Switch-Mode ICs Promote Efficient Power Management, Part 2: DC-DC Controllers And Converters

Dec. 8, 2008
Switch-mode ICs play a dominant role in optimizing the efficiency and minimizing the size of power-conversion subsystems used in consumer, computer, and industrial equipment.

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 DC-DC Integrated Switch Converter ICs

Only requiring external components, dc-dc integrated switch converters usually are just passive output voltage devices. Power switches may be either a bipolar or MOSFET capable of handling the required current and power. Typically, the power semiconductor switch turns on and off at a frequency that may range from 100 kHz to 2 MHz, depending on the IC type. Most power switches employ PWM to control the output voltage, so the duty cycle varies according to the desired output voltage.

Switching frequency determines the physical size and value of filter inductors, capacitors, and transformers. The higher the switching frequency, the smaller the physical size and component value. To optimize efficiency, magnetic core material for the inductor and transformer should be consistent with the switching frequency. That is, the transformer/inductor core material should be chosen to operate efficiently at the switching frequency.

Maxim’s MAX15036 and MAX15037 high-frequency dc-dc converters include an integrated n-channel power MOSFET that can handle a 3-A load. The MAX15036 includes an internal power MOSFET to enable the design of a non-synchronous buck or boost topology power supply (Fig. 5). The MAX15037 is intended for the design of a synchronous buck topology power supply.

These ICs operate from a 4.5- to 5.5-V or 5.5- to 23-V input voltage and offer the ability to set the switching frequency from 200 kHz to 2.2 MHz with an external resistor. The voltage-mode architecture with a peak switch current-limit scheme provides stable operation up to a 2.2-MHz switching frequency.

The MAX15036 includes a clock output for driving a second dc-dc converter 180° out-of-phase and a power-on-reset (POR) output. The MAX15037 includes a power-good output and a synchronous rectifier driver for an external low-side MOSFET in the buck converter configuration.

Both devices protect against overcurrent conditions by utilizing a peak current limit as well as overtemperature shutdown, providing a reliable and compact power source for point-of-load regulation applications. They also offer synchronization, internal digital soft-start, and an enable input. Both come in a thermally enhanced, space-saving 16-pin thin quad flat no-lead (TQFN) 5- by 5-mm package and operate from –40°C to 125°C. Additional features include:

• Output voltage adjustable down to 0.6 V (buck) or up to 28 V (boost)

• Synchronous rectifier driver output (MAX15037) for higher efficiency

• Resistor-programmable switching frequency from 200 kHz to 2.2 MHz

• External synchronization and enable (on/off) inputs

• Clock output for driving a second converter 180° out-of-phase (MAX15036)

• Integrated 150-mΩ high-side n-channel power MOSFET

• Power-on-reset output (MAX15036)/power-good output (MAX15037)

• Short-circuit protection (buck)/maximum duty-cycle limit (boost)

• Thermal-shutdown protection

Battery-Based Power-Supply Controller ICs

Virtually all battery-based systems are intended for portable operation. As such, their power supplies have unique requirements. First, they must operate from one- or two-cell lithium-ion (Li-ion) batteries or three- or four-cell nickel-cadmium (NiCd) or nickel-metal-hydride (NiMH) packs. They also must provide the appropriate voltage and current for the load, as well as high efficiency for maximum battery run time.

Furthermore, such power supplies must be light and small to minimize overall system size. They need to be thermally efficient to prevent overheating. Also, they have to minimize assembly and component cost for consumer-based systems. Finally, they must provide high-reliability, carefree service.

These requirements dictate the associated power-supply controller IC configurations. This also means that the controller ICs should require very few external components and any that are used should be low in cost. To minimize size and weight, the IC should come in a small-outline package as well. In addition, the application will determine whether the controller should provide a step-up, a step-down, or some other topology.

One tradeoff in selecting a controller IC lies in whether it employs external or on-chip power MOSFET switches. On-chip devices minimize external components but can increase the junction temperature and degrade thermal performance. Depending on the package, this could also reduce the current-carrying capacity of the IC. Some controller ICs have on-chip power MOSFETs, while others require external MOSFETs.

Designers must consider reducing the power dissipated by the power supply, which increases battery run time. Most controller ICs have a shutdown pin that disables the power supply, cutting battery drain. This can be done in many systems that have a normal “sleep” mode. When the IC comes out of the shutdown mode, it has to do so without upsetting the system.

Also available in most battery-based controller ICs, undervoltage lockout (UVLO) shuts down the power supply if the input voltage drops below a specific threshold. Another characteristic of these controller ICs is protection against overcurrent, which protects both the controller IC and the system components. This is accomplished by sensing current to the load and cutting power for an overload condition.

The LTC3528B-2 is a battery-based 1-A dc-dc converter IC from Linear Technology (Fig. 6). It is a synchronous, fixed-frequency step-up converter with output disconnect. High-efficiency synchronous rectification, in addition to a 700-mV startup voltage and operation down to 500 mV once started, provides longer runtime for single- or multiple-cell battery-powered products.

The IC’s 2-MHz switching frequency minimizes solution footprint by allowing the use of tiny, low-profile inductors and ceramic capacitors. The current mode PWM is internally compensated, simplifying the design process. The LTC3528B-2 also features continuous switching at light loads. Anti-ringing circuitry reduces electromagnetic interference (EMI) by damping the inductor in discontinuous mode.

The LTC3528B offers a low shutdown current, open-drain power-good output, short-circuit protection, and thermal overload protection. It comes in an eight-lead, 2- by 3- by 0.75-mm dual-flat no-lead (DFN) package and delivers 3.3 V at 200 mA from a single alkaline/NiMH cell or 3.3 V at 400 mA from two cells. Additional features include:

• 0.50- to 5.5-V input range

• 1.6- to 5.25-V VOUT range

• Up to 94% efficiency

• Output disconnect

• VIN > VOUT operation

• Integrated soft-start

• Low-noise PWM operation

• Logic controlled shutdown less than 1 µA Single-Output Off-Line DC-DC Converter ICs

Off-line dc-dc converters operate from the rectified ac powerline voltage, so they are optimized for a high voltage input. Isolated topologies must be used because these systems require galvanic isolation from the powerline. These self-contained converter ICs are usually oriented toward use as ac adapters in battery-based systems and in computer peripherals, such as printers and scanners. They’re usually rated at 100 W and below.

The VIPer17 off-line switch-mode power IC from STMicroelectronics can be configured as a flyback converter (Fig. 7). It is intended for off-line power supplies up to 6 W with a wide-range input voltage and up to 10 W with the European input voltage range. The IC includes both the power section and controller in one package.

The power section is an avalanche rugged MOSFET with 800-V breakdown, which ensures robustness and reliability for power supplies. The power section includes a SenseFET, temperature sensor, and high-voltage startup circuit.

The controller operates with current-mode control. It features circuits to minimize current consumption in the normal mode and reduces consumption even further in standby. Also, it can produce a power supply with a standby consumption under no-load conditions of less than 50 mW, which complies with energy-saving standards.

Furthermore, this IC offers functions that guarantee high performance with a lower component count: the soft startup function, simple feedback management, and frequency modulation jittering, which reduces EMI and helps meet standards regarding electromagnetic disturbance.

The VIPer17 has two possible fixed operating frequencies: 60 kHz (VIPer17L) or 115 kHz (VIPer17H). Both versions are available in DIP-7 (VIPer17LN and VIPer17HN) and SO-16N packages (VIPer17LD and VIPer17HD). Additionally, the IC offers:

• PWM operation with frequency jittering for low EMI

• Standby power less than 50 mW at 265 V ac

• Limiting current with adjustable set point

• Adjustable and accurate overvoltage protection

• On-board soft start

• Safe auto restart after a fault condition

• Overload protection (OLP)

• Brownout protection

• Hysteretic thermal shutdown protection

Finishing The Design

Capacitors employed with the controller are another design consideration that affects the switch-mode converter. The model for a capacitor is actually a resistor (ESR) in series with an ideal capacitor. ESR should be low to eliminate high-frequency switching noise from the output of the converter. Also, a low-ESR capacitor at the converter’s input prevents switching noise from being conducted through the input supply.

The designs of dc-dc controllers usually employ a tantalum or electrolytic capacitor in parallel with a low-ESR ceramic capacitor at the input and output. The tantalum capacitor filters lower-frequency noise, and the ceramic capacitor is better for filtering high-frequency switching noise. However, a very low-ESR capacitor may cause the converter to oscillate. Therefore, it is best to use the components suggested by the IC manufacturer.

Another output capacitor characteristic affects the controller’s output ripple—filtered output voltage variations. Typically, this ripple is a sawtooth at the switching frequency and may contain voltage spikes. Ripple can heat the output tantalum or electrolytic capacitor, which can shorten its life.

Board Layout

Component layout on a printed-circuit board (PCB) is an important system consideration. Use wide copper power traces and a ground plane. IC manufacturers usually supply a copper etch pattern on their datasheet. Large copper areas also provide good heat transfer to the ambient. Other thermal management factors include board size, shape, thickness, position, location, and board temperature.

Board layout and circuit components can also affect EMI conduction or radiation. Filter the converter input with low-ESR capacitors to prevent noise from entering the IC. Layout affects performance because fast-switching currents and associated wiring inductance can generate EMI. Also, keep the digital ground separate from the ground of the dc-dc converter. Connect the two grounds together at the output of the primary dc voltage source.

When designing high-frequency switch-mode converters:

• Avoid capacitive and inductive coupling of the switching waveform into high-impedance circuitry such as the error amplifier, oscillator, and current-sense amplifier.

• Keep printed-circuit traces associated with the converter IC as short as possible. Avoid long pc- board traces and component leads.

• External components should be as close to the converter IC. Also, locate oscillator and compensation circuitry near the IC.

• Use high-frequency decoupling capacitors on the reference voltage (VREF), and, if necessary, on VDD.

• Return high-di/dt currents directly to their source and use large-area ground planes.

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