This example takes the driving relay as an example to describe the relevant circuit design, MOS tube selection, and understand the chip manual.
The drive circuit is as shown below
The role of D1 is to discharge the reverse electromotive force of the relay
Relay parameter
The 24V relay has a large load of 25A/250VAC and a coil resistance of 640 ohms.
By simple calculation, the current flowing through the coil is 37.5mA.
Here is a background knowledge. The coil of the relay is an inductive load. At the moment of power-on and power-off, a reverse electromotive force will be generated. When the power is on, this electromotive force is in the same direction as the power supply.
When the power is off, the direction of the electromotive force is opposite to that of the power supply, and the bottom is positive and the top is negative. At this time, the diode D1 is turned on, and the electromotive force is short-circuited and discharged. For low-voltage circuits, 1N4007 diodes can be used, with a withstand voltage of 1000V and a current of 1A, which is cheap and easy to use. Pay attention to fast recovery diodes can be used, the performance is better. The leakage current is slightly larger and can be ignored here.
MOS tube (hereinafter collectively referred to as chip) selection
The selection of this MOS tube is very simple. The key parameters only involve voltage and current, and the others can be ignored.
Withstand voltage: 1.2~1.5 times the power supply voltage or higher
Current: 5 times the working current is enough, that is, more than 150mA is enough.
Power consumption: The Rds of the general tube is only tens of milliohms, and the power consumption must meet the demand.
Switching frequency: Don't think about it, it must be enough. The operating frequency of the relay cannot exceed 100Hz, and any tube is enough.
Sort by sales volume, and then scroll down to find a price that the brand can accept, such as the one in the picture below. N-channel is generally cheaper than P-channel, so pay attention to this selection.
Check the chip documentation to determine parameter matching.
For today's application, the information on the first page of the chip is enough, 30V, 5.2A, 27 milliohms, can meet the needs.
The SOT23 package is also small, saving board space. Try to choose SMD components, so that the cost can be reduced in mass production.
Why read English documents
It's very simple, because chip manufacturers generally provide English documents, even domestic manufacturers.
Some engineers are not good at English, and they are very reluctant to read the English version of the chip manual, and try to find Chinese documents translated by others on the Internet. That's how I started too. However, I gradually found that these translated documents, except those provided by the original factory (for example, TI has done a good job in this area), are generally relatively old, and often the words do not convey the meaning, and the manuals are incomplete. Later, they gradually become Go see the English manual.
Quickly understand the chip manual
The chip manual is generally divided into several parts, and the order is not necessarily the same, but basically includes the following parts:
brief introduction
The first page is generally a brief introduction of the chip, which will explain the chip performance, key parameters, and packaging, so as to facilitate the rapid selection of engineers.
Table of contents
The next step is the directory. There is nothing to say about this, it is to allow engineers to quickly locate the specific details they want to find. Simpler chips don't have this. For example, the MOS tube mentioned today has no catalog
Pin Description and Typical Application Circuit
The pin description is as shown above.
For multi-pin chips, there are generally. For simple components such as triodes and MOS tubes, generally there is no application circuit.
Limit parameter
Briefly explain:
VDS Maximum drain and source voltage 30V
VGS Gate and source voltage maximum ±12V
Drain current: maximum 5.2 and 4.6A, respectively corresponding to different temperatures
Maximum drain pulse current: 20A
Maximum power consumption 1W
Storage temperature range: -55 to 150
Working environment temperature: -55 to 150
The simple understanding is that, beyond this range, the components will be damaged, so do not exceed these specified values.
Electrical parameter table
This table contains the most basic parameters, such as voltage, on-resistance, gate turn-on voltage, capacitance between electrodes, etc. You
can check it carefully, and it will be clear at a glance when you look at it later.
chart
The relevant parameter is not a constant value, but changes with the change of other parameters, and the manufacturer uses some charts to show these changes. Look at the horizontal and vertical axes, and then look at the description in bold, and you will understand.
Packaging and Layout
Line 1 states that the package is SOT-23, and each package is 3000/reel.
The table on the lower right gives the dimensional parameters of each part of the component, with metric units (mm) on the left and imperial units (inches) on the right.
This manual does not give Layout suggestions, I found a screenshot of the manual of another MOS tube, as follows:
Each dimension has 2 values, the upper unit is inches and the lower unit is millimeters.