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Like the LT before it, the only thing that can destroy.

Lilanka Udawatta. The Lens serves almost all the patents and scholarly work in the world as a free, open and secure digital public good, with user privacy a paramount focus. The new devices were also easy to use, and inexpensive. LT Design Manual. A switched- mode power supply switching- mode power supply, switch- mode power supply, switched power supply, SMPS, or switcher is an electronic power supply that incorporates a switching regulator to convert electrical power efficiently.

AN19 is the LT design manual, rich with examples:. The LT is in flyback mode during normal start- up until the feedback pin rises above. A switched- mode power supply also switching- mode power supply and SMPS is an electronic power supply unit PSU that incorporates a switching regulator. This manual contains a comprehensive discussion. Com and others, December. Switchmode Power Supply Troubleshooting SAFETY The primary danger to you is from the input side of the supply which is directly connected to the AC line and will have large electrolytic capacitors with V or greater DC when powered often, even if the supply does not work correctly and for some time after being unplugged especially if the power.

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The free energy generator finds a no. This low dropout design allows the input voltage to vary from 3 to 60 V. Lt design manual. The Ripple BlockerTM family of devices is easy to use and improves the signal- to- noise ratio in a number of leading edge noise sensitive applications. This low dropout design allows the input voltage to. Application Note LT In other words, the part is working on the boundary between continuous conduction mode where current is always flowing in either the primary or the secondary and discontinuous mode where there is a region of zero current in both windings before the MOSFET starts charging the primary again.

We will design for maximum load, therefore if the load current decreases, there will be a delay between the secondary current ramping down to zero and the primary current charging again discontinuous mode. FIG 3 shows our outline architecture. FIG 3.

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Referring to FIG 2 earlier in the text, we see that the secondary current is triangular ramping down from a peak value to nearly zero. The area under this red waveform needs to have an average of mA to support our load. However, the secondary current is only present for a certain amount of time dictated by the duty cycle , so the current needs to be considerably greater than this.

A flyback converter has a duty cycle of. Earlier we calculated the duty cycle ignoring the diode drop. We can now include it without too much hardship. If we include a diode drop of 0. Obviously the lower the output voltage, the more influential the diode drop is in our duty cycle equation. Therefore to get the desired peak secondary current, we need to divide the peak mA current calculated above , by Thus the peak desired current in the secondary is.

For a more mathematical derivation of the above procedure, Click Here. A mA peak current in the secondary, with a turns ratio of means a peak primary current of 1. So we need to pick a transformer with a primary inductance of approximately 10uH, with a saturation current rating of at least 1. This is quite a specific requirement and the reader will probably find it impossible to find a transformer with such characteristics.

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All is not lost however, as magnetics companies are now introducing general purpose transformers with 6 windings on one bobbin that can be configured in any way. Since we have more current on the primary than secondary, it makes sense if our design has 3 windings in parallel for the primary and 3 windings in series for the secondary.

This ensures each of our 3 windings shares the 1. The Wurth is a suitable device. We must ensure that the transformer has a current rating of at least 1. If we exceed this current, the ferrite that the transformer is wound on will saturate and lose its magnetic properties. Thus the inductance value of the primary will collapse and from the equation.

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Now, parallel inductors wound on the same ferrite do not follow the same laws as separate inductors placed in parallel. They keep the same overall inductance value, but share the current. This is explained the following Wurth document:. Transformer windings in series and parallel. Thus the has an inductance of As they share the current, the saturation current of 0.

Rsense Choice. The current sense threshold on the LTC is 95mV, so a current sense resistor of 63mOhms ensures a peak current of 1. A current sense resistor of 50m Ohms should suffice. With an output voltage of 12V, the voltage on the secondary winding is Now, it is worth choosing a MOSFET with a drain source rating much higher than this since any transformer will not perfectly couple the primary energy into the secondary.

This term is called leakage inductance and can be modelled as an inductance in series with the primary that is not coupled to the secondary. Thus it stores energy that is not dumped into the secondary during the flyback cycle. This spike increases with primary current and if it is high enough, a snubber network may be required to reduce it. The Qg of the MOSFET will also have an impact on the heat dissipation of the chip, especially if the input voltage to the chip is high. Charge is dictated by the equation:. Since Frequency is the inverse of Time, we can write. So we can calculate the current needed to flow into the chip, just to charge the gate capacitance of the FET.

Again, this needs to be as low as possible. Thus, like connecting resistors in parallel, the ON resistance comes down with more parallel paths. However, in connecting Drain Source paths in parallel, a negative effect is that the Gate Source capacitance Qg is also connected in parallel, so a low ON resistance and hence low conduction loss sometimes implies a high gate source capacitance hence high switching loss.

In addition, high current MOSFETs tend to come in much larger packages, so meeting the ideals of low ON resistance and low Qg might violate a space requirement spec, so the selection process has to start over. Engineering, as ever, is a compromise. Failing that, download all the results to a spreadsheet and sort from there. The Fairchild FDS represents a good compromise between low ON resistance and low gate charge, but its SO8 package is large and therefore might be unsuitable for compact designs.

FDS Datasheet. Rectifier Diode Choice. Schottky diodes also have a much lower forward voltage drop 0.

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When choosing a Schottky diode, the key parameters are: forward voltage drop should be as low as possible , forward current this should be greater than the peak secondary current and reverse voltage rating. When the FET is charging the primary, there will be a voltage across the primary equal to the input voltage. Get A Copy. Kindle Edition , 33 pages. More Details Friend Reviews.

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