How to Choose the Right Power Supply for your CarComputer System.

When designing your CarComputer system (CarPC and CarMac systems) one of the most important elements to consider, as well as one of the least understood, is the power supply. We will discuss the various types of power sources and how to select the best one for your CarComputer system.

The Car Power System

In order to properly design your CarComputer system you need to know a little about the car’s “12 volt” electrical system.

The typical passenger car electrical system provides “12 volts” to power the various electrical elements of the car. However, the real definition of “12 volts” is important to understand when designing a CarComputer system. The actual voltage available to power accessories within a car can have the following range:

  • During Engine Cranking
    • As low as 6-5 volts if the battery is old, partially discharged, damaged, and/or very cold
    • Typically 7 – 9 volts if the battery is in relatively good condition
  • When the Engine Is Off (see battery chart)
    • 12.7 volts if the battery is “fully charged” (100%)
    • 11.9 volts if the battery is at 40% of its full charge
    • 11.58 – 11.75 volts if the battery is nearly discharged (30-40% charged)
    • 10.5 – 11.3 volts if the battery is “fully discharged”.

Here is a summary of battery voltage vs state of charge.

  • When the Engine Is Running
    • 14+ voltage spikes (noisy alternator or other anomalies like electric door locks, electrical window operation, etc.)
    • 13.8 volts (the typical output from the alternator)

As you can see the “12 volt” electrical system is anything but that. It ranges from about 7 volts during engine cranking to well over 14 volts when electrical impulses are present. Your CarComputer power supply must be tolerant of these input variations and must continue to provide a well regulated output, even at the extremes. Otherwise your CarComputer will “crash” and perhaps sustain damage on one or more of its components.

How are Computers Powered In a “ Normal ” Enviroment?

It is also important to understand how a typical computer is powered under normal (indoor) conditions so that you can better judge how to power your computer in an automotive environment.

Below is a diagram of the typical power process for a computer used in a home or office environment.

The primary input power is provided by the “mains” (110VAC or 220VAC). This “alternating current” (AC) is converted by the PSU (Power Supply Unit) to the voltages and currents required by the various components of the computer. For example, most motherboards require several different DC voltages to run them. Typical voltages include:

  • 3.3 volts DC
  • 5 volts DC
  • 5 volts standby DC
  • 12 volts DC

Notice that all of the voltages produced by the PSU are “DC” (direct current). Since the PSU converts the incoming “AC” (mains) voltage to the appropriate “DC” voltages, the typical PSU is an “AC to DC converter”.

The computer manufacturing community has established limits on both the voltage tolerances (ie regulation), and maximum currents for each “rail” (the 3.3V, 5V, 5VSB, and 12V PSU outputs are typically referred to as “rails”). The specification that governs these tolerances (as well as many other aspects of a PSU) is called the ATX12V Power Supply Design Guide. The latest version is V2.2 published in March 2005 and available here (http://www.formfactors.org/developer/specs/ATX12V_PSDG_2_2_public_br2.pdf)

The typical voltage tolerances defined by the ATX12V are as follows:

Notice that the +12V1DC and +12V2DC tolerances are +/-5%. This means that motherboard manufacturers design their motherboards to accept a maximum input on the +12V rail of 12 x 1.05 = 12.6V, and a minimum input on the +12V rail of 12 x .95 = 11.4V. Thus, a PSU that directly provides a +12V rail to the motherboard must fall with the voltage limits of 11.4 volts to 12.6 volts. This is one very important criterion for choosing a PSU for automotive use. We will explain this in more detail in the following sections.


How Are Computers Powered in an Automotive Environment?

You have seen above how the typical computer is powered in a “normal” (indoor) environment using mains power. However, if you are designing a power system for a computer to be used in an automotive environment (CarComputer), you must make sure that the motherboard, as well as all other components within the computer system, receive the same voltage and current as they would have received in a normal environment. Again, these include the voltages and currents specified in the ATX12V design guide. Since the computer must be powered by car’s electrical system, and the car’s electrical system is based on “+12V DC”, you must convert the car’s +12V power to something the CarComputer can use for all of its components. These are several alternatives that can provide these voltage/current sources. We will discuss each below. These solutions fall into two main categories:

  • DC to AC to DC Conversion (+12V-to-110/220VAC-to-ATX12V rails)
  • DC to DC Conversion (+12V-to-ATX12V rails)


DC-to-AC-to-DC Conversion

This technique converts the car’s +12VDC electrical system first to AC mains, then to the various DC rails specified by ATX12V document. This technique typically uses an “Inverter”.

DC to AC Inverter

Perhaps one of the simplest techniques for powering the CarComputer is the DC to AC Inverter.  They are relatively cheap, readily available, and allow you to use the existing AC PSU that may have come with your CarComputer. They also allow you to use various AC adapters (wall warts) that come with other components such as USB hubs, external sound cards, WiFi adapters, etc.

The major downside to Inverters is the electrical “noise” they produce while in operation. This noise quite often will make its way into the CarComputer audio system and produce a very annoying hum, buzz, or other undesirable effect. In addition, the typical inverter does not include a startup/shutdown controller, which means you must manually turn on or off your CarComputer system. Over time this becomes cumbersome and annoying. And finally, converting your power from +12VDC to 110/220VAC then back to DC is somewhat inefficient.


DC to DC Conversion

The DC to DC conversion technique is typically a more efficient, and generally more elegant solution than the DC-AC-DC approach. There are basically two categories of DC to DC conversion.

  • DC-DC ATX PSUs
  • DC-DC Regulators

DC-DC ATX PSUs

There are several well know suppliers of  DC-DC ATX PSUs (ie Opus Solutions and Mini-Box) as well as a few newcomers and on-going development projects. The diagram below illustrates the fundamental functional blocks of a DC-DC ATX PSU.

The typical DC-DC PSU takes the incoming +12V battery voltage and steps it down to 3.3V, 5V, and 5VSB for these rails. This is a fairly straight forward process that uses standard “buck” switching converter technology.

In a 12 volt automotive environment the most difficult PSU output rail to design and produce is the +12VDC output rail. Since, as was mentioned in the Car Power System section above, the car’s electrical system can produce a voltage that varies from 7 volts during engine cranking to 14+ volts during electrical spikes, the +12V output rail must produce a well regulated output (+/-5%) with an input that is either ABOVE  or BELOW its output voltage. And, it must do this at full load over the entire specified temperature range. This calls for unique and innovative designs, especially at higher load currents. Some products do a good job of this, others don’t.

It is important to understand the load current requirements of your CarComputer so that you don’t overload or damage your ATX PSU as some manufacturers do not provide adequate overcurrent protection on this circuitry.

The DC-DC ATX PSU is a popular solution for many users with full-size motherboards, or with cases or motherboards that can accept a 20-pin ATX power supply connector.

If your motherboard has a 20-pin ATX PSU connector you are probably NOT a good candidate for a CarNetix DC-DC regulator (next section).


CarNetix DC-DC Regulators

CarNetix specializes in high-efficiency, feature rich DC-DC regulators for automotive CarComputer applications. Our models P1260, P1290, and P1900 are conservatively designed specifically for the harsh automotive environment and can accommodate many different computer types and configurations. CarNetix DC-DC regulators are generally useful when you have one of the following CarComputer systems.

  • CarComputer with an internal DC-DC PSU
  • Laptop CarComputer
  • CarComputer case that includes a “12V input “ DC-DC PSU
  • Motherboard with On-board  PSU
  • Non-standard motherboard power connectors

CarComputer with Internal PSU

Perhaps the largest category of regulator candidates is the CarComputer that includes a built-in DC-DC PSU. Many of the newer, smaller computers now include a built-in DC-DC PSU. Examples include the Mac mini, Cappuccino, Xenarc, Sumicom, IWill, EzGo, and many others. In these systems the internal DC-DC PSU is designed to be powered by an AC to DC wall adapter. These AC wall adapters typically produce well regulated voltages of +18, +19, or +20 volts DC. The current requirements vary with the type of system. On the low end the average power requirements begin around 40 watts (Mac mini) and go to the high end of 120 watts (P4-based systems). Below is a functional block diagram of power supply for this type of system.

This type of CarComputer system typically includes an LCD monitor and one or more USB devices. The CarNetix P1290 and P1900 DC-DC regulators were designed specifically to power this type of system. They provide a second and optional third output that allow the user to power the CarComputer, screen, and USB devices all from the same intelligent power source.

In addition, the CarNetix regulators include an sophisticated Startup/Shutdown Controller (SSC) which will automatically start (boot) and stop (shutdown or sleep) your CarComputer when you turn your ignition on or off. The more you use your CarComputer system, the more you will come to appreciate this SSC feature. To use the SSC in your system you will need to solder two small wires to either side of your power switch and run them to the regulator “ACPI” control output. See our Installation Manual for details.

Laptop CarComputer

A CarNetix regulator can also be used to provide power to the typical laptop computer since its power functionality is similar to the example above (CarComputer with internal PSU). Many users install a docking station which receives power from an AC adapter. In this case the P1260, P1290 or P1900 could potentially replace this AC adapter. Take a look at the label on your laptop AC adapter.

If it says 12V, 18V, 19V, or 20V at XX amps, you are a good candidate for a regulator. You will need to make sure the regulator you select can provide adequate current for your laptop and system components.

One benefit of using a CarNetix regulator with you laptop is that standard laptop adapters do not include a Startup/Shutdown Controller. As with the System Computers mentioned above, to use the Startup/Shutdown Controller in your system you will need to solder two small wires to either side of your power switch and run them to the regulator “ACPI” control output. See our Installation Manual for details.

CarComputer case that includes DC-DC PSU

Many of today’s enclosures (cases) are designed to accommodate a mini-ITX motherboard come with a DC-DC ATX PSU included. Examples include the popular Travla and Morex cases. Two of the more popular DC-DC PSUs that are included in these cases are the Travla C4-CT-DC150L and P4-CT-DC150 show below.

             

These DC-DC ATX PSUs require a regulated input voltage. The CarNetix regulators can provide this voltage at power levels suitable for P4 PC applications.

Below is a diagram that illustrates the function blocks of the power system for this configuration:

Below is a compatibility matrix for some popular cases.

Motherboard with On-board PSU

Several of the newer motherboards are now including the ATX PSU regulator functionality on-board. Below is a diagram of where these functions reside in a CarComputer system.

Typical examples of this type of motherboard include:

  1. Commell LV-671
  2. EPIA TC10000 & 6000

Non-standard motherboard power connectors

Some motherboards do not have ATX standard power connections. In these rare situations a DC-DC regulator may be able to be used to provide power. One example we know of is the iCue book PC (BkMVP4) however, each non-standard application is different and will require some diligent homework by the user.

©2006 CarNetix, LLC