Transformerless AC-DC constant current LED driver circuit

Ac-DC constant current LED driver circuit powered by industrial frequency power supply. Regardless of the topology, a bridge rectifier, transformer or inductor is generally required. Zywyn has launched a new type of AC-DC constant current LED driver IC. Using a unique linear control structure system, it integrates a full-wave bridge rectifier, linear regulator, control circuit, and overvoltage and over-temperature protection circuits. No external rectifier, transformer, and inductor are required. Only a few components are needed to form an AC-DC constant current LED driver circuit with 85-240V universal AC input. It can drive an LED string consisting of dozens of LEDs (forward voltage Vf=3.3v for each LED, operating current Iled=30mA) or even 117 LEDs (Vf=1.8V for each LED).

The AC-DC constant current LED driver circuit based on ZD832 is shown in Figure 1. When the AC input is 110V (85~125V), the IC pin VSEL (pin 15) should be connected to HVCC (pin 16); when the input is 220V (range is 180-240V), the pin VSEL is left floating. Therefore, the IC pin VSEL acts as an AC power selection switch. The NTC thermistor RT is used to limit the inrush current during startup. The AC input voltage is rectified inside the IC and output from the pin HVCC. The ripple is filtered out by the capacitor Cout and applied to the LED string. Cvcap on the IC pin VCAP (pin 7) is the IC regulator output decoupling capacitor. The LED current Iled is set by the ground resistor Rset on the IC pin Iset (pin 8), and the calculation formula is Iled=2200V/Rset. The IC output current range is 3~30mA, and the Rset resistance value range is 733kΩ~73kΩ.

IC pin VOUT is the open drain output of the internal MOSFET. To prevent the internal power MOSFET of the IC from being damaged due to excessive power dissipation, a series limiting resistor Rext should be added when the voltage Vout on pin VOUT is high. The Vout voltage range is 7 to 20V, and Rext is set only when Vout ≥ 20V. If the LED string has N LEDs, and the forward voltage of each LED is VF, the resistance value of Rext is:

Rext=(Vhvcc-N・Vf-Vout)/Iled

If the AC input is 220V, leave the IC pin VSEL floating, and the DC output Vhvcc is about 286V. If the LED's Vf=3.3V, Iled=30mA, and there are 78 LEDs connected in series, the Rset value is 2200V/30mA=73kΩ. If Vout=20.5V is selected, the Rset value is:

Rset=(Vhvcc-N・Vf-Vout)／Iled

=(286V-78x3.3V-20 5V)／30mA=270Ω

If the AC input is 220V, the number of LEDs connected in series driven by ZD832 is the minimum (such as 40), then an RC network should be added to the input end, as shown in Figure 2. The 220V AC voltage is stepped down by capacitor Cac (0.68uF/250V) to become AC110V input, so the IC pin VSEL needs to be connected to HVCC. Rac in parallel with Cac is a bleeder resistor. The voltage Vhvcc on the IC pin HVCC is about 150V, the Vf of the LED is 3.3V (@30mA), and the voltage on the IC pin VOUT is: Vout=Vhvcc-N・Vf=150V-40x3.3V=18V<20V, so there is no need to set a limiting resistor Rext

When the input voltage range is 180-240V, a voltage regulator circuit consisting of transistor Q (ZN3439), R and Zener diode D2 (220V, 3W) can be added, as shown in the dotted box in Figure 3. The IC output voltage Vhvcc = 227-310V, and the output voltage VO = 219-230V after the voltage regulator circuit drives 65 3.3V series LEDs, Vout < 18V, so there is no need to add a limiting resistor.

Adding a 50Hz~5kHz PWM signal to the IC pin PWM can achieve PWM dimming. Author: Mao Xingwu