Isolation describes the electrical separation between the input and output of a DC-DC converter. Isolated DC-DC converters use a transformer to eliminate the DC path between the input and output. In contrast, non-isolated DC-DC converters have a DC path between the input and output. Non-isolated DC-DC converters are usually designed with dedicated ICs.
Why do system power supplies need isolation?
For safety reasons, electronic systems must be isolated between the AC input and DC output. All systems that operate on the AC line have isolation requirements, including front-end ac-dc power supplies, isolated "brick" dc-dc converters, and non-isolated point-of-load converters. Depending on the specific application, the typical isolation voltage for DC-AC and DC-DC power supplies ranges from 1500V to 4000V.
What are the typical applications for non-isolated DC-DC converter ICs?
Battery systems that do not use the AC line are the mainstream application for non-isolated DC-DC converters. In addition, point-of-load DC-DC converters, such as bus converters, where input power is provided by an isolated DC-DC converter, represent another widespread application area for non-isolated DC-DC converters. What is the circuit of a typical non-isolated DC-DC converter IC?
The following figure shows a simple schematic of a non-isolated buck converter IC. Resistors R1 and R2 form a voltage divider that adjusts the output voltage of the buck converter. Non-isolated DC-DC converters can have different output voltages by using precision resistors to set the output voltage to 1.8, 2.5, 3.3, or 5.0 V, etc.
What are the typical component configurations of isolated switch-mode DC-DC converter ICs?
An isolated DC-DC converter uses an inductor-capacitor output filter and a switching transformer with a diode rectifier or synchronous rectifier as the secondary circuit to generate the DC output voltage. It often uses an optocoupler to provide the isolated feedback voltage. The typical topology is a flyback or forward converter. The advantage of this configuration is that multiple output voltages can be generated by adding secondary transformer windings.
What are the typical component configurations of non-isolated switch-mode DC-DC converter ICs?
Most of these DC-DC converter ICs use internal or external synchronous rectifiers. The only magnetic component is usually the output inductor. Compared with isolated DC-DC converters, they are often less expensive, have fewer components, and occupy less PCB area for the same power and voltage levels. Also, since these DC-DC converters have only one magnetic component, an inductor, they are less susceptible to EMI. This non-isolated buck converter IC uses an external synchronous converter consisting of an n-channel power MOSFET that feeds back a non-isolated DC voltage. However, isolated converters require an optocoupler for feedback.
Why is the input capacitance of a non-isolated DC-DC converter IC important?
The dominant factor in the size of the input capacitor is its current handling capability. This capability is usually determined by the package size and the equivalent series resistance (ESR) of the capacitor. If both criteria are met, the capacitor is often large enough to prevent impedance interaction with the input voltage source. Generally, ceramic capacitors are used for input filtering because they offer both low impedance and a small footprint. In the figure, CIN is the input capacitor.
Why is the output filter of a non-isolated DC-DC converter IC important?
The output filter consists of an inductor (L1 in the figure) and a capacitor (COUT in the figure). Since the output capacitor is part of the feedback loop of the DC-DC converter, its size and ESR are important. The inductor characteristics affect the output ripple and circuit stability. The important parameters of the inductor include its inductance, saturation current and core material. The saturation current should be greater than the maximum output current so that it can handle any ripple or transient overcurrent events.
Is soft start necessary in a non-isolated DC-DC converter IC?
The internal soft start limits the inrush current and enables controlled startup of the DC-DC converter. Without soft start, the DC-DC converter may produce startup transient effects that affect other circuits in the system. Most IC soft start circuits use an external capacitor to provide startup time.
Use a non-isolated conversion power supply, which is characterized by the ground connection between the input and output ends. It is low cost and poor reliability. When a fault occurs, it will cause all low-voltage electrical appliances in the vehicle to burn out.
Therefore, the isolated DC converter is the best solution. It can reduce the higher battery voltage to a stable DC low-voltage output. Combined with the development of mainstream electric vehicles in China, it adopts internationally advanced digital analog hybrid control technology, and the input and output are completely electrically isolated, which is safer and more reliable. At the same time, it has the characteristics of small size, easy installation, high conversion efficiency; it can run at full load for a long time, low static power consumption, stable output voltage, full protection function, high reliability, and long service life.
