FROM EDN EUROPE: Smart power switches simplify low-voltage systems

From the battery charger in a notebook PC to the solenoids in a control process, delivering fully protected dc power presents complex challenges. Basic parameters, such as current, temperature, and voltage, traditionally require independent circuits that you individually fine-tune to provide a timely shutdown under adverse conditions. Such shutdown actions must be fast enough to protect the load and its wiring without overreacting to the transients that typify capacitive and inductive loads. Harsh automotive and industrial environments have additional demands, such as transient-overvoltage protection. The resulting balancing act is difficult for seasoned power-system designers and generally out of a digital designer's range. But by providing multiple integrated protection functions, today's IPSs (intelligent power switches) put most power-control requirements within a nonspecialist's reach.

The automotive and PC markets represent two of the fastest growing power-control areas. Vehicles require protected load control ranging from power-train-management systems to body functions, such as electric-seat and -window control. PCs require load protection for peripherals such as USB devices. These diverse requirements create natural divisions for IPS voltage and current levels. Automotive and industrial applications must often accommodate 60V or more at tens of amps, but the PC market requires at most 12V and a couple of amps. And, although power-system designers continue to argue the merits of high-side versus low-side switching—that is, where the switch connects the load to the positive supply rail or to return—circuits that suit either configuration are now commonly available. For a comprehensive selection, check the application guides that appear on vendors' Web sites (see sidebar "For more information").

Before considering what's available, it's worth revisiting the basics. Standard MOSFETs require driver circuits to enhance their gates by applying a voltage that's typically 5 to 20V either side of the supply rail. Accordingly, the easiest way to build a switch is to use a low-side configuration and an n-channel device; similarly, the simplest high-side switch uses a p-channel device. But p-channel devices are less efficient and less rugged than their n-channel cousins, so power-system designers traditionally use discrete circuits, such as bootstrap supplies, on top of the supply rail. A bootstrap supply provides sufficient gate-enhancement voltage for n-channel devices. You also need to consider the device's input capacitance and provide sufficient drive current to cleanly switch the device on and off. This action is necessary not only to meet switching-speed requirements but also to minimise switching losses as the device passes through its linear conduction region. Such switching losses dissipate as heat and can significantly add to the device's steady-state losses due to its inherent drain-source on-resistance (RDS_ON). Even when you use dedicated gate-driver ICs, designing high-performance gate-driver circuits isn't a trivial task—and still won't give you bulletproof protection functions.

Many vendors offer MOSFETs that ruggedise standard components by including many of the on-chip protection circuits that typify IPS devices. And by providing direct access to the gate terminal, devices such as STMicroelectronics' OmniFET-II family can replace standard n-channel MOSFETs in high-frequency switching applications, such as PWM control at 25 to 50 kHz. The company built this second-generation family from a proprietary vertical-MOS process that integrates analogue and digital circuitry with the power device (Figure 1). Family members range from the VNN1NV04, which has a 250-mΩ on-resistance and a 1.7A current limit, to the newly announced 10-mΩ VNB35NV04, which self-limits at 30A. Nominal current-handling abilities are 0.5 and 15A, respectively. Common protection features include short-circuit protection with linear current limiting, junction-temperature thermal shutdown with hysteresis, and 4-kV human-body-model ESD protection. The VNN1NV04 costs 18 cents (100,000), and the VNB35NV04 costs $1.64 (100,000)

These and many other IPS devices actively clamp their gate-drain voltages below their avalanche-threshold voltages to further increase device ruggedness. That is, the power MOSFET turns on to clamp transients when a device exceeds its threshold voltage. Loads must be able to withstand the transient overvoltages that can result from switching inductive components, such as motors and solenoids. Clamping voltages are device-specific and typically lie within 36 to 70V. The power MOSFET's intrinsic source-drain diode helps clamp positive excursions when the device turns off, but you should ensure that the resulting energy flow doesn't exceed data-sheet specifications. Without external clamping, the load terminal will swing well below the ground rail, which can take the device beyond its maximum ratings. If in doubt, add a suitably rated fast-recovery diode from a vendor such as General Semiconductor or Philips.

Notice, too, that the OmniFET-II's direct gate connection suits linear control, just as for a standard device. But to obviate the need for a separate power supply for the chip's protection circuits, a bias current of less than 200 µA flows into the gate when the device is on; when it's off, the voltage across the load supplies this bias current. Interestingly, these features also facilitate using the gate terminal to provide overtemperature-status feedback to driver logic. When the device gets too hot, the on-chip shutdown logic tries to sink a diagnostic current from its gate to its internal ground. If you're driving the device from a low-impedance source, such as TTL gate, you'll need to sense this extra current flow to recognise the shutdown. But if you use an appropriate gate resistor, the device's gate terminal voltage will drop to zero. Neither of these conditions affects device operation; it returns to normal when the chip temperature drops 15°C below the typical 170°C shutdown threshold. OmniFET-II device packaging options include SO-8, SOT-223, DPAK, IPAK, PowerSO-10, and TO-220 packages. Other vendors with broadly similar devices include Infineon, International Rectifier, and Philips, with variants that suit linear gate-drive voltages or 5V logic-level control. Packaging options include three-pin devices that replicate a standard-device footprint and dedicated five-pin low-side drivers. Low-side- driver types include protection-supply and status pins that ease control-logic interfaces.

Suitable for low-frequency switching applications, high-side drivers span current levels from a few hundred milliamps to as much as 170A. Representative examples of low-power devices for the PC market come from vendors such as Advanced Analogic Technologies, Analog Integrations, Advanced Monolithic Systems, and Texas Instruments. Higher power devices for automotive and industrial use include Infineon's PROFETs, International Rectifier's IPS line, Philips' TOPFETs, and the VN-series from STMicroelectronics. Motorola is also developing a line of devices for the automotive market. Such devices are popular because they're so easy to use, and the high-side configuration is natural in automotive applications, in which most vehicle bodies act as the electrical system's ground return. And because high-side drivers aren't required to replace standard MOSFETs, vendors routinely dedicate pins to internal power supplies and provide various status flags that interface with external circuitry. In some cases, vendors include a current-sense terminal that mirrors a small percentage of the current flowing through the source-drain path, making it possible to construct monitoring or feedback circuits. There's also a huge range of available packaging options, from tiny SOT-223s to TO-220 variants to multichannel surface-mount power packages.

High-side drivers that employ n-channel power MOSFETs require a charge pump to raise the gate-enhancement voltage above the positive supply rail (Figure 2). This charge pump typically comprises a ring oscillator that switches one terminal of a silicon capacitor between ground and the positive supply rail. The doubled voltage that results at the capacitor's opposite terminal passes through a diode to a storage capacitor that supplies the gate-driver switch. Charge-pump-design enhancements can include bootstrapping the positive supply to the gate-driver supply to reduce the storage capacitor's charging time and soft-switching the voltage-doubling capacitor to reduce peak ground currents and ground-plane noise. Crucially, when the gate-driver switch turns on, the storage capacitor must charge several nanofarads of the power MOSFET's gate capacitance. As a result, devices may switch on and off in tens of or a few hundred microseconds for a single operational cycle, but the charge pump's ability to replenish the gate-enhancement voltage limits the device's switching frequency to tens of or a few hundred hertz. In general, the higher the device's current-handling ability, the slower its maximum switching frequency.

Allan Askey, technical support manager at International Rectifier, observes that the charge pump has to wind itself up after the input signal becomes active and wind down when the input becomes inactive.

"If you attempt to switch the input at a fast rate," Askey notes, as you would do in a typical motor PWM operation, "the PWM signal fights with the charge pump's characteristics and puts the IPS into an indeterminate state—usually ending in a burnt-out part."

At first sight, you may think that a high-side driver's relatively low switching frequency is a serious limitation. But Steve Sellick, international product manager at Philips Automotive Power Semiconductor Division, reports that his customers are requesting lower switching speeds to combat EMC generation in vehicle harnesses. Citing daylight running lights as an example, Sellick explains that the 100- to 150-Hz switching strategy that vehicle manufacturers use can easily interfere with AM-band radios—still a crucial consideration in the United States.

"You could go down to 25 to 50 Hz without noticeable lamp dimming, but the longer bulb-cooling time leads to greater inrush currents." Instead, he notes, Philips limits voltage rise times to 1V/µsec or less to limit harmonic generation.

Also, notice that high-side drivers require undervoltage-lockout protection to ensure that the charge pump's conditions are within operational limits. Although a data sheet may specify a device as having an operational range of, say, 5 to 40V, its RDS_ON can vary by as much as 4 to 1 across the power-supply range. Worst-case conductance most likely occurs under low-voltage, high-junction-temperature conditions.

As well as overvoltage, overtemperature, and short-circuit protection, many high-side drivers include protection from reverse voltages and the loss of a ground connection. These features especially suit automotive environments, in which sustained reverse voltages appear when technicians incorrectly install peripherals, such as entertainment systems. Loss of ground is particularly problematic in areas such as rear-light clusters, which rely on spring-loaded terminals or a single vulnerable wire to provide a ground return. In a normal steel-framed car, that ground return may also have to traverse several welded seams back to the battery, creating ground shifts that can move by several volts under transient conditions. Reverse voltages can also appear due to inductive load-switching reflections on the power rail or during load-dump conditions. Load dumps occur when the alternator is charging and a battery connection fails, leaving the undamped alternator to dissipate its stored energy directly into the power rail. The transients that result can reach several hundred volts peak-to-peak and last several milliseconds (see the ISO-7637 specifications for guidance). Transient-protection diodes help protect vulnerable loads and simplify circuit behaviour under stress, including IPS overvoltage clamping. One example is General Semiconductor's surface-mount SM8A27. It has a zener voltage of 27V and handles a 10/10,000-µsec profile pulse at peak levels of 130A.

Some high-side drivers also offer a choice of latched or nonlatched shutdown modes. Christian Arndt, an application engineer at Infineon, explains that when an overload triggers the protection circuits, a latched device switches off and remains off until you reset it externally. This reset mechanism generally requires cycling the drive signal. Alternatively, a nonlatched device responds to overloads by switching off for a predetermined period, and then trying to turn on. Arndt comments that this automatic restart behaviour is usually required if you are to overcome nonpermanent fault conditions without any external triggering. Temporary fault conditions can occur when you try to start a motor that's temporarily overloaded, such as a jammed wiper mechanism. But for a permanent fault, such as a wiring-loom short-circuit, an ideal nonlatched device will cycle indefinitely. If you specify a nonlatching part for stand-alone operation, make sure your device is rated for repetitive fault cycles, and test this parameter for yourself. More generally, latched or nonlatching types require logic to monitor their status. Open-drain status pins allow individual monitoring or a wire-OR connection to report a problem in a subassembly. Notice that, by externally decoding the status-pin information and the input-control pin and output-load states, many devices allow you to obtain information, such as overtemperature, undervoltage lockout, short-circuit protection tripped, and low load current. Some devices include a dedicated open-load detection pin that's similarly useful for detecting potentially hazardous conditions, such as brake-light failure or open-circuit solenoids.

In making your selection, also examine a device's drive requirements. Choices include logic-level or current-sink drive together with external protection resistors. Logic-level drive is conventionally active-high but may require a series resistor to limit the device's internal current flow under conditions such as reverse voltage. Such an error current is also likely to find its way into your logic. A series resistor of a few kilohms protects without affecting drive-current levels or device switching speed. Alternatively, devices with current-sink control inputs require an open-collector transistor for control. Here, the control-input terminal floats at or near the supply-rail voltage; pulling this node to ground turns on the device. Notice that this configuration demands that the driving transistor withstand the maximum transient overvoltage that your system is likely to experience. Some devices also require external diodes for reliable reverse-voltage protection, so check the application data.

Other differences among devices include construction methods. Because of the high current densities that vertical-MOS processes provide, virtually every high-side driver for moderate-to high-power use employs some such process. The main alternative is a lateral process, such as BCD (bipolar-CMOS-DMOS), which suits lower power levels but more easily integrates analogue and digital control circuitry. Accordingly, many device designers prefer a co-chip approach in which the control circuitry is fabricated on a separate slice and glued on top of the vertical- power MOSFET (Figure 3). This approach allows the manufacturer to run each element through separate fabrication processes to provide an economic benefit that's particularly appropriate at high power levels. Other device designers prefer monolithic construction, arguing that the protection circuits are in closer contact with the power element and that using fewer bond wires increases reliability. In either case, the power device mounts to a heavy copper lead frame that dissipates heat, and heavy or multiple bond wires take the high-current connections to device pins.

High-current devices often employ multiple power-I/O pins, and with devices handling tens of amps, your pc-board connections come under scrutiny. In the worst case, connections must withstand temporary overloads while the device shuts down without vaporising tracking. Surface-mount devices with vias through to power planes are particularly vulnerable. To help lay adequate copper area, turn off the thermal relief control in your pc-board-design package and use multiple oversized vias. Other strategies include using 70-micron-thick copper (2 oz/ft²) for tracks rather than the de facto standard 35 microns. Some pc-board manufacturers also offer 105 microns, but it's unusual; the best you're likely to get is 70 microns for outer layers and 35 microns for inner layers. A glance at standard pc-board tables confirms the current-carrying capacity for a 20°C temperature rise between 0.01 in. of these thicknesses: 35 microns=6A, 70 microns=10A, and 105 microns=14A. Thus, increasing copper thickness is unlikely to help the tracking meet many requirements, leaving options such as copper fills and parallel tracks to get the current offboard. Contemporary pc-board-design packages, such as Pulsonix, include a "remove-redundant-loops" feature that's on by default. Such normally useful features prevent you from making parallel connections; turn it off, and you can mirror tracks on either side of the board or even through ground and power planes.

Typically, the device's tab provides an alternative high-current connection to the supply rail. You can use this connection to run wiring to a tab-mounted ring terminal or mount multiple devices on a bus bar. In some cases, you may be able to use a heat sink to carry power to the tab. But, because most heat sinks employ highly resistive black anodising to increase their efficiency by as much as 30% over natural aluminum finishes, ensure that the tab contact area and your connections to the heat sink are truly low-impedance. Don't neglect the wire and the terminals you use. Compared with commercial PVC-jacketed wire, an airframe-quality wire, such as Raychem's Spec 55, is far more resilient to overloads and withstands temperatures to 200°C. And if you use wires in bundles, remember that they require derating. As a rule of thumb, a single 16-gauge conductor in free air carries about 15A for a 20°C temperature rise, three similar conductors in a bundle carry 0.8×15A each, and six-wire bundles carry 0.6×15A each. Soldering wires directly into the pc board is the most efficient way to make the board-to-wire connection. When this setup isn't permissible, check with a specialist, such as Anderson Power Products, for ranges of modular pc-board-mounting power connectors, such as the company's 55A-rated Powerclaw.

Several recently announced devices illustrate the diversity that's now available to provide high-side supply switching. Starting with low-voltage/low-current devices, Advanced Analogic Technologies' AAT4250 family targets applications such as USB and Firewire ports in laptops and handheld products. Costing just 32 cents (1000), this device has a 1.7A maximum current rating across its 1.8 to 5.5V operating voltage. This voltage range suits all of today's logic families and is made possible by using a p-channel MOSFET to switch power. The undervoltage-lockout threshold is typically 1.5V. Other distinguishing features include 120-mΩ RDS_ON with a 5V supply; active slew-rate control for 1.5-msec turn-on and 10-µsec turn-off times; and 2-µA quiescent current that drops to around 100 nA when disabled. The device works across the industrial –40 to +85°C temperature range and features 5-kV human-body-model ESD protection. Packaging is either in five-pin SOT-23s or eight-pin SC70s, with four active pins for power in and out, ground, and switch control; there's no status-flag output provision.

Many automotive and industrial loads require comprehensive protection at levels from 2 to 6A, and you have a wide choice of single-channel and multichannel devices. Single-channel examples include Infineon's BTS409L1, International Rectifier's IPS511, STMicroelectronics' VN02 series, and Philips' new BUK219-50Y. This monolithic Philips device has 180-mΩ RDS_ON at 25°C that yields 2A continuous rating with 6A available before shutdown. The device handles as much as 50V before clamping and tolerates reverse voltages of 32V peak. Two external series resistors limit the input-control and status-pin error currents. The output stage also tolerates negative transients as large as –30V that dissipate as much as 75 mJ. With help from external logic, the device can decode seven operational and error states, including load not present. Available in a five-pin TO-220 package, the BUK219-50Y costs around 88 cents in production volumes.

At the low-power level, you can also select from a range of multichannel devices. Examples include dual-, quad-, and eight-channel devices from Infineon that range from the 8×0.625A rated BTS4880R to the 2×12A BTS840S2. Motorola, too, is becoming active in this area, with the new MC33888FB. This device combines four high-side drivers with eight low-side drivers in a 64-pin PQFP. The high-side drivers comprise two 10-mΩ and two 40-mΩ channels that suit applications such as automotive-lighting controls. The low-side drivers are 0.5Ω channels to drive LEDs or relay coils. Available for sampling now, the guide price is $4 (100,000). Another new device is STMicroelectronics' VNQ05XSP16 quad-channel high-side driver, with each channel continuously rated for 2.5A. The device includes an addressable four-channel multiplexer that switches between the output channels to provide load-current feedback. Thus, a single external resistor and ADC channel can monitor the device's outputs. Now available in production volumes, the VNQ05XSP16 comes in a PowerS0-16 package and costs approximately $1.50 (100,000).

Midrange power devices that handle currents greater than 12 to 50A are exclusively single-channel products and include several members of Infineon's PROFET family, various VN-series devices from STMicroelectronics, and International Rectifier's 35A-rated IPS5451. Available in five-pin TO-220 and surface-mount packages, the IPS5451 also detects seven operational and error states. Protection circuits latch on overcurrent but cycle on overtemperature. The IPS5451 costs about $2 (10,000). International Rectifier also offers its IPS5551, a three-pin device that's rated at 35A continuously with a 100A shutdown threshold. Targeting supplies reaching 18V, this device clamps overvoltages at 40V. The input is a current-sink, design and overtemperature- and overcurrent-protection circuits are latching types. The $3.60 (1000) IPS5551 comes in TO-220 or surface-mount package.

Getting more current limits your choices. Infineon's BTS650P and BTS555 enjoy 70 and 165A continuous ratings, making the BTS555 the highest-power IPS device available today. The BTS650P is a 6-mΩ device with a 130A overcurrent limit; the BTS555 is a 2.5-mΩ high-side switch with a massive 520A overcurrent limit. Each device employs extensive protection, including "Reversave," a proprietary reverse-battery-protection scheme. The control inputs are current-sink types, with latching short-circuit and cycling overtemperature protection for the BTS650P; the BTS555 also latches off on overtemperature. Both devices include a current-sense output that reflects a small fraction of the device's load current. You can convert this current to a voltage via a ground-referenced resistor to provide a measure of load current. More significantly, you can use the current-sense feature to implement an external control loop. This option lets you set arbitrary current limits below the devices' preset shutdown thresholds. You can now use the very low on-resistance of these devices to limit power dissipation in your design with a current limit that's safe for your pc-board tracks, the load, and its wiring. And by including retry circuitry within your control loop, you can limit the inrush current to a motor to provide a soft-start function (Figure 4). Here, the start-up waveform self-limits until the motor picks up speed, when its current consumption typically falls to around one-third of the starting value (Figure 5). The BTS650P occupies a seven-pin TO-220 package, and the BTS555 uses a TO-218 package. Both devices are available now, with small-quantity distributor pricing starting at around $3 and $5, respectively, dropping to $1.50 and $2.50 in 1 million-unit volumes.