8. Filter / Autoranging Rectifier Module (FARM) Design Guide & Applications Manual For Maxi, Mini, Micro Family DC-DC Converters and Configurable Power Supplies The Filter / Autoranging Rectifier Module (FARM) provides an effective solution for the AC front end of a power supply built with Vicor DC-DC converters. This highperformance power-system building block satisfies a broad spectrum of requirements and agency standards. In addition to providing transient / surge immunity and EMI filtering, the FARM contains all of the power switching and control circuitry necessary for autoranging rectification, inrush current limiting, and overvoltage protection. This module also provides converter enable and status functions for orderly power up / down control or sequencing. To complete the AC front-end configuration, the user only needs to add hold-up capacitors, and a few discrete components. POWER-DOWN SEQUENCE (Figure 8–2) When input power is turned off or fails, the following sequence occurs as the bus voltage decays: 1.2 Bus OK is de-asserted when the bus voltage falls below 205 Vdc (Typical). 2.2 The converters are disabled when the bus voltage falls below 190 Vdc. If power is reapplied after the converters are disabled, the entire power-up sequence is repeated. If a momentary power interruption occurs and power is re-established before the bus reaches the disable threshold, the power-up sequence is not repeated, i.e., the power supply “rides through” the momentary interruption. FUNCTIONAL DESCRIPTION (Figure 8–1) +OUT Initial Condition.The switch that bypasses the inrush limiting PTC (positive temperature coefficient) thermistor is open when power is applied, as is the switch that engages the strap for voltage doubling. In addition, the converters are disabled via the Enable (EN) line, and Bus OK (BOK) is high. PTC Thermistor Strap L Strap EMI Filter –OUT N POWER-UP SEQUENCE (Figure 8–2) EN Microcontroller 1.1 2.1 3.1 Upon application of input power, the output bus capacitors begin to charge. The thermistor limits the charge current, and the exponential time constant is determined by the hold-up capacitor value and the thermistor cold resistance. The slope (dv/dt) of the capacitor voltage versus time approaches zero as the capacitors become charged to the peak of the AC line voltage. If the bus voltage is less than 200 V as the slope nears zero, the voltage doubler is activated, and the bus voltage climbs exponentially to twice the peak line voltage. BOK EMI GRD Figure 8–1 — Functional block diagram: FARM Power Up 90–132 V AC Line Output Bus (Vdc) If the bus voltage is greater than 200 V, the doubler is not activated. The converters are enabled ~150 ms after the thermistor bypass switch is closed. 5.1 Bus OK is asserted after an additional ~150 ms delay to allow the converter outputs to settle within specification. 400 300 200 100 0 1.1 2.1 Strap PTC Thermistor Bypass If the bus voltage is greater than 235 V as the slope approaches zero, the inrush limiting thermistor is bypassed. Below 235 V, it is not bypassed. 4.1 Power Down Converter Enable Bus OK 3.1 ~150 ms ~150 ms 4.1 5.1 2.2 1.2 Figure 8–2 — Timing diagram: power up/down sequence Timing Diagram, Power Up/Down Sequence Maxi, Mini, Micro Design Guide Page 36 of 88 Rev 4.9 Apps. Eng. 800 927.9474 vicorpower.com 800 735.6200 8. Filter / Autoranging Rectifier Module (FARM) Design Guide & Applications Manual For Maxi, Mini, Micro Family DC-DC Converters and Configurable Power Supplies OFF-LINE POWER SUPPLY CONFIGURATION The FARM maintains the DC output bus voltage between 250 and 370 Vdc over the entire input-voltage range, which is compatible with the Maxi, Mini, Micro 300 V input converters as well as VI-260 family and VI-J60 family DC-DC converters. The FARM automatically switches to the proper bridge or doubler mode depending on the input voltage, eliminating the possibility of damage due to improper line connection. The FARM1xxx is rated at 500 W in the low range (90 – 132 Vac input), and 750 W in the high range (180 – 264 Vac input). The FARM2xxx is rated for 750 W and 1,000 W for the low and high input ranges respectively. Either of these modules can serve as the AC front end for any number and combination of compatible converters as long as the maximum power rating is not exceeded. Strap (ST) Pin. In addition to input and output power pin connections, it is necessary to connect the Strap pin to the center junction of the series hold-up capacitors (C1, C2, Figure 8–3) for proper (autoranging) operation. Metal-oxide varistors, V1 and V2 provide capacitor protection. The bleeder resistors (R1, R2, Figure 8–3) discharge the hold-up capacitors when power is switched off. Capacitors C7 and C8 are recommended if the hold-up capacitors are located more than 3 inches (75 mm) from the FARM output pins. Enable (EN) Pin. (Figure 8-4) The Enable pin must be connected to the PC or GATE IN pin of all converter modules to disable the converters during power up. Otherwise, the converters would attempt to start while the hold-up capacitors were being charged through an un-bypassed current-limiting thermistor, preventing the bus voltage from reaching the thermistor bypass threshold, thus disabling the power supply. The Enable output (the drain of an N channel MOSFET) is internally pulled up to 15 V through a 150 kΩ resistor. A signal diode should be placed close to and in series with the PC or GATE IN pin of each converter to eliminate the possibility of control interference between converters. The Enable pin switches to the high state (15 V) with respect to the negative output power pin to turn on the converters after the power-up inrush is over. The Enable function also provides input overvoltage protection for the converters by turning off the converters if the DC bus voltage exceeds 400 Vdc. The thermistor bypass switch opens if this condition occurs, placing the thermistor in series with the input voltage, which reduces the bus voltage to a safe level while limiting input current in case the varistors conduct. The thermistor bypass switch also opens if a fault or overload reduces the bus voltage to less than 180 Vdc. CAUTION: There is no input to output isolation in the FARM, hence the –Out of the FARM and thus the –In of the downstream DC-DC converter(s) are at a high potential. If it is necessary to provide an external enable / disable function by controlling the DC-DC converter’s PC or GATE IN pin (referenced to the –In) of the converter an opto-isolator or isolated relay should be employed. C3 N EMI GND Z1 C1 F1 +IN C10 C7** V1 BOK FARM C9 Filter/Autoranging ST Rectifier Module EN N/C L L F3 R1 + N Vicor DC-DC Converter D3 C8** V2 PC (GATE IN) PR –IN – R2 * C2 R3 PE C4 D1 Part C1,2 C3-C6 C7,8** C9 C10,C11 D1, 2 D3, 4 F1, F2 R1, 2 R3, 4*** V1,2 Z1 Vicor Description Part Number Hold-up capacitors 4,700 pF (Y2 type) 01000 Film Cap., 0.61 µF 34610 0.47 µF 03047 0.001 µF Diode 00670 1N5817 26108 Use recommended fusing for specific DC-DC Converters 150 KΩ, 0.5 W 250 Ω 220 V MOV 30234-220 MOV 270 30076 C5 Sizing PCB traces: All traces shown in bold carry significant current and should be sized accordingly. N/ST/L +/– In F2 +IN 10 A rms at 90 Vac and 500 W 4 A DC at 190 Vdc and 750 W R4 FARM2-xxx N/ST/L 20 A rms at 90 Vac and 750 W +/– In 8 A DC at 190 Vdc and 1000 W C11 Vicor DC-DC Converter D4 PR * See Agency Approvals on FARM data sheet. –IN **Required if C1 & C2 are located more than 3 in (75 mm) from output of the FARM. ***Not used with VI-260/VI-J60 Not used with VI-260/VI-J60 C6 To additional modules Figure 8–3 — Offline power supply configuration Maxi, Mini, Micro Design Guide Page 37 of 88 PC (GATE IN) D2 Rev 4.9 Apps. Eng. 800 927.9474 vicorpower.com 800 735.6200 8. Filter / Autoranging Rectifier Module (FARM) Design Guide & Applications Manual For Maxi, Mini, Micro Family DC-DC Converters and Configurable Power Supplies Bus OK (BOK) Pin. (Figure 8–5) The Bus OK pin is intended to provide early-warning power-fail information and is also referenced to the negative output pin. CAUTION: There is no input-to-output isolation in the FARM. It is necessary to monitor Bus OK via an optoisolator if it is to be used on the secondary (output) side of the converters. A line-isolation transformer should be used when performing scope measurements. Scope probes should never be applied simultaneously to the input and output as this will damage the module. Filter. (Figure 8–6) An integral input filter consists of a common-mode choke and Y-capacitors (line-ground) plus two X-capacitors (line-line). This filter configuration provides common-mode and differential- mode insertion loss in the frequency range between 100 kHz and 30 MHz. Hold-up Capacitors. Hold-up capacitor values should be determined according to output bus voltage ripple, power fail hold-up time, and ride-through time. (Figure 8–7) Many applications require the power supply to maintain output regulation during a momentary power failure of specified duration, i.e., the converters must hold up or ride through such an event while maintaining undisturbed output voltage regulation. Similarly, many of these same systems require notification of an impending power failure to allow time to perform an orderly shut down. The power fail warning time (∆t) is defined as the interval between Bus OK and converter shut down (EN) as illustrated in Figure 8–7. The Bus OK and Enable thresholds are 205 V and 190 V, respectively. A simplified relationship between power fail warning time, operating power, and bus capacitance is obtained by inserting these constants: C = 2P∆t / (2052 – 1902) C = 2P∆t / (5,925) It should be noted that the series combination (C1, C2, Figure 8–3) requires each capacitor to be twice the calculated value, but the required voltage rating of each capacitor is reduced to 200 V. Allowable ripple voltage on the bus (or ripple current in the capacitors) may define the capacitance requirement. Consideration should be given to converter ripple rejection and resulting output ripple voltage. For example, a converter whose output is 15 V and nominal input is 300 V will provide 56 dB ripple rejection, i.e., 10 V p-p of input ripple will produce 15 mV p-p of output ripple. (Figure 8–11) Equation 3 is again used to determine the required capacitance. In this case, V1 and V2 are the instantaneous values of bus voltage at the peaks and valleys (Figure 8–7) of the ripple, respectively. The capacitors must hold up the bus voltage for the time interval (∆t) between peaks of the rectified line as given by: The energy stored in a capacitor which has been charged to voltage V is: ε = 1/2(CV ) ε = stored energy 2 where: ∆t = (π – θ) / 2πf (1) where: f = line frequency θ = rectifier conduction angle (Figure 8–7) C = capacitance V = voltage across the capacitor The approximate conduction angle is given by: Energy is given up by the capacitors as they are discharged by the converters. The energy expended (the power-time product) is: ε = P∆t = C(V 1 2– V22) / 2 (2) where: P = operating power ∆t = discharge interval θ = cos-1(V2 / V1) Irms = 2P / Vac V2 = capacitor voltage at the end of ∆t Rearranging Equation 2 to solve for the required capacitance: C = 2P∆t / (V12 – V22) Page 38 of 88 (3) (5) Another consideration in hold-up capacitor selection is their ripple current rating. The capacitors’ rating must be higher than the maximum operating ripple current. The approximate operating ripple current (rms) is given by: V1 = capacitor voltage at the beginning of ∆t Maxi, Mini, Micro Design Guide (4) (6) where: P = total output power Vac = operating line voltage Calculated values of bus capacitance for various hold-up time, ride-through time, and ripple-voltage requirements are given as a function of operating power level in Figures 8–8, 8–9, and 8–10, respectively. Rev 4.9 Apps. Eng. 800 927.9474 vicorpower.com 800 735.6200 8. Filter / Autoranging Rectifier Module (FARM) Design Guide & Applications Manual For Maxi, Mini, Micro Family DC-DC Converters and Configurable Power Supplies EXAMPLE In this example, the output required from the DC-DC converter at the point of load is 12 Vdc at 320 W. Therefore, the output power from the FARM would be 375 W (assuming a converter efficiency of 85%). The desired hold-up time is 9 ms over an input range of 90 to 264 Vac. Determining Required Capacitance for Power Fail Warning. Figure 8–8 is used to determine capacitance for a given power fail warning time and power level, and shows that the total bus capacitance must be at least 820 µF. Since two capacitors are configured in series, each capacitor must be at least 1,640 µF. NOTE: The warning time is not dependent on line voltage. A hold-up capacitor calculator is available on the Vicor website, at http://asp.vicorpower.com/calculators/calculators.asp?calc=4 Determining Ride-through Time. Figure 8–9 illustrates ride-through time as a function of line voltage and output power, and shows that at a nominal line of 90 Vac, ridethrough would be 68 ms. Ride-through time is a function of line voltage. Determining Ripple Voltage on the Hold-up Capacitors. Figure 8–10 is used to determine ripple voltage as a function of operating power and bus capacitance, and shows that the ripple voltage across the hold-up capacitors will be 12 V p-p. Determining the Ripple on the Output of the DC-DC Converter. Figure 8–11 is used to determine the ripple rejection of the DC-DC converter and indicates a ripple rejection of approximately 60 dB for a 12 V output. Since the ripple on the bus voltage is 12 Vac and the ripple rejection of the converter is 60 dB, the output ripple of the converter due to ripple on its input (primarily 120 Hz) will be 12 mV p-p. Not used with VI-260/VI-J60 + N N +IN + 15 Vdc EMI GND 15 Vdc 150 k FARM N/C ST Vicor DC-DC Converter ST EN PR – –IN N/C L EMI GND 330 µH PR – –IN + L1 0.47 µF EN Figure 8–5 — Bus OK (BOK) isolated power status indicator Figure 8–4 — Enable (EN) function N 4.7 nF CM 0.099 µF BOK ST N/C EN 4.7 nF L – Figure 8–6 — Internal filter Maxi, Mini, Micro Design Guide Page 39 of 88 Vicor DC-DC Converter Secondary referenced Microcontroller Microcontroller L +IN PC BOK PC (GATE IN) BOK EMI GND +5 Vdc Rev 4.9 Apps. Eng. 800 927.9474 vicorpower.com 800 735.6200 8. Filter / Autoranging Rectifier Module (FARM) Design Guide & Applications Manual For Maxi, Mini, Micro Family DC-DC Converters and Configurable Power Supplies Hold-up Time Ripple (V p-p) π–θ Power Fail Warning θ 254 V 205 V 190 V Ride-Through Time Bus OK Power Fail Converter Shut down 40 100 35 90 * 30 25 (FARM2XXX) Ride-through Time (ms) Power Fail Warning Time (ms) Figure 8–7 — General timing diagram of bus voltage following interruption of the AC mains (FARM1XXX) 20 15 10 5 0 250 * * 500 750 Total capacitance 820 µF 80 60 50 40 30 20 10 0 250 1000 500 80 * 25 75 Ripple Rejection (dB) P-P Ripple Voltage (Vac) 1000 Figure 8–9 — Ride-through time vs. operating power 30 * 15 10 5 1100 µF 820 µF 1300 µF 1600 µF 500 65 60 55 50 45 680 µF (FARM1XXX) * 2200 µF (FARM2XXX) 750 70 40 2 1000 5 15 Figure 8–10 — Ripple voltage vs. operating power and bus capacitance, series combination of C1, C2 (Figure 8–3) Maxi, Mini, Micro Design Guide 30 50 Output Voltage Operating Power (W) Page 40 of 88 750 Operating Power (W) Figure 8–8 — Power fail warning time vs. operating power and total bus capacitance, series combination of C1, C2 (Figure 8–3) 0 250 115 Vac 70 Operating Power (W) 20 90 Vac Figure 8–11 — Converter ripple rejection vs. output voltage (Typical) Rev 4.9 Apps. Eng. 800 927.9474 vicorpower.com 800 735.6200