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ML4812CP Datasheet(PDF) 10 Page - Micro Linear Corporation |
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ML4812CP Datasheet(HTML) 10 Page - Micro Linear Corporation |
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10 / 16 page  ML4812 10 Choose 4.75k Ω, 1%. One more critical component in the voltage regulation loop is the feedback capacitor for the error amplifier. The voltage loop bandwidth should be set such that it rejects the 120Hz ripple which is present at the output. If this ripple is not adequately attenuated it will cause distortion on the input current waveform. Typical bandwidths range anywhere from a few Hertz to 15Hz. The main compromise is between transient response and distortion. The feedback capacitor can be calculated using the following formula: C RBW C kHz F F F = ´´ = ´´ =m 1 3142 1 3142 356 2 044 1 . . . Ω (19) OVERVOLTAGE PROTECTION (OVP) COMPONENTS The OVP loop should be set so that there is no interaction with the voltage control loop. Typically it should be set to a level where the power components are safe to operate. Ten to fifteen volts above VOUT is generally a good setpoint. This sets the maximum transient output voltage to about 395V. By choosing the high voltage side resistor of the OVP circuit the same way as above i.e. R4 = 356K then R5 can be calculated as: R VR VV Vk VV k REF OVP REF 5 4 5356 395 5 4564 = ´ - = ´ - = Ω Ω . (20) Choose 4.53k Ω, 1%. Note that R1, R2, R4 and R5 should be tight tolerance resistors such as 1% or better. CONTROLLER SHUTDOWN The ML4812 provides a shutdown pin which could be used to shutdown the IC. Care should be taken when this pin is used because power supply sequencing problems could arise if another regulator with its own bootstrapping follows the ML4812. In such a case a special circuit should be used to allow for orderly start up. One way to accomplish this is by using the reference voltage of the ML4812 to inhibit the other controller IC or to shut down its bias supply current. OFF-LINE START-UP AND BIAS SUPPLY GENERATION The ML4812 can be started using a “bleed resistor” from the high voltage bus. After the voltage on VCC exceeds 16V, the IC starts up. The energy stored on the 330µF, C15, capacitor supplies the IC with running power until the supplemental winding on L1 can provide the power to sustain operation. The values of the start-up resistor R10 and capacitor C15 may need to be optimized depending on the application. The charging waveform for the secondary winding of L1 is an inverted chopped sinusoid which reaches its peak when the line voltage is at its minimum. In this example, C9 = 0.1µF, C15 = 330µF, D8 = 1N4148, R10 = 39k Ω, 2W. ENHANCEMENT CIRCUIT The power factor enhancement circuit shown in Figure 12 is described in detail in Application Note 11. It improves the power factor and lowers the input current harmonics. Note that the circuit meets IEC 1000-3-2 specifications (with the enhancement) on the harmonics by a large margin while correcting the input power factor to better than 0.99 under most steady state operating conditions. CONSTRUCTION AND LAYOUT TIPS High frequency power circuits require special care during breadboard construction and layout. Double sided printed circuit boards with ground plane on one side are highly recommended. All critical switching leads (power FET, output diode, IC output and ground leads, bypass capacitors) should be kept as small as possible. This is to minimize both the transmission and pick-up of switching noise. There are two kinds of noise coupling; inductive and capacitive. As the name implies inductive coupling is due to fast changing (high di/dt) circulating switching currents. The main source is the loop formed by Q1, D5, and C3–C4. Therefore this loop should be as small as possible, and the above capacitors should be good high frequency types. The second form of noise coupling is due to fast changing voltages (high dv/dt). The main source in this case is the drain of the power FET. The radiated noise in this case can be minimized by insulating the drain of the FET from the heatsink and then tying the heatsink to the source of the FET with a high frequency capacitor (CH in Figure 12). The IC has two ground pins named PWR GND and Signal GND. These two pins should be connected together with a very short lead at the printed circuit board exit point. In general grounding is very important and ground loops should be avoided. Star grounding or ground plane techniques are preferred. TYPICAL APPLICATIONS (Continued) |
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