Power supply step-down control circuit module design

Electronic circuits usually operate at positive regulated output voltages, and these voltages are generally provided by buck regulators. If a negative output voltage is also required, the same buck controller can be configured in a buck-boost topology. A negative output voltage buck-boost, sometimes called negative inversion, operates with a 50% duty cycle and provides an output voltage equal to the input voltage but with opposite polarity. It can adjust the duty cycle as the input voltage fluctuates to - buck or - boost the output voltage to maintain a stable voltage.

Figure 1 shows a simplified buck-boost circuit and the switching voltage appearing across the inductor. The similarity of this circuit to a standard buck converter will immediately become clear. In fact, it is exactly the same as a buck converter except that the output voltage is opposite to ground. This layout can also be used for synchronous buck converters. This is where it's similar to the buck or synchronous buck converter side, because the circuit operates differently than a buck converter.

Transfer files for PCB proofing

The voltage that appears across the inductor when the FET switches is different from the voltage in the buck converter. As in a buck converter, it is necessary to balance the volt-microsecond (V-μs) product to prevent inductor saturation. When the FET is on (ton interval as shown in Figure 1), the full input voltage is applied to the inductor. This positive voltage on the "spot" side of the inductor causes the current to ramp up, which results in a V-μs product of the inductor's turn-on time. During the time the FET is off (toff), the voltage polarity of the inductor must be reversed to maintain current flow, so that the pull point side is negative. The inductor current ramps down, flows through the load and output capacitor, and returns through the diode. The V-μs product when the inductor is off must be equal to the V-μs product when it is on. Since Vin and Vout are constant, it is easy to get the expression for duty cycle (D): D=Vout/(Vout “Vin). This control circuit maintains the output voltage by calculating the correct duty cycle Voltage regulation. The above expressions and the waveforms shown in Figure 1 assume operation in continuous conduction mode.

The buck-boost inductor must operate at a higher current than the output load current. Just the input current and the output current are added. For a negative output voltage equal to the input voltage (D = 0.5), the average inductor current is 2 times the output. Interestingly, there are two ways to connect the return terminal of the input capacitor, which will affect the rms current of the output capacitor.

Typical capacitor layout is between +Vin and Gnd, and vice versa. Using this input capacitor configuration reduces the rms current in the output capacitor. However, since the input capacitor is connected to Vout, a capacitive voltage divider is formed across Vout. This creates a positive peak in the output at the turn-on time just before the controller begins to function. To minimize this effect, the best approach is usually to use an input capacitance that is much smaller than the output capacitance, see the circuit shown in Figure 2. The input capacitor current alternates between providing dc output current and sinking the average input current. The rms current level is worst at low input voltages with the highest input current. Therefore, pay more attention when selecting capacitors so that their ESR is not too high. Ceramic or polymer capacitors are often suitable choices for this topology.

You must choose a controller that can power up with a minimum input voltage minus the diode drop, and must also be able to withstand the voltages of Vin plus Vout during operation. The FETs and diodes must also be rated for this voltage range. Regulation of the output voltage is achieved by connecting a feedback resistor to output ground, since the controller references the negative output voltage. With just a few carefully selected component values ​​and minor circuit modifications, a buck controller can do double duty in a negative-output buck-boost topology.