Drive Piezoelectric Actuators With Fast, High-Power Op Amps

COMPUTING THE MAX DISSIPATED POWER PER MODULE The load impedance of the piezoelectric cartridge is given by the expression:

See equation

assuming that R = 1 Ω, C = 1 nF, and ω = 80 kHz.

To compute the maximum power per module, we can devise the equivalent circuit shown in Figure 3. To do this, the Figure 1 circuit is split into two parts, with each part comprising a 2-nF capacitor and a 0.5-Ω resistor, while assuming a virtual ground denoted by the dotted line and symbol. Because the real part of the impedance (1 Ω) is negligible compared with the total capacitive reactance of 1989 Ω, we shall neglect it.

In our equivalent circuit, the applied voltage will be one half the total potential applied to each module:

See equation

The circuit for each half will drive half the capacitive reactance, which is 1989 divided by 2 each, or 994.5 Ω. Determining power dissipation begins with knowing the phase difference between V and I in the load. This is a simple case because we've modeled our load as a pure capacitor, so the phase angle φ is 90°. The formula for determining the maximum power dissipated when there's a reactive load for a phase angle greater than 40° is given by:³

See equation

where V_S is the magnitude of each supply and Z_L is the magnitude of the load impedance.

Therefore,

See equation

Because the load is totally reactive, the 5.18 W are dissipated by each PA78 amplifier IC and none by the load. We can then go on to select a heatsink and confirm that the maximum allowable junction temperature of each PA78 won't be exceeded.

DEALING WITH THE HEAT An HS27 heatsink has been selected for mounting each PA78 IC. The thermal resistance of each is 5.3°C/W and, as we have determined, the dissipation of each amplifier will be 5.18 W.

We must confirm that the junction temperatures of the MOSFET devices in the PA78 won't exceed a safe value. The familiar thermal resistance equation is:

See equation

We can modify the above equation by substituting the thermal resistance of the heatsink θ_HS for θ_CA:

See equation

We want to solve this for T_J to confirm that we won't exceed the maximum junction temperature. Rearranging the terms, we have:

See equation

In our case the power per device is 5.18 W and the θJC, according to the PA78 data sheet, is 5.5°C/W. The uHS for the heatsink is 7.8°C/W, and the rise in temperature above the ambient is 48.2°C. (For the graphs that show the heatsink's thermal resistance as a function of power, and the temperature rise at the interface, go to www.elecdesign. com and see Drill Deeper 11366.)

Thus, the maximum junction temperature will be:

See equation

Therefore, the actual T_J will never rise above 93.9°C. This is far below the maximum permissible value of 150°C specified in the PA78 data sheet.

It's essential when applying high power to a highly reactive load, such as a piezoelectric actuator, to check the dissipation and the safe operating area. The former is discussed in the Application Note " General Operating Conditions,"³ and the latter is covered in the PA78 data sheet.

In the past, industrial-grade power amplifiers have traded off bandwidth to ensure unity-gain stability. Bipolar designs haven't always met the linearity requirementsof demanding applications⁴, such as the piezoelectric actuator design in this article. But with the availability of a MOSFET-based architecture in devices, the possibilities have changed. Now new standards for bandwidth and linearity can be created for IC power amplifiers.

References:

1. Application Note 20, "Bridge Mode Operation of Power Operational Amplifiers," Apex Microtechnology Corp., www.apexmicrotech.com.
2. Application Note 21, Section 3.1, "Single Supply Operation of Power Operational Amplifiers," ibid.
3. Application Note 1, Section 7.2, " General Operating Considerations," ibid.
4. Application Note 17, "Wide Band Low Distortion Techniques," ibid.

COMPUTING THE MAX DISSIPATED POWER PER MODULE The load impedance of the piezoelectric cartridge is given by the expression:

See equation

assuming that R = 1 Ω, C = 1 nF, and ω = 80 kHz.

To compute the maximum power per module, we can devise the equivalent circuit shown in Figure 3. To do this, the Figure 1 circuit is split into two parts, with each part comprising a 2-nF capacitor and a 0.5-Ω resistor, while assuming a virtual ground denoted by the dotted line and symbol. Because the real part of the impedance (1 Ω) is negligible compared with the total capacitive reactance of 1989 Ω, we shall neglect it.

In our equivalent circuit, the applied voltage will be one half the total potential applied to each module:

See equation

The circuit for each half will drive half the capacitive reactance, which is 1989 divided by 2 each, or 994.5 Ω. Determining power dissipation begins with knowing the phase difference between V and I in the load. This is a simple case because we've modeled our load as a pure capacitor, so the phase angle φ is 90°. The formula for determining the maximum power dissipated when there's a reactive load for a phase angle greater than 40° is given by:³

See equation

where V_S is the magnitude of each supply and Z_L is the magnitude of the load impedance.

Therefore,

See equation

Because the load is totally reactive, the 5.18 W are dissipated by each PA78 amplifier IC and none by the load. We can then go on to select a heatsink and confirm that the maximum allowable junction temperature of each PA78 won't be exceeded.

DEALING WITH THE HEAT An HS27 heatsink has been selected for mounting each PA78 IC. The thermal resistance of each is 5.3°C/W and, as we have determined, the dissipation of each amplifier will be 5.18 W.

We must confirm that the junction temperatures of the MOSFET devices in the PA78 won't exceed a safe value. The familiar thermal resistance equation is:

See equation

We can modify the above equation by substituting the thermal resistance of the heatsink θ_HS for θ_CA:

See equation

We want to solve this for T_J to confirm that we won't exceed the maximum junction temperature. Rearranging the terms, we have:

See equation

In our case the power per device is 5.18 W and the θJC, according to the PA78 data sheet, is 5.5°C/W. The uHS for the heatsink is 7.8°C/W, and the rise in temperature above the ambient is 48.2°C. (For the graphs that show the heatsink's thermal resistance as a function of power, and the temperature rise at the interface, go to www.elecdesign. com and see Drill Deeper 11366.)

Thus, the maximum junction temperature will be:

See equation

Therefore, the actual T_J will never rise above 93.9°C. This is far below the maximum permissible value of 150°C specified in the PA78 data sheet.

It's essential when applying high power to a highly reactive load, such as a piezoelectric actuator, to check the dissipation and the safe operating area. The former is discussed in the Application Note " General Operating Conditions,"³ and the latter is covered in the PA78 data sheet.

In the past, industrial-grade power amplifiers have traded off bandwidth to ensure unity-gain stability. Bipolar designs haven't always met the linearity requirementsof demanding applications⁴, such as the piezoelectric actuator design in this article. But with the availability of a MOSFET-based architecture in devices, the possibilities have changed. Now new standards for bandwidth and linearity can be created for IC power amplifiers.

References:

1. Application Note 20, "Bridge Mode Operation of Power Operational Amplifiers," Apex Microtechnology Corp., www.apexmicrotech.com.
2. Application Note 21, Section 3.1, "Single Supply Operation of Power Operational Amplifiers," ibid.
3. Application Note 1, Section 7.2, " General Operating Considerations," ibid.
4. Application Note 17, "Wide Band Low Distortion Techniques," ibid.