Cascode Gate Driver Staging for Low-EMI Current Sensing
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Solution Overview
Problem
Conventional cascode switches in power converters face challenges with inaccurate current sensing due to non-nominal drain-source resistance, leading to improper functioning of associated circuitry like overcurrent protection, and rapid turn-on increases electromagnetic interference (EMI).
Innovation Solution
Implementing a multiple stage gate drive that initially turns on the switch with a weak signal to reduce EMI, followed by a strong turn-on signal after a delay to achieve accurate current sensing, ensuring the control switch is fully enhanced with nominal resistance.
Engineering Contradictions & Design Principles
Engineering Contradiction Analysis
1Speed
If a rapid turn-on signal is applied to the cascode switch, then the switching speed is improved, but electromagnetic interference increases and current sensing accuracy deteriorates
Solution Approach 1:
The gate drive signal is segmented into multiple stages: a first stage with a first slope rate that limits dV/dt to reduce EMI, followed by a second stage with a second slope rate that achieves full enhancement. This segmentation allows the switch to turn on in controlled steps rather than abruptly, reducing electromagnetic interference while maintaining acceptable switching speed.
Solution Approach 2:
The first stage of the gate drive signal performs a preliminary action by initially turning on the cascode switch with a limited slope rate before the full turn-on signal is applied. This preliminary activation reduces the rate of change of voltage (dV/dt) during the critical transition period, thereby reducing electromagnetic interference before the switch reaches full conduction.
2Object-affected harmful factors
If a weak turn-on signal is applied to reduce EMI, then electromagnetic interference is reduced, but current sensing accuracy deteriorates due to non-nominal resistance
Solution Approach 1:
The gate drive is segmented into two distinct stages: the first stage uses a weaker signal with limited slope rate to reduce EMI during initial turn-on, while the second stage applies a stronger signal to fully enhance the switch and achieve nominal drain-source resistance for accurate current sensing. The segmentation allows both requirements to be satisfied at different times in the switching sequence.
Solution Approach 2:
The gate drive signal dynamically changes its characteristics over time, transitioning from a first slope rate in the initial stage to a second slope rate in the second stage. This dynamic adjustment of the drive signal strength and rate allows the system to first prioritize EMI reduction, then prioritize current sensing accuracy as the switch transitions through different conduction states.
3Measurement precision
If a strong turn-on signal is applied to achieve accurate current sensing, then current sensing accuracy is improved, but electromagnetic interference increases
Solution Approach 1:
The gate drive signal is divided into sequential stages where the strong turn-on signal (second stage) is applied only after the initial turn-on (first stage) has already reduced the dV/dt. This temporal segmentation allows the strong signal to achieve accurate current sensing without causing excessive EMI, as the critical high-dV/dt period has already passed.
Solution Approach 2:
The first stage performs a preliminary action by initially activating the switch with a limited slope rate, which reduces electromagnetic interference before the strong turn-on signal is applied in the second stage. This preliminary EMI mitigation allows subsequent strong signaling to achieve accurate current sensing without compounding the interference problem.
Data Source
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AI summary
A controller for use in a power converter includes a current sense circuit to generate a current limit signal and an overcurrent signal in response to a source signal, a first sense finger signal, and a second sense finger signal. A control circuit is coupled to generate a control signal in response to the current limit signal and the overcurrent limit signal. A drive circuit is coupled to generate a drive signal with a multiple stage gate drive in response to the control signal. The drive signal in a first stage of the multiple stage gate drive is a weak turn on drive signal to turn a switch on slowly to reduce electromagnetic interference (EMI). The drive signal in a second stage of the multiple stage gate drive is a strong turn on drive signal to fully turn on the switch quickly to enable accurate current sensing of the switch.