1027 SoftStart Techniques
Some applications need the input current to be controlled, to prevent high current spikes when power is first applied. This could be to reduce damage to switch contacts by the risk of sparking. Clearly the inrush techniques just described could be used, but sometimes it is necessary to control the output power instead. For example, a circuit for driving one or two power LEDs from the AC mains could use a double-buck topology. But typical applications for this circuit are inside lamp housings,...
75 Conclusions
The boost-buck is an ideal topology where the LED load voltage can be higher or lower than the supply voltage. It should also be used when the supply voltage is no more than 20 difference worst case from the LED load voltage. So if the LED voltage maximum is 20 V and the supply voltage minimum is 23 V, the difference is 3 V, and 3 20 0.15 or 15 , so a Cuk or SEPIC should be used. If the supply voltage is more than 20 higher, use a buck topology. If the supply voltage is more than 20 lower, use...
1026 Inrush Limiters
Because almost all circuits have decoupling capacitors, when a power source is connected there will be an inrush current. This current can be very high, causing temporary heating in the capacitor and possible damage to switch contacts or components connected in series. Inrush current limiting using passive or active components can be provided to reduce this risk. For AC mains applications, an NTC thermistor designed to carry high current is often used. In the active state, the flowing current...
56 Double Buck
The double buck is an unusual design, as shown in Figure 5.9. It uses one MOSFET switch, but two inductors L2 and L3 in series. Diodes steer the current in L2, which must operate in discontinuous conduction mode DCM for correct operation. The double buck is used when the output voltage is very low and the input voltage is high. An example is driving a single power LED from an AC supply line. A single buck stage cannot work easily because the on-time of the buck converter is too small, unless a...
526 Choosing the Sense Resistor R2
The sense resistor value is given by This is true if the internal voltage threshold of 0.25 V is being used. Otherwise, substitute the voltage at the LD pin instead of the 0.25 V into the equation. Note that the current limit is set to 15 above the maximum required current, due to the total 30 ripple specified. For this design, R2 0.625 U. The nearest standard value is R2 0.62 U. If a standard value is not close to the value calculated, or if a lower power dissipation in the sense resistor is...
125 Magnetic Saturation
Magnetizing core losses are also present and are due to the energy required to make the magnetic fields in the core to align with each other. In a switching circuit these losses are continuous and can cause core heating. These losses increase rapidly if the magnetization is forced to operate outside its linear region. Generally, the magnetic flux density should be limited to about 200 mT 200 Weber m2 . If an inductor or transformer has a large discontinuous current flow, as in certain fly-back...