Series of articles catalog
Base element 1.
The circuit design 3.PCB design 4. The welding element 6. programming 9. testing standards
Article Directory
Preface
For those who can't find the direction of struggle after graduating from university (updated from time to time every week) the
actual design problems and considerations are much more than those listed here. Rome was not built in a day, so it needs to be built over time.
Embedded System Designer Exam
1. Thinking of abnormal situation
Thinking of abnormal situations
1. Current backflow
The typical model of integrated circuit is as follows:
1. D1 plays an anti-static function in most CMOS integrated circuits. At the same time, it also assists in limiting the input end. But there is no such diode in ABT, LVT, LVC and AHC/AHCT integrated circuits.
2. D2 is a parasitic diode (present in all digital integrated circuits) produced by semiconductor integration. Its auxiliary function is to limit the undershoot signal reflected by the line and provide some discharge protection functions.
3. D3 is used to protect CMOS circuit from interference during discharge. This diode is also present in most bipolar devices, but it is a parasitic diode. There is no such diode in bipolar devices with open collector and tri-state output.
4. D4 has this diode in all integrated circuits. It is the diode of the collector or drain of the device. A Schottky diode is added to the bipolar device to limit the undershoot signal reflected by the line. A diode is added to the CMOS circuit to increase the anti-static function .
Causes of current backflow:
When using a CMOS device as an interface chip in the circuit shown in the following figure, if Vcc2 is powered off, Vcc1 continues to supply G1, and the high-level output current of G1 will charge the capacitor on Vcc2 through D1 (the charging current will Overload and damage D1 quickly. In CMOS devices, D1 can only withstand a current of 20mA) and establish a voltage on Vcc2, which makes other circuits powered by Vcc2 work abnormally, especially for programmable devices .
Solution:
As shown in figure (a): Add a current-limiting resistor of several ohms to the signal line to prevent overcurrent from damaging the diode D1, but it cannot solve the problem of establishing a voltage on Vcc by the perfusion;
Figure (b): Add a diode D3 and a pull-up resistor R to the signal line. D3 is used to block the perfusion path. When R solves the high level output of the previous stage, the input of G1 remains high. This method can solve the problem of damaging the diode D1 by the perfusion, and can solve the problem that the perfusion establishes a voltage on Vcc. The disadvantage is that the addition of diode D3 reduces the low-level noise tolerance of G1;
Figure (c): Add diode D7 to the power supply of G1. The disadvantage is that when the front stage outputs a high level, G1 obtains the voltage through D1 and outputs the high level to the subsequent stage circuit. At the same time, the power supply voltage of G1 is reduced, so that the high-level output voltage is reduced during normal use.
The most effective solution is to use a bipolar device (such as a triode) as an interface. Since the bipolar device does not have a protective diode D1, the above-mentioned perfusion path does not exist. It should be noted that pull-up resistors cannot be added to the input and output signal lines of the interface at this time (the input of a bipolar device is suspended and treated as a high level).
2. Hot plug design
Ø The impact of hot swap on the power supply
When the circuit board is powered on or hot swapped, a large starting current will be drawn from the power supply and cause fluctuations in the power supply voltage. Improper control of this phenomenon will affect the normal use of other circuits in the system, and even cause damage to the entire system.
Any board card has a certain load capacitance. When the board card is inserted into the normal working backplane, the backplane power supply will use a large instantaneous current to charge the load capacitance of the inserted board card; when the board card is pulled out from the normal working backplane , Due to the discharge of the load capacitance on the board, a low-resistance path will be formed between the board and the backplane, and a large instantaneous current will also be generated. The surge phenomenon can cause the backplane power supply to fall instantaneously, cause the system to reset unexpectedly, and even damage the interface circuit. The research on the hot-swappable protection circuit will become the key to the promotion and application of the backplane structure equipment .
The minimum requirement of the hot-swap circuit is to provide inrush current limit to prevent damage to the entire system when a large capacitive load is energized. The current limiting function also helps reduce the size of the power supply and prevents arcing when the connector contacts. Other hot-swappable features include: low equivalent series resistance, circuit breaker, status indication, dual insertion point detection and power ready indication.
At present, our company's products use PTC to limit the load overcurrent, except for individual processors that take power-on and current-limiting measures, but there is no power-on and current-limiting measures.
①. The simplest current-limiting component is a fuse, which can be used alone or in conjunction with other protection components. Because fuses can effectively prevent the impact of overcurrent, they are both necessary in the system (such as UL standards) and The ultimate line of defense when the system encounters a catastrophic failure. The main drawback of standard fuses is that they can only be used once. Another alternative small device is multiple fuses. The physical size of this fuse can be expanded or shortened according to the heat generated by the current flowing through it. Multiple fuses work The voltage range is limited by temperature, but it can reset itself, which is the biggest advantage over standard fuses.
② The ordinary hot-swap circuit is composed of capacitor, Zener and FET, as shown in the figure below. The purpose of limiting the surge current is achieved by charging the capacitor C1 connected between the gate and source of Q1. If C1 discharges during power-up, the gate and source of Q1 are equivalent to a short circuit, and Q1 will remain open. When C1 is charging, Vgs increases and Q1 turns on slowly. The size of C1 and the Vgs index of Q1 determine the turn-on time of Q1 and the charging time of the load capacitor C2. Zener ZD1 is used to prevent the gate-source voltage from exceeding its maximum rating.
③. In terms of mechanical structure, the ground pin and power pin of the USB plug are designed to be relatively long, while the two data pins are relatively short, so that when plugged into the socket, first connect the power and ground, and then turn on Two data lines. This can ensure that the power is turned on before the data line to prevent latch-up.
④. Examples of hot- swappable protection circuit design process Examples of
hot-swappable protection circuit design process
Ø Hot swap of interface IC
If the circuit board is powered on or hot-swapped improperly, it will affect the normal use of other circuit boards in the system through the signal line, and may also cause soft or hard damage to the interface IC. Therefore, the following requirements should be met as much as possible in the system and circuit board design:
When the circuit board is hot-plugged, it must be ensured that the ground terminal is connected first, which is the basis for the normal operation of the circuit board . In a multi-power system, especially a system with negative power supplies used at the same time, if you cannot ensure that the ground terminal of the circuit board is connected first during hot plugging, you should try not to use large-capacity capacitors on the negative power supply of the circuit board, because here Under circumstances, the ground potential of the circuit board may deviate to a negative potential, causing the input and output pins of the interface IC to exceed the tolerance range of the ground voltage, causing damage to the interface IC pins.
1. Use ICs without protective diodes on the input or output terminals;
2. Use an IC with power-on tri-state function.
The correct circuit board power sequence should be:
first connect the ground of the circuit board;
secondly connect the power supply of the
circuit board ; connect the reset terminal of the
circuit board ; and finally connect the signal terminal of the circuit board .
3. Overcurrent protection
Overcurrent protection technology is commonly used in power supply design and can be used for reference in circuit board design. The circuit board power supply may be over-current due to the abnormal operation of the device or failure or damage. If this is not restricted, it may bring catastrophic consequences to the system.
A small resistance PTC element connected in series at the power inlet of the circuit board can effectively protect the power supply. When the circuit board generates overcurrent, the current flowing through the PTC increases, which increases the temperature of the PTC and increases its resistance. The further increase of the limiting current limits the current entering the circuit board to a smaller range, which can effectively protect the circuit board without affecting the normal operation of other circuit boards. Another advantage of using PTC is repeatability. When the over-current condition does not exist, the temperature of the PTC will drop and the resistance will return to the normal state, which will not affect its normal use .
When choosing PTC, you need to pay attention to its withstand voltage, non-operating current, static resistance and operating time. Case: Our OC output, control solenoid valve. One end of the solenoid valve is connected to 12V, and one end is connected to OC output. However, during the installation process, it often appears: due to careless construction, the OC is directly short-circuited with 12V, causing the triode or MOS tube to fail directly, causing a large number of triodes on the scene to burn out (this text, the blogger said that it did not understand)
After optimization, Through the following circuit, to prevent over-current, resulting in damage to the transistor. (This text, the blogger said that he did not understand)
