astra_conv:conversion:dcdc_converter:dcdc_converter
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astra_conv:conversion:dcdc_converter:dcdc_converter [2014/03/18 19:47] richard |
astra_conv:conversion:dcdc_converter:dcdc_converter [2014/06/20 13:02] (current) richard |
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| * On/off control input: If such an input were available, and if the "off" primary power drain were acceptable, one would not need a primary side driver or input relay. Even if the feature had it's price, it would probably pay if you consider your own effort realistically. | * On/off control input: If such an input were available, and if the "off" primary power drain were acceptable, one would not need a primary side driver or input relay. Even if the feature had it's price, it would probably pay if you consider your own effort realistically. | ||
| - | The implementation of a primary side driver (no control input available on the DC/DC converter) is described in [astra_conv:conversion:distribution_boxes:distribution_boxes#dc_dc_converter_primary_side_driver_and_output_relay]] | + | The implementation of a primary side driver (no control input available on the DC/DC converters that I used) is described below. |
| * Secondary idle reverse current: This is current that the unit will draw from the 12V grid when it is switched off. Two of the devices I have been testing had considerable idle reverse current of 60mA and even 200mA. This makes necessary an output side switch to isolate the device from the grid when idle. Unfortunately, it is not so easy to integrate a power FET here - it would be difficult to drive and would not cover all possible operative constellations. A relay, if driven without special precaution, tends to "stick" since the relay contact gets overloaded if closed too early. My solution is using a mechanical relay, but switching it on only when the output voltage of the converter has already approached the grid voltage. | * Secondary idle reverse current: This is current that the unit will draw from the 12V grid when it is switched off. Two of the devices I have been testing had considerable idle reverse current of 60mA and even 200mA. This makes necessary an output side switch to isolate the device from the grid when idle. Unfortunately, it is not so easy to integrate a power FET here - it would be difficult to drive and would not cover all possible operative constellations. A relay, if driven without special precaution, tends to "stick" since the relay contact gets overloaded if closed too early. My solution is using a mechanical relay, but switching it on only when the output voltage of the converter has already approached the grid voltage. | ||
| - | The implementation of an output relay with delayed switch-on is described in [[astra_conv:conversion:distribution_boxes:distribution_boxes#dc_dc_converter_output_relay]]. | + | The implementation of an output relay with delayed switch-on is described below. |
| - | Anyhow, a properly designed DC/DC converter should not require an output relay and thus save a lot of integration effort. | + | Anyhow, a properly designed DC/DC converter should not require an output relay at all and thus save a lot of integration effort. |
| * Overload behaviour: Best behaviour will be a simple U/I characteristics - so if the output current reaches its rated maximum, the output voltage will just dip a bit to prevent further increase of the current. | * Overload behaviour: Best behaviour will be a simple U/I characteristics - so if the output current reaches its rated maximum, the output voltage will just dip a bit to prevent further increase of the current. | ||
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| The bottom center building block is the current limiter, that avoids overcurrent shutdown. It is required only for the second device that I integrated, and the device really should feel ashamed for it's deficiencies. You may notice that I placed the shunt resistor and the power MOSFET into the return line, so that the negative output of the DC/DC converter is below ground potential. Reason is simply because this will work with an n channel FET, which particularly has lower "on" resistance than it's p channel counterparts. The control circuit based on an old-fashioned operational amplifier allows to use a very small shunt resistor (10 mOhms), that gives a voltage drop of only 300mV at 30Amps. Despite my concerns, the circuit is reacting quick enough to prevent shutdown of the converter, and so far proved to run stable without oscillations. | The bottom center building block is the current limiter, that avoids overcurrent shutdown. It is required only for the second device that I integrated, and the device really should feel ashamed for it's deficiencies. You may notice that I placed the shunt resistor and the power MOSFET into the return line, so that the negative output of the DC/DC converter is below ground potential. Reason is simply because this will work with an n channel FET, which particularly has lower "on" resistance than it's p channel counterparts. The control circuit based on an old-fashioned operational amplifier allows to use a very small shunt resistor (10 mOhms), that gives a voltage drop of only 300mV at 30Amps. Despite my concerns, the circuit is reacting quick enough to prevent shutdown of the converter, and so far proved to run stable without oscillations. | ||
| - | On the right side of the diagram, you find the output relay already mentioned above (and also discussed in the chapter "distribution boxes" - since it is located in the front 12V distribution box). Delayed switch-on is achieved by sensing the output voltage of the DC/DC converter (after the current limiter). | + | On the right side of the diagram, you find the output relay already mentioned above (and also in the chapter "distribution boxes" - since it is located in the front 12V distribution box). Delayed switch-on is achieved by sensing the output voltage of the DC/DC converter (after the current limiter). The relay will only be triggered after this voltage has exceeded around 10V. Without this delay, there would be a high reverse inrush current from the 12V battery to the DC/DC converter, which would cause the relay contact to "stick". |
| The output relay is controlled by a separate diode combiner located on the DC/DC primary side driver board. | The output relay is controlled by a separate diode combiner located on the DC/DC primary side driver board. | ||
astra_conv/conversion/dcdc_converter/dcdc_converter.1395172071.txt · Last modified: 2014/03/18 19:47 by richard
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