Your Electronics Open Source

By Editorial Staff [ DESIGN IN ]

The LED using, in lighting applications, is becoming more and more attractive and the possibility of using the powerline directly, as a source of power supply, is becoming more and more interesting. Moreover, if the circuital solution allows a compact implementation that can be placed in the same volume of a standard lamp screw, it becomes more interesting.

Switching obviously
The solution is a switching converter, with buck topology, that is able to work with high input voltages (compared to the one required for LEDs driving ). Among lots of specific solutions proposed by the market for LED driving, the one proposed by Micronix (a IXYS company), with the MXHV9910 device, seems to be able to answer to the mentioned requirements ( its block diagram is shown on Fig.1).

Peculiar feature of this controller is the internal voltage regulator that can operate directly from the powerline high voltage without any resistive dividers or external capacitives in order to reduce the power supply voltage. The last ones, apart from occupying space, dissipate energy decreasing the power supply operative efficiency.

The chip works with input voltages that can change from 8 to 450 Volt and it can be configured as buck regulator, boost, or buck-boost operation even if the buck topology is the best one. It also allows to check the brightness both the linear control of the threshold switching through the LD pin and a PWM driving (Pulse Width Modulation) on PWMD pin. The high input operating voltage allows to drive more than a hundred of high-brightness LEDs serially placed, with the ability of exceeding efficiency levels of 90%. The device, as mentioned, has a regulator that provides a voltage of 7.8 volts to the internal circuitry, with the ability to provide up to 1 mA to possible external circuits. The ability of driving a power MOSFET gate directly and the entire device, enclosed in an extremely compact 8 pins SOIC package, complete the device description.

Figure 2 shows the application circuit proposed by Micronix and it is also available as evaluation board with the following code :MXHV9910DB3. The R4 resistance, which works as current sensor on LEDs, is used to set the current peak value, taking account that the circuit forces the MOSFET switching at the threshold value of 250 mV. In the circuit showed on Fig. 2 the peak current has been defined at 446mA that is 250 mV/0,56 Ω.

If it is used as switching the high speed MOSFET IXTA8N50P, by IXYS, is characterized by a maximum Vds of 500 V with a Rds (on) of 0.8 Ω and a maximum current value of 8 A at a 25 ° C. The L1 inductance value is determined by the switching frequency, by the input voltage and by the required voltage values on the diodes array.
The switching frequency is set to ~ 64 kHz by R1 value. Higher frequencies could cause a dissipation increase in the MOSFET, while lower values would lead to higher inductances for L1 with higher costs and volumes.