User Tools

Site Tools


Sidebar

astra_conv:conversion:dcdc_converter:dcdc_converter

This is an old revision of the document!


DC/DC converter

This chapter is about the “main” DC/DC converter, that powers the 12V grid from the traction grid (note that there is also a couple of small DC/DC converters, a few Watts each, too, within the circuitry). It has an output voltage of 13,8V and delivers up to 36Amps.

At a voltage of 13,8 Volt, a 12V lead-acid battery can be completely charged, provided that the charging time is sufficient. Therefore, the DC/DC converter is powered not only when ignition is on, but also during charging of the traction battery. An extra battery charger is then not required any more. The 12V battery sure would appreciate an “equalisation” charge at higher voltage now and then, but in a conventional car, it won't be coddled either.

I learnt that it is quite hard to find a DC/DC converter that is reliable, meets the technical requirements and has enough functionality by itself so that one need not spend effort in additional gadgets. My first converter failed after TUEV had already done a first inspection of the car. Since the manufacturer did not make the impression that they would shortly deliver a replacement unit, I bought and integrated a different brand. This cost me dozens of hours of integration time. Needless to say, the replacement unit of the first converter then arrived just before the next TUEV inspection date, after only two months …. In the following, you will find pictures and peripheral circuitry of both converters.

Requirements on a DC/DC converter are e.g.

* Water-Proofness: A device with air cooling and a noisy blower wont go anywhere easily - it is too loud for the interior, and too sensitive for the engine bay.

* Output voltage: The output voltage must be 13.8V to 14V (under load as well) so that the 12V battery can be charged and is not depleted during driving. A grid voltage near 14V is also expected by the headlights, which will otherwise not appear “bright”.

* Idle primary power consumption: Especially when the converter is also operated when the car is not running (though perhaps intermittingly), the idle power consumption plays a role. Since the 12V idle current drain of an EV vehicle is usually high and the 12V battery does not charge quickly below 14V, one will normally choose as much “on” time of the converter as possible.

* 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]]

* 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 dc_dc_converter_output_relay. Anyhow, a properly designed DC/DC converter should not require an output relay 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. The most stupid behaviour to implement, in turn, would be a shutdown at maximum current, without automatic recovery. Needless to say, one of the converters I integrated really did have this behaviour. To get this device to start up reliably, an external current limiter circuit is required. Otherwise, it will shut down immediately after switch on, if the battery is just a bit discharged.

The sealed converter (the light grey box at the left edge of the first picture) mounted onto the "controller" auxiliary frame. The mounting position is close to the radiator grill to ensure good ventilation.

Both primary and secondary side are protected by fuses, see 100V distribution box - fuses and 12V distribution box - fuses.

*** back to top of this page ***

astra_conv/conversion/dcdc_converter/dcdc_converter.1395085031.txt · Last modified: 2014/03/17 19:37 by richard