Definition of Buck and Boost DC-DC Converters
A DC-DC converter is a device that converts DC power from one voltage level to another. It is widely used in power electronics, communications, industrial control and other fields. According to different working principles and conversion methods, dc power regulator can be divided into many types, among which buck dc power regulator and boost dc power regulator are the most basic and common types.
Buck dc power regulator
A buck dc power regulator, also known as a step-down converter, is a device that reduces the input voltage to a lower output voltage. It uses an inductor to store energy and periodically switches current to achieve step-down conversion.
Buck dc-dc charger are usually used in situations where high voltage needs to be converted to low voltage, such as converting the DC power of a high-voltage battery pack into a low-voltage power supply for on-board electronic equipment.
The working principle of a buck high power dc dc converter is to control the on and off of a switching element (such as a MOSFET) so that the inductor stores energy when the switch is on and releases energy when the switch is off, thereby reducing the input voltage to the required output voltage.
At the same time, the output capacitor is used for smoothing and filtering to reduce the fluctuation of the output voltage.
Boost dc power regulator (Boost Converter)
The boost high power dc dc converter, also known as the step-up converter, is a device that increases the input voltage to a higher output voltage. It also uses inductance to store energy and periodically switches current to achieve boost conversion.
Boost dc - dc battery charger are usually used in situations where low voltage needs to be converted to high voltage, such as the boost circuit in a solar photovoltaic system.
The working principle of the boost high power dc dc converter is similar to that of the buck type, but the control method of the switching element is different.
In the boost converter, the switching element is periodically turned on and off during the cycle of the input voltage, so that the inductor accumulates energy when the switch is turned on, and releases the energy to the output when the switch is turned off, thereby achieving a voltage increase. Similarly, the output capacitor is also used for smoothing and filtering to reduce the fluctuation of the output voltage.
Differences in output ripple between buck and boost dc power regulator
Output ripple is the fluctuating component in the output voltage of a power converter dc , which may be caused by a variety of factors, such as switching action, inductor energy storage and release process, capacitor smoothing and filtering effect, etc.
There are significant differences in output ripple between buck dc - dc battery charger and boost dc - dc battery charger, which mainly stems from the differences in their working principles and circuit structures.
Output ripple of buck DC-DC converters
Buck dc power regulator directly output the inductor current that fluctuates due to the ripple current, and the output capacitor smoothes and filters the fluctuating current. The output current is "DC current + inductor ripple current", and the output capacitor only smoothes and filters the inductor ripple current (i.e., AC component).
The ripple voltage ΔV generated by the output capacitor is basically the voltage fluctuation caused by the charge Q coulomb charged and discharged by the output capacitor through smoothing and filtering action. That is, ΔV=Q/C, where C is the capacitance value of the output capacitor.
The ripple current increases during the high-side switch on time and decreases during the low-side switch on time, and the average value of the increase and decrease currents is the DC output current. When the current exceeds the average value, the output capacitor is charged; when the current is lower than the average value, the output capacitor is discharged.
Since the output voltage of a step-down dc power regulator is generally low and the inductor ripple current does not depend on the load current, the output ripple voltage remains constant in most cases.
However, during light loads (such as when the output current is less than half the ripple current value), products with a light load mode designed to improve efficiency during diode rectification and light loads may have their charge and discharge charges vary due to factors such as on-time control or intermittent switching operation, causing the ripple voltage to also vary.
Output ripple of a step-up DC-DC converter
In a step-up dc power regulator, the actions of accumulating energy by increasing the inductor current when the low-side switch is on and reducing the inductor current by releasing the energy when the high-side switch is on are repeated.
Current is supplied to the output capacitor only during the on-time of the high-side switch, so the supplied current is an intermittent pulse-like charging current.
In a step-up dc power regulator, the output capacitor needs to smooth the pulse-like supply current output. Therefore, compared with a step-down converter, a step-up converter needs to smooth a larger amount of charge, and a larger output capacitor is required for the same degree of ripple voltage.
In addition, since the output voltage of a step-up converter is usually high, the actual effective capacitance of an output capacitor such as a multilayer ceramic capacitor may be greatly reduced (DC bias characteristics) when a high voltage is applied, and may even be only a fraction of the nominal capacity.
This causes an increase in the output ripple voltage and may cause problems in transient response characteristics and negative feedback control safety.
In a dc dc step up converter, the waveform and size of the output ripple voltage are also affected by various factors, such as inductance value, switching frequency, load current, etc. Increasing the inductance value can reduce the inductor ripple current, but the peak value of the charging current pulse to the output capacitor will also become smaller.
Since the average current value does not change, the amount of charge charged and discharged to the capacitor will not change, so the ripple voltage will not decrease. In addition, when the load current changes, the inductor current needs to be increased or decreased to follow the change. However, if the ripple current is small, the inductor current will increase or decrease more slowly, and the speed of following the load current increase or decrease will also be slower.
