EV Fundamentals: DCDC Converter
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DCDC Converter
04.48
| 00:00 | A key component in all electric vehicles is the DC-DC converter. |
| 00:04 | These can be thought of as the alternator of an EV as it's essentially performing the same task of charging the low voltage system, except that instead of getting electricity from a spinning pulley, the DC-DC converter takes electricity from the high voltage battery and converts it or steps it down into a lower voltage to be stored in the 12 volt battery. |
| 00:22 | Because the DC-DC converter only gets power when the main battery is online, that is when the main contactors are closed, the DC-DC can only charge the low voltage battery when the car is on. |
| 00:32 | Some EVs are always onand some, especially older electric vehicles, can kill the 12 volt battery just like a normal gas car can if you leave the vehicle on without the high voltage system online. |
| 00:43 | A modern EV though will generally automatically wake up on its own accord and bring the high voltage system up to charge the low voltage battery and occasionally do other system checks. |
| 00:52 | DC-DC converters are typically controlled via CAN with a command message that contains the target voltage and sometimes the maximum current. |
| 01:00 | The converter will then typically broadcast signals like its input voltage and current, output voltage and current, as well as its internal temperature and a status message or two. |
| 01:09 | Using this information, clever decisions can be made by the VCU like lowering the target voltage to reduce the current load on the DC-DC, triggering it to turn on, or ramping up a water pump if the DC-DC is liquid cooled. |
| 01:21 | In addition, if a power distribution module is being used or a current sensor is mounted on the 12 volt battery, the current going into and out of that battery can be recorded and its charging can be current limited to prolong its life. |
| 01:33 | This is particularly useful if a lightweight lithium low voltage battery is being used. |
| 01:37 | It's important to be aware that the DC-DC is often built into other high voltage power electronics. |
| 01:42 | For example, the Tesla Model 3 combines the DC-DC with the onboard charger and there are a number of aftermarket products that combine an onboard charger and a DC-DC into one package. |
| 01:52 | Looking towards the higher end of the market, some DC-DC converters even have multiple outputs at different voltages. |
| 01:59 | This is helpful since the DC-DC converter has an input voltage in the hundreds of volts and a 24 or 48 volt output for things like power steering or other high current loads is a great way to reduce component size and wiring weight. |
| 02:11 | It's also now becoming common to use the DC-DC bi-directionally to pre-charge the vehicle's high voltage battery. |
| 02:17 | This means that instead of using a pre-charged contactor and a pre-charged resistor to slowly charge the vehicle's high voltage bus and fill those capacitors in the inverter, the DC-DC can take energy from the 12 volt battery and boost it up to the high voltage battery's voltage. |
| 02:32 | The BMS then sees that both sides of the contactors are at the same voltage and closes them bringing the high voltage system online. |
| 02:38 | This is a lightweight and reliable method to pre-charge the system and it's something that some OEMs and race teams are now utilizing. |
| 02:45 | If you're shopping for a DC-DC converter, you'll first need to ensure that it's compatible with the voltage range that your HV battery will operate in and that it matches your output voltage. |
| 02:54 | Most systems are designed for 14 volts, but there are also 24 volt systems out there. |
| 02:59 | You'll also need to decide between air-cooled and liquid-cooled version. |
| 03:02 | Liquid-cooled components are always more work to integrate but are usually the best way to go as they'll typically be more reliable due to operating in lower temperature and they're often lighter as well because fewer heat sinks are required. |
| 03:12 | Beyond that, we need to confirm that both the power and current ratings will satisfy the needs of the vehicle. |
| 03:18 | Typical sizes for aftermarket DC-DC converters are 1 and 1.5 kilowatts and while 1 kilowatt might sound like a lot, that works out to about 60 to 70 amps at 14 volts. |
| 03:28 | By the time you consider a few radiator fans, water pumps, and an ABS module, we can easily be over that. |
| 03:34 | If we then add electric power steering or an electric brake assist or vacuum pump, we've quickly blown past 70 amps so it's crucial to properly estimate the current load that a vehicle will draw before selecting a DC-DC. |
| 03:45 | Of course, the 12 volt battery will act as a buffer, meaning that if the current limit is momentarily exceeded, the 12 volt battery will still support the system. |
| 03:53 | Some DC-DCs do a better job of handling overcurrent events than others though so it's always best to pick a DC-DC that has enough overhead that it can comfortably handle the peak power load, especially if you don't have a fancy VCU that can properly manage the 12 volt power load by turning off certain components if it sees that the DC-DC is near its maximum output. |
| 04:12 | That's just about everything you need to know about DC-DC converters so let's do a quick recap. |
| 04:16 | In this module, we learned that DC-DC converters are like the alternator of an EV system, taking high voltage current from the main battery and stepping it down to 12 or sometimes 24 volts to be fed into the low voltage battery which in turn powers the car's low voltage devices. |
| 04:29 | The DC-DC is typically controlled by the VCU via CAN and like a VCU, their capabilities and complexity vary depending on price and application. |
| 04:37 | Aftermarket examples can sometimes be found paired with an onboard charging unit , which we'll be looking at next in the following module. |
