Switch-mode dc-dc converters employ three major topologies: buck (step-down), boost (step-up), and flyback. Other topologies include SEPIC, Cuk, and forward converters. In addition, there are full-bridge and half-bridge output versions of these converters. This article will focus on buck, boost, and flyback converters.
A switch-mode converter employs ICs as well as a power semiconductor switch. This can take one of two different forms: a separate controller IC and an external discrete MOSFET, or a control IC with an integrated power MOSFET in a single package. Typically, the discrete MOSFET will have a lower on resistance than an integrated MOSFET.
Buck and Boost Converters
In the simplified buck regulator shown in Figure 1, the IC accepts a dc input and converts it to a pulse-width modulation (PWM) switching frequency that controls the output of the power MOSFET (Q1). An external synchronous rectifier, an inductor, and output capacitors produce the regulated dc output.
This regulator IC compares a portion of the rectified dc output with a voltage reference (VREF) and varies the PWM duty cycle to maintain a constant dc output voltage. If the output voltage tends to increase, the PWM reduces its duty cycle, reducing the output and maintaining the required regulated output voltage. Conversely, if the output voltage tends to go down, the feedback causes the PWM duty cycle to increase and maintain the proper output.
A MOSFET switch controls the circuit in the simplified boost IC circuit (Fig. 2). Turning the switch on causes current to build up through the inductor. Turning the switch off forces current through the diode to the output. Multiple cycles of this switching cause the output capacitor voltage to build due to charge it stores from the inductor current. The result is a higher output voltage than its input.
A PWM control circuit drives the MOSFET. Without feedback, the PWM duty cycle determines the output voltage, which is twice the input for a 50% duty cycle. Stepping up the voltage by a factor of two causes the input current to be twice the output current. The input current will be slightly higher in a real circuit with losses.
For both converters, the input capacitor should be a low-ESR (equivalent series resistance) aluminum, tantalum, or ceramic type connected between the input pin and power ground. This capacitor prevents large voltage transients from appearing at the input, so its value depends on the circuit’s rms current and voltage requirements.
Output capacitor selection depends on the maximum allowable output voltage ripple. Usually, the capacitor’s ESR also plays a role in determining the output voltage ripple. Most circuits require a low-ESR aluminum electrolytic or tantalum capacitor.
An electrolytic capacitor is not recommended for temperatures below −25°C since its ESR rises dramatically at cold temperature. A tantalum capacitor has a much better ESR specification at cold temperatures and is also preferred for low-temperature applications.
The critical parameters for the inductor are its inductance, peak current, and dc resistance. The inductance value affects the peak-to-peak inductor ripple current and the input and output voltages. A high ripple current reduces inductance, but increases the conductance loss, core loss, and current stress for the inductor and associated switch devices.
Additionally, a high ripple current requires a bigger output capacitor for the same output voltage ripple requirement. A reasonable value is setting the ripple current to 30% of the dc output current. Because ripple current increases with the input voltage, the maximum input voltage affects the choice of inductance value.
The flyback converter employs a transformer to provide dc isolation between its input and the output (Fig. 3). It is similar to the boost converter with the inductor split to form a transformer. When the semiconductor switch turns on, the primary of the transformer is directly connected to the input voltage source. This results in an increase of magnetic flux in the transformer.
The voltage across the secondary winding is negative, so the diode is reverse-biased. The output capacitor supplies energy to the output load. When the semiconductor switch turns off, the energy stored in the transformer is transferred to the output of the converter. The output voltage remains constant because of the feedback from the output to the control circuit.
Because it is an isolated power converter, the control circuit also requires isolation. The two prevailing control schemes are voltage mode control and current mode control. Both require a signal related to the output voltage. Therefore, an opto-coupler provides the isolation for the control circuit, which obtains a portion of the output voltage fed back from the R1/R2 resistive voltage divider.
The flyback is often used in multiple output circuits because of the cost-effective regulation of multiple outputs. By holding one output at a constant voltage and transferring current, whichever output is lowest (relative to the turns ratio of the transformer) will receive the most current, bringing it back up in voltage. When an output is too high, it receives less current, and the loading brings it back down.
Single-Output DC-DC Controller ICs
Efficiency is one of the important characteristics of a dc-dc controller. One aspect of a controller IC’s efficiency is the input voltage applied to it. An IC may work properly with a +5- to +40-V dc input, but a higher input voltage causes higher power dissipation, which means lower efficiency. Using a synchronous rectifier instead of a diode rectifier can improve efficiency.
National Semiconductor’s LM5118 wide-voltage-range, switch-mode buck-boost controller features all the functions necessary to implement a cost-efficient buck-boost regulator using a minimum of external components (Fig. 4). The buck-boost topology maintains output voltage regulation when the input voltage is either less than or greater than the output voltage, making it especially suitable for automotive applications.
The LM5118 operates as a buck regulator while the input voltage is sufficiently greater than the regulated output voltage. It gradually transitions to the buck-boost mode as the input voltage approaches the output. This dual-mode approach maintains regulation over a wide range of input voltages with optimal conversion efficiency in the buck mode and a glitch-free output during mode transitions. This controller includes drivers for the high-side buck MOSFET and the low-side boost MOSFET.
The regulator’s control method is based upon current mode control utilizing an emulated current ramp. Emulated current mode control reduces the noise sensitivity of the PWM circuit, allowing reliable control of the very small duty cycles necessary in high input voltage applications. Additional protection features include current limit, thermal shutdown, and an enable input.
The device is available in a power-enhanced TSSOP-20 package featuring an exposed die attach pad to aid thermal dissipation. It also offers:
• Ultra-wide input voltage range from 3 to 75 V
• Emulated peak current mode control
• Smooth transition between step-down and step-up modes
• Switching frequency programmable to 500 kHz
• Oscillator synchronization capability
• Internal high-voltage bias regulator
• Integrated high-side and low-side gate drivers
• Programmable soft-start time
• Ultra-low shutdown current
• Enable-input wide-bandwidth error amplifier
• 1.5% feedback reference accuracy