DC Circuit Design Skills and Device Selection Principles

01

Concept and functions

1

Concept

DC-DC refers to a direct current (direct current) power supply. This is a device that converts electrical energy of one voltage value into electrical energy of another voltage value in a DC circuit. For example, DC voltage (5.0V) can be converted to other DC voltages (1.5V or 12.0V) with a converter, we call this converter DC/DC converter or switching power supply or switching adjustment device .

DC-DC converters typically consist of control ICs, inductors, diodes, transistors, and capacitors. When discussing performance of DC/DC converters, if we only focus on control IC, we cannot judge its pros and cons. The characteristics of peripheral circuit components and connection method of substrate can change characteristics of power circuit, so a comprehensive decision should be made.

The use of DC-DC converters helps to simplify design of power circuits, shorten development cycle and achieve best performance. Widely used in power electronics, military industry, scientific research, industrial control equipment, communication equipment, instrumentation, switching equipment, equipment access, mobile communications, routers and other areas of communication and industrial control, automotive electronics, aerospace and other fields. Power modules, which are highly reliable and easy to upgrade system, are increasingly being used. In addition, DC/DC converters are also widely used in mobile phones, MP3s, digital cameras, portable media players and other products. It belongs to breaker circuit in classification of circuit types.

2

Features

Its main feature is high efficiency: Compared with linear regulator LDO, high efficiency is a significant advantage of DCDC. Usually, efficiency is more than 70%, and high efficiency can reach more than 95%. Second, adapt to a wide voltage range.

A: Modulation

1: PFM (Pulse Frequency Modulation)

The switching pulse width is constant, and output voltage can be stabilized by changing pulse output frequency. Even if PFM control type is used for a long time, it has advantage of low power consumption, especially under light load.

2: PWM (Pulse Width Modulation)

The switching pulse frequency is constant, and output voltage can be stabilized by changing output pulse width. The PWM type has high efficiency and good output voltage ripple and noise.

B: In general, performance differences of DC/DC converters using two different modulation methods, PFM and PWM, are as follows.

PWM frequency, PWM duty cycle selection method. The PWM/PFM conversion type realizes PFM control under light load, and automatically switches to PWM control under heavy load.

02

Classification of structures

1

Three General Principal Structures

A, Buck (DC/DC buck converter)

Picture 1

B, Boost (DC/DC boost converter)

Picture 2

C, Buck-Boost (DC-DC converter)

Picture 3

2

A detailed explanation of how BUCK circuit works

Picture 4

Volt-second balance principle: for a steady state inductor, positive volt-second product at both ends of inductor is equal to negative volt-second product, that is, volt-second product at both ends of the inductor must be balanced within one switching cycle.

Picture 5

When switch is turned on: input voltage Vin is applied to input terminal of LC filter, and current in inductor rises linearly with a fixed slope. As shown below

Picture 6

When switch is off: Since current in inductor cannot suddenly change, energy stored in inductor is released into load, while current in inductor continues to flow through diode. At this stage, shape of current curve is a slope with a negative slope. As shown below

Picture 7

03

Design skills and requirements for selection of key technical parameters

The design of a DC circuit must at least take into account following conditions:

A. External power supply input voltage range and output current.

B. DC output voltage, current, maximum system power.

1

Input/output voltage (input and output voltage): Vin/Vout

Select according to device's recommended operating voltage range, and consider actual voltage fluctuation range to ensure that device's specifications cannot be exceeded.

2

Output current: Iout

The continuous output current of device is an important parameter to refer to when choosing, and a certain margin should be left.

When this option is selected, it is also necessary to evaluate instantaneous peak current and heat generation in circuit, determine them comprehensively, and comply with derating requirements.

3

Output ripple: Vpk-pk

Ripple is an important parameter for measuring fluctuations in output voltage of a circuit. Pay attention to ripple under light and heavy load, usually ripple under light load is greater. Pay attention to whether light load ripple will exceed requirements in nuclear power and other applications. Real load test of various scenarios. Typically, bandwidth of oscilloscope is 20m for testing.

4

Efficiency

Notice both light and heavy loads. Light load will affect standby power, and heavy load will affect temperature rise. Usually look at 12V input, 10mA efficiency at 5V output is usually over 80%.

5

Transitional response

Transient characteristics reflect ability of system to adjust in time to ensure stability of output voltage during sudden changes in load. The less fluctuation of output voltage is required, better, generally in accordance with requirement of less than 10% of peak value.

In fact, pay attention to choice of feedback capacitor according to the recommended value. Common values ​​are between 22p and 120pF.

Picture 9

6

Switching frequency: fsw

The most commonly used switching frequencies are above 500 kHz. A higher switching frequency of 1.2m to 2m is also available. Since high-frequency switching losses increase heat dissipation of IC, they are mainly concentrated in low-voltage input and low-current 5V products. Switching frequency is related to choice of inductors and capacitors, and with it also deals with other issues such as EMC and light load noise.

7

Feedback voltage and output error: Vref

The feedback voltage should be compared with internal reference voltage, interact with external resistor of feedback divider to output different voltages. The reference voltage of different products will be different, such as 0.6 ~ 0.8V, pay attention to adjustment of feedback resistor when replacing.

Feedback resistor should be selected with 1% accuracy if selected according to manufacturer's recommendations, as a rule, do not select too large, so as not to affect stability.

The accuracy of reference voltage will affect accuracy of output. The normal accuracy is lower than 2%, such as 1%~1.5%, and cost of products with high accuracy will be different. Choose according to your needs.

8

Line/load control

Linearity stability depends on input voltage variations and output voltage stability. Response to load regulation Output load change Output voltage stability. The general requirement is 1%, and maximum should not exceed 3%.

9

English level

EN high and low levels must meet device specification requirements, and some ICs cannot exceed a certain voltage range; when resistor divides voltage, pay attention to timely shutdown, and consider maximum voltage fluctuation range. .

Due to need for time control, this pin will increase capacitance to adjust level and turn off discharge, at same time there must be resistance to ground.

10

Protection effectiveness

There must be over-current protection OCP, over-temperature protection OTP, etc., and it can automatically recover after state disappears after protection.

11

Other

Requires soft start, temperature resistance and packaging, operating temperature range must cover high and low temperatures, etc.

04

General principles for device selection

✔ Versatility

✔ High profitability

✔ Easy Procurement has a long life cycle

✔ Compatibility and replacement

✔ Saving resources

✔ Derating

✔ Easy production and normalization

05

Peripheral Device Selection Requirements

1. Input capacitance: It must meet requirements for voltage withstand and input ripple. Generally, withstand voltage requirement is 1.5-2 times input voltage. Note that actual capacitance of ceramic capacitors will decrease with DC bias.

2. Output Capacitor: It must meet voltage withstand and ripple requirements of output. The general withstand voltage requirement is 1.5~2 times.

Relationship between ripple and capacitance:

3. Capacitor BST: according to recommended value in specification. Typically 0.1uF-1uF. The withstand voltage is usually higher than input voltage.

4. Inductance: Different output voltages require different inductances; Please note that temperature rise and saturation current must meet limit requirements, usually more than 1.2 times maximum current (or saturation current of inductor must be greater than maximum output current + 0.5 * inductor ripple current). Typically, an appropriate value of inductance L is chosen so that ΔIL is between 30% and 50% of output current. Calculation formula:

5. Capacitor VCC: take value according to requirements of specification, it should not be reduced, and it should not be too large. Pay attention to withstand voltage.

6. Feedback Capacitor: Accept a value as required by specifications. Different manufacturers have different chip ratings, and different output voltages will have different requirements.

7. Feedback Resistor and Voltage Divider Resistor EN: Required to accept value according to specification, accuracy is 1%.

06

PCB design requirements

1. The input capacitor is located near Vin input of chip and PGND of power supply to reduce presence of parasitic inductance, because input current is discontinuous, and noise caused by parasitic inductance will affect withstand voltage of chip and negatively affect logic block. Add vias to ground end of capacitor to reduce impedance.

2. The power circuit should be as short and thick as possible, so that loop area is small and noise emission is reduced. SW is a source of noise, keep area as small as possible while still providing current, and keep it away from sensitive and susceptible to interference. For example, inductor should be close to SW pin and away from feedback line. The output capacitor sits next to inductor and ground terminal adds ground.

3. The VCC capacitor should be placed between chip's VCC pin and chip's signal ground, as far as possible on first layer, without through holes.

4. FB is most sensitive part of chip, most susceptible to interference and most common cause of system instability.

1) The FB resistor connected to FB pin can be short and placed close to IC to reduce noise coupling; voltage divider resistor under FB is usually connected to signal ground AGND;

2) Keep away from sources of interference, short circuit points, inductors, diodes (asynchronous tanks), ground OS tracks;

3) The large current load FB is taken at far end of load, and feedback capacitor wiring should be taken nearby.

5 The BST capacitor wiring should be as short as possible and not too thin.

6. The heat dissipation of chip should be designed according to design requirements, and try to increase heat dissipation through holes at bottom.