To use my USB charger boards in the car, I need to convert the nominal 12 volt supply to 5 volts at up to 2 amps (a 10 watt output). The first choice in voltage regulation is often the ubiquitous 78xx (where xx is the desired output voltage). For low power loads, this is an effective, inexpensive choice, but for higher power applications, these regulators don't work so well.
The input current to a linear regulator will be equal to the output current at the lower voltage plus a negiliable amount to operate the regulator itself. In this case, the output is 2 amps at 5 volts, equaling a 10 watt output. The input is therefore 2 amps at 14 volts (when the car is running), which equals 28 watts of input power. 28 watts input for 10 watts output??? Where does the extra 18 watts go? HEAT. A nightlight bulb is 4 watts for comparison and that's more than enough to burn your fingers, so 18 watts is a huge waste! It would take a large heat sink to dissipate this much heat to keep a linear regulator at a safe temperature.
There are two important equations to keep in mind when using a linear voltage regulator. The first consideration is the power dissipated by the regulator in the form of heat.
Two factors determine the power dissipation: the output current and the voltage drop across the regulator. Consider a 5 volt regulator supplying 500 mA from a 12 volt source.
Thinking of that 4 watt nightlight bulb, that's a lot of heat to dissipate! If we could reduce the supply voltage, say to 7 volts, what would the result be?
That's a nice reduction in wasted power. Many dev boards use a 5 volt regulator with a higher voltage wall wart. Even though a 7805 regulator can supply 1.5 amps, it's still limited by the power dissipation. Using a supply closer to the desired voltage can prevent overheating problems.
The second equation to note is the efficiency of a linear regulator. It is almost entirely controlled by the voltage drop across the regulator; the quiesent current of the regulator reduces this slightly but it's largely insignificent with any large load.
So for the first example,
Only 42% of the power is going to our circuit with 58% going to heat! Reducing the input voltage from 12 volts to 7 really improves the situation.
Getting back to the iPad/iPhone car charger, the supply voltage will range from 12 - 14 volts or more and I need to draw 2 amps of current. Clearly, a linear regulator is not a good choice for this application.
Switch mode power supplies are fundamentally different than linear regulators. Instead of dissipating energy as heat to control voltage, a switch mode power supply converts the low voltage DC into a high frequency AC voltage and converts it back to DC at the desired voltage. There are many different topologies for switch mode power supplies and a detailed discussion is beyond the scope of this article.
For the car charger, I'm using a switch mode module that accepts 12 - 24 volts input and outputs 5 volts at up to 3 amps. There are a variety of these available from eBay for under $10.
So how does this $10 module compare to 7805 linear regulator? This module claims to take up to 24 volts, and provide a 3 amp output at 5 volts. From the above equations, we can calculate that a linear regultor would be dissipating 57 watts! Since there's no massive heatsink, we can guess this module is better than a linear regulator.
To evaluate this module's performance, I connected a 12 volt supply andmy variable load through my power monitor and measured input voltage and current and output voltage and power. From these measurents, I calculated input and output power and efficiency.
The efficiency ε is calculated by
The results are shown in this graph. The efficiency is greater than 80% over most of the operating range. Very little power is wasted as heat. With no load connected, the module draws about 10 mA, an important parameter for operating in the car.
I elected to use pre-built modules in this case but the design and layout of a switching power supply is simple with modern components. Many vendors offer on-line calculators to aid in circuit layout and component selection. Depending on output parameters, modern switching power supplies are nearly as simple as inefficient linear regulators.
One additional difference between linear and switching circuits is worth noting. The input power to a linear regulator increases with supply voltage, resulting in increased power dissipation. With a switching circuit, the input power remains essentially constant with supply voltage. This is generally a good thing but bear in mind, as the input voltage is decreased, the current must increase to maintain the power. If the voltage is too low, the required current can overload the components of the regulator circuit. I managed to burn up an switching LED driver module by starting at a low supply voltage (outside the normal operating range). Because I was being cautionous and starting slowly, the excessive current draw blew up the module!