Offline led driver design

The proposed offline LED driver has been demonstrated in a 7 W, V rms experimental prototype, which provides high efficiency and low LED current ripple with high power factor. Article :. DOI: Need Help? For reducing differential mode EMI, a pi filter can be used by means of two 47nF capacitors and a suitable inductor. Due to Boundary Conduction Mode switching, the buck inductance value will influence the converter switching frequency. For smaller size coil, a small inductance value could be selected, but the limitations are set by the IC minimum ON time 0.

To calculate the inductance, we first need to calculate the maximum peak current I peak at the top of the rectified sine wave V peak :. The factor a can be calculated at the peak of the rectified sine wave:. From Figure 3 or formula 1 we can derive,. The range of inductance can now be calculated from:.

The frequency at the top of the sine wave can be calculated:. After the inductor current has reached zero, a resonance will occur between the inductor and the total capacitance at the switch node, which is mainly determined by the MOSFET drain-source capacitance. In order to minimize the MOSFET switching losses, RT provides the flexibility to adjust the delay time of next switch-on cycle in order to switch-on at the maximum point of the resonance, which corresponds to the minimum drain-source voltage value.

The delay time from zero current point to the maximum of the switch resonance T delay in figure 4 which can be calculated from: where C SW is the capacitance at the switch node, mostly determined by the MOSFET drain-source capacitance, which in this application equals 38pF. The resonance delay becomes:. The total required delay time for optimal resonant switching needs to be chosen a bit larger to include zero current detection delay around nsec in this case.

Figure 5 below shows the switching waveform with optimal resonant switch-on point. When the power MOSFET turns off, the path for the current is through the diode connected between the switch output and ground.

This forward biased diode must have low forward voltage drop and fast recovery times. To achieve high power factor and low THDi, the inductor current contains considerable low frequency ripple. The output capacitor will filter the switching and low frequency ripple current to deliver a low ripple voltage to the LED string. The amount of output ripple voltage together with the differential resistance of the LED string will determine the ripple current through the LEDs.

To reduce this ripple, a larger value output capacitor is required. Figure 6 shows the input and output voltage and current waveforms.

The average output LED current is accurately set at mA. Figure 7 below shows the switching waveforms. To achieve low THDi, the current peak value is around 4x higher than the average current. Many switching regulators cannot turn on the internal power N-mosfet indefinitely. Since the minimum off time is fixed, the maximum duty cycle that can be achieved decreases with increasing switching frequency.

This is because those ns consume a larger and larger portion of the switching cycle. Typical switching frequencies range from 50kHz to 1MHz, and kHz is often a good balance between the physical size of the power components, such as the inductor — which is smaller when switching frequency is higher — and power efficiency, which is higher when the switching frequency is lower.

In this case, kHz is not possible, so kHz will be used. This ensures that the LED driver has the smallest components possible while still being able to properly drive all 10 LEDs during worst-case input and output voltage conditions. Christopher Richardson is systems applications engineer for lighting at National Semiconductor. Your email address will not be published.

By using this website you are consenting to the use of cookies. Electronics Weekly is owned by Metropolis International Group Limited, a member of the Metropolis Group; you can view our privacy and cookies policy here. Design challenges with buck regulators When using a buck regulator to drive LEDs, the principal design challenge is the case when input voltage is at a minimum and output voltage is at a maximum.

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