In the previous article, absolute maximum ratings and recommended operating conditions relating to output currents were explained. Here, in order to secure the needed service lifetime and ensure safety when using a motor driver, rather than limiting the discussion only to output current ratings, I would like to talk about how derating may be necessary.
Motor Driver Output Currents and Standards
The table below is the same as that presented in the previous column "Absolute Maximum Ratings of Motor Drivers". This motor driver IC incorporates power MOSFETs to form an H-bridge. From these conditions, while the output current has a maximum rating of 3 A, the maximum recommended output current is 2.4 A, and if Tjmax = 150°C is not exceeded, we can see that use under these conditions should not cause problems.
High Operating Temperatures Shorten Service Lifetimes
In actuality, if the device is used in conformance with the recommended operating conditions, essentially no problems should arise; but the reliability (operating lifetime) of semiconductor components such as transistors and ICs will differ depending on usage conditions. Temperature is a major factor determining the acceleration factor that is used when deriving such indices of reliability as MTBF and FIT. Put simply, the higher the temperature, the greater is the acceleration, meaning that degradation occurs more rapidly and the lifetime is shortened, or in other words, reliability suffers.
Where component and material lifetimes are concerned, there is an empirical rule called the "10°C doubling rule (10°C halving rule)". This means that if the temperature is 10°C higher (lower), the lifetime is half (or double) the ordinary lifetime; it is based on the Arrhenius law used to calculate the acceleration factor related to reliability. For aluminum electrolytic capacitors, which require careful consideration of lifetime, the lifetime may be indicated as "105°C/2000 hours", where a 10°C difference means a halving, or doubling, of the lifetime. Semiconductor devices have far longer service lifetimes than do capacitors, so this may not strike the reader as being too serious, but the principle is the same. Apart from temperature, such influences as humidity and chemical reactions are also related to the acceleration factor, and setting aside actual numerical values, the harsher the conditions, the shorter the lifetime becomes.
Derating is Necessary to Secure Reliability and Safety
Derating involves providing a margin for a specification rating. It is used not only for temperature, but for withstand voltages and other quantities as well. In the above example, a 2.4 A driver current can be passed continuously, but instead of using the device at close to Tj = 150°C, for example the current could be limited to 2 A (after having performed thermal calculations), and the ambient temperature Ta held to under 60°C. These measures are related to reliability, of course, but also to safety.
In particular, where power devices and motor drivers are concerned, it is important for safety that the output-stage transistors operate within the range of safe operation (SOA or Safe Operating Area, also called ASO or Area of Safe Operation).
In the above example, the driver IC incorporates four MOSFETs in an H-bridge, and so derating is performed, that is, the margin is considered not in terms of the separate transistors, but relating to the ratings as an IC, the allowable losses (PD) of the package, and ultimately the conditions under which Tjmax is not exceeded. However, when a controller-type motor driver IC is used and the H-bridge is configured externally, the safe operating area must be studied when selecting and evaluating the transistors.
Safe Operating Area（SOA・ASO）
Below is an example of a diagram of safe operating area characteristics for a MOSFET. The safe operating area is the area on the inside of the blue curve (the area in which the voltage and current are smaller).
The safe operating area is simply on the inside of the VDS and ID ratings, but to this are added the limitations of the allowable loss (heat) and secondary breakdown*1. In addition, there is a limit imposed by the on-resistance of the MOSFETs (when VDS is low, Ohm's law stipulates that ID cannot flow at the rated value), but this is omitted from the graph (please see this for more information).
- ①：This is the limit imposed by the VDS rating, in this example 20 V.
- ④：The ID rating limit, in this example 2 A.
- ③：While on the inside of the VDS and ID ratings, this is the limit imposed by the allowable loss (heat). This is the boundary beyond which heat generation（VDS×ID×package thermal resistance) ＋Ta ＝ Tj exceeds Tjmax.
- ②：Limit imposed by secondary breakdown*1. If this is exceeded, thermal runaway may occur, culminating in degradation or failure.
To confirm that operation is within the safe operating area, the actual voltage and current must be measured. In motor driving, because the coils are an inductive load, phase differences occur between voltage and current, and this must be considered when determining the voltage (VDS) and the current (ID).
*1: "Secondary breakdown" was originally a term used to refer to a thermal runaway state in a bipolar transistor, in which, when an area of high voltage and large current is entered, concentration of current occurs and hot spots appear, causing the impedance to fall and resulting in still greater currents. Strictly speaking, this term is not used for MOSFETs. However, in MOSFETs also, heat may cause the gate threshold voltage to fall and the channel resistance to fall so that the current increases, causing a further rise in temperature so that the current increases further in thermal runaway. Hence there are not a few cases in which what is referred to as secondary breakdown occurs in MOSFETs as well in this area, and so we have used the term "secondary breakdown".