Picking the Right Transistors

Strangely enough picking the correct transistor for the job can sometimes be the most challenging. A number of factors affect the choice, operating voltage, current, cost, etc. and all of those effect each other in often unpredictable ways. If you imagine that each choice is an axis in some multi-axis space, the volume the choices encompass is called the "design space." Since we're building an H-bridge, not a nuclear reactor, we can be a bit loose with our design constraints.

Pretty arbitrarily I've chosen to design an H-bridge that lets you control motors that take a DC voltage between 3 and 24 volts, and when stalled do not consume more than 5 or 6 amps of current. So that is motors that consume somewhere between 15 and 150 watts of power. The good news is that there are literally thousands of motors in this category. They are often found in radio controlled toys you might buy on clearance or at a garage sale, and a number of places like Jameco, Scientifics Online, and others sell them as well.

The canonical example motor is the Mabuchi RC-280SA. This motor typically runs at .75 - 1.5 A and stalls at under 7 amps. It provides good torque when geared down and is found in a variety of toys and models. This motor is "too big" for the single chip H-bridges like the SN754410 and L293D.

Factors to Consider

The two major factors that control this design are current capacity and cost. I wanted to have a design that was inexpensive to build but controlled a wide variety of easily accessible motors.

There are H-bridge chips that control 3 to 6A but those are generally expensive and you don't learn much by soldering them on to a board :-). So, low cost was key but so was capability.

Transistor Packages and Heat

As we learned in the theory pages, one of the limiting factors on transistors is how much heat they can dissipate before the silicon inside them melts.  There are "high power" packages such as the TO-3 and "low power" packages such as the TO-92, the "midrange power" package is the TO-220. The TO-220 doesn't take up as much volume as the TO-3 but can dissipate over twice the power of the TO-92 without a heatsink and it is easy to add a heatsink for more power capacity.

Package	Power Characteristics
	The TO-92 package is fairly small and comes in either a metal or plastic version.  They can dissipate about 200 mW before you need to provide some form of heat sink.
	The TO-220 package is a good compromise. By itself it can dissipate a couple of watts but with an attached heat sink it can dissipate 4 - 5 watts. There are a wide variety of sinks available for this part. Note however that the collector is electrically connected to the metal tab so if you want to put multiple transistors on the same heat sink they should be isolated with a mica insulator and use non-conducting hardware.
	The TO-3 has even better power dissipation but this comes at the cost of taking up space. Designed to be attached to a heat sink by the end tabs, the TO-3 can dissipate close to 3 watts without a specific sink, but by connecting the case to a large copper pad on your circuit board it can use the circuit board as a heat sink. Like the TO-220 the case is connected to the collector.

Forward Breakdown Voltage

Transistors are like any semiconductor, when the voltage across them gets too large the silicon can't resist the flow of current any longer and they go into "avalanche" mode. Not pretty and it usually results in the destruction of the device. Unlike FETs, most BJTs have plenty of headroom here. I've picked a couple that can do 100V. So if you decided you really had to use 90V DC motors you could. Note however that the design that is discussed later in this series is not designed to work with high voltage motors.

Hfe and Drive Current

Finally, transistors are current driven devices. What that means to this design is that for a given output current of say 5 amps, and the ability of a microprocessor pin to source at most 20 milliamps (less required is better), the H_fe of our transistors must be high.

Alternatively we can use a transistor to drive a transistor. This was pretty common in the early days when the H_fe spec for a power transistor was 40. (Given an H_fe of 40, a 5A collector current requires 125 mA of drive!) Fortunately for us, there are power darlington transistors that incorporate the base drive transistor in the same package as the power transistor. Sort of a 3 pin, two transistor integrated circuit. This configuration is called a Darlington configuration after its inventor.

The TIP102 and TIP107

Given all of the constraints, the best choice for this project was the TIP102 and TIP107 power transistors. These transistors are packaged in the TO-220 package which can dissipate 2W without a heat sink. The transistors have a breakdown voltage of 100V (which easily covers our 3 - 24V requirement). Finally, because they are internally a darlington pair, they have extraordinarily high H_fe values (as high as 20,000). Finally, they are pretty darn cheap at less than $1 each in single quantities from a number of places.

These transistors are sold as a "complimentary pair" which means they are designed to be used together and many of their characteristics such as voltage drop, current gain, and breakdown voltage are identical. That further simplifies their use as the switching element in the BJT H-bridge.

As an added bonus the folks who make these transistors include a freewheel diode in the package. That can make driving inductive loads such as motors with them simpler still. But before we talk about that, lets talk about implementing an H-bridge with the TIP102 and TIP107 transistors.

Next: Implementing an H-bridge ...

Transistor Theory		Bridge Implementation