CMC-05

We’ve got the Power. HIGH POWER WHITE LIGHT CONSTANT CURRENT LED-DRIVERS DROP IN REPLACEMENT FOR 3-PIN REGULATORS (78 SERIES) SWITCHING REGULATORS EFFICIENCIES UP TO 97% NEW! 100% POWER. NO DERATING. HIGH EFFICIENCY HIGH TEMP. DC-DC POWER MODULES MICROSIZE PCB MOUNT SAFETY APPROVED AC-DC MODULES Application Notes 2009 POWERLINEPLUS 100% DC-DC Power. No derating. From -45°C up to 100°C Full Load Operating Temperature Built-in EN55022/FCC Class B EMC Filter Up to 3kVDC Isolation Compact Size, Standard Pinouts Efficiency above 90% Single & Dual Outputs 2:1and 4:1 Input Voltage Ranges 20W, 30W, 40W or 50W DC-DC Power Packages The POWERLINE PLUS uses ICE Technology. A combination of techniques to minimise internal heat dissipation and maximise the heat transfer to ambient to create a new converter series which offers high end performance at a price which is significantly lower than conventional specialist converters. RECOM - Green high-efficiency power solutions. SAVE ENERGY. NOW. Application Notes CHOOSING THE RIGHT CONVERTER: A GUIDE for DC/DC Converters Step 1: Do you need Isolation? (An isolated converter has outputs that are floating and not connected to the inputs i.e. they are galvanically isolated) No isolation needed: Check our Innoline parts first (R-78 series, R-5xxx, R-6xxx and R-7xxx series) Isolation needed: decide whether you need 1kVDC/1 sec (standard) or 1.6VDC/ 1 sec, 2kVDC/1 sec, 3kVDC/1 sec, 4kVDC/1 sec, 5.2kVDC/1 sec or 6kVDC/1 sec. Step 2: Decide on the output voltage and number of outputs: single, dual bipolar (+/-), dual isolated or triple. It is also important to decide whether the output voltage needs to be regulated or unregulated. Unregulated converters are offered standard without short circuit protection or optionally with short circuit protection (option /P) All Series are available with single outputs. Please note that a dual output converter can be used as a single output by leaving the common pin unconnected i.e. +/-5V = 10V, +/-12V = 24V, +/-15V = 30V, etc. Step 3: Decide on the output current. The output voltage times the output current gives the output power of the converter in Watts. DC/DC converters are designed to run at full load, so only round up the power if a suitable converter is not available. e.g. 5V @ 150mA = 0.75W = 1W converter. e.g. +/-15V @ +/-1A = 30W = 30W converter. Step 4: Decide on the input voltage. Standard input voltage ranges are: 3.3, 5, 9, 12, 15, and 24VDC with +/-10% tolerance 4.5 ~ 9V, 9 ~ 18V, 18 ~ 36V and 36 ~ 72VDC with 2:1 input voltage range 9 ~ 36V and 18 ~ 72VDC with 4:1 input voltage range. Step 5: Decide on the case style and pin-out. Many Recom series are available in either through hole or surface mount styles and with several pin-out options, including Remote On/Off Control. Step 6: Use either the Selection guide or Contents guide at the start of each section to find the most appropriate converter. REMEMBER: THERE IS AN INTERACTIVE SELECTION GUIDE ON OUR WEBSITES Go to WWW.RECOM-INTERNATIONAL.COM then click on PRODUCT SEARCH Go to WWW.RECOM-POWER.COM then click on PRODUCT SEARCH: SPEC BY STEP www.recom-international.com 2009 A-1 Application Notes SPECIFICATION CHECKLIST Use this checklist to help you define your specification. If you can’t find a converter that fulfils your needs then call us, fax us, e-mail us or use our Web Sample Enquiry link and we will find the best match for you. 1.
Non-Isolated
Isolation Required _____kVDC or ____kVAC 2.
Single Output
Dual Bipolar Output
Other: Output Voltages (V) _____ /______/______ Output Currents (A) _____/______/______ 3. Total Output Power (V x A) _________________ 4.
Regulated 5. Short Circuit Protection:
No
Yes 6. Remote Control Pin:
No
Yes 7. Output Voltage Trim:
No
Yes 8. Input Voltage: minimum ______ maximum ______ 9. Mounting Style:
Through Hole
Unregulated
SMD Single-In-Line Pins
SIP4
SIP6
SIP7
SIP8
SIP12 Dual-In-Line Pins
DIP8
DIP14
DIP16
DIP24 - Pinout? A_B_C_ Standard Brick
1” x 1”
1” x 2”
1.6” x 2”
2” x 2” Case Style:
Metal Case
Plastic Case
Open Frame 10. Certifications Required:
None
EN 60950-1
EN 60601-1
UL 60950-1 11. Operating Temperature Range: minimum ______ maximum ______ 12. Heatsink required :
No
Yes 13. Other Requirements:________________________________________ REMEMBER: THERE IS AN INTERACTIVE SELECTION GUIDE ON OUR WEBSITES Go to WWW.RECOM-INTERNATIONAL.COM then click on PRODUCT SEARCH Go to WWW.RECOM-POWER.COM then click on PRODUCT SEARCH: SPEC BY STEP A-2 2009 www.recom-electronic.com Application Notes Contents ECONOLINE (General Apps.) ● No Load Over Voltage Lock-Out POWERLINE DC-DC Terminology Long Distance Supply Lines ● EMC Filter Suggestion Input Range LCD Display Bias ● General Test Set-Up Load Regulation Pre- and Post Regulation ● Input Voltage Range Line Voltage Regulation EIA-232 Interface ● PI Filter Output Voltage Accuracy 3V/5V Logic Mixed Supply Rails ● Output Voltage Accuracy Input and Output Ripple and Noise Isolated Data Acquisition System ● Voltage Balance EMC Considerations ● Line Regulations Insulation Resistance Power Supply Considerations ● Load Regulation Efficiency at FulI Load Interpretation of DC-DC Converter EMC Data ● Efficiency Temperature Drift Conducted and Radiated Emissions ● Switching Frequency Switching Frequency Line Impedance Stabilisation Network (LISN) ● Output Ripple and Noise No Load Power Consumption Shielding ● Output Ripple and Noise (continued) Isolation Capacitance Line Spectra of DC-DC Converters ● Transient Recovery Time Input to Output Isolation ● Mean Time Between Failure (MTBF) ● Temperature Performance of DC-DC Converters ● Current Limiting Noise ● Transfer Moulded (SMD) DC-DC Converters ● Fold Back Current Limiting Operating Temperature Range Production Guideline Application Note ● Isolation Calculation of Heatsinks Component Materials ● Break-Down Voltage Isolation Component Placement ● Temperature Coefficient Isolation Voltage vs. Rated Working Voltage Component Alignment ● Ambient Temperature ● Isolation mode in IGBT Driver Circuits Solder Pad Design ● Operating Temperature Range ● Connecting DC-DC Converters in Series Solder Reflow Profile ● Storage Temperature Range ● Connecting DC-DC Converters in Parallel Recommended Solder Reflow Profile ● Output Voltage Trimming ● Chaining DC-DC Converters Adhesive Requirements ● Heat Sinks ● Filtering Adhesive Placement Output Filtering Calculation Cleaning ● Limiting Inrush Current Vapour Phase Reflow Soldering ● Maximum Output Capacitance ● Settling Time ● Isolation Capacitance and Leakage Current INNOLINE ● Application Examples ● EMC Filter Suggestion Overload Protection ● Soft Start Circuit Input Voltage Drop-Out (brown-outs) ● Positve - to - Negative Converters ● ● POWERLINE AC/DC ● Input Fuse ● Earthing ● External Filter ● Paralleling AC/DC Converters ● Chaining Converters ● DC Inputs Tin Whisker Mitigation BLOCK DIAGRAMS Transport Tubes & Reels www.recom-international.com 2009 A-3 DC-DC Converter Applications Terminology The data sheet specification for DC-DC converters contains a large quantity of information. This terminology is aimed at ensuring that the user can interpret the data provided correctly and obtain the necessary information for their circuit application. Input Range The range of input voltage that the device can tolerate and maintain functional performance over the Operating Temperature Range at full load. Load Regulation The change in output voltage over the specified change in output load. Usually specified as a percentage of the nominal output voltage, for example, if a 1V change in output voltage is measured on a 12V output device, load voltage regulation is 8.3%. For unregulated devices the load voltage regulation is specified over the load range from 10% to 100% of full load. Line Voltage Regulation The change in output voltage for a given change in input voltage, expressed as percentages. For example, assume a 12V in-put, 5V output device exhibited a 0.5V change at the output for a 1.2V change at the input, line regulation would be 1%/1%. Output Voltage Accuracy The proximity of the output voltage to the specified nominal value. This is given as a tolerance envelope for unregulated devices with the nominal input voltage applied. For example, a 5V specified output device at 100% load may exhibit a measured output voltage of 4.75V, i.e. a voltage accuracy of –5%). Input and Output Ripple and Noise The amount of voltage drop at the input, or output between switching cycles. The value of voltage ripple is a measure of the storage ability of the filter capacitors. The values given in the datasheets include the higher frequency Noise interference superimposed on the ripple due to switching spikes.The measurement is limited to 20MHz Bandwidth. Input to Output Isolation The dielectric breakdown strength test between input and output circuits. This is the isolation voltage the device is capable of withstanding for a specified time, usually 1 second (for more details see chapter “Isolation Voltage vs. Rated Working Voltage”). A-4 Insulation Resistance The resistance between input and output circuits. This is usually measured at 500V DC isolation voltage. Efficiency at FulI Load The ratio of power delivered from the device to power supplied to the device when the part is operating under 100% load conditions at 25°C. Temperature Drift The change in voltage, expressed as a percentage of the nominal, per degree change in ambient temperature. This parameter is related to several other temperature dependent parameters, mainly internal component drift. Switching Frequency The nominal frequency of operation of the switching circuit inside the DC-DC converter. The ripple observed on the input and output pins is usually twice the switching frequency, due to full wave rectification and the push-pull configuration of the driver circuit. No Load Power Consumption This is a measure of the switching circuits power cunsumption; it is determined with zero output load and is a limiting factor for the total efficiency of the device. Isolation Capacitance The input to output coupling capacitance. This is not actually a capacitor, but the parasitic capacitive coupling between the transformer primary and secondary windings. Isolation capacitance is typically measured at 1 MHz to reduce the possibility of the on-board filter capacitors affecting the results. Mean Time Between Failure (MTBF) RECOM uses MIL-HDBK-217F standard for calculation of MTBF values for +25°C as well as for max. operating temperature and 100% load. When comparing MTBF values with other vendor's products, please take into account the different conditions and standards i.e. MILHDBK-217E is not as severe and therefore values shown will be higher than those shown by RECOM. (1000 x 10³ hours =1000000 hours = 114 years!) These figures are calculated expected device lifetime figures using the hybrid circuit model of MIL-HDBK-217F. POWERLINE converters also can use BELLCORE TR-NWT-000332 for calculation of MTBF. The hybrid model has various accelerating factors for operating environment (πE), maturity (πL), screening (πQ), hybrid function (πF) and a summation of each individual component characteristic (λC). The equation for the hybrid model is then given 2009 by: λ = Σ (NC λC) (1 + 0.2πE) πL πF πQ (failures in 106 hours) The MTBF figure is the reciprocal of this value. In the data sheets, all figures for MTBF are given for the ground benign (GB) environment (πE = 0.5); this is considered the most appropriate for the majority of applications in which these devices are likely to be used. However, this is not the only operating environment possible, hence those users wishing to incorporate these devices into a more severe environment can calculate the predicted MTBF from the following data. The MIL-HDBK-217F has military environments specified, hence some interpretation of these is required to apply them to standard commercial environments. Table 1 gives approximate cross references from MIL-HDBK217F descriptions to close commercial equivalents. Please note that these are not implied by MIL-HDBK-217F, but are our interpretation. Also we have reduced the number of environments from 14 to 6, which are most appropriate to commercial applications. For a more detailed understanding of the environments quoted and the hybrid model, it is recommended that a full copy of MIL-HDBK217F is obtained. It is interesting to note that space flight and ground benign have the same environment factors. It could be suggested that the act of achieving space flight should be the determining environmental factor (i.e. missile launch). The hybrid model equation can therefore be rewritten for any given hybrid, at a fixed temperature, so that the environmental factor is the only variable: λ = k (1 + 0.2 πE) The MTBF values for other environment factors can therefore be calculated from the ground benign figure quoted at each temperature point in the data book. Hence predicted MTBF figures for other environments can be calculated very quickly. All the values will in general be lower and, since the majority of the mobile environments have the same factor, a quick divisor can be calculated for each condition. Therefore the only calculation necessary is to devide the quoted MTBF fig. by the divisor given in table 2. www.recom-electronic.com DC-DC Converter Applications Environment Ground Benign πE Symbol GB Ground Mobile GM Naval Sheltered NS Aircraft Inhabited Cargo AIC Space Flight SF Missile Launch ML MIL-HDBK-271F Description Non-mobile, temperature and humidity controlled environments readily accessible to maintenance Equipment installed in wheeled or tracked vehicles and equipment manually transported Sheltered or below deck equipment on surface ships or submarines Typical conditions in cargo compartments which can be occupied by aircrew Earth orbital. Vehicle in neither powered flight nor in atmospheric re-entry Severe conditions relating to missile launch Commercial Interpretation or Examples Laboratory equipment, test instruments, desktop PC's, static telecomms In-vehicle instrumentation, mobile radio and telecomms, portable PC's Navigation, radio equipment and instrumentation below deck Pressurised cabin compartments and cock-pits, in flight entertainment and non-safety critical applications Orbital communications satellite, equipment only operated once in-situ Severe vibrational shock and very high accelerating forces, satellite launch conditions Table 1: Interpretation of Environmental Factors Environment Ground Benign Ground Mobile Naval Sheltered Aircraft Inhabited Cargo Space Flight Missile Launch πE Symbol GB GM GNS πE Divisor Value 0.5 1.00 4.0 1.64 4.0 1.64 AIC 4.0 1.64 SF ML 0.5 12.0 1.00 3.09 Table 2: Environmental Factors Noise Input conducted noise is given in the line conducted spectra for each DC-DC converter (see EMC issues for further details). Noise is affected significantly by PCB layout, measurement system configuration, terminating impedance etc., and is difficult to quote reliably and with any accuracy other than via a spectrum analysis type plot. There will be some switching noise present on top of the ripple, however, most of this is easily reduced by use of small capacitors or filter inductors, as shown in the application notes. DC GND 0V a) Single Output DC VIN +VO 0V -VO DC GND b) Dual Output VIN GND V O1 0V1 VO 2 0V2 DC DC c) Twin Isolated Outputs Figure 1: Standard Isolated Configurations Operating temperature range: Operating temperature range of the converter is limited due to specifications of the components used for the internal circuit of the converter. The diagram for temperature derating shows the safe operating area (SOA) within which the device is allowed to operate. At very low temperatures, the specifications are only guaranteed for full load. Up to a certain temperature 100% power can be drawn from the device, above this temperature the output power has to be less to ensure function and guarantee specifications over the whole lifetime of the converter. These temperature values are valid for natural convection only. If the converter is used in a closed case or in a potted PCB board, higher temperatures will be present in the area around thermal converter because the convection may be blocked. If the same power is also needed at higher temperatures either the next higher wattage series should be chosen or if the converter has a metal case, a heatsink may be considererd. Please refer to the Powerline Application Notes Section for more information on thermal impedance and heatsinking. www.recom-international.com VO DC VIN 2009 VCC +VO DC DC 0V -VO GND a) Non-lsolated Dual Rails VCC DC DC +VO 0V -VO GND b) Non-lsolated Negative Rail VCC DC DC +VO (VO+VIN) 0V GND c) Dual Isolated Outputs (U/T) Figure 2: Alternative Supply Configurations A-5 DC-DC Converter Applications Isolation One of the main features of the majority of Recom DC-DC converters is their high galvanic isolation capability. This allows several variations on circuit topography by using a single DC-DC converter. bias, resistor feed). Having an alternative return path can upset the regulation and the performance of the system may not equal that of the converter. These configurations are most often found in instrumentation, data processing and other noise sensitive circuits, where it is necessary to isolate the load and noise presented to the local power supply rails from that of the entire system. Usually local supply noise appears as common mode noise at the converter and does not pollute the main system power supply rails. The isolated positive output can be connected to the input ground rail to generate a negative supply rail if required. Since the output is isolated from the input, the choice of reference voltage for the output side can be arbitrary, for example an additional single rail can be generated above the main supply rail, or offset by some other DC value (see figure 2). Regulated converters need more consideration than the unregulated types for mixing the reference level. Essentially the single supply rail has a regulator in its +Vout rail only, hence referencing the isolated ground will only work if all the current return is through the DC-DC and not via other external components (e.g. diode Isolation Test Voltage (kV) The basic input to output isolation can be used to provide either a simple isolated output power source, or to generate different voltage rails, and/or dual polarity rails (see figure 1). Isolation Voltage vs. Rated Working Voltage The isolation voltage given in the datasheet is valid for 1 second flash tested only. If a isolation barrier is required for longer or infinite time the Rated Working Voltage has to be used. Conversion of Isolation Voltage to Rated Working Voltage can be done by using this table or graph. 12 10 8 6 4 2 0 0 1 2 3 4 5 6 7 Rated Working Voltage (kV) Figure 5: IEC950 Test Voltage for Electrical Strength Tests Isolation Test Voltage (V) Rated Working Voltage (V) 1000 130 1500 230 3000 1100 6000 3050 Table 2: Typical Breakdown Voltage Ratin gs According to IEC950 The graph and table above show the requirements from IEC950. According to our experience and in-house tests, we can offer the following conversion tables: Isolation Test Voltage (1 second) 500 VDC 1000 VDC 1500 VDC 2000 VDC 2500 VDC 3000 VDC 4000 VDC 5000 VDC 6000 VDC Isolation Test Voltage (1 minute) 400 VDC 800 VDC 1200 VDC 1600 VDC 2000 VDC 2400 VDC 3200 VDC 4000 VDC 4800 VDC Isolation Test Voltage (1 minute) 250 VAC 500 VAC 750 VAC 1000 VAC 1250 VAC 1500 VAC 2000 VAC 2500 VAC 3000 VAC Table 1 : D.C. Isolation Voltage test vs different conditions A-6 2009 www.recom-electronic.com DC-DC Converter Applications Isolation Test Voltage (1 second) 500 VAC 1000 VAC 1500 VAC 2000 VAC 2500 VAC 3000 VAC 4000 VAC 5000 VAC 6000 VAC Isolation Test Voltage (1 minute) 350 VAC 700 VAC 1050 VAC 1400 VAC 1750 VAC 2100 VAC 2800 VAC 3500 VAC 4200 VAC Isolation Test Voltage (1 minute) 565 VDC 1130 VDC 1695 VDC 2260 VDC 2825 VDC 3390 VDC 4520 VDC 5650 VDC 6780 VDC Table 2 : A.C. Isolation Voltage test vs different conditions Isolation mode in IGBT driver circuits An application for DC/DC converters is to isolate driver circuits for IGBT stacks. In these applications, the maximum DC voltage applied across the isolation gap is not the only factor to be considered because the highly dynamic switching waveforms are an additional stressing factor (typical switching transients can exceed 20kV/µs.) Taking into account that both factors mean a permanent stress on the converter, it is recommended to over specify the converter in terms of isolation voltage and coupling capacitance. Even if a 3kVDC product seems to be appropriate if you just look at the rated working voltage that is required, it is still recommended to choose a product which is specified to 5.2kVDC or 6kVDC to also cover the high dv/dt rates. The higher the isolation voltage rating for a DC/DC converter is, the lower the coupling (isolation) capacitance and a low coupling capacitance is essential in AC or highly dynamic switched DC usage. This will ensure a safe usage and avoid a shortened lifetime in such a highly demanding situation. In the example below, A RP-0524S is used to provide a 5200V isolated supply for the high side drivers and a second, non-isolated converter is used to boost the 5V supply voltage up to 15V for the low side drivers. www.recom-international.com 2009 A-7 DC-DC Converter Applications Connecting DC-DC Converters in Series Galvanic isolation of the output allows multiple converters to be connected in series, simply by connecting the positive output of one converter to the negative of another (see figure 3). In this way non-standard voltage rails can be generated, however, the current output of the highest output voltage converter should not be exceeded. When converters are connected in series, additional filtering is strongly recommended, as the converters switching circuits are not synchronised. As well as a summation of the ripple voltages, the output could also produce relatively large beat frequencies. A capacitor across the output will help, as will a series inductor (see filtering). Vcc DC DC DC DC If two or more converters are operated from a common supply voltage (inputs in parallel), then input decoupling via LC-filters is recommended (see input filters in figure 5). This helps to avoid hard-to-handle conducted EMI caused by totally un-synchronized oscillators. Also inrush current peaks are lowered. Having several smaller filters, one for each converter, is recommended instead of using one common filter for all converters, as this helps to reduce the possibility of the converters beating against each other. Chaining DC-DC Converters Connecting the output of one DC/DC converter to the input of a second converter is sometimes a very useful technique. For example, the benefits of the very wide input voltage range of the innoline series can be combined with the high isolation of the econoline series to create A-8 +V DC Balance Link 2Vo +Vo DC 0V DC GND 0V Figure 4: Paralleled DC-DC Converters with Balance Function. Figure 3: Connecting DC-DC Converters in Series +Vout DC/DC Converter 1 +Sense RADJ -Sense RSENSE -Vout ADJ Sense VCC Load Share Control +Share Connecting the outputs of DC/DC converters in parallel is possible but not recommended. Usually DC/DC converters have no possibility to balance out the output currents. The only possibility to balance out the individual currents is to use a special balance function (like in R-5xxx) or use converters with SENSE function and additional load-share controllers (as can be done for the RP40-SG and RP60-SG, for example). Refer to figure 5 below. DC 0V GND Connecting DC-DC Converters in Parallel So there is potential danger that if the loading is asymmetrical, that one of the converters starts to be overloaded while the others have to deliver less current. The over-loaded converter may then drop out of circuit leading to power supply oscillation. Vcc +Vo -Share +Vout DC/DC Converter 2 +Sense System VCC RADJ -Sense RSENSE -Vout ADJ System GND -Share Connected to System GND Sense VCC Load Share Control +Share -Share Figure 5: Paralleled DC-DC Converters using Load Share Controllers a combination converter which is both isolated and with an exceptionally wide 7:1 input voltage range. Vcc = 5V +12V REC5-0512SRW/H6/A DC Similarly, an isolated DC/DC converter can be used to power a R-78 switchning regulator to provide dual positive outputs with nonstandard voltages. In every case, some care has to be taken concerning the inrush current of the second converter in the chain. If the peak inrush current is too high, then then the first converter in the chain may not start up. DC -Vo 10µ R-783.3 -0.5 +15V Vcc = 9-39V RP15-2415SAW DC DC GND +3.3V 0V GND The solution to this problem is to add some capacitance to supply the peak inrush current and/or to delay the start-up of the second converter in the chain. Figure 5a shows some typical examples. +Vo RP08-1205DA +Vo C1 -Vo DC CTRL DC +5V 0V -5V RP08 starts up after delay C1 provides inrush start up current Figure 5a: Chained DC-DC Converter Examples 2009 www.recom-electronic.com DC-DC Converter Applications Filtering When reducing the ripple from the converter, at either the input or the output, there are several aspects to be considered. Recom recommend filtering using simple passive LC networks at both input and output (see figure 6). A passive RC network could be used, however, the power loss through a resistor is often too high.The self-resonant frequency of the inductor needs to be significantly higher than the characteristic frequency of the device (typically 1OOkHz for Recom DC-DC converters). The DC current rating of the inductor also needs consideration, a rating of approximately twice the supply current is recommended. The DC resistance of the inductor is the final consideration that will give an indication of the DC power loss to be expected from the inductor. Output Filtering calculation: Calculating of the filtering components can be fc = Figure 6: Input and Output Filtering Common Mode Chokes Better results in filtering can be achieved if common mode chokes are used instead of a single choke. Common mode chokes are multiple chokes sharing a core material so the common mode rejection (Electrical noise which comes through one power line and returns to the noise source through some type of ground path is common mode noise.) is higher. 1 2π L OUT C0 done using This frequency should be significant lower than the switching frequency of the converter. Please refer to our page "Common Mode Chokes for EMC" also part of these application notes. These can be used for input filtering as well as for the output side. Limiting Inrush Current Example - RC series: Operating frequency = 85kHz max. then, fc =10 % of 85 kHz = 8,5 kHz fc = Using a series inductor at the input will limit the current that can be seen at switch on (see figure 7). 1 V i =_ R 2π L OUT C0 fc = 8,5 kHz = –t Voltage : V = Vin (1 – exp __ ) RC ( ) 1 2π L OUT C0 VIN for: L OUT = 470 µH V Current : i = _ exp R ( –t__ ) RC ⎛ ⎞ ⎛ ⎞ 1 1 ⎟ =⎜ ⎟ = 745 nF C0 = ⎜⎜ 2 ⎟ 2 ⎟ ⎜ ⎝ (2 π fc) L OUT ⎠ ⎝(2 π 8,5 kHz) 470 uH ⎠ time This would imply that for a 5V input, with say 50mOhm track and wire resistance, the inrush current could be as large as 1OOA. This could cause a problem for the DC-DC converter. A series input inductor therefore not only filters the noise from the internal switching circuit, but also limits the inrush current at switch on. A typical value for an input inductor used to reduce the inrush current is 1mH or higher. A typical value for an inductor used to filter the input is 50-300µH. So although the circuit diagram may look similar, the input inductors have very different functions and different values. If a common mode choke is used as an inrush current limiter, it has the added advantage over a single inductor that the inrush currents flowing in the two windings cancel out and the ferrite is less likely to go into saturation. Short Circuit Protection in 0.25W - 2W Econoline converters In the low wattage, unregulated converter Portfolio we offer continuous short circuit protection (option /P). Especially in applications where the output of converters is connected via a plug and socket to an external module, the chances of having a short circuit across the output is quite high. A conventional unregulated converter can withstand a short circuit across the outputs for only a limited time. The same condition can occur with high capacitive loads if they have a low ESR. Figure 7: Input Current & Voltage at Switch On However, depending on your application design and loadsituation may interfer with the calculated filter so testing in the final application and re-adjustment of the component’s values may be necessary. When choosing a value for the filtering capacitor please take care that the maximum capacitive load is within the specifications of the converter. www.recom-international.com If we consider the circuit without the series inductor, then the input current is given by; ( ) i = V exp – t R RC When the component is initially switched on (i.e. t=O) this simplifies to; i=V R 2009 RECOM uses balancing between transformer core saturation ratings and the maximum electrical ratings of the switching transistors in the primary side oscillator to create a converter that can withstand a continuous short circuit (