[Power module knowledge] Hands-on teach you DC-DC converter do anti-interference

We do not need to introduce the importance of DC converter in circuit design. Its main function is to convert the voltage into a fixed and effective voltage based on different functions,DC-DCConverter also has a variety of categories. Its main application fields are distributed in digital cameras and mobile phone portable products. A large amount of use makes some common problems in the DC-DC converter gradually exposed, this article we will mainly discuss the DC-DC converter whenInterferenceThe problem, some experts said, DC-DC converter when the problem is largely out of the DC itself, why to say so? Let's take a look.

In fact, in a complete circuit system, the energy flowing between various components and conductors is actually an energy conversion. Energy is the ability to do work, which exists in two forms: 1) potential energy and 2) kinetic energy. Potential Energy is an inactive energy storage (such as voltage between battery terminals ).

Kinetic energy is the energy generated when the potential energy changes to the active State (for example, the current passes through the bulb ). Electronics is simply a science of converting Potential Energy (voltage) into kinetic energy (current) by controlling the current in various conductors! Ohm's 'DC law' must always meet the energy conversion before it can work! Therefore, whether the AC function is available or not, the DC structure of the circuit must be well designed to effectively support energy conversion in any form. In other words, it is impossible to achieve AC performance if the circuit DC is poorly designed.

Linear Regulator

Linear Regulators are the most fundamental device for all DC-DC converters. A linear regulator is a regulator that works in a "linear region" as opposed to a switch regulator that operates in a "non-linear" switch mode area (which we will discuss later. The linear regulator must provide rated power for the load (low noise to reach acceptable levels), while reducing the output impedance so that the voltage gain is not affected by the load impedance value. The linear regulator acts as a variable resistance and adjusts the partial pressure network to maintain a constant output voltage while providing various load currents.

 

Figure 1

Figure 1 shows the linear regulator schematic. As shown in the figure, the linear regulator circuit is "connected" because the regulator (transistor Q) is connected to the load R2. The circuit adjusts the DZ output voltage of the zoneer diode (because the base current of the transistor is a small portion of the zoneer-to-R1 offset current ). The output voltage of the transistor emission pole is lower than that of the zoneer tube. A diode voltage drop has sufficient current gain to drive the high output value Iout (via R2 ). Although the circuit has good output voltage adjustment capability (as long as Q works in a linear area), it still senses load, power variables (VS), noise and power ripple. Some problems can be solved by using the negative feedback circuit output. In other cases, this circuit is often used as a voltage reference and supports more advanced Linear Regulator design. When designing or selecting a linear regulator, you must also carefully consider the electrical noise, the ripple produced by the power supply vs Vout, and the common mode voltage that may be coupled in the regulator output.

 

For example, when selecting a linear regulator, you must carefully determine the circuit power requirements and the output characteristics of the voltage regulator. Taking the National Semiconductor Company lm340/lm78xx series three-end Positive Pressure regulator as an example, this type of linear regulator is a standard device with basic design elements in the industry. In general, some devices define the fixed output voltage (generally vs-Vout 2 v) under fixed input voltage conditions and the maximum fixed output load current Iout.

The Load Adjustment defines the changes in the output voltage (Vout) within the given output current range. As the output voltage is close to the vs input voltage, the output voltage regulation transistor (Q1) is in near saturation state and the voltage/current gain declines, which leads to worse load regulation. This situation also applies to line adjustment. Line adjustment is to change the output voltage (Vout) within the specified input voltage (VS) range ). Similarly, Vo line adjustment generally defines low-level vs at the MV level. As the input voltage changes, the MV level can be increased by ten times (compared with the output voltage ), when the output voltage adjustment transistor is close to the breakdown point, its gain will decrease accordingly. Line adjustment can also implement Ripple Suppression (VIN /? Vout ratio), and should be greater than 60 dB, to avoid AC ripple through the input power line access Linear Regulator DC input voltage. Ripple Suppression is critical to a simulation system that requires precise gain and dc accuracy. For the power supply ripple entering the linear regulator, you can also add the necessary power supply decoupling capacitor, further filter out non-expected ripple in the input and output of the Linear Regulator for improvement (we will discuss the problem of Power Supply decoupling later ).

Decoupling (Vout is connected to the L-ground through two capacitors)

 

Figure 2

Some important design concepts of correct decoupling to reduce noise 2 are shown. Place a large-capacity electrolytic capacitor C1 (generally 10 μF-100 μF) near the output end of the Linear Regulator (less than 2 inch ). This capacitor is used as the charge library to instantly supply current to the load without the need to provide charge through the regulator/inductor. The position of the small capacity capacitor C2 (generally 0.01 μF-0.1 μF) should be as close as possible to the load, the purpose of this capacitor is to reduce the high-frequency noise of the load. All decoupling capacitors should be connected to a large area of low-Impedance Ground layer to reduce the impedance. Linear Regulator output-end inductor L1 (typically using small ferrite magnetic beads) limits noise in the system and suppresses high-frequency noise from external loads, while avoiding internal noise (from the load) transmitted to other parts of the system.

 

(As mentioned above), the actual noise bandwidth will be very high.

Finally, although the linear regulator is easy to use (generally three terminals, namely input, ground and output), it has excellent DC and AC features in most circuit environments, however, it has great limitations in terms of thermal characteristics. Because the input voltage vs in the internal circuit of the linear regulator is higher than the output voltage Vout (vs-Vout 2 v), this difference (vs-Vout) is multiplied by the power value given by the output current (Iout, eventually it becomes the thermal dissipation of Linear Regulators and systems. This heat conversion factor must be carefully considered. Correct heat dissipation and airflow around the system must be considered throughout the design. For example, if the maximum junction temperature of a linear regulator is 150 °C (and there is no radiator or airflow in the system), the system ambient temperature can reach 125 °C; If Θ ja is close to 50 °C/W, the maximum power output of the linear regulator should be less than W to keep it within the acceptable junction temperature limit. This is why linear regulators have significant disadvantages for systems that require high power and thermal efficiency. In the following article, we will discuss the switch regulators that solve these two problems.

Linear Regulators are still the key to the design of electronic devices and systems, whether driving independent circuits of other devices or driving sub-units of other on-chip circuits. To ensure the maximum performance of the entire system, you must carefully design and comply with the technical specifications.

Switch Voltage Regulator

The switch regulator is the most efficient regulator for all DC-DC converters. Switching voltage regulators are much more efficient than linear voltage regulators. Of course, the disadvantage is that the switching process produces high output noise. However, the switching voltage regulator topology is widely used in a variety of applications, including step-up (boost), step-down (step-down) and conversion Voltage Adjustment (boost/step-down ).

The switch voltage regulator has built-in power switches (usually vertical metal oxide semiconductors (VMOs), but bipolar devices can also be used ). Power switch ON/off the working cycle to determine how much energy is stored, and then power the load. Unlike linear voltage regulators that use low-energy-efficient voltage drops between resistors to regulate voltage, switching voltage regulators have almost no power consumption! The secret lies in the power switch. When the switch is turned on, the two ends of the switch are high voltage, and the current is zero. When the switch is closed, the switch output high current, and the voltage at both ends is zero! Because the voltage and current from the inductor have a 90 degree phase difference (and there is no DC Pressure Drop), the switching voltage regulator can reach an extremely high level of energy efficiency.

 

Figure 3 step-up switching voltage regulator (Boost Converter)

Next, we will take the boost converter as an example to briefly introduce the functions of the step-up switch voltage regulator (see figure 3 ). Figure 1 shows a simple boost converter consisting of an inductor, a power switch, a rectifying diode, and a capacitor. The main function of the inductor is to store energy and limit the current change rate (otherwise, the peak current can only be restricted by the switch resistance ). In a stable state, the switch is turned on and the inductor is charged as a capacitor until + Vout is equal to + VIN (the diode current is zero ). When the switch is closed, the input voltage + Vin acts on the inductor because the diode prevents + Vout (still equal to + VIN) from discharging to the ground. The current through the inductor increases linearly with the ratio of + VIN/L, DI/dt (with the switch closure time ). When the switch is turned on again, the inductor current is charged by the rectified diode as a capacitor, and the voltage increases by the ratio of DV to DT (with the switch opening time ).

 

If the working cycle (D = tclosed/(tclosed + topen) of the power switch is equal to 50%, + Vout can reach Vin + Vin under ideal conditions, that is, twice the applied input voltage (because the average inductance voltage is definitely equal to zero in a stable State )! Of course, the work cycle DV will change accordingly, and adjusting the output voltage can result in vout = VIN/(1-D. This uses the Boost Converter topology to double the DC output voltage under the DC input voltage (+ VIN) limit, driving Circuit loads provide great flexibility within a given overall energy efficiency range.

Of course, although the ideal boost converter has significant advantages in terms of efficacy, it also needs to consider the actual limitations of the circuit. The maximum power consumption factor of the boost converter is the rectifier diode. The simple power consumption calculation method is (in Hot State). The forward voltage drop is multiplied by the current passing through the rectification diode. To maximize efficiency, you can replace the diode with another power switch. When the main switch and Closed Switch are enabled, the switch can be switched on in the First-off mode to prevent both switches from being turned on at the same time. With this configuration, the efficacy can reach more than 90%.

Taking the lm2578a/lm3578a switching voltage regulator of National semiconduas an example, this switching voltage regulator uses a bipolar transistor as a power switch device. It contains an on-board oscillator that can use an external capacitor below 1Hz to 100 kHz to set the switching frequency. The output current can reach up to 750 Ma, with throttling and hot shutdown functions. When lm2578a/lm3578a is configured by the boost converter (for example, if Iout = 150 mA, Vin = + 5 V, vout = + 15 V ), the device load is adjusted to 14 mV (30 mA Vin 8.5 V ). Similarly, linear adjustment changes the output voltage (Vout) within a given input voltage range (VIN ). Linear adjustment can be easily controlled in a DC power supply control system. But when using the switching voltage regulator, the designer needs to be careful because the constant changes inherent in the Iout current of the device will cause the Load Adjustment output voltage to be unstable. 14 MV Load Adjustment will cause about 1% fluctuation in the 15 V system and switch noise (from the circuit load) without correct decoupling) the Vout of the boost converter (which will be described later) will be sensed backward, which will make the Circuit load performance very difficult to manage.

 

The heat generated is small, which eliminates the cost of complicated heat dissipation design and saves space. Note that the output voltage ripple of the switch voltage regulator and its impact on the circuit drive can significantly improve the design level.

Only excellent DC-DC circuit design can be efficient and complete completion of energy replacement, it is because of this, it is necessary to multi-discipline integrated knowledge as the basis, so this step is hard to fall a lot of power designers. In contrast to various input signals, in order to convert DC (DC) with low noise under various voltage conditions, you must first select the correct DC-DC converter. DC-DC converters are available in a variety of sizes and types: the converters include linear, switching mode and magnetic and other different types. In addition, the boost and step-down functions use different types of energy conversion circuits. Correct understanding of these circuit types can avoid performance degradation during use. Later, we can analyze the devices launched by industry leaders, such as the power supply modules recently launched by national semiconductor companies.

Therefore, if you want to design the circuit correctly and efficiently, let's first think about what problems will occur on the DC, whether this problem is caused by the DC, and consider this problem before each design, it will lay the foundation for us to design a good and efficient circuit.

 

[Power module knowledge] Hands-on teach you DC-DC converter do anti-interference