DC-DC Circuit Design Skills and Device Selection Principles
Time:2022-11-20
Views:1550
Concept and characteristics
1 Concept
DC-DC refers to direct current power supply. It is a device that changes electrical energy of one voltage value into electrical energy of another voltage value in DC circuit. For example, a converter can convert a DC voltage (5.0V) to other DC voltages (1.5V or 12.0V). We call this converter DC-DC converter, or switching power supply or switching regulator.
DC-DC converter is generally composed of control chip, inductance coil, diode, triode and capacitor. When discussing the performance of DC-DC converter, it is impossible to judge its advantages and disadvantages if only focusing on the control chip. The component characteristics of the peripheral circuit and the wiring mode of the substrate can change the performance of the power circuit, so a comprehensive judgment should be made.
The use of DC-DC converter is conducive to simplifying the power circuit design, shortening the development cycle, achieving the best indicators, etc. It is widely used in power electronics, military industry, scientific research, industrial control equipment, communication equipment, instruments, switching equipment, access equipment, mobile communications, routers and other communication fields, as well as industrial control, automotive electronics, aerospace and other fields. With the characteristics of high reliability and easy system upgrade, power modules are used more and more widely. In addition, DC-DC converters are also widely used in mobile phones, MP3, digital cameras, portable media players and other products. It belongs to chopper circuit in circuit type classification.
2 Features
Its main feature is high efficiency: compared with LDO of linear regulator, high efficiency is a significant advantage of DCDC. Generally, the efficiency is more than 70%, and the high efficiency can reach more than 95%. The second is to adapt to a wide voltage range.
A: Modulation mode
1: PFM
The switching pulse width is fixed, and the output voltage is stabilized by changing the frequency of the pulse output. Even if the PFM control type is used for a long time, especially when the load is small, it has the advantage of low power consumption.
2: PWM (pulse width modulation mode)
The switching pulse frequency is fixed, and the output voltage is stabilized by changing the pulse output width. PWM control type has high efficiency and good output voltage ripple and noise.
B: In general, the performance differences of DC-DC converters using PFM and PWM are as follows.
PWM frequency, PFM duty cycle selection method. The PWM/PFM conversion type implements PFM control when the load is small, and automatically switches to PWM control when the load is heavy.
Schema Classification
1. Three common principle architectures
A. Buck (buck DC/DC converter)
Figure 1
B. Boost (boost DC/DC converter)
Figure 2
C. Buck-Boost (step-up and step-down DC/DC converter)
Figure 5
Figure 6
Figure 7
Figure 8
Figure 3
2 Working principle of BUCK circuit
Figure 4
Figure 4
Volt second balance principle: for an inductor in a stable state, the positive volt second product at both ends of the inductor is equal to the negative volt second product, that is, the volt second product at both ends of the inductor must be balanced within a switching cycle.
Figure 5
Figure 6
When the switch is on: the input voltage Vin is applied to the input end of the LC filter, and the current on the inductor rises linearly with a fixed slope. As shown below
Figure 7
When the switch is turned off: because the current on the inductor cannot jump, the energy stored in the inductor is released to the load, and the inductor current flows continuously through the diode. At this stage, the current waveform is a diagonal line with a negative slope. As shown below
Figure 8
Design skills and requirements for selection of main technical parameters
DC-DC circuit design shall at least consider the following conditions:
A. The range of external input power supply voltage and the size of output current.
B. Maximum voltage, current and system power of DC-DC output.
1 Input&Output Voltage: Vin/Vout
It shall be selected according to the recommended working voltage range of the device, and the fluctuation range of the actual voltage shall be considered to ensure that the device specification cannot be exceeded.
2 Output Current: Iout
The continuous output current capability of the device is an important parameter, which should be referred to when selecting, and a certain margin should be reserved.
The selection of this parameter also needs to evaluate the instantaneous peak current and heating of the circuit, determine them comprehensively, and meet the derating requirements.
3 Output ripple: Vpk pk
Ripple is an important parameter to measure the output voltage fluctuation of a circuit. Attention should be paid to light load and heavy load ripple. Generally, light load ripple should be large. Pay attention to whether the ripple under light load in nuclear power and other occasions will exceed the requirements. The actual situation under various scenario loads. The oscilloscope 20M bandwidth is usually selected for testing.
4 Efficiency
Pay attention to both light load and heavy load. Light load will affect standby power, and heavy load will affect temperature rise. Generally, the efficiency of 10mA under 12V input and 5V output is more than 80%.
5 Transient response
The transient response characteristics reflect whether the system can be adjusted in time to ensure the stability of the output voltage when the load changes dramatically. The smaller the output voltage fluctuation is, the better. Generally, the peak to peak value is less than 10%.
In fact, the feedback capacitor shall be selected according to the recommended value. The common values are 22p to 120pF.
Figure 9
6 Switching Frequency: fsw
Most of the commonly used switching frequencies are above 500kHz. There are also higher switching frequencies of 1.2M to 2M. Because the high switching frequency increases the switching loss, the IC cooling design is better, so it is mainly focused on products with low input current at 5V low-voltage. Switching frequency is related to the selection of inductance and capacitor, and other problems such as EMC and noise under light load are also related to it.
7 Feedback Voltage&output accuracy: Vref
The feedback voltage shall be compared with the internal reference voltage and output different voltages in coordination with the external feedback voltage divider. The reference voltage of different products will be different, such as 0.6~0.8V. When replacing, pay attention to adjusting the feedback resistance.
The feedback resistor should be selected with 1% accuracy, as long as it is selected according to the manufacturer‘s recommendation, generally it should not be too large, so as not to affect the stability.
The accuracy of the reference voltage affects the output accuracy. The common accuracy is below 2%, such as 1%~1.5%. The cost of products with high accuracy will vary. Select as needed.
8 Linear stability and load stability (line/load regulation)
Linear stability reflects the stability of input voltage and output voltage. The load stability reflects the stability of output voltage when the output load changes. Generally, 1% is required, and the maximum value shall not exceed 3%.
9 EN level
The EN high and low levels shall meet the device specification requirements, and some ICs shall not exceed the specific voltage range; The resistance shall be switched off in time during voltage division, and the maximum range of voltage fluctuation shall be considered.
Due to the need of timing control, this pin will increase capacitance. For level adjustment and off discharge, there must be a resistance to ground.
10 Protection performance
There should be over-current protection OCP, overheating protection OTP, etc., and the condition can be recovered automatically when the protection disappears.
11 Others
Soft start is required; Thermal resistance and encapsulation; The operating temperature range should cover high and low temperatures.
General principles of device selection
✔ universality
✔ High cost performance
✔ Long life cycle of easy procurement
✔ Compatible and replaceable
✔ Resource conservation
✔ Derating
✔ Easy to produce and normalize
Requirements for peripheral device selection
1. Input capacitance: meet the requirements of withstand voltage and input ripple. Generally, withstand voltage is required to be more than 1.5~2 times of input voltage. Note that the actual capacitance of the porcelain chip capacitor will decrease with the influence of DC voltage bias.
2. Output capacitance: meet the requirements of withstand voltage and output ripple. The general withstand voltage requirement is 1.5~2 times.
Relation between ripple and capacitance:
3. BST capacitance: according to the recommended value in the specification. Generally 0.1uF-1uF. The withstand voltage is generally higher than the input voltage.
4. Inductance: different output voltages require different inductances; Note that the temperature rise and saturation current should meet the margin requirements. Generally, the maximum current should be more than 1.2 times (or the saturation current of the inductor must be greater than the maximum output current+0.5 * the ripple current of the inductor). Usually, select the appropriate inductance value L, so that Δ IL accounts for 30% to 50% of the output current. Calculation formula:
5. VCC capacitance: it shall be taken according to the requirements of the specification, and shall not be reduced or too large. Pay attention to withstand voltage.
6. Feedback capacitance: take the value according to the requirements of the specification. Different manufacturers have different chip values, and different output voltages have different requirements.
7. Feedback resistance and EN voltage divider resistance: the value shall be taken according to the specification, with an accuracy of 1%.
PCB design requirements
1. The input capacitor is placed near the input Vin and power PGND of the chip to reduce the presence of parasitic inductance. Because the input current is discontinuous, the noise caused by parasitic inductance has a negative impact on the withstand voltage of the chip and the logic unit. The capacitance ground terminal is provided with via to reduce impedance.
2. The power loop should be as short and thick as possible, with a small loop area and less noise radiation. SW is the noise source, which ensures the current and keeps the area as small as possible, far away from sensitive and easily disturbed locations. For example, the inductance is close to the SW pin and far away from the feedback line. The output capacitance is close to the inductance, and the ground terminal is provided with a ground via.
3. The VCC capacitor should be placed between the VCC pin of the chip and the signal ground of the chip as close as possible, on the same layer, without via.
4. FB is the most sensitive and easily disturbed part of the chip and the most common cause of system instability.
1) The FB resistor may be short when connected to the FB pin, and placed close to the IC to reduce noise coupling; The voltage divider under FB is usually connected to the signal ground AGND;
2) Keep away from noise source, SW point, inductor, diode (asynchronous buck); FB wiring and grounding;
3) The FB of high current load shall be taken at the remote end of the load, and the feedback capacitor wiring shall be taken nearby.
5. The capacitance wiring of BST shall be as short as possible and not too thin.
5. The capacitance wiring of BST shall be as short as possible and not too thin.
6. The chip heat dissipation shall be in accordance with the design requirements, and the vias shall be added at the bottom as much as possible for heat dissipation.
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