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The negative voltage shown (CrM) flyback controller intended for LED Lighting corresponds to the on-time of the power FET which then applications where high power factor is required. There are becomes positive after the FET turns off. The transformer several demo boards available from ON Semiconductor does not demagnetize due to the shorted output and the bias which illustrate how the NCL30000 can be used in practical winding voltage remains above 2 V which is above the ZCD power supply applications. Some LED driver power supply threshold. Since no ZCD event is detected, after designs have special requirements or need additional approximately 170 ms, the start timer within the NCL30000 features not covered in the existing demo boards and initiates a new switching cycle. So normally under a short application notes. This document covers some ideas to circuit event, the controller effectively operates in a skip enhance performance in practical LED driver solutions. mode resulting in a low duty cycle and reduced component stress. If and when the short is removed, the controller Tip 1: Overload and Transformer Leakage Inductance returns to normal operation. Under certain application situations, an offline LED driver may inadvertently have the outputs shorted together. This may also occur as part of a safety qualification regime. Depending on the transformer parameters, a short circuit may result in high component stress. LED drivers based on the NCL30000 controller operating in CrM rely on information derived from the bias winding to signal when it is time to start the next switching cycle. The NCL30000 maintains CrM operation by monitoring the bias winding via the Zero Current Detection (ZCD) input. Typically the voltage at this pin falls to a low level at the end of a switching cycle when the transformer has demagnetized and the power switch is turned on once again. Depending on the design of the transformer and the magnitude of the leakage current, it may be necessary to add a minimum off time delay circuit to further limit the power under a short circuit condition. Figure 1. Bias Winding Voltage for Low Leakage Leakage inductance, or uncoupled flux, associated with Inductance Transformer. the transformer secondary winding slows the rate of rise of If the transformer leakage inductance is high, the ringing current as a function of the load on the winding. Under short on the bias winding could be interpreted as transformer circuit conditions the bias winding is lightly loaded demagnetization by the ZCD pin thus initiating another compared to the main secondary winding and as such switching cycle. Since the transformer is not actually experiences a higher rate of voltage rise. The leakage demagnetized the current in the power switch will rise inductance between these two secondary windings rapidly to the over current threshold. When the current limit introduces a resonant behavior due to the difference in function turns the power switch off the transfer of energy to relative voltage. As the windings normalize a ringing the secondary begins at a higher level stimulating ringing in waveform can appear on the bias winding where the the bias winding. The cycle repeats itself at a very high rate. amplitude is dependant on leakage inductance. The high current and fast switching rate results in increased Figure 1 below shows the bias winding voltage of a low power dissipation. Figure 2 shows the bias winding voltage leakage inductance transformer with the LED driver under © Semiconductor Components Industries, LLC, 2011 1 Publication Order Number: January, 2011 − Rev. 1 AND8462/D AND8462/D of a high leakage inductance transformer under short circuit. Low transformer leakage inductance minimizes the Note the higher switching frequency. resonance and allows the bias winding to dampen very quickly without improperly triggering the ZCD function. Unfortunately producing a transformer with low leakage inductance is not always possible. In these circumstances, some measure to prevent the power switch from turning back on prematurely can ensure proper CrM operation by allowing proper detection of transformer demagnetization. The circuit shown in Figure 3 below performs a masking or minimum off-time function which blocks the ZCD function during the ringing on the bias winding immediately after the power switch turns off. Capacitor Ca charges quickly through resistor Ra and the lower Da diode while power switch Q3 gate drive signal is on. When the gate drive turns off, capacitor Ca retains charge and keeps the voltage on the ZCD above the trigger threshold during the period when there may be ringing on the bias winding. The capacitor quickly discharges through resistor Rb and Figure 2. Bias Winding Voltage for High Leakage restores normal bias winding monitoring of transformer Inductance Transformer demagnetization. T1A T1D T1B R16 47K T1E Da MMBD7000 T1C U1 1 8 Ra MFP Vcc 2.2K R19 C8 2 7 Comp DRV 10uF 3 6 Ct GND Rb Q3 4 5 CS ZCD 10K NCL30000 Ca 100pF R20 C9 Minimum off− time circuit Figure 3. Minimum Off-Time Circuit http://onsemi.com 2 AND8462/D Typical masking time is 1 to 4 ms which is well within Tip 2: Enhanced Over Voltage Protection operating times of LED drivers based on the NCL30000. A constant current source LED driver requires some way Values in the circuit can be adjusted to fit particular to limit the output voltage at no load. The NCL30000 transformer leakage characteristics. Figure 4 shows the application circuit includes protection in the event the output same high inductance transformer operating in short circuit is left open or if the output opens due to a fault in the LED with the minimum off-time circuit shown above. Note the string. The open load protection utilizes the current ringing does not erroneously activate the ZCD function and feedback opto-coupler. In some circumstances, additional the converter operates at a lower switching frequency with protection may be required in the event the opto-coupler lower dissipation. fails. Note that isolation is required for redundant over voltage protection. A second opto-coupler could be used, but there is another alternative. The bias winding voltage is proportional to the output voltage. Coupling the bias winding to the Multi Function Pin through a zener diode will provide redundant over voltage protection in the event of feedback opto-coupler failure. The zener diode should be selected such that it not interfere with normal operating voltages and yet provide a limit on output voltage