H-Bridge

The H-bridge is one of the simplest and most widely used piezo drive topologies. It consists of four semiconductor switches, typically MOSFETs, arranged in an H configuration:

By turning on diagonal pairs of switches, the polarity of the voltage across the piezo can be reversed. This allows bi-directional operation and enables techniques like pulse-width modulation (PWM) to control the amplitude.

Advantages

• Simple design
• Efficient (low losses)
• Enables PWM control

Disadvantages

• Limited to a single supply voltage
• High peak current during switching
• Piezo is shorted in the off state

Push-Pull

The push-pull topology uses two transistors to alternately source and sink current through the primary of a step-up transformer. The secondary of the transformer connects to the piezo load.

By varying the duty cycle of the drive waveform, the amplitude of the high voltage signal on the secondary can be adjusted. Typically a resonant tank circuit is used on the secondary to generate a sinusoidal waveform.

Advantages

• Simple, low cost design
• High step-up ratio possible
• Sinusoidal output

Disadvantages

• Bulky transformer
• Limited bandwidth
• High voltage spikes during switching

Flyback Converter

A flyback converter uses a single switch to periodically store energy in the primary of a coupled inductor and then release that energy to the secondary and load. It can generate output voltages much higher than the input.

The flyback is well-suited for high voltage, low current applications. The output voltage can be controlled by varying either the duty cycle or switching frequency. A variable frequency flyback may be used to track the resonance of the piezo.

Advantages

• Simple topology
• Wide input voltage range
• High step-up ratio

Disadvantages

• Requires coupled inductor
• High voltage stress on switch
• Asymmetric charge/discharge current

Multilevel Converter

Multilevel converters generate a stepped waveform approximating a sine wave by selectively connecting the piezo across a stack of DC voltage sources. The more levels, the better the approximation.

By using a sufficiently high number of levels (5+) and a switching pattern that follows a sine wave, a low distortion drive signal can be generated without a resonant output circuit.

Advantages

• Low distortion
• Arbitrary waveform generation
• No output filter needed

Disadvantages

• Many components (caps, switches)
• Complex control
• Limited voltage range

FAQ

What are the key specifications of a Piezo Drive Circuit?

The important specs to consider for a piezo driver are:
– Output voltage range
– Output current capability
– Power efficiency
– Switching frequency
– Ability to handle the piezo’s capacitive load

How do I choose the right topology for my application?

Selecting the best piezo drive circuit depends on factors like:
– Required voltage and current
– Desired waveform (square wave, sine wave, arbitrary)
– Input voltage range
– Budget and space constraints

Generally, the H-bridge is a good choice for low-cost, bi-directional control. Flybacks and push-pull circuits work well for high voltage sine wave generation. And multilevel converters offer low-distortion arbitrary waveforms.

What are some common failure modes?

Piezo drive circuits can fail due to:
– Overvoltage/overcurrent events
– Excessive temperature rise
– Poor circuit board layout leading to noise coupling
– Mechanical stress on components

Careful component selection, layout, and the use of snubbers and protection devices is critical for reliable operation.

How do I control the amplitude, frequency of the output?

Piezo driver amplitude can be adjusted by:
– Varying supply voltages
– Changing switching duty cycle
– Modulating switching frequency

The drive frequency is typically set by the switching frequency of the converter. Variable frequency control and external modulators may be used to adjust the operating frequency.

What are some emerging piezo drive technologies?

Some newer piezo drive technologies include:
– Active damping circuits to reduce ringing
– Digital control techniques like sigma-delta modulation
– Piezo energy harvesting circuits
– High frequency (MHz+) drivers for ultrasonic applications

As piezo applications expand, drive circuits continue to evolve to meet new requirements for efficiency, precision, and integration.