Introduction to LED drive circuit using inductive boost switch converter

1. Basic circuit topology and working principle

LED drive circuits based on inductive boost switching converters are widely used in the backlighting of battery-powered consumer portable electronic devices. The basic circuit topology of the inductive boost converter is mainly composed of a boost inductor (L1), a power switch MOSFET (VT1), a control circuit, a boost diode (VD1) and an output capacitor (C0), as shown in Figure 1(a).

Figure 1 Basic circuit and working principle diagram of inductive boost converter

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) together, thus minimizing the number of external components.

When the controller drives VT1 to turn on, VD1 is turned off, and the current in L1 cannot change suddenly, but can only increase slowly and linearly from zero. The power supply stores energy in L1 and flows through L1 and VT1 back to the negative terminal of the power supply. In this process, the output capacitor C. supplies power to the load (Z), as shown in Figure (b).

When the controller turns off VT1, the induced electromotive force on L1 (negative on the left and positive on the right) makes VD1 forward biased and turned on, L1 releases the stored energy, and the current gradually decreases. In this process, the energy stored in inductor L1 and the forward polarity input voltage jointly 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 voltage VIN is:

Where: D is the switch duty cycle.

From equation (1), we know that since D <1, (1 -D) <1, Vo>VIN. The larger the duty cycle D, the higher Vo. The ratio of Vo to VIN is called the boost ratio of the converter.

2. Main advantages and disadvantages

LED driver 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 first and then in parallel). Their main advantages are as follows:

First, the efficiency is high, generally reaching 80% to 85%, which is about 20% higher than the capacitor boost converter.

Secondly, when driving a series-connected LED string, the current passing through each LED is equal, thereby ensuring that the brightness of the LEDs is consistent.

Third, the inductive boost switching regulator is usually set as a constant current source, which can keep the LED brightness constant over a wide battery voltage range.

However, there are some shortcomings of multi-inductor boost converters, which are specifically manifested in the following aspects:

First, the size is too large and the price is relatively high.

Second, when driving a series-configured LED string, the number of LEDs is limited. Usually, its maximum output voltage of about 40V can support at most 10 to 13 white light LEDs. Not only does the current density on the PCB copper wire become a problem, but as long as one LED is open-circuited, all other LEDs will go out.

Third, EMI is quite serious.