High-Voltage Optocoupler Amplifier for Electrostatic Actuators

I. INTRODUCTION

• Larger actuator electrode gaps require higher drive voltages, and kilovolt-rated devices are expensive and uncommon.Series-stacked drive designs can also require complex gate drivers and isolators, while capacitive loads store energy proportional to voltage squared.Source: §I · p. 1
• A general-purpose actuator amplifier should produce waveforms around 10 kV, from DC to kilohertz frequencies.DC operation suits capacitive actuators, while kilohertz operation serves electrostatic induction machines.Source: §I · p. 1
• Earlier high-voltage FET and switched-capacitor designs can involve large component counts and substantial design complexity.Prior optocoupler amplifiers used PWM and microcontroller feedback, adding complexity and reducing potential bandwidth.Source: §I · p. 1
• The proposed linear optocoupler amplifier is compared with earlier designs on output voltage and power, and is presented as offering low component count and true analog control.Using high-voltage optocouplers permits higher voltages than single FETs; linear operation also yields lower harmonic distortion and higher bandwidth than PWM-based work.Source: Figure 1 · p. 1, §I · p. 2

II. AMPLIFIER DESIGN

• A pair of OC200HG optocouplers forms a push-pull, class-B amplifier with analog feedback to correct distortion and capacitive-load phase shifts.The optocouplers modulate photodiode reverse current; their built-in optical isolation simplifies drive compared with a FET requiring gate-side isolators.Source: §II · p. 2
• Feedback compensates for capacitive-load phase shift and attenuation, while slewing through crossover distortion to maintain low harmonic distortion.Without feedback, current-source optocouplers would make the amplifier act as an integrator, and nonlinear LED inputs would create crossover distortion.Source: §II · p. 2
• A high-resistance resistor divider provides low-component-count voltage feedback, but removes isolation between the low-voltage input and high-voltage output.An open failure of the low-side resistor could damage low-voltage circuitry; a high-impedance buffer prevents measurement and feedback from loading the divider.Source: §II · p. 2

III. OPTOCOUPLER MODELING

• The OC200HG model treats the optocoupler as a current-dependent current source with output parasitics.Its measured parameters include βopto = 0.00256, Rpd = 5 MΩ, and Rled = 3 Ω.Source: §III · p. 2
• The photodiode capacitance was measured as Cpd = 5.5 pF using the optocoupler gain frequency response and a 100 kΩ test load.The −3 dB cutoff and test-load resistance were used to determine the capacitance.Source: §III · p. 3, Figure 5 · p. 3
• The design limits maximum output current to 128 µA, setting the slew-rate limit for capacitive loads.The limit follows from the input resistor and is below the optocoupler’s 500 µA rating.Source: §III · p. 3
• For sinusoidal excitation, maximum frequency depends on output amplitude and load capacitance; exceeding the limit causes waveform distortion and attenuation.The −3 dB cutoff is specified for conditions above the slew-rate limit.Source: §III · p. 3

IV. FEEDBACK DESIGN

• Analog feedback maintains the desired output voltage across varying loads and removes crossover distortion without requiring digital signal processing.The optocouplers provide current control, while the actuators require voltage control.Source: §IV · p. 3
• Direct op-amp feedback with capacitive loads has low stability because the amplifier and op-amp contribute phase shifts in the return ratio.The resulting poorly damped oscillations can propagate to the output and distort the waveform.Source: §IV · pp. 3–4
• A Type-II lead compensator is used to increase stability and reduce distortion by boosting return-ratio phase margin at crossover.The design targets a 50 kHz crossover frequency and an 85° phase margin.Source: §IV · p. 4

V. IMPLEMENTATION

• The tested implementation separates encapsulated high-voltage components from a low-voltage carrier board, allowing the high-voltage module to be reused with different control boards or replaced independently.The design was intentionally oversized to minimize high-voltage creep and interference during measurements.Source: §V · p. 4, Figure 6 · p. 4
• The alternative six-phase PCB omitted encapsulation and feedback and sensing buffer stages, achieving up to 10 kV with performance similar to the tested design.It was explored without an encapsulated module or buffer components.Source: §V · p. 4, Figure 7 · p. 4
• The unencapsulated alternative experienced dielectric breakdown above 10 kV.All reported test data used the encapsulated implementation.Source: §V · p. 4

VI. RESULTS

• At a 20 kV target into 33 pF, outputs are sinusoidal at 10 and 50 Hz, begin distorting at 100 Hz, and are triangular and significantly attenuated at 500 Hz.The waveform changes at higher frequencies are attributed to the amplifier's slew-rate limit.Source: §VI · pp. 4–5, Figure 8 · p. 5
• Below 100 Hz, THD is 1%; as slew-rate distortion dominates, it rises to 12%, characteristic of a triangle wave, and increases further with attenuation at higher frequencies.At the 20 kV target, higher capacitive loads raise THD by reducing the slew-rate limit; higher drive amplitudes also cause distortion at lower frequencies.Source: §VI · p. 5, Figure 9 · p. 5
• The measured −3 dB frequency limit varies with target drive amplitude and total capacitance; beyond it, attenuation follows a −20 dB/decade fall-off, with greater attenuation at higher capacitance.The tests include parasitic capacitances, and the limit is attributed to slew rate.Source: §VI · p. 5, Figure 10 · p. 6, Figure 11 · p. 6
• The opamp output approximates a square wave as it slews through the LED forward-voltage dead zone and leads the output to correct capacitive-load phase shift; compensation yields 50 kHz crossover and 83° phase margin.Without compensation, the phase margin would be near zero; the stable response also appears as a lack of ringing in intermediate voltage steps.Source: §VI · p. 5
• The measured return ratio matches the model near crossover, while low-frequency deviations from limited opamp open-loop gain are described as inconsequential because crossover dictates closed-loop response.The low-frequency deviation is not accounted for in the model.Source: §VI · p. 5
• The closed-loop small-signal bandwidth is approximately 50 kHz and is useful for quick small adjustments, but the amplifier cannot drive a full-voltage waveform at that frequency.The full-voltage limitation is associated with the lower large-signal bandwidth.Source: §VI · pp. 5–6, Figure 11 · p. 6

• The amplifier provides 20 kV voltage capability, 128 mA current capability, and up to 500 Hz bandwidth for electrostatic actuators.Source: §VII · p. 6
• Linear optocoupler drive in feedback improves bandwidth and lowers component count versus PWM designs.Source: §VII · p. 6
• Its output serves near-DC HASEL actuators and AC electrostatic devices such as motors.Source: §VII · p. 6
• The present 50 mA LED drive yields 128 mA; near-rated 100 mA drive could reach 256 mA, and parasitics constrain bandwidth.Source: §VII · p. 6
• Step response reveals slew-rate limits across capacitive loads; output shows crossover distortion as a quasi-square intermediate voltage, while compensation boosts crossover phase to prevent ringing.Source: Figure 12 · p. 6, Figure 13 · p. 6, Figure 14 · p. 7
• Parallel optocouplers could raise current, while series devices could raise voltage but require circuitry to protect device ratings.Source: §VII · p. 7
• The amplifier is presented as a replicable, off-the-shelf, low-component-count option for research on low-power, high-voltage electrostatic actuators.Source: §VII · p. 7

• Data and design files are posted at the project GitHub repository.Source: §VIII · p. 7