Keep The Ramp right

Posted on Feb 6, 2014

If the ramp driving your PWM-based controller isn`t linear or monotonic, or just plain noisy, your DC/DC converter isn`t going to work up to its capabilities. Here`s what you need to know to get the best performance from your system. Today`s typical DC/DC power system relies on the slope of a ramp to set the duty cycle of a pulse width modulating(PWM) controller. This is true for

Keep The Ramp right
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both voltage-mode controllers, which use a separately generated ramp for setting the pulse width; and current-mode controllers, which use a ramp generated by a current sensor monitoring the current in the switching element. Figure 1 shows a typical block diagram of a voltage-mode power controller, in this example the UC2823. A simplified schematic of the control circuit in a forward converter topology is shown in Fig. 2. When the MOSFET power switch is turned on by the control circuit signal OUT, voltage VIN is applied across the primary winding of transformer T1. Thus voltage VTOUT appears at the secondary of T1. It`s applied to output inductor LOUT through D1. This voltage is greater than the voltage on the output, VOUT. As a result, there`s current buildup in LOUT. When the power MOSFET turns off, the voltage on at the input of LOUT drops to zero. However, current is drawn through diode D2 and continues to flow through LOUT, though it decreases in a linear manner. For this analysis we set the design parameters of a forward converter for an input-voltage range from 36 to 57 volts, and an output of 100 watts. The output voltage is set at 5 volts, and the switching frequency is 100 kHz. We want to minimize the output ripple ”no more than 50 mV. We size inductor LOUT for a peak-to-peak ripple current of 20 percent. We select the output capacitor so that half the output ripple is a function of the capacitance, and half is a...

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