Isolated PWM Driver Circuit for Optocoupler Current Accuracy
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Solution Overview
Problem
Existing isolated driver circuits for mains power, such as lighting drivers, face challenges in accurately transferring current set points across optocouplers due to high tolerance in current transfer ratios, leading to increased tolerance in load output, particularly in LED lighting where current accuracy must be maintained within 5%, and existing solutions are either costly or less accurate.
Innovation Solution
A driver circuit that uses a primary side controller to provide a PWM load drive level signal via an optocoupler, with a sensing circuit to measure power consumption and a comparator to adjust the PWM frequency and duty cycle to compensate for errors introduced by the optocoupler, ensuring accurate load drive levels by monitoring power consumption changes.
Engineering Contradictions & Design Principles
Engineering Contradiction Analysis
1Measurement precision
If a dedicated transformer is used to provide current sensing feedback signal from secondary side to primary side, then measurement precision is improved, but device complexity and cost increase
Solution Approach 1:
The patent extracts the current sensing function from the primary side and relocates it to the secondary side, where it can directly measure the actual output current. This eliminates the need for complex transformer-based sensing on the primary side while maintaining measurement accuracy.
Solution Approach 2:
The patent introduces an optocoupler as an intermediary device to transfer the error signal from the secondary side back to the primary side controller. This provides galvanic isolation while enabling feedback, avoiding the need for direct electrical connection or complex transformer arrangements.
2Device complexity
If optocoupler is used to transfer PWM signal from primary side to secondary side, then device complexity is reduced, but manufacturing precision deteriorates due to current transfer ratio tolerance
Solution Approach 1:
The patent implements a feedback control loop where the actual load current is measured on the secondary side and compared with the desired current level. The resulting error signal is transmitted back to the primary side controller via optocoupler, which continuously adjusts the PWM duty cycle to compensate for optocoupler transfer ratio variations and maintain precise current control within 5% tolerance.
3Speed
If PWM frequency is increased to improve response time, then speed is improved, but use of energy increases due to optocoupler bandwidth limitations
Solution Approach 1:
The patent dynamically adjusts the PWM frequency based on the specific operating conditions and optocoupler characteristics. Rather than using a fixed high frequency that would always consume excessive power, the system optimizes the frequency to achieve the necessary response time while minimizing power consumption, adapting to the actual bandwidth limitations of the optocoupler in use.
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
This solution effectively compensates for errors introduced by the optocoupler, maintaining accurate load output by adjusting the PWM signal's frequency and duty cycle, thereby improving the precision of LED current control within the required tolerance, reducing the need for bulky filtering components and minimizing processing resources.
Implementation Method 1
an optocoupler between the primary side circuit and the secondary side circuit; the primary side circuit comprises: a controller for providing a PWM load drive level signal, for setting a load drive level, to the secondary side circuit via the optocoupler
Implementation Method 2
a transformer between the primary side circuit and the secondary side circuit for delivering power to the load
Data Source
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AI summary
An isolated driver generates a PWM load drive level signal at the primary side and provides this to the secondary side via an optocoupler. Power consumption of the load is also sensed at the primary side. A drive level is sensed at the secondary side and compared at the secondary side with the load drive level to provide an error signal for the primary side controller. A frequency of the PWM load drive level signal is adjusted and the power consumption of the load is monitored in response to the frequency adjustment. The duty cycle of the PWM load drive level signal can then be adapted to compensate for errors introduced by the optocoupler.