Power OP Amps

Nov. 15, 2002
Power operational amplifiers (POPs) are becoming increasingly common in control circuitry.

Power operational amplifiers (POPs) are becoming increasingly common in control circuitry. Their primary advantages over conventional amplifiers are reduced part counts, increased reliability, and simplified design. And while the individual components of a power op amp may cost more than alternatives, total cost, including design time, logistics, and production costs, may be less.

There are other advantages as well. When used as linear motor drivers, operational amplifiers produce lower electrical noise than their fast-switching digital counterparts.

An op amp is considered to be a power device if it has an output current greater than ±50 mA and a supply voltage greater than 44 V (±22 V for a dual-supply device). Some power op amps have internal power dissipations of up to 500 W, and can deliver up to 1,000 W of peak power in audio applications. So far, most of these devices are found in submarine sonar units.

Some devices can dissipate 500 W continually and deliver 30 A of current. The power and current handling capability is primarily the result of new packaging and protection techniques.

POP designers give close consideration to thermal characteristics. Manufacturers use copper-alloy packages and lead frames over steel because of its superior thermal conductivity. Copper conducts heat over five times faster than steel. To isolate localized substrate heating, and prevent cracking, multiple substrates are used. New soldering processes eliminate voids under the die and substrate, allowing higher safe operating areas (SOA).

Some op amps incorporate a voltage-current limiter circuit to protect the amplifier from SOA damage such as a short to ground. The limiter senses both output current and voltage, and increases current limit value as output voltage approaches supply voltage. The circuit is activated by connecting a programming resistor.

Power op amps often use circuits that minimize the number of power-handling components. In many power amps, the output devices may take some 70% of the silicon die area and, thus, bear appreciably on device cost. Older designs frequently used at least three power components, a quasi-complementary pair of output transistors and a standoff diode. These components were required to provide class AB operation.

The alternative to power op amps in most applications is to build comparable circuits with discrete power transistors. The discrete approach has often been less expensive in applications that do not demand high performance. Typically, discrete designs must use class C power outputs (where output transistors are off for much more than half the cycle) rather than class B or AB complementary output used on hybrids.

One reason discrete designs are often built class C is because their outputs cannot be thermally matched sufficiently to obtain a class B output. The problem is that the voltage needed to drive a given amount of current through a transistor drops appreciably with temperature. Thermal tracking circuits that compensate for the effect are difficult to construct with discrete components because the circuits must precisely register the temperature of the base-emitter junction of the power transistor. On the other hand, hybrid circuits (and monolithic devices) can provide tracking more easily because of the close proximity of components.

Discrete power amps based on class C operation commonly use a feedback circuit to obtain adequate response. Manufacturers say that such designs are often acceptable where loop response time is less than 1 msec and where amplified signals are below 1 kHz. But these designs may be difficult to stabilize at higher speeds. Feedback in class C designs is nonlinear with rapid changes in stage gain as the circuit goes from threshold to full on. Because of such difficulties, discrete power circuits may produce greatly distorted outputs when operating at frequencies of around 10 kHz.

Automatic test equipment manufacturers often use power op amps in programmable power supplies. They are also used in phased-array sonar because of their accurate phase response and linearity. And head-up displays for fighter planes are simplified because of the high slew rates of POPs.

Many POP designs ultimately drive some type of dc motor. Manufacturers warn of several considerations that must be taken into account for such circuits, particularly back electromotive force. When the motor suddenly stops or reverses, a large voltage develops across the amplifier output stage. The output transistor also current limits, and this combination must be checked against the SOA.

Other inductive loads such as lead wires or coils can also drive a POP out of its SOA. To guard against transients, some amplifiers have built-in protection diodes that clip flyback voltages greater than Vs. In other designs, external diodes must be used.

Large capacitive loads can cause POPs to oscillate. Usually, critical capacitance depends on the amount of feedback. The lower the gain, the lower the capacitance that can start oscillations. Op amp manufacturers can suggest ways to deal with capacitive loads and prevent ringing.

Because POPs operate at high currents, manufacturers have issued precautions for prototype designs.

  • When a circuit is powered up for the first time, power supplies should be set to the minimum operating level indicated on the data sheet.
  • Current limit levels should be set by current limiting resistors, not the lab power supply. A low current setting on the supply does not always provide surge protection from the filter capacitors. Current limiting resistors ensure operation in the SOA, and prevent breakdown of the bipolar output stages.
  • Once basic circuit operation is verified, the current limit can be raised. After the worst case operating conditions have been checked, supply voltages can be increased.
  • Improper grounding also causes POP performance problems, since large output currents can cause ground loops. In general, loops can be avoided by returning grounds (for load, output compensation, and low-level signals) separately to the same point. Improper grounding of test equipment also causes difficulties. A common recommendation is to eliminate any direct ground connection between the signal generator and oscilloscope timing input.
  • Although a POP may be rated to dissipate 90 W continuously, it cannot do so unless properly mounted on a heat sink. One manufacturer reports that customers sometimes drill a single hole encompassing both pins in the heat sink to mount a TO-3 package. Because most heat comes from inside the pin circle, a long thermal path and poor operation result.

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