500V 10A 150W High Voltage Power Op Amp

PAD126 Rev – E

DESCRIPTION

The PAD126 high voltage operational amplifier is constructed with surface mount components to provide a cost effective solution for many industrial applications. With a footprint only 80mm square the PAD126 offers outstanding performance that rivals much more expensive hybrid component amplifiers or rack-mount amplifiers. User selectable external compensation tailors the amplifier’s response to the application requirements. The PAD126 also features a substrate temperature reporting output and overtemp shutdown and is also compatible with the PAD125 Current Limit Accessory Module. The amplifier circuitry is built on a thermally conductive but electrically insulating substrate mounted to an integrated heat sink and fan assembly. No BeO is used the amplifier. The resulting module is a small, high performance turn-key solution for many industrial applications.

A NEW CONCEPT

A critical task in any power amplifier application is cooling the amplifier. Until now component amplifier manufacturers often treated this task as an after-thought, left for the user to figure out. At Power Amp Design the best heat sink and fan combination is chosen at the start and becomes an integral part of the overall amplifier design. The result is the most compact and volumetrically efficient design combination at the lowest cost. In addition, this integrated solution concept offers an achievable real-world power dissipation rating, not the ideal rating usually cited when the amplifier case is somehow kept at 25°C. The user no longer needs to specify, procure or assemble separate components.

CIRCUIT & CONNECTIONS

EQUIVALENT CIRCUIT

AMPLIFIER PINOUT & CONNECTIONS

ABSOLUTE MAXIMUM RATINGS

NOTES:

1. Unless otherwise noted: TC=25°C, compensation Cc=470pF, DC input specifications are ± value given, power supply voltage is typical rating.
2. Derate internal power dissipation to achieve high MTBF.
3. Doubles for every 10°C of case temperature increase.
4. Rating applies if the output current alternates between both output transistors at a rate faster than 60Hz.
5. Limited by fan storage characteristics. During operation, even though the case may be at 85°C the fan will be at a lower temperature.
6. L10 refers to the time it takes for 10% of a population of fans to fail. MTBF (Mean Time Before Failure), on the other hand, refers to a 50% failure
   rate. The MTBF of fans used by Power Amp Design is 210 kHrs at 50C

OPERATING CONSIDERATIONS

SAFETY FIRST

The operating voltages of the PAD126 are potentially deadly. When developing an application circuit it is wise to begin with power supply voltages as low as possible while checking for circuit functionality. Increase supply voltages slowly as confidence in the application circuit increases. Always use a “hands off” method whereby test equipment probes are attached only when power is off. See application note AN-16 for PCB layout guidelines.

EXTERNAL CIRCUIT COMPONENTS

The output of the PAD126 can swing up to +/- 240V and this may stress or destroy external components that are often not seriously considered when developing circuits with small signal op amps. High voltage rated resistors may be purchased for the feedback circuit or, alternately, several ordinary resistors may be placed in series to obtain the proper voltage rating. The compensation capacitor C_C is a NPO type and is rated for 500V. See AN-16 for PCB layout guidelines.

COOLING FAN

The PAD126 relies on its fan for proper cooling of the amplifier. Make sure that air flow to the fan and away from the heat sink remains unobstructed. To eliminate electrical noise created by the cooling fan we recommend a 47µF capacitor placed directly at the point where the fan wires connect to the PCB. See application note AN-24 for further details.

CURRENT LIMIT

The current limiting function of the PAD126 is a versatile circuit that can be used to implement a four-wire current limit
configuration or, in combination with some external components can be configured to implement a fold-over current limit circuit. The four-wire current limit configuration insures that parasitic resistance in the output line, Rp, does not affect the programmed current limit setting. See Figure 1. The sense voltage for current limit is 0.65V. Thus approximately:

Where IL is the value of the limited current and RS is the value of the current limit sense resistor. In addition, the sense voltage has a temperature coefficient approximately equal to –2.2mV/°C. The fold-over function reduces the available current as the voltage across the output transistors increases to help insure that the SOA of the output transistors is not exceeded. Refer to Application Circuits for details on how to connect the current limit circuitry to implement either a four-wire current limit or current limit with a fold-over function (Figures 1 and 5).

MOUNTING THE AMPLIFIER

The amplifier is supplied with four 4-40 M/F hex spacers at the four corners of the amplifier. Once the amplifier is seated, secure the module with the provided 4-40 nuts and torque to 4.7 in lb [53 N cm] max. See “Dimensional Information” for a detailed drawing. It is recommended that the heat sink be grounded to the system ground. This can easily be done by providing a grounded circuit board pad around any of the holes for the mounting studs.

TEMPERATURE REPORTING

An analog output voltage is provided (pin 6, TMP) relative to ground (pin 8, GND) and proportional to the temperature in degrees C. The slope is approximately -10.82mV/°C. The output voltage follows the equation:

T = (2.127 ─ V) (92.42)

Where V is the TMP output voltage and T is the substrate temperature in degrees C. This high impedance output circuit is susceptible to capacitive loading and pickup from the output of the amplifier. TMP is internally filtered, but when monitoring TMP the filter as shown in Figure 3 may be useful. See Applications Circuits.

THERMAL SHUTDOWN

The temperature monitoring circuit turns off the amplifier when the substrate temperature reaches 110°C. When the substrate cools down 10°C the amplifier is enabled once again. The thermal shutdown feature is activated either by amplifier overloads or a failure of the fan circuit.

EXTERNAL SHUTDOWN

When pin 7 ( SD ) goes low (ground) or is forced low the amplifier is turned “off” and remains “off” as long as pin 7 is low. When pin 7 is monitored with a high impedance circuit it also functions as a flag, reporting when the amplifier is shut down. A “high” (+5V) on pin 7 indicates the temperature is in the normal range. A “low” (ground) indicates a shutdown condition. See Application Circuits (Figure 2) for details. To avoid loading the internal circuit any external circuit must be an open collector or open drain connection.

PHASE COMPENSATION

The PAD126 must be phase compensated. The compensation capacitor, CC, is connected between pins 4 and 5. The compensation capacitor must be an NPO type capacitor rated for the full supply voltage (500V). On page 2, under Amplifier Pinout and Connections, you will find a table that gives recommended compensation capacitance value for various circuit gains and the resulting slew rate for each capacitor value. Consult also the small signal response and phase response plots for the selected compensation value in the Typical Performance Graphs section. A compensation capacitor less than 10pF is not recommended.

PAD125 ACCESSORY MODULE

Precision current limiting can be achieved connecting the optional PAD125 Current Limit Accessory Module (See Figure 3) that provides a precision 150mV temperature compensated set point for the current limit as well as other programmable features. Please refer to the PAD125 datasheet for details on the operation of the module.

TYPICAL PERFORMANCE GRAPHS

SAFE OPERATING AREA

The safe operating area (SOA) of a power amplifier is its single most important specification. The SOA graph presented above serves as a first approximation to help you decide if the PAD126 will meet the demands of your application. But a more accurate determination can be reached by making use of the PAD Power™ spreadsheet which can be found in the Power Amp Design website. While the graph above adequately shows DC SOA and some pulse information it does not take into account ambient temperatures higher than 30°C, AC sine, phase or non-symmetric conditions that often appear in real-world applications. The PAD Power™ spreadsheet takes all of these effects into account.

DIMENSIONAL INFORMATION

APPLICATION CIRCUITS

With the three current limit function pins (pins 38-40) dual slope current limiting can be implemented that more closely approximates the SOA curve of the amplifier than can be achieved with standard current limiting techniques. Values for resistors R1-R7 and RS can be calculated using the PAD Power™ spreadsheet that can be downloaded from the Power Amp Design web site.