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| This page contains some valuable engineering information to help you in your design. You may also contact us directly at LMC@linearmagnetics.com with any specific questions. | |
| For our convenience, you may also save or print this as an Adobe Acrobat PDF File (113 Kb): |
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| Some of the more common Power Supply Topologies | |
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In the following approximate equations for some of the more
common Power Supply Topologies, Linear Magnetics Corporation
allows 1-volt drop for each conducting diode in the current path.
Some topologies have 1 diode in the circuit at a time while others
have 2 diodes in the circuit. VDC is the maximum DC voltage required and IDC is the maximum current. VA (Volt Amps) determines the Core size and turns required to build the transformer. A prototype is recommended to verify the results as iterations are sometimes needed. "F*" is the Form Factor that typically varies between 1.6 to 1.8 and is a function of flux density and the storage capacitor. The larger the storage Capacitor, the lower the ripple but the form factor must be larger due to the higher peak current pulses and a smaller conduction angle. A larger factor gives way to larger wire diameters and higher VA, which equates to a larger, heavier, and a higher cost transformer. |
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| 1. | Dual Complementary Rectifier |
| A dual Complementary Rectifier circuit is the best choice when a positive and negative DC output of the same voltage is required. The two secondary windings are wound Bifilar to give equal voltages, winding resistance, coupling and capacitance. Only one diode is in the circuit for each half cycle of each supply side. | |
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| 2. | Full Wave Bridge Rectifier |
| The full wave bridge rectifier is the most cost-effective circuit for DC voltages if that voltage is well above the voltage drop of the two conducting diodes. Since the entire transformer secondary is used on each half cycle, no center tap is required. Two diodes are in the circuit at the same time. | |
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| 3. | Full Wave Center Tapped Rectifier Circuit |
| A full wave center tapped rectifier circuit is commonly used in high current, low voltage drop applications, since there is only one diode voltage drop in the circuit. However, since only one secondary winding is used at a time, the transformer power rating has to be about 30% greater than the Full Wave Bridge transformer. | |
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| 4. | Full Wave Center Tap with Choke Input |
| Choke input filters are used in higher current applications requiring lower ripple and better transformer efficiency. F* approaches unity here so it does not apply. This topology has a constant current through L equal to the load current. The line current is a square wave. This reduces the storage capacitor value and the transformer to its minimum size. This also reduces the input EMI filter size, if required. Minimum DC current determines the Inductance value, but the highest DC current determines the Inductor's physical size. The inductor resistance, RL, voltage drop must be added back to determine the required VAC value. The line frequency, fline, is used in these equations. Of course, LMC can also provide the inductor, L, and the EMI filter inductors along with the entire box. | |
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| 5. | Regulated Linear Power Supply |
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A simplified formula to determine the required AC voltage and AC current to design the transformer is as follows. Where V is in volts, I is in Amps, C is the filter capacitor value in Farads, and fline is the line frequency in Hertz - not the ripple frequency. Vdc = Output DC voltage. |
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| 6. | Double Primaries - Transformers used for both Europe and the United States |
| This is determined by Phase dots. Connect the phase dotted wires together and also the two non-dotted together and connect the dotted and non-dotted to the power line. For Europe, connect one on the non-dotted to the power, connect that winding's dotted end to the non-dotted of the second winding. Then power that winding's dotted end. Also, for Europe the transformer must be specified for 50 Hz operation. Each Primary carries half the power. | |
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| 7. | How to Calculate the Total VA required |
| Calculate the VA of each of each secondary using the Equations in this section then total them. Then divide this by the transformer efficiency - typically .9 (90%). This Total VA is the value required value for sizing the transformer core and this also determines the Primary wire size. Each individual secondary wire size is a function of that current (IAC) only. | |
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| 8. | Multiple Primary Taps |
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These two Primaries can support many different line voltages.
100 volts by paralleling the two zeros and the two 100 volt taps
for Japan. 108 by doing the same technique, but LMC is not aware
of this requirement. Again, 120 by paralleling these two taps.
200-volt for Japan is available by placing the two 100 volt windings
in series. 208 volts output by using one primary at 100 volts
and hooking up the 108 of the other in series. By using 120 volts
on one primary and 108 on the other in series for a total of
228 volts. This should work well for a 230-volt input. Then series
the two 120 taps on each for 240 volt input. Many tap arrangements
are available. Note again, for Europe and Japan 50 Hz is required. Our customers have two options. 1) Specify the quantity, topology, and all the required variables then we will provide a transformer quotation. 2) Specify the quantity, topology, Primary AC voltages, DC outputs with currents, then we will also provide a quotation. |
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Richard Lee (Oz) Ozenbaugh VP of Engineering Linear Magnetics Corporation |
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www.emi-filter-design.com. Publications, design software and consulting services are available. |
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| Linear Magnetics Corporation • www.linearmagnetics.com • email |
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