Low-Side Driver Circuit With Anti-Serial MOSFET Diodes for EMC Immunity
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Solution Overview
Problem
Existing driver circuits lack protection against mis-wiring and electromagnetic interference (EMC) disturbances, particularly due to the presence of intrinsic parasitic diodes that can lead to uncontrolled current flow and EMC instability, and require expensive external filters for noise suppression.
Innovation Solution
The proposed driver circuit employs anti-serially coupled parasitic diodes of transistors to block unwanted currents and uses dual control loops to manage current flow symmetrically during EMC disturbances, ensuring immunity to mis-wiring and transient overvoltage pulses without external filters, while maintaining precise output voltage control.
Engineering Contradictions & Design Principles
Engineering Contradiction Analysis
1Ease of operation
If a single NMOS output transistor is used in a classical low side driver, then the circuit can provide basic drive functionality, but the intrinsic drain bulk diode allows uncontrolled current flow when output voltage goes below ground, causing mis-wiring and EMC vulnerabilities
Solution Approach 1:
The single output transistor is segmented into two series-connected transistors (first and second transistors), each with its own control loop. This segmentation allows independent control of each transistor's parasitic diode, enabling the anti-serial connection strategy where one diode blocks current in the negative direction while the other blocks in the positive direction, thus resolving the vulnerability to uncontrolled current flow.
Solution Approach 2:
The two transistors are connected in anti-serial configuration, creating asymmetric current blocking characteristics. The first transistor's parasitic diode blocks negative-going currents while the second transistor's parasitic diode blocks positive-going currents. This asymmetric arrangement provides bidirectional protection without requiring additional external diodes or complex protection circuits.
2Reliability
If a diode is added in series with the output transistor to block output current when output pin is below ground, then mis-wiring protection is improved, but the diode prevents the use of this solution for drivers with desired output voltages closer to the supply than one diode threshold voltage
Solution Approach 1:
The transistors' own parasitic diodes are utilized for protection functionality rather than adding external diodes. The control loops actively manage the transistors to make their parasitic diodes perform the protection function, allowing the output voltage to swing close to the supply voltage without being limited by external diode threshold drops. This self-service approach eliminates the voltage range limitation.
3Measurement precision
If traditional transconductance regulators are used, then voltage regulation can be achieved, but the circuits are not EMC safe and require expensive external filters to filtering out EMC noise
Solution Approach 1:
The parasitic diodes, which are normally considered harmful or unwanted elements in transistor operation, are converted into beneficial protection mechanisms. By actively controlling the transistors, their parasitic diodes become EMC protection elements that block transient currents and prevent disturbance propagation, eliminating the need for external filtering components while maintaining voltage regulation.
Solution Approach 2:
Separate control loops are implemented for each transistor, providing active feedback control that monitors and adjusts the transistor states in response to output conditions. This feedback mechanism enables the control loops to detect and respond to EMC disturbances, maintaining stable operation and preventing instability that would otherwise require external filters.
4Power
If the output transistor size is increased to handle higher current during EMC disturbances, then current capability is improved, but the area occupied by the transistor increases
Solution Approach 1:
The current handling capability is segmented between two transistors working in series. Each transistor can be sized for moderate current capability, but together they provide bidirectional current control. The segmentation allows each transistor to be optimized for specific current ranges while sharing the total current handling responsibility, reducing the area requirement compared to a single large transistor.
Solution Approach 2:
Each transistor is optimized for specific local requirements: the first transistor is optimized for blocking negative currents while the second is optimized for blocking positive currents. This local optimization allows each transistor to be sized appropriately for its specific function rather than requiring both to handle the full range of potential currents, reducing total area while maintaining overall current capability.
Data Source
AI summary
An output driver circuit has an input, an output node, and first and second transistors coupled in series between the output node and a first source of operating potential. Parasitic diodes of the first and second transistors are anti-serially coupled. The output driver circuit has first and second control circuits coupled to control the first and second transistors respectively. The first transistor is controlled as a controlled current source depending on a signal at the input during normal conditions when the current that flows through the output is in a first direction, and the second control circuit controls the second transistor to prevent unwanted DC current at the output from flowing through the output in a second direction. The first and second transistors are also controlled to limit unwanted transient currents during an EMC disturbance substantially symmetrically.