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TEA1566 Datenblatt(PDF) 7 Page - NXP Semiconductors |
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TEA1566 Datenblatt(HTML) 7 Page - NXP Semiconductors |
7 / 24 page 1999 Apr 20 7 Philips Semiconductors Preliminary specification GreenChip ™; SMPS module TEA1566 Multi frequency control The oscillator is also capable of working at a lower frequency (see fosc-l in Chapter “Characteristics”). A ratio of 1 : 2.5 is maintained between high and low frequency of the oscillator. Low frequency operation is invoked if the power supply is working at or below one ninth of its peak power. By working at a lower frequency, the switching losses in the power supply are reduced. A novel scheme is used to ensure that the transfer of high to low frequency and vice versa has no effect on the regulation of the output voltage. Gate driver The gate driver has a totem-pole output stage that has current sourcing capability of 120 mA and a current sink capability of 550 mA. This is to enable fast turn on and turn off of the power device for efficient operation. A lower driver source current has been chosen in order to limit the ∆V/∆t at switch-on. This is advantageous for EMI (ElectroMagnetic Interference) and reduces the current spike across Rsense. Demagnetization protection This feature guarantees discontinuous conduction mode operation for the power supply which simplifies the design of feedback control and gives faster transient response. Demagnetization protection is an additional protection feature that protects against saturation of the transformer/inductor. Demagnetization protection also protects the power supply components against excessive stresses at start-up, when all energy storage components are completely discharged. The converter is cycle by cycle protected during shorted output system fault condition due to the demagnetization protection. The value of the demagnetization resistor (Rdem) can be calculated with the formula given in Section “Sample and hold”. Negative clamp The negative clamp circuit does not let the voltage on pin Dem go below −0.4 V, when the auxiliary winding voltage goes negative during the time that the power device is turned on, to ensure correct operation of the IC. Over voltage protection An Over Voltage Protection (OVP) mode has been implemented in the GreenChip ™ series. This circuit works by sensing the Vaux voltage. If the output voltage exceeds the preset voltage limit, the OVP circuit turns off the power MOSFET. With no switching of the power device, the Vaux capacitor is not re-supplied and discharges to UVLO level and the system goes into the low dissipation safe-restart mode described earlier. The system recovers from the safe-restart mode only if the OVP condition is removed. Over current protection Cycle by cycle Over Current Protection (OCP) is provided by sensing the voltage on an external resistor which is connected to the source of the power MOSFET. The voltage on the current sense resistor, which reflects the amplitude of the primary current, is compared internally with a reference voltage using a high speed comparator. This threshold voltage is specified as Vth(Imax) in the chapter “Characteristics”. The maximum primary (protection) current is therefore: [A] If the power device current exceeds the current limit, the comparator trips and turns off the power device. The power device is typically turned off in 210 ns (see tD in Chapter “Characteristics”). The availability of the current sense resistor off-chip for programming the OCP trip level increases design flexibility for the power supply designer. An off-chip current sense resistor also reduces the risk of an OCP condition being sensed incorrectly. At power MOSFET turn-on the ∆V/∆t limiters capacitance discharge current does not have to flow through the sense resistor, because this capacitor can be connected between drain and source of the power MOSFET directly. The Leading Edge Blanking (LEB) circuit works together with the OCP circuit and inhibits the operation of the OCP comparator for a short duration (see tLEB in Chapter “Characteristics”) when the power device is turned on. This ensures that the power device is not turned off prematurely due to false sensing of an OCP condition because of current spikes caused by discharge of primary-side snubber and parasitic capacitances. LEB time is not fixed and it tracks the oscillator frequency. I prot V th Imax () R sense ------------------------ = |
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