11 D8 D9 + C12 Q1 G C10 WIDE INPUT RANGE FLYBACK POWER SUPPLY ITC109B ITC109B R7 C13 1 D6 R10 R8 C6 C9 CAUTION HIGH VOLTAGES INDUSTRIAL TECHNOLOGY CENTER R11 R4 D2 H1 GND R3 C16 R13 R14 R15 + C17 R17 C15 D7 Figure 6. Printed Circuit Board Layout http://onsemi.com 5 C14 + + R18 R19 J2 R20 U2 + U3 O/P + 12V + 5V R21 GND AN1327/D AN1327/D Table 1. Parts List Designator Quantity Value/Rating C1, C4 2 0.1 mF, 1.0 kV Capacitor, Ceramic Description C2, C3 2 0.0047 mF, 3.0 kV Capacitor, Ceramic C5, C6 2 100 mF, 450 V Capacitor, Electrolytic C7 1 220 pF, 50 V Capacitor, Ceramic C8 1 1000 pF, 50 kV Capacitor, Ceramic C9 1 1000 pF, 1.0 kV Capacitor, Ceramic C10 1 10 mF, 25 V Capacitor, Tantalum C11, C12 2 100 mF, 20 V Capacitor, Tantalum C13, 14 2 100 mF, 10 V Capacitor, Tantalum C15 1 1500 pF, 50 V Capacitor, Ceramic C16 1 1.3 mF, 50 V Capacitor, Mylar C17 1 0.22 mF, 35 V Capacitor, Ceramic D1-D4 4 1.0 A, 1.0 kV Diode, 1N4007 1N4007 D5 1 3.3 V, 500 mW Zener Diode, 1N5226B 1N5226B D6 1 1.0 A, 1.0 kV Diode, UF, MUR1100E MUR1100E D7 1 4.0 A, 300 V Diode, UF, MUR430E MUR430E D8 1 3.0 A, 70 V Diode, Schottky, MBR370 MBR370 Diode, UF, MUR130E MUR130E D9 1 1.0 A, 300 V D10 1 - Diode, SIGNAL, 1N4148 1N4148 J1 1 - Connector J2 1 - Connector L1 1 - Coil Craft P/N E3493 E3493 Q1 1 3.0 A, 1.2 kV MOSFET, MTB3N120E MTB3N120E R1-R4 4 470 kW, 1/2 W Resistor R5-R9 5 82 kW, 1/2 W Resistor R10 1 1.8 kW, 1/4 W Resistor R11 1 27 kW, 1/4 W Resistor R12 1 10 W, 1/4 W Resistor R13 1 1.0 kW, 1/4 W Resistor R14 1 1.2 W, 1/2 W Resistor R15 1 680 W, 1/4 W Resistor R16 1 100 kW, 1/2 W Resistor R17 1 7.5 kW, 1/4 W Resistor R18 1 3.57 kW, 1/4 W Resistor R19 1 32.4 kW, 1/4 W Resistor R20 1 120 W, 1/4 W Resistor R21 1 2.49 kW, 1/4 W Resistor T1 1 Transformer U1 1 - IC, UC3845B UC3845B U2 1 - Optoisolator, MOC8102 MOC8102 U3 1 - IC, TL431CLP TL431CLP http://onsemi.com 6 Cramer, CSM, 3015-027 AN1327/D AN1327/D L1 H1 C1 90 VAC- 0.1 600 VAC 1 kV D1-D4 1N4007s C4 0.1 1 kV L1 +Vin H2 C3 0.0047 3 kV EARTH GND C2 0.0047 3 kV R4 R3 R2 + C5 100 mF 450 V + 470 k 1/2 W 470 k 1/2 W R1 C6 100 mF 450 V 470 k 1/2 W 470 k 1/2 W INPUT GND Figure 7. AC Input/Filter Circuit Section T1 100 mF 20 V C11 D8 100 mF MBR370 MBR370 10 V +Vin R9 82 k, 1/2 W R8 D9 MUR430 MUR430 R7 R6 R5 R16 Vaux 100 k 1/2 W 10 mF 25 V D10 + LL R11 1.8 k D5 3.3 V 6 4 1 C7 220 pF C9 C13 MUR130 MUR130 MUR1100 MUR1100 D6 D7 7 UC3845BN UC3845BN R10 27 k 1 nF 3 kV 2 U2 1/2 MOC8102 MOC8102 5 3 MTB3N120E MTB3N120E R12 10 W + C8 1000 pF C14 Vaux R14 1.2 W 1/2 W INPUT GND Figure 8. DC/DC Converter Circuit Section 7 C12 U2 MOC8102 MOC8102 C17 2.2 nF Q1 http://onsemi.com +12V + +5V + R13 1k R15 680 W + R20 120 W C15 1.5 nF R19 32.4 k 1.3 mF 7.5 k U3 TL431 TL431 C16 R17 R21 2.49 k GND AN1327/D AN1327/D Design of the Wide Input Range Flyback Converter The number of turns needed to produce the required primary inductance is: Predesign Considerations Npri + 1000 Output Power: Po = (5.0 V)(1.0 A) + (12 V)(1.0 A) = 17 Watts DC Input Voltages: Vin(low) = 1.414 · Vin-ac(low) = 1.414(90 VAC) = 127 VDC Vin(hi) = 1.414 · Vin(hi) = 1.414(600 VAC) = 854 VDC Maximum Average Input: Current: Iin-av(max) = Pout/(eff · Vin(min) = (17 W)/(0.8)(127 VDC) = 167 mA + 1000 N sec + N() 5) + (eq. 10) The minimum length of the airgap for the core is then: + Co [ f 2 8 0.4 p Lpri i pk10 ACB2 max (eq. 14) Sizing the Output Filter Capacitors Since this is a variable frequency system, all the calculations for the value of the output filter capacitors will be done at the lowest frequency since the ripple voltage will be greatest at this frequency. Since capacitor values are determined by the output current, and both the +5.0 V and the +12 V outputs have the same maximum output current rating, their capacitance values will be equal. (eq. 9) Using equation 6, one gets the maximum operating flux density at the low input voltage of: Ig + (5 V ) 0.5 V)(8T) + 4 turns (12.9 V) The amount of error between the actual transformer output voltages and the required output voltages are: +5 V: +5.0 V +12 V: +10.1 V after the rectifier drop. Add one turn (9 turns) to the +12 V outputs. The resulting output voltage, including the rectifier drop is 11.5 volts. The physical winding of the transformer is extremely important (refer to Figure 9). First, there are the creepage requirements (space between windings over surface) of the safety agencies. Secondly, with 850 volts across the primary winding at the high input voltage, the interlayer voltage could cause arcing between layers of the primary winding. A layer of Mylar tape must be placed between adjacent layers of the primary. The final transformer construction is given below. 0.5(127 V) + 553 mH (0.82 A)(140 kHz) Bmax(lo) [ 3500 G + 1237 G 2 2 (eq. 13) The auxiliary winding to power the control IC is also +12 V, so it will have the same number of turns. The number of turns needed for the +5.0 volt winding is: Designing the Transformer After reviewing the core sizing information provided by the various core manufacturers, it is decided that an E-E core of about 1.2 inches (30.5 mm) on a side will adequately fit the windings and insulation needed by this application. This corresponds to a Magnetics Inc. part number 43007-EC 43007-EC core (or Philips 782E272 782E272 (E 30). The core material should be a Magnetics P, F or N material or Philips 3C85 or 3F3 material. A Magnetics part number F-43007-EC F-43007-EC will be used. Calculating the primary inductance needed for this application: + (Vout ) Vfwd)(1 * max) · Npri max · Vin(min) (12 V ) 0.9 V)(0.5)(74T) + + 7.5 turns, make 8 turns (0.5)(127 VDC) Nominal Peak Current: Ipk = 5.5 · Pout/Vin(min) = 5.5(17 W)/127 V = 0.74 Amps The desired maximum frequency of operation is 140 kHz. max Vin(min) Ipkf max (eq. 12) 0.553 mH + 74.4 turns, make 74 turns 100 mH The number of turns needed by the +12 V secondary, and assuming ultrafast recovery rectifier is: NOTE: The primary winding's AWG should be #30 AWG. Lpri + LApri L Io(max) est(min) · Vripple(max) (eq. 15) (1 A) [ + 142 mF (70 kHz)(0.1 V) (eq. 11) Make the output capacitors two 100 mF capacitors placed in parallel for each output (C11 and C13, C13 and C14). 0.4 p (553 mH)(0.82 A)2 108 + 0.046 cm or 18 mils (0.6 cm2)(1300 G)2 An airgap that produces an AL of 100 mH/1000T is larger than this, so that is what is used. http://onsemi.com 8 AN1327/D AN1327/D Designing the Voltage Feedback Section The internal error amplifier in the UC3845 UC3845 (U1) will not be used. The inverting input pin should be grounded to ensure that the output will be always high. The error amplifier function will be provided by a TL431 TL431 (U3) on the secondary being connected to the primary side via an optoisolator, the MOC8102 MOC8102 (U2). The collector of the MOC8102 MOC8102 optoisolator represents the key control node for power supply. The value of the voltage at this node sets both the frequency of operation and the peak current flowing through the power switch during each cycle. The collector of the U2 will be connected to the compensation pin of the UC3845 UC3845 which will directly set the peak current. Then a 3.3 volt zener diode (D5) will elevate this voltage to a higher voltage to set the frequency of the voltage controlled oscillator. Choosing the maximum current from the output of the MOC8102 MOC8102 optoisolator to be 5.0 mA, an external resistor (R11) from the VCC of the IC to the VCO input is needed. Its value is set when the MOC8102 MOC8102's output is at saturation and is: R11 + (12 V * 3.3 V)5 mA + 1740 ohms, make 1.8 k (eq. 16) The MOC8102 MOC8102 has a Ctrr of 100 percent. That makes the LED current 6.0 mA (5.0 mA from R10, 1.0 mA from pin 1 of U1). A margin of 30 percent should be added for variations in the gain of the optoisolator. That would make the LED current 8.0 mA. The value of the current limiting resistor for the optoisolator LED (R20) is: R20 + [5 V * (VU3 ) VLED)]8 mA + 138 ohms, make 120 k (eq. 17) BOARDER TAPE 5 mm +5 V AND +12 V WINDINGS (SELF RATED) 3 LAYERS TAPE EA. +12 V WINDING (Vaux) KAPTON INSULATED 1 LAYER TAPE PRIMARY BOARDER TAPE 4 mm Figure 9. Construction of the Transformer The lower resistor of the voltage sensing network (R21) is set by assuming a sense current. One milliamp yields 1.0 k ohm per volt, which is easy. So: R21 + VrefIsense + 2.5 V1 mA be at its highest and the duty cycle will be 50 percent. The VCO control node will be at its highest linear value which is 7.7 volts. One starts by selecting the size of the timing capacitor (Ct, C7). This is done by referring to UC3845 UC3845 data sheet, Figure 2 "deadtime vs. frequency.'' It is desired that the deadtime be a minimum, since the UC3845 UC3845 is already 50 percent duty cycle limited. At an oscillator frequency of 280 kHz, which is divided by 2 for an operating frequency of 140 kHz, the largest capacitor that yields the least deadtime is approximately 220 pF. Using Figure 1 from the UC3845 UC3845 data sheet "Timing Resistor vs. Oscillator Frequency'' and knowing the VCO control voltage will be 2.2 V higher than the +5.0 V reference assumed by the chart, one can "scale'' Figure 1 so that the same charging current is flowing through the timing resistor (R10) but from the higher voltage source. So by multiplying the ratio of 7.7 V divided by 5.0 V by the value of the resultant resistor value from Figure 1, one gets the approximate final resistor value. Figure 1 results in a value of 18 k ohms for the timing resistor for a timing capacitor of 220 pF. The final value of R10 is then: (eq. 18) + 2.5 k make 2.49 k 1% Splitting the output voltage sensing between more than one output will improve the cross regulation of all the outputs. The +5.0 V output is usually connected to MCUs which are voltage sensitive. Usually, the loads connected to the +12 V output are less susceptible to voltage variations. Select the proportion of sense current to be 70 percent from the +5.0 V and 30 percent from the +12 V outputs. The value of the +5.0 V sense resistor (R18) is: R18 + (5 V * 2.5 V) + 3.57 k, 1% 0.7(1 mA) (eq. 19) The +12 V sense resistor (R19) is: R19 + (12.2 V * 2.5 V) + 32.3 k 0.3(1 mA) (eq. 20) make it 32.4 k, 1% Designing the Voltage Controlled Oscillator One designs the VCO component values when the power supply is at the lowest input voltage. Here the frequency will Rt + http://onsemi.com 9 7.7 V (18 k) + 27.7 k make 27 k 5.0 V (eq. 21) AN1327/D AN1327/D The lowest frequency that the power supply is capable occurs when the error amplifier is at its lowest output which is about 0.8 V. The lowest operating frequency would then be: flow + Rst + I PD + The Current Sense Resistor Determining the value of the current sense resistor (R14), one uses the peak current determined in the predesign considerations and at the minimum input voltage. To keep the current-mode operation linear, the peak currents must be kept less than 1.0 volt in normal operation. So to find the current sensing resistor (R14) value: (Vin(max)2 (854)2 + + 1.66 W (eq. 26) Rst 4(110 k) This makes the power dissipated in each resistor 0.41 watts. This is a little high, so place five resistors (R5-R9) each having a value of 82 k ohms and each will dissipate 0.36 watts which is below a 25 percent derating point. The Voltage Feedback Loop Compensation The output that is most heavily sensed is the +5.0 V output, so that will be the output that is used as the reference input for the feedback loop analysis. The output filter pole at light load (0.1 A) of this output is: V Rsc + sc + 1.0 V + 1.35 W make 1.2 ohms, 12 W Ipk 0.74 A (eq. 23) 1 2pRoCo (eq. 27) 1 + + 15.9 Hz 2p(50 W)(200 mF) The Current Ramp Time Delay Circuit This circuit is very important to the operation of the overall circuit. Aside from providing the usual spike elimination to the current comparator, it also provides a time delay function from the current sense resistor to the input to the current comparator. Although a wide range of resistor and capacitor values will work, some minimum time delay is required to avoid instabilities due to too short an on-time at the high range of input voltages. One starts by selecting a value for the capacitor (C8). The common range of values for this function are 470 pF to 1000 pF. For this particular application a value of 1,000 pF was selected for C8. A good estimate of the time delay needed would be approximately 0.7 mS. This is also the approximate minimum on-time at the high input voltage. The value of the resistor (R15) would then be: (700 nS) + + 700 W make 680 ohms (1000 pF) (eq. 25) Each resistor is then 110 k. The power dissipated is: (0.8 V ) 3.3 V) · 140 kHz + 75 kHz (eq. 22) 7.7 V T Rf + d Cf Vin(min) + 127 V + 423 k 0.3 mA start(min) ffp() 5) + The +5.0 V output filter pole at rated load (1.0 A) is 159 Hz. The zero contributed by the ESR of the output filter capacitors will be approximately 15 kHz. The gain exhibited by the open loop power supply at the high input voltage will be: ADC + (Vin * Vout)2 N sec Vin · Ve · Npri (eq. 28) (854 V * 5 V)2 4T + + 48.3 (854 V)(1)(70T) or GDC = 33.6 dB. This is the highest DC gain that will be exhibited by the open loop power supply and will reduce to 16.5 dB at the low input line. This will reduce the bandwidth of the closed loop power supply by almost a decade when going from high input line to low input line. This is marginally acceptable. By setting the widest allowable bandwidth at the high input line, then one can be assured of a reasonable bandwidth at the low input line. The maximum recommended bandwidth is approximately: (eq. 24) The Start-Up Circuit A passive start-up circuit is used. That is, resistors will bring current from the input line to start-up the control IC. It is desired that a "hiccup'' mode of overcurrent protection be implemented, which means that the amount of current that flows through the start-up circuit must be less than the current needed to run the control IC. The UC3845B UC3845B uses approximately 10 mA during normal operation and draws between 0.3 to 0.5 mA in standby. The start-up energy will be stored in the 10 mF filter capacitor. To meet the breakdown rating of the half watt resistors (approximately 250 V), a minimum of four resistors in series will be needed to accommodate the 854 VDC maximum input voltage. The total start-up resistance will be: fxo + fsw(min) + 75 kHz + 15 kHz 5 5 (eq. 29) The gain needed to be contributed by the error amplifier to achieve this bandwidth is calculated at rated load because that will yield the widest bandwidth condition which is: Gxo + 20 Log ffxo * GDC fp (eq. 30) Gxo + 20 Log 15 kHz * 33.6 dB + 5.9 dB 159 Hz The gain in absolute terms (needed later) is: http://onsemi.com 10 AN1327/D AN1327/D Axo + 10 Gxo 20 fc + fsw · 10 Att 40 (eq. 31) Axo + 10(5.9 dB20) + 1.97 where Att is the attenuation needed at the switching frequency in negative dB. Now the compensating circuit elements can be calculated. 1 2 · p · Axo · R19 · fxo 1 + 2 · p (1.97)(3.57 k)(15 kHz) (eq. 32) + (1.97)(3.57 k) + 7033 ohms, make R20 7.5 k The compensating zero must be placed at or below the light load filter pole. 1 2 · p · R20 · fze 1 + + 1.3 mF 2 · p(7.5 k)(15.9 Hz) L+ C+ + 0.1 mF Post Design Modifications It was found that the control circuitry drew more quiescent current than anticipated. That made the start-up voltage higher than desired. It was also found that the optoisolator's dark collector leakage current and the timing capacitor's leakage current were responsible for this behavior. D10 and C17 which isolated these elements behind a low reverse leakage diode during the start-up process were added. Toff · Iin * av(max) (5 mS)(0.167 A) [ + 42 mF 20 V (eq. 37) 1 1 + (2pfc)2L [2p (18.8 kHz)]2(598 mH) Coilcraft offers off-the-shelf common-mode filter chokes (transformers) and the part number closest to this value is E3493 E3493. With this filter design a minimum of -40 dB between the frequencies of 500 kHz and 10 MHz can be expected. If later during the EMI testing stage, additional filtering is needed, a third order to the filter design will be added by using a differential-mode filter. The Bulk Input Filter Capacitor The approximate value of capacitance needed is: Vripple RL · z (50)(0.707) + + 598 mH p · fc p (18.8 kHz) (eq. 33) Design of the Input Rectifier/Filter Circuit This circuit provides EMI filtering, rectification and bulk energy storage for the power supply. It has some severe operating conditions it must withstand, such as very high AC and DC voltages at the high input voltage range. High voltage ratings for the rectifiers and bulk filter capacitors are needed. Also large creepage distances, the distance an arc must travel over a surface, must be maintained to meet the requirements of the safety regulatory agencies. 1N4007 1N4007's will be used as the input rectifiers because of their 1000 V reverse voltage ratings and the average input current of the power supply is less than 1.0 amp. Cin [ (eq. 36) A damping factor of 0.707 or greater is good and it provides a -3.0 dB attenuation at the corner frequency and does not produce ringing in the filter reactances. Assume that the input line impedance is 50 ohms since the regulatory agencies use a LISN (Line Impedance Stabilization Network) which makes the line impedance equal this value. Calculating the values needed in the common-mode inductor (L1) and "X'' capacitors (C1 and C4): R17 + Axo · R18 C16 + fc + (75 kHz) 10 * 24 + 18.8 kHz 40 C15 + + 1500 pF (eq. 35) (eq. 34) Performance of the Sample Design Output Regulation: +5.0 V -1.2% +12 V -1.5% make this two 100 mF, 450 VDC capacitors in series (C5 and C6). Input Regulation: +5.0 V +0.4% +12 V +0.5% The EMI Filter A second order, common-mode filter is used. The lowest frequency of operation occurs at the low input voltages. The estimated lowest frequency of operation is 75 kHz. This is important to attenuate the switching noise sufficiently to pass EMI testing. A good starting point is to assume that 24 dB of attenuation at 75 kHz is needed. That makes the corner frequency of the common-mode filter as: Output Ripple: +5.0 V 100 mV (@ 90 VAC) +12 V 130 mV (@ 90 VAC) http://onsemi.com 11 AN1327/D AN1327/D Figures 10 through 13 graphically represent some of the parameters of the evaluation board that are important to its operation. These parameters were measured at full-rated load for all of the outputs. 3.5 140 3.0 120 ON-TIME (mS) FREQUENCY (kHz) 160 100 80 60 40 2.0 1.5 1.0 0.5 20 0 120 2.5 200 300 400 500 600 700 800 0 120 854 200 300 400 500 600 700 800 INPUT VOLTAGE (VDC) Figure 10. Frequency vs. Input Voltage Figure 11. On-Time vs. Input Voltage 0.6 0.5 0.5 PEAK CURRENT (A) 0.6 PEAK CURRENT (A) 854 INPUT VOLTAGE (VDC) 0.4 0.3 0.2 0.1 0 120 200 300 400 500 600 700 800 0.4 MEASURED PEAK CURRENT 0.3 0.2 0.1 0 120 854 ACTUAL PEAK CURRENT INPUT VOLTAGE (VDC) 200 300 400 500 600 700 800 854 INPUT VOLTAGE (VDC) Figure 12. Actual Peak Current vs Input Voltage Figure 13. Actual and Measured Peak Current vs. Input Voltage ON Semiconductor and are registered trademarks of Semiconductor Components Industries, LLC (SCILLC). SCILLC reserves the right to make changes without further notice to any products herein. SCILLC makes no warranty, representation or guarantee regarding the suitability of its products for any particular purpose, nor does SCILLC 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 special, consequential or incidental damages. "Typical" parameters which may be provided in SCILLC 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. SCILLC does not convey any license under its patent rights nor the rights of others. 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