Embedded Systems Design - July 2008 - (Page 31)

feature Operation of this PWM controller (U41) makes use of optically isolated feedback from the transformer secondary to the primary. +12V T2 +3V6 D58 C49 R126 5V U41 8 4 VREF R125 5V VCC OUT 7 6 R128 M2 U45 C43 C42 R129 R132 2.5V C46 RT/CT 1 2 COMP VFB UC3845 ISENSE GND 3 1V 5 R133 C53 R134 R135 Figure 3 the width of the control pulse narrows of the main controller plus that of sate for possible delays caused by the to store less energy in the inductor. For the voltage- and current-feedback input current (Iref = 0.5 µA) and the networks. input capacitance (this problem can be solved usThe main advantage of PWM control is its Topology of the ing capacitive dividers). ﬁxed switching frequency, . . . its main feedback loop The output filtering capacitor (C47) is large, so you drawback is the current draw under no-load Voltage feedback is produced by routmay need to choose a lowand light-load conditions. ing current from ESR electrolytic type (such the phototransistor as tantalum, Os-Con, and low-current loads, the power supply (of optocoupler U45) through R135. organic aluminum). If so it must also operates in discontinuous mode and The value of R135 must be large to be a low-leakage type, because current the main current draw is the power minimize power, but must also be leakage can be significant, especially at supply itself. small enough to supply the minimum higher temperatures. (For a 16-V The main advantage of PWM con- current needed for operation of the Kemet T495 100 µF capacitor, IL equals trol is its fixed switching frequency, phototransistor. 16 µA at 25°C and 160 µA at 85°C.) which simplifies the circuit design Current feedback is obtained with regard to controlling EMI and though the voltage drop in R134. To Control circuit minimize loss, we use R125 and R133 The most common power-supply con- optimizing efficiency for heavy loads. Its main drawback is the current draw to divide the drop between this voltage trol scheme is current-mode pulseunder no-load and light-load condiand the reference (VREF = 5 V, pin 8), width modulation (PWM), in which tions, because the oscillator in the reg- thereby allowing a 1-V equilibrium at variable-width pulses control the inulator chip has a fixed frequency (a ductor’s charging current. When the ISENSE (pin 3). These connections lightly loaded UC3845, for example, load is heavy, the width of the applied improve efficiency by reducing the pulse increases to store more energy in draws Icc = 17 mA). Figure 3 shows a voltage drop in R135. The divider rethe inductance, as Figure 2 shows. Un- typical UC3845 configuration in sistors must have high values to minider no-load or light-load conditions, mize power consumption, but you which the total current drawn is that www.embedded.com | embedded systems design | JULY 2008 31 http://www.embedded.com

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Embedded Systems Design - July 2008 Contents #Include Parity Bit Programming Pointers Interactive C-code Cleaning Tool Supports Multiprocessor SoC Design Building a Power Supply for Discontinuous Transmission Wireless Networks An Exception Primer Advertising Index Break Points Marketplace

Embedded Systems Design - July 2008

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