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astra_conv:conversion:dcdc_converter:dcdc_converter [2014/03/17 22:30]
richard [how to spare the weight of the 12V lead-acid battery]
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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-On the left side of the diagram, the primary side DC/DC driver is shown. It is located in the front 100V distribution box, and has already been discussed ​in the chapter "​distribution boxes"​.+On the left side of the diagram, the primary side DC/DC driver is shown. It is located in the front 100V distribution box, and has already been mentioned ​in the chapter "​distribution boxes"​.
  
 The DC/DC driver has two parallel control inputs, one of which is served by the DC/DC control unit (a microcontroller circuit that has already been described in the chapter "BMS peripherals"​). This circuit takes care that the 12V battery never runs low. The DC/DC driver has two parallel control inputs, one of which is served by the DC/DC control unit (a microcontroller circuit that has already been described in the chapter "BMS peripherals"​). This circuit takes care that the 12V battery never runs low.
 +
 +The actual switching is done by an N channel MOSFET. The transistor may appear overdimensioned with it's 500V 44A rating, but upon tragic experience with a 14A type I know that it needs some reserves to take up with the inrush current of the converter. ​
 +
 +The MOSFET is controlled via a small solid state relay, that provides the required isolation between 12V grid and traction grid.
 +
 +A thermo switch next to the source of the MOSFET interrupts the current when the heat sink temperature exceeds 75°C. I have experienced that such situations may occur if water gets into the distribution box and leakage currents elevate the gate voltage into an "​intermediate"​ range. Then the MOSFET will switch incompletely and produce a lot of heat.
 +
 +In the case of the DC/DC primary side driver, it would have been difficult to make a "​water-proof"​ control circuit that drives the MOSFET'​s gate with with low impedance, has a hysteresis that avoids intermediate states and still draws no quiescent current. For some of the other drivers, this might be a possible improvement. ​
 +
 +Below a view of the DC/DC converter primary side driver.
 +
 +{{:​astra_conv:​conversion:​wiring:​p1100347_dcdc_f.jpg?​600|}} {{:​astra_conv:​conversion:​wiring:​p1100348_dcdc_r.jpg?​600|}}
 +
 +
  
 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.
  
  
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 In principle, it would be possible to keep the DC/DC converter on permanently,​ or to add a second, smaller device that feeds the 12V grid while the car is idle.  In principle, it would be possible to keep the DC/DC converter on permanently,​ or to add a second, smaller device that feeds the 12V grid while the car is idle. 
  
-An additional, small DC/DC converter could be optimized for prolonging the battery life by doing all that ctek rituals of running ​different charging phases at different voltages. I am imagining to use a small 15V output DC/DC converter module as a power supply, followed by a microcontroller circuit that fine tunes the output voltage.+An additional, small DC/DC converter could be optimized for prolonging the battery life by doing all that ctek rituals of switching between ​different charging phases at different voltages. I am imagining to use a small 15V output DC/DC converter module as a power supply, followed by a microcontroller circuit that fine tunes the output voltage.
  
 With a permanent power supply, the battery is not drained any more while the car is idle, and it's size can be substantially reduced. With a permanent power supply, the battery is not drained any more while the car is idle, and it's size can be substantially reduced.
astra_conv/conversion/dcdc_converter/dcdc_converter.1395095447.txt · Last modified: 2014/03/17 22:30 by richard