The basic principle of switching power supply is to use PWM square wave to drive power MOS tube

As a power R&D engineer, naturally I often deal with various chips. Some engineers may not know the internals of the chip very well. Many students directly turn to the application page of the Datasheet when applying a new chip, and build the peripherals according to the recommended design. That’s it. In this way, even if there is no problem with the application, more technical details are ignored, and no better experience has been accumulated for its own technological growth.

As a power R&D engineer, naturally I often deal with various chips. Some engineers may not know the internals of the chip very well. Many students directly turn to the application page of the Datasheet when applying a new chip, and build the peripherals according to the recommended design. That’s it. In this way, even if there is no problem with the application, more technical details are ignored, and no better experience has been accumulated for its own technological growth. Today, take a DC/DC step-down power supply chip LM2675 as an example, try to explain the internal design principle and structure of the next chip in detail, students in the IC industry just have a look at it, welcome to advise!

Typical application circuit of LM2675-5.0

The basic principle of switching power supply is to use PWM square wave to drive power MOS tube

Open the DataSheet of LM2675, first look at the block diagram

  The basic principle of switching power supply is to use PWM square wave to drive power MOS tube

This picture contains all the internal unit modules of the power chip. We already understand the BUCK structure very well. The main function of this chip is to drive the MOS tube, and through the FB pin to detect the output state to form a loop to control the PWM drive power MOS tube , To achieve voltage regulation or constant current output. This is a non-synchronous mode power supply, that is, the freewheeling device is an external diode instead of an internal MOS tube.

Let’s analyze how each function is implemented.

1. Reference voltage

Similar to the reference power supply for 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 requires high accuracy, good stability, and low temperature drift. The reference voltage inside the chip is also called the band gap reference voltage, because this voltage value is similar to the band gap voltage of silicon, so it is called the band gap reference. This value is about 1.2V, a structure as shown in the figure below:

The basic principle of switching power supply is to use PWM square wave to drive power MOS tube

Here is going back to the textbook to talk about the formula, the current and voltage formula of the PN junction:

It can be seen that it is an exponential relationship, and Is is the reverse saturation leakage current (that is, the leakage current of the PN junction due to the minority carrier drift). This current is proportional to the area of ​​the PN junction! That is Is-》S.

In this way, Vbe=VT*ln(Ic/Is) can be derived!

Going back to the figure above, from the op amp analysis VX=VY, then I1*R1+Vbe1=Vbe2, so that: I1=△Vbe/R1, and because the gate voltages of M3 and M4 are the same, so 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 to Q2!

Going back to the figure above, from the op amp analysis VX=VY, then I1*R1+Vbe1=Vbe2, so that: I1=△Vbe/R1, and because the gate voltages of M3 and M4 are the same, so 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 to Q2!

In this way, we finally get the benchmark Vref=I2*R2+Vbe2, the key point: I1 is a positive temperature coefficient, and Vbe is a negative temperature coefficient, and then adjust it through the N value, but achieve good temperature compensation! Obtain a stable reference voltage. N is generally designed in the industry according to 8. To achieve zero temperature coefficient, Vref=Vbe2+17.2*VT is calculated according to the formula, so it is about 1.2V. At present, the reference of less than 1V can be achieved in the low voltage field, and in addition to the temperature coefficient, There are problems such as power supply ripple suppression PSRR, which cannot be deepened due to the limited level. The final sketch is like this, and the design of the op amp is of course very particular:

 The basic principle of switching power supply is to use PWM square wave to drive power MOS tube

As shown in the figure, the temperature characteristic simulation:

The basic principle of switching power supply is to use PWM square wave to drive power MOS tube

2. Oscillator OSC and PWM

We know that the basic principle of a switching power supply is to use a PWM square wave to drive a power MOS tube, so an oscillation module is naturally needed. The principle is very simple, which is to use the charge and discharge of the capacitor to form a sawtooth wave and a comparator to generate an adjustable duty cycle Square wave.

 The basic principle of switching power supply is to use PWM square wave to drive power MOS tube

The final detailed circuit design drawing looks like this:

 The basic principle of switching power supply is to use PWM square wave to drive power MOS tube

There is a technical difficulty here is the slope compensation in the current mode. For the purpose of stabilizing the slope when the duty cycle is greater than 50%, an additional compensation slope is added. I also have a rough understanding, and interested students can learn in detail.

Three, error amplifier

The function of the error amplifier is to ensure the output constant current or constant voltage, and to sample the feedback voltage. So as to adjust the PWM driving the MOS tube, as shown in the diagram:

 The basic principle of switching power supply is to use PWM square wave to drive power MOS tube

Fourth, the drive circuit

The final driving part has a very simple structure, which is a large-area MOS tube with strong current capability.

 The basic principle of switching power supply is to use PWM square wave to drive power MOS tube

Five, other module circuits

The other module circuits here are to ensure that the chip can work normally and reliably. Although it is not the core of the principle, it really occupies an important position in the design of the chip.

Specifically, there are several functions:

1. Start the module

The function of the start-up module is naturally to start the work of the chip, because the current of all transistors may be 0 and remain unchanged at the moment of power-on, so that it cannot work. The function of starting the circuit is equivalent to “light a fire” and then turn it off. As shown in the figure:

At the moment of power-on, S3 is naturally opened, and then S2 is opened to open M4, Q1, etc., then M1 and M2 are opened, the constant current source circuit on the right works normally, S1 is also opened, and S2 is closed to complete the startup. If there is no S1 S2 S3, the current of all transistors will be 0 at the moment.

 The basic principle of switching power supply is to use PWM square wave to drive power MOS tube

2. Overvoltage protection module OVP

It is well understood that when the input voltage is too high, the output is turned off by the switch tube to avoid damage, and a protection point can be set through the comparator.

The basic principle of switching power supply is to use PWM square wave to drive power MOS tube

3. Over temperature protection module OTP

The temperature protection is to prevent the chip from being damaged by abnormal high temperature. The principle is relatively simple. It uses the temperature characteristics of the transistor and then sets the protection point through the comparator to shut off the output.

The basic principle of switching power supply is to use PWM square wave to drive power MOS tube

4. Overcurrent protection module OCP

For example, in the case of an output short circuit, the state of the output tube can be fed back and controlled by detecting the output current, which can be turned off or current limited. As shown in the current sampling, the current of the transistor is proportional to the area for sampling. Generally, the area of ​​the sampling tube Q2 will be one-thousandth of the area of ​​the output tube, and then the voltage comparator is used to control the driving of the MOS tube.

The basic principle of switching power supply is to use PWM square wave to drive power MOS tube

There are some other auxiliary module designs.

Six, constant current source and current mirror

In the IC, how to set the working state of each transistor is through the bias current. The constant current source circuit can be said to be the cornerstone of all circuits. The bandgap reference is also generated for this, and then the current mirror is used for each functional module. To provide current, the current mirror is to set the required current through the area of ​​the transistor, similar to mirroring.

The basic principle of switching power supply is to use PWM square wave to drive power MOS tube

The basic principle of switching power supply is to use PWM square wave to drive power MOS tube

Seven, summary

The above is probably the entire internal structure of a DC/DC power chip LM2675, which can be considered as a review of the previous fur knowledge. Of course, this is only a basic structure in principle. A lot of parameter characteristics must be considered in the specific design, a lot of analysis and simulation are required, and a deep understanding of semiconductor process parameters must be required, because the manufacturing process determines the transistor’s Many parameters and performance, accidentally produced chips have defects or even cannot be used at all. The entire chip design is also a relatively complex system engineering, requiring good theoretical knowledge and practical experience. Finally, learn and learn from time to time, not to mention.

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