System structure design of DC-DC power supply chips
The DC-DC switching power conversion system with a monolithic integrated control chip is known as a high-efficiency and energy-saving power management system. It represents a development direction of DC regulated power supply, and has now become a mainstream product in the DC regulated power supply market. Its main feature is that the power tube works in the switching state, the power tube is constantly turned on and off, and the DC energy passes through the switch tube intermittently, temporarily stored in the inductor in the form of magnetic field energy, and then filtered by capacitance to transmit energy to the load in a continuous way. In this way, the switching power conversion system realizes the DC-DC conversion by taking advantage of the energy storage characteristics of the inductive and capacitive elements. With the increasing popularity of portable equipment in people's daily life, switching power conversion technology continues to advance, and the power conversion performance is greatly improved. The system control mode has gradually developed from voltage control mode to current control mode. The integration of control chips is getting higher and higher, and the integration of monolithic chips has been gradually realized. These make the switching power conversion system has made great progress in both performance and structure.
The topology of the DC-DC power conversion system
The DC-DC switching power conversion system is mainly composed of two parts: the main circuit and the control circuit. The components that make up the main circuit include: input power supply, switches, rectifiers, as well as energy storage inductors, filter capacitors, and loads. They complete the conversion and transfer of electrical energy, which is collectively called the power stage. The control circuit is a collection of all circuits that control the on/off of the power switch and adjust the output voltage stability, and most of them are now integrated into the control chip. Under normal circumstances, when the input voltage and load change within a certain range, adjust the on-off time of the switch, and the load voltage can be maintained at an approximately constant value. The power switching element and the energy storage element can be converted into a variety of different output voltages by using different configurations or different connection methods. The specific configuration and connection relationship between the switching element and the energy storage element is called the topology of the switching power conversion system.
Figure 1-1 Basic topology of the DC-DC switching power conversion system.
Among the many circuit topologies, typical are buck (as shown in Figure 1-1(a)), boost (as shown in Figure 1-1(b)), and reverse (as shown in Figure 1-1(c)). At present, many complex topologies in circuit applications are expanded and optimized on the basis of these three basic topologies.
For a DC-DC power conversion system with a monolithic integrated control chip, the switching devices in Figure 1-1 are typically implemented with transistors, and they are sometimes integrated inside the control chip.
The reverse output structure is usually implemented by means of a charge pump. Only the first two circuit topologies will be analyzed below.
Step-down switching power conversion system
The step-down switching power supply conversion system is named because the output voltage vout is less than or equal to the input voltage vin. It is characterized by an LC filter immediately following the output rectifier on the secondary side of the power tube or transformer. The working process can be divided into two stages.
When the switch S is closed, the current IL flowing through the inductor L tends linearly upward. Ignoring the voltage drop on S has the following relationship:
where vin and vout can be considered as relatively stable constants, and the energy stored on the inductor is:
When the switch S is disconnected, the current on the inductor cannot be abruptly changed, and the inductor current Il depends on the diode D to continue the current. At this point, the IL gradually decreases and a portion of the energy stored in the inductor is released to the load. Ignoring the on-voltage drop of the diode, there is the following relationship:
The inductor current IL rises when the switch S is closed and decreases when S is off, and the cycle changes. By changing the duty cycle of the switch through the control circuit, the output voltage can be kept constant. The advantage of this structure is that the ripple peak-to-peak ratio of the output voltage is low, and the output power can be high.
Step-up switching power supply
Step-up switching power supply conversion systems have the same components as step-down switching systems, except that they are relocated. When the switch is turned on, the current loop consists only of the inductor, the switch and the input voltage source, and the diode is blocked in the opposite direction. The inductor current rises linearly and can be described by the following formula:
When the switch is disconnected, the diode is immediately turned on because the current in the inductor cannot be abruptly changed. In this case, the voltage at the end of the inductor connected to the switch is clamped by the output voltage, which is called the flyback voltage, and its amplitude is the output voltage minus the forward conduction voltage drop of the diode. During the period when the switch is turned off, the current on the inductor can be expressed as follows:
If the magnetic flux in the inductor drops to zero completely before the next switching cycle, the circuit is said to be operating in current intermittent mode. The voltage and current waveforms are shown in Figure 1-2.
Figure 1-2 Inductor current discontinuous mode waveform.
If the magnetic flux in the inductor does not completely drop to zero before the next switching cycle, and there is still some remanence, the circuit is said to be operating in continuous current mode. The voltage and current waveforms are shown in Figure 1-3.
Figure 1-3 Waveform of continuous mode inductor current.
Because the output voltage of the step-up switching power supply conversion system is higher than the amplitude of the input voltage, the circuit operation in current continuous mode has an inherent instability problem. Typically, step-up switching power supply conversion systems are limited to intermittent current operation. This type of conversion topology uses fewer components and is generally popular in small and medium-power applications.
In the above analysis process, we can see that no matter which way of working, their basic principle is a process of energy conversion, and the specific relationship can be expressed by the following formula:
Among them, ton is the time when the switch is turned on each time, toff is the time when the switch is turned off, and eout and esup are the energy supplied to the load and the energy supplied by the power supply, respectively. The switching power supply changes the output voltage by changing the ratio of the switching on time and the duty cycle. In practice, the load voltage and the input supply voltage are variable, as long as the ratio of the switch on time ton to the whole duty cycle is appropriately adjusted, the output voltage vout can be maintained. With the progress of switching power supply conversion technology, the control method of adjusting the duty cycle of switching on time is constantly evolving and becoming more and more mature.
Modulation mode of DC-DC switching power supply
In order to stabilize the switching power supply conversion output voltage at the set voltage value, the system needs the control chip to sample the output signal, and control the switch tube to be turned on and off according to the analysis of the sampled signal. The way to adjust the on-on and off-off of the control switch tube is called the conversion modulation mode of the switching power conversion system. At present, the mainstream modulation methods mainly include: pulse width modulation and pulse frequency modulation. Each modulation has its own advantages and disadvantages. The designer can choose the appropriate modulation mode according to the actual application needs, or use the two methods together or add some optimization control to a certain modulation mode to promote the strengths and avoid the weaknesses.
Pulse width modulation (PWM).
Pulse width modulation is to adjust the pulse width of the switch control under the condition of a certain pulse frequency, so as to change the duty cycle of the switch control pulse. The basic implementation method is to generate a sawtooth wave with a constant frequency by the internal oscillator, compare it with the sawtooth wave generated internally through the feedback signal at the load end, and then output a set of square wave signals with constant frequency and widening to control the power switch, adjust the on-time of the switch in real time according to the load condition, and stabilize the output voltage. The working waveform is shown in Figure 1-4.
Figure 1-4 Schematic diagram of PWM modulation mode.
The advantages of this method are high efficiency at large loads, good tracking of load changes, and constant noise spectrum, which is conducive to EMC design. The disadvantage is that when the load is small, the proportion of the working current of the control circuit to the total working current increases, resulting in a decrease in the conversion efficiency of the system.
Pulse Frequency Modulation (PFM).
Pulse frequency modulation is to adjust the frequency of the control pulse when the control pulse width of the switch tube is constant. Pulse frequency modulation can be achieved in two ways:
The first method is to fix the opening width of each pulse cycle inside the chip, and change the adjustment pulse frequency (i.e., change the length of the pulse period) to control the duty cycle of the control pulse. In this case, the inductor current changes with the input voltage, which is not conducive to inductor selection.
The second is that the pulse width is not fixed inside the chip, but the width of the switch control pulse is set by inductor current control. Inside the chip, the inductor current is simply set to a maximum value to detect the current on the switching device in real time. When the switching current reaches a set maximum, the switching device is turned off. After the switch is turned off for a certain period of time (generally the time of the US level), the output voltage of the detection system is vout, and the size of the output voltage determines whether the switch device needs to be turned on again. In this way, if the pulse width is turned on or the peak current of the switching device is too large, the conduction loss of the system will be increased. If the operating frequency is too high, it will increase the switching loss of the system. However, high switching frequencies can reduce the size of power devices and energy storage inductors. Therefore, the design should be optimized in various aspects, and the circuit is slightly more complex than the first method.
At present, the PFM control mode is widely used in the switching power supply conversion system, and has the following advantages: the conversion efficiency is higher in the case of light load applications; The switching device can operate at a very high frequency; The frequency characteristics are better; Good voltage adjustment performance, etc. The PFM modulation mode can be implemented in either current or voltage control mode, and its operating waveform is shown in Figure 1-5.
Figure 1-5 Schematic diagram of pulse-frequency modulation waveform.
Mixed modulation method
Hybrid modulation refers to the mixing of PWM and PFM, and the pulse width and switching frequency are not fixed, and they can be changed from each other. It also has the advantages of higher PFM conversion efficiency at light load and PWM mode at heavy load. At light loads, the circuit selects pulse frequency modulation and skips pulses when needed. When the load is heavy, the circuit selects pulse width modulation to provide high efficiency over the widest possible load range. The hybrid modulation method has many advantages in theory, but when it is actually implemented, the design of control and detection circuits is more complicated.
Feedback control mode for switching power supplies
In order for the entire DC-DC switching power supply conversion system to operate stably, the output signal needs to be sampled and negative feedback introduced. According to the type of feedback sampling, the feedback control mode of switching power supply can be divided into two types: voltage feedback control mode and current feedback control mode.
Voltage feedback control mode
The basic principle of the voltage feedback control mode is to sample the output voltage and compare it to a voltage reference. Analyze and judge the results of the comparison and decide the on/off of the power switch. Figure 1-6 shows the functional structure of a voltage-feedback controlled pulse-width modulated DC-DC conversion system.
Figure 1-6: Schematic diagram of a PWM-modulated DC-DC converter in voltage control mode.
In Figure 1-6, the system output voltage (VOUT) is sampled to obtain the VFB in the figure. VFB is compared with the reference voltage VREF, the error is amplified to obtain VE, and the PWM comparator compares VE with a fixed frequency sawtooth wave (VRAMP) to output a set of control pulses. The width of these pulses varies with the error signal ve, and they determine the amount of output energy. When the energy consumed by the load increases, the pulse width increases; When the load consumes less energy, the output pulse width decreases. This keeps the output voltage relatively stable. This voltage feedback controlled switching power supply conversion system only needs one voltage feedback signal to achieve negative feedback of the entire circuit. There is only one feedback loop in the whole control circuit, which is a single-loop control system. However, a voltage feedback-controlled switching power supply conversion system is a second-order system that has two state variables, the voltage on the output filter capacitor and the current in the output filter inductor. Since the second-order system is a conditionally stable system, the closed-loop system can only work stably if the control loop is carefully designed and certain conditions are met. We know that the current of the switching power conversion system has to pass through the inductor, and there is a phase delay of 90° for the voltage signal. The whole regulated power supply system adapts to the input voltage and load change requirements through the change of magnetic flux on the inductor, and keeps the output voltage stable. This method of sampling the output voltage has a certain lag in the adjustment process, the response speed is slow, the stability is not high, and it is even easy to oscillate when the large signal changes.
Figure 1-7: Schematic diagram of PFM-modulated DC-DC converter in voltage control mode.
Figure 1-7 shows the functional structure schematic diagram of a voltage-feedback controlled pulse-frequency modulation DC-DC conversion system. The output voltage is sampled and added to the inverting input of the error comparator. When the output voltage vout is below a certain set value, the error comparator outputs a high level. This high level allows the square wave output from the oscillator to pass through the trigger to drive the power switch. If the output voltage vout is higher than the set value, the error comparator outputs a low level. This low level enters the flip-flop, which locks the flip-flop, the square wave output of the oscillator cannot pass through the flip-flop, and the power switch is turned off. In this way, the pulse width of the control output signal remains the same, but in fact, the switching cycle becomes longer, the duty cycle is reduced, and the output is controlled stably.
Current feedback control mode
Due to some shortcomings of the voltage feedback control DC-DC conversion system, the current feedback control technology has been well developed in the past ten years. The current feedback control DC-DC switching conversion system is based on the traditional voltage feedback control conversion system, and the current feedback loop is added to make it a double-loop control system. In this way, the inductor current is no longer an independent variable, and the unknown parameter of the inductor current is removed from the second-order model of the DC-DC switching converter and becomes a first-order system. Figure 1-8 shows the schematic diagram of the current feedback-controlled PWM modulated DC-DC conversion system.
Figure 1-8: Schematic diagram of a current-controlled mode PWM-modulated DC-DC converter.
In Figure 1-8, the current sampling signal VS is compared with the output level VE of the error amplifier, and the PWM comparison output and the oscillator pulse signal jointly control the power switch tube. When the voltage of the current on the sensing resistor (RS) reaches the VE level, the output state of the PWM comparator is flipped and the power tube is shut off. The entire circuit detects and adjusts the switching current pulses one by one, so that the system achieves a stable output.
The PFM modulation system in current feedback mode is similar to the PWM system in current control mode, and the circuit schematic diagram is shown in Figure 1-9.
Figure 1-9: Schematic diagram of PFM-modulated dc-dc converter in current control mode.
The circuit is still made up of two feedback loops. One is a loop that monitors the output voltage through the sampled voltage, and the other is a current limit loop of the power switch. The current-controlled mode PFM modulation system has its own unique advantages. The working principle of the system shows that it is a stable system in itself, and no other additional system stabilization measures are required. At high inductor currents, the internal loop directly enforces the current limit, which greatly reduces the on-time of the power switch. When the output voltage is high, its operating frequency will automatically change, and the system output voltage adjustment range can be very wide. Therefore, in the case of light load or no load, the PFM modulation control chip has lower power consumption and higher conversion efficiency. Its output voltage accuracy depends primarily on the accuracy of the internal voltage comparator and the output voltage characteristics of the reference.
DC-DC power chip system structure design
The DC-DC switching power conversion control chip requires a high-frequency, high-performance, and constant-current step-up DC-DC power conversion system, which is mainly used in battery-powered portable electronic products to drive white LEDs or similar current-type loads.
Chip system architecture design ideas
At this stage, most mainstream portable electronic products are powered by two ordinary batteries or one lithium-ion battery. Therefore, for the DC-DC power conversion system, it is often necessary to adapt to 24v～3.6V input supply voltage. In order to be compatible with the standard interface voltage of 3V, 5V and 12V, the DC-DC power conversion system is designed with a wide input voltage range and the output voltage is adjusted according to the load. For battery-powered circuit systems, the load is generally light in order to extend the life of the battery. Therefore, PFM modulation, which has higher efficiency at light loads, is preferred. Moreover, the PFM modulation mode is relatively mature in theory and has many advantages, and has been widely used in DC-DC switching power supply conversion systems. For the system design of the circuit, the PFM modulation method is theoretically less risky.
For the system feedback control mode, the current feedback control mode has developed relatively mature in recent decades, and it has many advantages compared with the voltage feedback control mode
1) Good voltage regulation;
2) Good loop stability and fast load response;
3) The inherent pulse amplitude detection and current limiting simplify the overload protection and short-circuit protection, and greatly improve the reliability of the work;
4) Effectively reduce the power loss of high-frequency power switching conversion circuit and improve the efficiency of switching power supply.
Taken together, the current feedback control mode PFM modulated DC-DC power conversion system can meet the practical application requirements of portable electronic products. Therefore, the DC-DC source conversion control chip designed by ** is based on the PFM boost system architecture of current feedback control mode.
Chip functional structure block diagram design
Based on the PFM architecture of the current feedback control mode, the functional block diagram of the chip designed by ** is as follows:
Figure 1-10 Block diagram of the internal functions of the chip.
As can be seen from Figure 1-10, in addition to the standard module for current feedback control step-up DC-DC conversion, a series of protection circuits are also designed, such as over-temperature protection, power supply under-voltage protection, output over-voltage protection, soft start, etc. In order to realize the control of LED brightness and the programmable control of the power supply system, a set of circuits for dynamic control of output current are designed in the circuit. In the mode of power shutdown, the load LED is disconnected from the ground, which effectively avoids the leakage current generated by the load.
Table 1-1 describes the functions of the external ports of the chip. The control pin CTRL has two functions, one is to control the start and stop of the entire chip system; The other is PWM mode control LED switch M2. If the CTRL port is not loaded with a PWM signal, the CTRL terminal is a standard enable control end. In order to strictly distinguish between the "start-stop control" and "output current PWM modulation" functions of the CTRL port, the system stipulates that the CTRL should be kept high for at least 500 s before the circuit starts. After the system is working normally, the system will shut down if the ctrl level remains low for more than 32ms.
Table 1-1 Functions of external ports on the chip.
After the system is powered on, the enabling control module outputs two sets of control signals. A set of reference currents used to start the bias module to establish the full circuit and connect to the load; The other set is used to start the soft-start circuit module and reset the RS latch. When the reference current source is activated, the bias module outputs an "enable allow" signal to start the subsequent circuitry. The error comparison module compares the reference voltage VREF with FB, and when FB is less than VREF, the RS latch is set and the switch M1 is turned on. When the on-current of M1 reaches the set value (500mA is set here), the current limit module resets the RS latch, M1 is turned off, and the circuit works in cycles. When the switching time reaches the limit value, the time control module generates corresponding actions to protect the circuit.
The CTRL port can load the PWM signal with a permissible frequency range of 100Hz 50kHz after the system works normally, and the PWM signal controls the on/off of the switch M2, which can modulate the load current appropriately. The magnitude of the payload is only related to the duty cycle of the PWM signal, not the frequency and amplitude.
When the circuit is started, it is easy to generate excessive inrush current, and even cause the entire system to shut down unexpectedly. For this purpose, a soft-start control circuit is designed in the chip. The soft-start circuit sets the allowable value of the switching current in two steps. During the period of time before the circuit starts, the switching current limit value increases step by step according to these two setting steps. In this way, the switching current transitions smoothly and avoid excessive current spikes.
During the use of the battery, the voltage will gradually decrease, which can easily lead to insufficient power supply voltage of the power conversion system. The whole conversion system circuit runs under the condition of weak power supply voltage, and the stability of the system operation will deteriorate. In order to protect the safety of the load and conversion circuit, the chip is designed with an under-voltage protection circuit. When the supply voltage is less than a certain set value (15V), the output protection signal of the under-voltage locking module turns off the subsequent circuit, and the whole circuit stops working. When the supply voltage returns to a normal value, the circuit restarts.
During the operation of the circuit, the temperature will increase with its own power consumption. If the temperature is too high, the electrical characteristics of the chip will change, and even affect the stability of the system. In order to prevent the adverse effects of changes in the working environment, an over-temperature protection circuit is designed in the system. When the temperature rises to a certain set point (CMOS circuits are typically set to 160), the overtemperature protection circuit outputs a logic level that shuts down subsequent module circuits. When the temperature is lowered to another set point (usually due to layout thermal balance issues, there is a hysteresis margin, such as 150 or below), the output level of the over-temperature protection circuit changes and the circuit resumes normal operation.
In addition, the requirement is to achieve a constant current source, and the circuit structure is a PFM step-up structure. If the load is accidentally disconnected, the output voltage may rise dramatically, causing damage to other circuits. The Overvoltage Protection Module (OVP) is designed to prevent such unexpected situations. When the output voltage is greater than the overvoltage protection threshold, the main switch is turned off until the output voltage returns below the overvoltage protection threshold, and the system returns to normal.