Switching regulator and integrated FETs works well. One of the most common switching regulator topologies is a step-down switching regulator. Step-down regulator ICs usually use built-in controllers and integrated FETs for step-down conversion. Not only that, the buck regulator IC can also be applied to various designs, such as inverting power supplies, bipolar power supplies, and isolated power supplies with single or multiple independent voltage outputs.
Step-down converter using step-down regulator IC
Renesas Electronics’ ISL8541x series of step-down regulator ICs have integrated upper and lower FETs, internal start-up diodes and internal compensation to minimize the number of external components and achieve a very small overall solution. In addition, this series of regulator ICs have a wide input voltage range of 3V to 40V, and can support multiple batteries and various regulated voltage outputs. This article will take the ISL85410 step-down regulator IC as an example to explain various application designs in detail.
In power supply design, when the required voltage is lower than the available voltage in the system, a step-down converter is required. For example, a system that uses a 12V battery as the input voltage needs to output a voltage of 5V, 3.3V, or 1.2V to power the microcontroller, I/O, memory, and FPGA. By effectively converting high voltage to low voltage, the buck converter can extend battery life in the system, reduce heat dissipation, and improve reliability. Figure 1 is a simplified schematic diagram of a buck converter using the ISL85410 buck regulator IC.
A brief analysis of what is a buck regulator and its design principles
Figure 1: Simplified schematic diagram of a buck converter
The output voltage has the same polarity as the input voltage, and the voltage conversion rate in continuous conduction mode (CCM) can be expressed as:A brief analysis of what is a buck regulator and its design principles(1)
Where D is the duty cycle, ranging from 0 to 1, which means that the output voltage (VOUT) is always less than or equal to the input voltage (VIN).
Inverting power supply using step-down regulator IC
Although Electronic systems usually use positive voltages, they sometimes need to use negative voltages. In this case, an inverting power supply is required to generate a negative voltage with a positive input. To meet these application requirements, one of the more common solutions is to use an inverting buck-boost converter.
Figure 2 compares the power stages of a buck converter and an inverting buck-boost converter, showing that an inverting buck-boost converter can be obtained by switching FET Q2 and Inductor L1. This topology change will produce different voltage conversion ratios and the inverted polarity of the output voltage:A brief analysis of what is a buck regulator and its design principles(2)
In an inverting buck-boost converter, the output voltage amplitude can be higher or lower than the input voltage, and the output voltage is negative with respect to the ground of the input voltage source.
A brief analysis of what is a buck regulator and its design principles
Figure 2: Power stage of a buck converter and an inverting buck-boost converter
The inverting buck-boost converter can be implemented with a highly integrated buck regulator IC. As shown in Figure 3, a simplified circuit using the ISL85410 buck regulator. When configuring a buck regulator as an inverting buck-boost converter, two important differences need to be noted.
First, the (VIN) return (RTN) connection of the input voltage. In the buck converter shown in Figure 1, the RTN of the input voltage is also the ground terminal (that is, the AGND/PGND pin of the buck regulator), while the RTN of the input voltage and the ground in the inverting buck-boost converter The ends are no longer the same. Therefore, when implementing an inverting buck-boost converter, an input voltage source must be applied to the VIN pin and RTN (not the AGND/PGND pin).
Second, the voltage stress on the VIN pin needs to refer to the AGND pin. Regardless of the output voltage, the voltage in the buck converter is always equal to the input voltage (VIN). In contrast, the VIN pin in an inverting buck-boost converter must be able to withstand the sum of the input voltage and the output voltage (V IN + V OUT). For example, in a design that converts 24V to -5V, the voltage stress on the VIN pin is 29V instead of 24V. It must be remembered that the voltage stress on the VIN pin should not exceed the absolute maximum rated voltage specified in the IC data sheet.
A brief analysis of what is a buck regulator and its design principles
Figure 3: Simplified inverting buck-boost converter
Bipolar power supply with step-down regulator IC
Many applications, such as operational amplifiers and data acquisition systems, require bipolar ±5V or ±12V power supplies. A common method is to use a single switching regulator and coupled inductor (also commonly referred to as a transformer) to generate negative and positive voltage outputs. Figure 4 shows how to use a buck converter and an inverting buck-boost converter to generate a bipolar power supply.
As shown in Figure 4(a), the ISL85410 buck regulator is first configured as a buck regulator that regulates the positive output VOUT+, and then generates a negative output VOUT- by adding an additional coupling winding. If the positive output VOUT+ is regulated as in a step-down converter, the negative output VOUT- has the same value as VOUT+ (for simplicity, the forward voltage drop of the rectifier diode D1 is ignored), but has the opposite polarity.
A brief analysis of what is a switching regulator and integrated FETs regulator as well as its design principles
Figure 4: Simplified schematic diagram of a bipolar power supply using the buck method (a) or the inverting buck-boost method (b)
Figure 5 shows the equivalent circuit of the bipolar power supply using the step-down method during the time interval between DT and (1-D)T. During DT, the upper FET Q1 is turned on, causing the rectifier diode D1 to be biased in reverse voltage, so no current flows in the secondary winding.
During the (1-D)T period, Q1 is disconnected, the current Ip flows through the down tube FET Q2, and the voltage (Vs) across the secondary winding corresponds to VOUT+, so D1 is turned on, charging the output capacitor COUT2, and supplying power to the load . It is recommended to configure the converter with forced CCM to achieve good voltage regulation of the negative output voltage (VOUT-).
A brief analysis of what is a switching regulator and integrated FETs regulator and its design principles
Figure 5: Equivalent circuit of bipolar power supply using step-down method
The following is a detailed description of the working principle of using the ISL85410 to establish and simulate the SIMPLIS Model of the bipolar power supply. The key parameters are shown in Table 1.
A brief analysis of what is a buck regulator and its design principles
Table 1: Key parameters of bipolar power supply
The simulation waveform is shown in Figure 6. During the (1-D)T period when Q2 is turned on, the coupling current of the secondary winding current (Is) causes the total primary current (Ip) to become negative. Through proper design, ensure that the negative current is low enough to avoid triggering the negative current limit of the buck regulator under normal operating conditions.
A brief analysis of what is a buck regulator and its design principles
Figure 6: Bipolar power supply simulation waveform using step-down method
Figure 4(b) shows another method that uses inverting buck-boost conversion to generate a bipolar power supply. Compared with the use of buck conversion, inverting buck-boost conversion configures the buck regulator IC as an inverting buck-boost to generate a negative voltage output, and uses a coupled winding to generate a positive voltage output.
Unlike bipolar power supplies that use buck conversion, when the input voltage is lower than the output, the inverting buck-boost conversion can regulate the output (boost conversion). However, the FET voltage stress in the inverting buck-boost conversion is higher than that in the buck conversion. Table 2 compares these two conversions and provides design guidance for choosing the best solution for a specific application.
A brief analysis of what is a buck regulator and its design principles
Table 2: Bipolar power supply comparing buck conversion and inverting buck-boost conversion
Isolated power supply with step-down regulator IC
Generally, an isolated voltage output is required to provide galvanic isolation and enhance safety and noise immunity. Common applications include programmable logic controllers (PLC), smart power metering, and IGBT drive power supplies. Flyback and push-pull converters are two common and economical solutions. However, flyback converters usually require optocouplers or auxiliary windings to adjust the output voltage.
In addition, flyback switches are affected by high voltage spikes, so RCD snubbers are usually required. Push-pull DC transformers operate at a fixed 50% duty cycle, which may affect the output voltage regulation, and sometimes an additional LDO is needed to achieve precise output regulation.
In the above-mentioned bipolar power supply (Figure 4), additional output voltage output is achieved by adding a magnetically coupled winding using an inductor in a buck or inverting buck-boost converter. By simply isolating the two output circuits, isolated voltage output can be achieved (see Figure 7), and this method is becoming more and more common.
Isolated power supply with a single isolated voltage rail
A brief analysis of what is a buck regulator and its design principles
Figure 7: Simplified single isolated voltage rail using the buck method (a) or the inverting buck-boost method (b)
Two methods of using a buck regulator to produce an isolated voltage output are shown in Figure 7. These configurations are similar to the bipolar power supply shown in Figure 4, except that the two output circuits (reference) are separated. Unlike the bipolar power supply whose transformer turns ratio is 1:1, this method can set the required output voltage on the secondary side by optimizing the turns ratio of the isolated power supply. In addition, you can also adjust the controller to run with the best duty cycle.
An isolated power supply with a buck regulator has many advantages. As shown in Figure 7 (a), take this step-down method as an example to illustrate its advantages. First, it removes the optocoupler and auxiliary freewheeling circuit required in the flyback converter. Secondly, compared to the flyback converter, the buck configuration provides low voltage stress on the primary side FET, and the low voltage FET means lower on-resistance and higher efficiency.
Third, the primary side output (VOUT1) is well regulated, and the isolated output (VOUT2) corresponds to VOUT1, which can provide good output voltage regulation on the secondary side within a wide input voltage range. Compared with the push-pull DC transformer without additional LDO, better voltage regulation can be achieved. Highly integrated step-down switching regulator and integrated FETs ICs, such as the ISL85410 with internal compensation, can easily implement the application of the above methods in power supply design.
In Table 2, the advantages and disadvantages of buck conversion and inverting buck-boost conversion design bipolar power supplies are also applicable to isolated power supplies using buck regulator ICs. Power supply designers should choose the most suitable for their specific applications. Methods.
Isolated power supply with multiple isolated voltage outputs
As shown in the two cases in Figure 2, multiple isolated voltage outputs realize via adding more coupled windings. Its working principle is similar to a single isolated voltage output.
A brief analysis of what is a buck regulator and its design principles
Figure 8: Multiple isolated voltage outputs using buck method (a) or inverted buck-boost method (b)
in conclusion
The highly integrated buck regulator IC can more easily realize different power conversions and meet different application requirements. This article explains how we use these switching regulator and integrated FETs ICs to generate inverting power supplies, bipolar power supplies, single or multiple isolated power supplies.
The highly integrated ISL8541x series of step-down regulator ICs have a wide input voltage range, integrated start diode and internal compensation. Inverting, bipolar and isolated power solutions are with these buck regulator ICs. They have many important advantages such as a small number of external components, small overall solution size, and ease of use.
