DESCRIPTION. The UC/3/4/5 family of control ICs provides the necessary features to implement off-line or DC to DC fixed frequency current mode control. The UTC UC/ are high performance fixed frequency current mode controllers that specifically designed for Off-Line and. DC to DC converter. DESCRIPTION. The UCxB family of control ICs provides the nec- essary features to implement off-line or DC to DC fixed frequency current mode control.
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The C RAMP is an AC-coupling capacitor that allows the voltage uc3844 datasheet of the oscillator to be used without adding an offset to the current sense; select datwsheet value to approximate high frequency short circuit, uc3844 datasheet as 10 nF as a starting point and make adjustments if required.
Based previous the analysis, the right half plane zero, f RHPzis located at 7. The uc3844 datasheet oscillation would uc3844 datasheet in an increase in the output voltage ripple and may even limit the power handling capability of the converter.
Datasheet set the output voltage to 12 V, 2. By setting the total loop gain equal to 1 at the desired crossover frequency and rearranging Equation 57the optimal value for R LED dayasheet be determined, as shown in Equation These devices are available an 8-pin dual-in-line plastic package as well as the pin plastic surface mount SO Uc3844 datasheet Application Design Example Schematic. A trade-off between switching loss and EMI performance must be carefully performed.
UC datasheet – High Performance Current Mode Controller
It is best for the timing capacitor to have a flat dtasheet coefficient, typical of most COG uc3844 datasheet NPO type capacitors. The error signal crosses the primary to secondary isolation boundary using an opto-isolator whose collector is connected to the VREF pin and the emitter is connected to VFB. R G is the gate driver resistor for the power switch, Q SW. Data, Signal Line ; Mounting Type: The uc3844 datasheet of slope compensation is to achieve an ideal quality u3c844, Q Pto be equal to 1 at half of the switching frequency.
A system is stable if its response to a perturbation is that the perturbation eventually dies out. Uc8344 this point the gain transfer function of the error amplifier stage, G EA s uc3844 datasheet, of the compensation loop can be characterized:. This technique still achieves current mode control with cycle-by-cycle over-current protection.
The operating frequency can be uc3844 datasheet based the curves in Application Curveswhere the timing resistor can be found once the timing capacitor is selected. Uc3844 datasheet total uc3844 datasheet capacitance is selected based upon the output voltage vatasheet requirement. For this design example, the transformer magnetizing inductance is selected based upon the CCM condition. Based on calculated inductor value and the switching frequency, the current stress of the MOSFET and output diode can be calculated.
The bode for the open-loop gain and phase can be uc3844 datasheet by using Equation For this example, pole f P1 is located at Output Voltage During 0. Based uc3844 datasheet the 0.
Devices with higher turnon or turnoff hysteresis are ideal choices for off-line power supplies, while the uc3844 datasheet with a narrower hysteresis datasyeet are suited for DC-DC applications.
A slower switching speed reduces EMI but also increases the switching uc3844 datasheet. Maximum Package power dissipation limits must be observed.
The transformer design starts with selecting a suitable switching frequency for the given application. For good uc3844 datasheet response, the bandwidth of the finalized design should be as large as possible.
The VCC capacitor is required to uc3844 datasheet enough energy to prevent its voltage from dropping below the UVLO-off threshold during start-up, before the output is able to reach its regulated level. For the entire input uc3844 datasheet range, the selected inductor has value larger than the critical inductor.
Note that the exact value of these internal resistors is not critical but the IC provides tight control of the resistor divider ratio, so regardless of the actual resistor value variations their relative value to each other is maintained. There is also a double pole placed at half the switching frequency of the converter, f P2 calculated with Equation The internally implemented circuits uc3844 datasheet an undervoltage lockout UVLOfeaturing a start-up u3c844 of less than 1 mA, dtasheet a precision reference trimmed for accuracy at the error amplifier input.
The forward voltage drop, Uc3844 datasheet Fof this diode is estimated to be equal to 0. The power uc3844 datasheet transfer function can be characterized uc3844 datasheet Equation Each of these stages is combined with the power stage to result in a stable robust system. In Equation 38 datashest, S e is the compensation ramp slope and the S n is the inductor rising slope. Once the power stage poles and zeros are calculated and the slope compensation is determined, the power stage open-loop gain and phase uc3844 datasheet the CCM flyback converter can be vatasheet as a function of frequency.
Therefore, the transformer inductance should be approximately 1. The UC is capable of switching up dtaasheet kHz but considerations such uc3844 datasheet overall converter size, switching uc3844 datasheet, core loss, system compatibility, and interference with communication frequency bands generally determine an optimum frequency that should be used.