Since I am designing and building a H-Bridge now, transistors and diodes have become very important components. I will therefore start with a little post about terminology related to transistors.
Transistors are switches with 3 pins that let current pass from the "collector" pin to the "emitter" pin only when there is sufficient current applied to the "base" pin. We often talk about "open" and "closed" states, when talking about transistors either conducting or not conducting respectively. The following are important characteristics for transistors:
- Type. There are several different ways that transistors are made, each with its own set of pros and cons. Each type has a name that often gets identified with the characteristics typical for that transistor type. This is the complete list, and here is a summary for the most common ones:
- MOSFET (metal oxide semiconductor field-effect transistor). This is the predominant type of transistors today.
- Darlington. When two transistors are connected in such a way that the current amplified by the first transistor is amplified further by the second one create a single transistor with a significantly higher current gain. A "darligton transistor" refers to a single IC that incorporates a pair of transistors in this configuration.
- BJT (Bipolar Junction Transistor). This used to be the predominant type of transistors.
- Power MOSFET. This is a MOSFET designed to handle significant power levels. Its main advantage over IGBT is higher commutation speed.
- IGBT (Insulated-gate bipolar transistor). Made to tolerate significant power levels. Pros include high efficiency and fast switching.
- Package. How does the transistor look like, how big is it, and what power dissipation characteristics does it have? (more on dissipation below). Common packages include: TO-92, TO-220 and TO-3.
- Voltage drop. Transistors are not perfect conductors, and there are two voltage drops, one between the base and emitter pins (when in the open state), and one between the collector and emitter pins. These two voltage drops are called Vce(sat) and Vbe respectively in the transistor data sheets. Typical values for these voltage drops are 0.1-0.2V and 0.7V respectively.
- Commutation speed. This is how long the transistor spends going from a "open" to a "closed" state and vice versa. Usually specified in the time per commutation (milliseconds/nanoseconds) or in number of commutations per second (Hz).
- Forward breakdown voltage. The voltage required before the transistors "give in" and let current flow even though it is not in the open state.
- Max operating temperature. How hot the transistor can become before it stops working temporarily or permanently.
- Power dissipation. How much power the transistor package is able to safely transform into heat during operation. Bridging the transistor to a surface that efficiently conducts away heat such as a heat sink can increase this power dissipation which in turn will allow for more current to pass through the transistor before it will overheat.
- Drive current. The amount of current that the transistor can pass in the open state.
- Current gain. The ratio between how much current is required to trigger the transistor and how much current can pass in the open state. Current gain is referred to as Hfe in transistor data sheets.