1. Basic circuit topology and working principle
LED drive circuits based on inductive boost switching converters are widely used in backlighting of battery-powered consumer portable electronic devices. The basic circuit topology of the inductive boost converter is mainly composed of the boost inductor (L1), power switching MOSFET (VT1), control circuit, boost diode (VD1) and output capacitor (C0), as shown in Figure 1 (a) .
In the DC / DC boost converters used in portable devices, the controller and power MOSFET (VT1) are generally integrated on the same chip, and some also integrate the boost diode (VD1), thus Minimize the number of external components.
When the controller drives VT1 to turn on, VD1 is turned off, the current in L1 cannot be abrupt, it can only increase slowly and linearly from zero, and the power stored in L1 returns to the negative terminal of the power through L1 and VT1. In this process, the output capacitor C. Supply power to the load (Z) as shown in (b).
When the controller turns off VT1, the induced electromotive force (left negative right positive) on L1 biases VD1 forward and turns on, L1 releases the stored energy, and the current gradually decreases. In this process, the energy storage in the inductor L1 and the input voltage of the forward polarity both supply power to the load and charge C1 at the same time, as shown in Figure 1 (c).
In an inductive boost switching converter, the relationship between the output voltage Vo and the input power VIN is:
Where: D is the duty cycle of the switch.
From equation (1), we know that because D "1, (1-D)" 1, so Vo "VIN. The larger the duty cycle D, the higher the Vo. The ratio of Vo to VIN is called the boost ratio of the converter.
2. Main advantages and disadvantages
LED drive circuits based on inductive boost converters are mostly used to drive several LEDs connected in series (some can also drive LED arrays that are connected in series and then in parallel). The main advantages are as follows:
One is high efficiency, generally up to 80% to 85%, about 20% higher than the capacitor boost converter.
Second, when driving LED strings arranged in series, the current through each LED is equal, so that the brightness of the LEDs can be guaranteed to be consistent.
The third is that the inductive boost switching regulator is usually set as a constant current source, which can keep the LED brightness unchanged in a wider battery voltage range.
However, there are some problems with multi-inductance boost converters, which are manifested in the following aspects:
First, the size is too high, the price is higher.
Second, when driving LED strings in series configuration, the number of LEDs is limited. Usually, the maximum output voltage of about 40V can support up to 10 to 13 white LEDs, which not only makes the current density on the PCB copper wire a problem, but only One LED appears to be open, and all other LEDs will go out.
Third, EMI is relatively serious.
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