Useful information sharing! Understand the internal structure of power chips in one article![Copy link]
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As a power supply R&D engineer, you will naturally deal with various chips frequently, but some engineers do not know much about the internals of chips. When applying a new chip, many students directly turn to the application page of the datasheet and build the peripheral according to the recommended design. This does not cause any problems in application, but ignores the technical details, which is not conducive to their own technical growth and accumulation of experience.
I searched through the information, but I was still confused. I was stunned! At this time, I found this article about the internal structure of power chips from various platforms. Today, I will share it with you!
This article will take a DC/DC step-down power supply chip LM2675 as an example and try to explain the internal design principle and structure of the next chip in detail.
This diagram contains all the internal unit modules of the power chip. We have already understood the BUCK structure. The main function of this chip is to drive the MOS tube and form a loop control PWM drive power MOS tube through the FB pin to detect the output state to achieve voltage regulation or constant current output. This is an asynchronous mode power supply, that is, the freewheeling device is an external diode, not an internal MOS tube.
Let us analyze how each function is implemented.
The reference voltage
Similar to the reference power supply of board-level circuit design, the internal reference voltage of the chip provides a stable reference voltage for other circuits of the chip. This reference voltage has high requirements, good stability and small temperature drift.
The reference voltage inside the chip is also called the bandgap reference voltage. Because this voltage value is close to the bandgap voltage of silicon, it is called the bandgap reference. This value is about 1.2V, as shown in the following structure:
It can be seen that it is an exponential relationship. Is is the reverse saturation leakage current (i.e. the leakage current caused by minority carrier drift in the PN junction). This current is proportional to the area of the PN junction! That is, Is->S.
In this way, we can deduce that Vbe = VT*ln(Ic/Is)!
Back to the figure above, from the analysis of the op amp, VX=VY, then I1*R1+Vbe1=Vbe2, so we can get: I1=△Vbe/R1, and because the gate voltages of M3 and M4 are the same, the current I1=I2, so the formula is derived: I1=I2=VT*ln(N/R1) N is the ratio of the PN junction area of Q1 Q2!
In this way, we get the reference Vref=I2*R2+Vbe2. The key point is: I1 has a positive temperature coefficient, while Vbe has a negative temperature coefficient. By adjusting the N value, we can achieve good temperature compensation! We get a stable reference voltage. N is generally designed according to 8 in the industry. To achieve a zero temperature coefficient, we can deduce Vref=Vbe2+17.2*VT according to the formula, so it is about 1.2V. At present, a reference of less than 1V can be achieved in the low-voltage field. In addition to the temperature coefficient, there are also problems such as power supply ripple suppression PSRR. Due to my limited level, I can't go into it in depth. This is the simple diagram. The design of the op amp is of course very particular:
We know that the basic principle of switching power supply is to use PWM square wave to drive power MOS tube, so naturally we need a module to generate oscillation. The principle is very simple, that is, to use the charge and discharge of capacitor to form sawtooth wave and comparator to generate square wave with adjustable duty cycle.
A technical difficulty here is the slope compensation in current mode. In order to stabilize the slope when the duty cycle is greater than 50%, an additional compensation slope is added.
Error Amplifier
The function of the error amplifier is to ensure constant output current or constant voltage and to sample the feedback voltage, thereby adjusting the PWM that drives the MOS tube, as shown in the diagram:
The other module circuits here are to ensure that the chip can work normally and reliably . Although they are not in principle, they do occupy an important position in the design of the chip.
Specifically, there are several functions:
1. Start the module
The function of the startup module is to start the chip. Because at the moment of power-on, the current of all transistors may be 0 and remain unchanged, so it cannot work. The function of the startup circuit is equivalent to "igniting" and then shutting down. As shown in the figure:
At the moment of power-on, S3 is naturally turned on, and then S2 is turned on to turn on M4 Q1, etc., which turns on M1 M2. The constant current source circuit on the right works normally, and S1 is also turned on, so S2 is turned off to complete the startup. If there is no S1 S2 S3, the current of all transistors is 0 instantly.
2. Overvoltage protection module OVP
When the input voltage is too high, the output is shut down by the switch tube to avoid damage, and a protection point can be set by the comparator.
Temperature protection is to prevent the chip from being damaged by abnormally high temperature. The principle is relatively simple. The temperature characteristics of the transistor are used and the protection point is set by the comparator to shut down the output.
In the event of an output short circuit, for example, the output current can be detected to feedback and control the state of the output tube, thereby shutting down or limiting the current.
As shown in the figure, current sampling uses the current and area of the transistor to sample. Generally, the area of the sampling tube Q2 will be one thousandth of the area of the output tube, and then the drive of the MOS tube is controlled by a voltage comparator.
There are also some other auxiliary module designs.
Constant current source and current mirror
Inside the IC, how to set the working state of each transistor is through bias current. The constant current source circuit can be said to be the cornerstone of all circuits, and the bandgap reference is also generated for this reason. Then the current is provided to each functional module through the current mirror. The current mirror sets the required current size through the area of the transistor, similar to a mirror image.
The above is roughly the entire internal structure of a DC/DC power supply chip LM2675, which can be regarded as a review of the previous superficial knowledge.
Of course, this is only the basic structure in principle. The specific design needs to consider many parameters and characteristics, and requires a lot of analysis and simulation. In addition, it is necessary to have a deep understanding of semiconductor process parameters, because the manufacturing process determines many parameters and performance of transistors. If you are not careful, the chip will have defects or even cannot be used at all. The entire chip design is also a relatively complex system engineering, which requires good theoretical knowledge and practical experience.
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The main reason is that there are more modular products now, and most of the time we just pick them up and use them. We are also busy at work and don’t have much time to do research.
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Published on 2024-3-5 10:42
After learning, it is true that as mentioned above, fewer and fewer people make hardware, and the internal principles of IC are also worth learning about embedded systems.
Thanks for sharing. After so many years, I have forgotten some circuit principles, formulas and algorithms. I can’t just do engineering, I have to do theoretical research too.
624801474 Published on 2024-3-5 07:25 Thank you for sharing. It's OK. Now it feels like fewer and fewer people are engaged in hardware. I hope the OP will stick to it.
The main reason is that there are more modular products now, and most of the time we just pick them up and use them. We are also busy at work and don’t have much time to do research.
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