Slew Rate Limiting FETs for MEMS Actuator Control
Find Innovative SolutionsGenerate Solutions
Solution Overview
Problem
In MEMS actuators, rapid voltage slew rates caused by high-voltage CMOS drivers lead to undesirable rapid structural changes and high transient contact forces, resulting in shortened lifespans and increased settling times due to excessive acceleration.
Innovation Solution
A driver circuit with slew rate limiting field-effect transistors (FETs) and diode-connected FETs is used to control the charging and discharging of MEMS actuators, setting a predictable and consistent slew rate through a current mirror configuration, thereby reducing actuation impact velocity and mitigating rapid structural changes.
Engineering Contradictions & Design Principles
Engineering Contradiction Analysis
1Speed
If a high-voltage CMOS driver is used to charge MEMS actuators, then the charging speed is improved, but the slew rate becomes excessively high causing rapid structural changes and high transient contact forces
Solution Approach 1:
The patent introduces slew rate limiting circuits as intermediary components between the high-voltage CMOS driver and the MEMS actuator. These circuits include current-limiting transistors and RC networks that mediate the charge transfer, controlling the rate at which voltage is applied to the MEMS capacitance. This intermediary mechanism allows the system to benefit from the high-voltage driver's capability while preventing excessive slew rates that would cause mechanical damage.
Solution Approach 2:
The patent dynamically changes the electrical parameters (voltage slew rate, current) during the charging process. By using slew rate limiting circuits that control dV/dt and incorporating current-limiting mechanisms, the system transforms the abrupt high-current charging into a controlled, gradual charge process. This parameter control prevents the harmful effects of rapid voltage changes while maintaining adequate charging speed.
2Loss of time
If the voltage slew rate is high, then the actuation time is reduced, but the mechanical stability deteriorates due to rapid acceleration and pull-in behavior
Solution Approach 1:
The patent implements dynamic control of the charging process through slew rate limiting circuits that adapt the voltage application rate. The circuits use RC time constants and controlled current sources to dynamically adjust the charging profile, allowing faster actuation when mechanical stability is not compromised, while preventing rapid acceleration that would cause pull-in instability. This dynamic approach optimizes the trade-off between speed and stability.
3Productivity
If the CMOS driver provides high drive current, then the charging rate is improved, but the transient contact forces increase causing high stresses on MEMS structures
Solution Approach 1:
The patent introduces current-limiting transistors and resistance networks as intermediary elements between the high-current CMOS driver and the MEMS actuator. These components mediate the current flow, transforming the high instantaneous current into a controlled, limited current profile. The intermediary circuits maintain the productivity benefit of high drive capability while preventing excessive transient forces that would stress the MEMS structure.
Solution Approach 2:
The patent employs slew rate limiting circuits that预先 (in advance) control the voltage application rate before the MEMS actuator is fully charged. By limiting the initial slew rate and gradually increasing the charging rate, the system cushions against the harmful effects of abrupt high-current charging. This prior cushioning prevents stress concentration and transient force spikes that would occur with direct high-current driving.
Applied Scientific Principles
This section explains which scientific principles are used to turn an abstract innovation direction into a practical engineering solution.
Function Achieved in This Case
The solution provides a controlled and reduced slew rate drive to MEMS actuators, minimizing high velocity pull-in and mechanical instability, thus extending the lifespan and reliability of MEMS devices by limiting acceleration and impact forces during actuation.
Implementation Method 1
one or more first slew rate limiting field-effect transistors (FETs) connected between the first circuit node and a node of the electronic device to be driven, wherein a gate of the one or more first slew rate limiting FET is connected to the first control circuit
Implementation Method 2
the first control circuit comprises: one or more diode-connected FETs, wherein a gate of each of the one or more diode-connected FETs is connected to the gate of a corresponding one of the one or more first slew rate limiting FETs
Implementation Method 3
When using electrostatic forces to control micro-electro-mechanical systems (MEMS) actuators or similar structures, a charge needs to be delivered to actuate the system(s)
Data Source
AI summary
Devices, systems, and methods for limiting a slew rate of a driven device. In some embodiments, the device for limiting a slew rate of the driven device includes one or more slew rate limiting field-effect transistors (FETS) connected between a first circuit node and a node of the driven device, and a first control circuit. In some embodiments, the one or more first slew rate limiting FETs and the first control circuit are configured to set a rate at which the driven device is charged or discharged. In some embodiments, the first control circuit is within a voltage divider and the current flowing through the voltage divider is proportionally mirrored to the one or more first slew rate limiting FETs wherein the current mirror ratio is selected to ensure that a rate at which a capacitance of the driven device changes over time is below a specified limit.