DC/DC Switching Regulators refer to those that convert direct current to direct current. By this definition, LDO (Low Dropout Linear Regulator) chips should also fall into the category of DC/DC power supplies. However, generally, only those power supplies that convert direct current to direct current through a switching mode are called DC/DC power supplies.
Buck Converter: Applicable in situations where the input voltage is higher than the output voltage. It achieves a step-down output of the input voltage through the turning on and off of the power switch transistor.
Boost Converter: Suitable for scenarios where the input voltage is lower than the output voltage. By means of the on-off operation of the power switch transistor, along with the cooperation of the inductor and capacitor, it realizes a step-up output of the input voltage.
Buck-Boost Converter: Possessing both step-down and step-up functions, it is appropriate for occasions where there is a significant difference between the input voltage and the output voltage.
To understand the working principle of DC/DC, one first needs to be familiar with a law and the three basic topologies of switching power supplies.
1. The Volt-Second Balance Law of Inductor Voltage
The working state of a power converter when the input, load, and control are all fixed values is called the steady state in switching power supplies. In the steady state, the inductor in the power converter satisfies the Volt-Second Balance Law of Inductor Voltage: For a DC/DC power converter that is already working in the steady state, the positive volt-second applied to the filter inductor when the active switch is on must be equal to the negative volt-second applied to the inductor when the active switch is off.
2. The Three Basic Topologies of Switching Power Supplies
2.1 BUCK Step-Down Type
When the PWM drives a high level to turn on the NMOS transistor Q1, ignoring the on-state voltage drop of the MOS transistor, the inductor current rises linearly. At this time, the positive volt-second of the inductor is: V * Ton = (Vin - Vo) * Ton.
When the PWM drives a low level to turn off the NMOS transistor Q1, the inductor current cannot change abruptly and forms a loop through the freewheeling diode (ignoring the diode voltage drop) to supply power to the output load.
At this time, the inductor current decreases, and the negative volt-second of the inductor is: V * Toff = Vo * (Ts - Ton).
According to the Volt-Second Balance Law of Inductor Voltage, we can obtain: (Vin - Vo) * Ton = Vo * (Ts - Ton), that is, Vo = D * Vin (where D is the duty cycle).
2.2 BOOST boost type
Using an analysis method similar to that of the BUCK circuit, when the MOS transistor is turned on, the positive volt-second of the inductor is: Vin * Ton; when the MOS transistor is turned off, the negative volt-second of the inductor is: (Vo - Vin) * (Ts - Ton).
According to the Volt-Second Balance Law of Inductor Voltage, we can obtain: Vin * Ton = (Vo - Vin) * (Ts - Ton), that is, Vo = Vin / (1 - D).
2.3 BUCK-BOOST Type with Polarity Reversal for Step-Up and Step-Down (The direction of the diode in this circuit is reversed.)
Similarly, using the same analysis method as that for the BUCK circuit, when the MOS transistor is turned on, the positive volt-second of the inductor is: Vin * Ton; when the MOS transistor is turned off, the negative volt-second of the inductor is: -Vo * (Ts - Ton). According to the Volt-Second Balance Law of Inductor Voltage, we can obtain: Vin * Ton = -Vo * (Ts - Ton), that is, Vo = -Vin * (D / (1 - D)).
Therefore, DC/DC chips mainly ensure the stability of the output voltage by comparing the feedback voltage with the internal reference voltage, thereby adjusting the duty cycle of the MOS transistor drive waveform.
Advantages and Disadvantages of DC/DC Converters
Advantages:
High Efficiency: DC/DC converters usually adopt advanced control techniques and components, enabling them to achieve relatively high power conversion efficiency.
Good Stability: Through precise control and feedback mechanisms, DC/DC converters can maintain the stability and reliability of the output voltage.
Disadvantages:
Large Ripple: In some cases, there may be significant ripple noise in the output voltage of DC/DC converters, and additional filtering measures need to be taken to reduce its impact.
Key Parameters of DC/DC Converters
Efficiency: It refers to the ability of a DC/DC converter to convert input electrical energy into output electrical energy, usually expressed as a percentage. A high-efficiency DC/DC converter can reduce power losses and improve the overall efficiency of the device.
Maximum Rated Power: It indicates the maximum power that a DC/DC converter can withstand under normal working conditions. Exceeding this power may lead to device damage or performance degradation.
Maximum Input Voltage Range: It represents the range of input voltages that a DC/DC converter can accept. Input voltages outside this range may cause device damage or performance instability.
Maximum Output Voltage: It refers to the maximum output voltage that a DC/DC converter can provide under normal working conditions.
Maximum Output Current: It denotes the maximum output current that a DC/DC converter can supply under normal working conditions.
Ripple Noise: It refers to the fluctuating component in the output voltage, usually expressed in peak-to-peak value or root-mean-square value. Excessive ripple noise can affect the performance and stability of the device.
Quiescent Power Consumption: It refers to the power consumption of a DC/DC converter when it is in no-load or standby state.
Temperature Coefficient: It indicates the extent to which the output voltage of a DC/DC converter changes with temperature.
Key Points for DC/DC Switching Regulators Selection
Output Voltage: Select an appropriate output voltage range according to the actual requirements. Make sure that the output voltage of the DC/DC converter can meet the working needs of the device.
Maximum Output Power: Choose a suitable maximum output power based on the actual requirements and reserve at least 30% margin to ensure the stable operation of the device.
Output Ripple: Pay attention to the output ripple specifications of the DC/DC converter to ensure that it meets the performance requirements of the device. If necessary, filtering measures can be taken to reduce the ripple noise.
Input Voltage: Ensure that the input voltage range of the DC/DC converter complies with the actual power supply conditions of the device. If the input voltage fluctuates significantly, a DC/DC converter with wide voltage input can be considered.
Conversion Efficiency: Conversion efficiency is an important indicator of the performance of a DC/DC converter. When making a selection, attention should be paid to the conversion efficiency data and a high-efficiency model should be chosen. Meanwhile, understand the changes in efficiency under different working conditions to provide accurate data support for the application.
The application fields of DC-DC power chips are extremely extensive, covering multiple areas such as automotive electronics, industrial control, smart homes, new energy, medical equipment, and aerospace. In these fields, various electronic devices all require the use of DC-DC power chips to provide a stable power supply. For example, in automotive electronics, components like car stereos, navigation systems, and dash cams all need to use DC-DC boost chips to ensure a stable power supply. In the field of new energy, in power generation systems such as solar and wind energy, DC-DC power chips can be used to convert the generated direct current into alternating current that can be connected to the grid.