Isolated Gate Driver With DC-Restore Circuit For Short Circuit Protection
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Solution Overview
Problem
Pulse-width-modulated switched-mode power supplies (SMPSs) face issues with fault conditions such as short circuits across their output, where the power supply fails to turn off output current as commanded, particularly due to inefficiencies in isolated switch drivers during overload or short circuit events, leading to potential MOSFET damage.
Innovation Solution
The proposed solution involves a modified SMPS circuit with a pulse-width-modulator (PWM) and a power section using an isolation transformer with dual secondary windings, a DC-restore capacitor, and timing circuitry to ensure proper gate driver operation, including a DC-restore switch module with an NPN or N-channel MOSFET transistor and timing components to manage voltage pulses and prevent MOSFET turn-on during shutdowns.
Engineering Contradictions & Design Principles
Engineering Contradiction Analysis
1Reliability
If traditional isolated gate drivers are used during overload or short circuit events, then the circuit structure is simple, but the power supply fails to turn off output current as commanded
Solution Approach 1:
The gate driver is segmented into two separate secondary windings (S1a and S1b) on the isolation transformer. The first secondary winding S1a charges the DC-restore capacitor C2 to turn on the power switch, while the second secondary winding S1b controls the discharge of C2 to turn off the power switch. This segmentation allows independent control of turn-on and turn-off functions, enabling reliable fault condition response.
Solution Approach 2:
The DC-restore capacitor C2 acts as an intermediary energy storage element between the isolation transformer and the power switch gate. It stores the turn-on voltage from S1a and provides controlled discharge to the gate through the timing circuit, ensuring the power switch turns off correctly during fault conditions. This intermediary mechanism decouples the PWM control from direct gate drive, improving reliability.
2Productivity
If PWM duty cycle is increased to improve power transfer, then efficiency improves, but MOSFET may remain on during shutdown causing damage
Solution Approach 1:
The timing circuit (RC network with transistor Q2 or MOSFET Q2) is configured to preemptively discharge the DC-restore capacitor C2 before the PWM shutdown occurs. The RC time constant is selected to ensure C2 discharges completely during normal operation, but the timing ensures complete discharge during fault conditions before the power switch can be damaged by residual voltage.
Solution Approach 2:
The second secondary winding S1b provides feedback control to the timing circuit, which monitors the PWM shutdown condition and triggers the discharge of C2 accordingly. This feedback mechanism ensures the power switch turns off reliably during fault conditions regardless of the PWM duty cycle, preventing MOSFET damage while allowing high efficiency operation during normal conditions.
3Device complexity
If minimal additional parts are used in the gate driver, then device complexity is reduced, but reliable fault protection may be compromised
Solution Approach 1:
The second secondary winding S1b serves multiple functions: it controls the discharge timing of the DC-restore capacitor C2, provides fault detection capability, and enables reliable turn-off during short circuit conditions. By making S1b multi-functional, the circuit achieves robust fault protection with minimal additional components beyond the dual-winding transformer and timing circuit.
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 configuration effectively manages short circuit currents, preventing MOSFET damage by ensuring the power switch turns off correctly during fault conditions, allowing the SMPS to operate from near zero to near 100% duty cycles with minimal additional parts and without integrated circuits, enhancing reliability and efficiency.
Implementation Method 1
an isolation transformer with dual secondary windings
Implementation Method 2
a DC-restore capacitor, and timing circuitry
Implementation Method 3
timing circuitry turns off the DC-restore capacitor switch module after a preset time
Data Source
AI summary
A switched mode power supply is provided comprising a pulse-width-modulator (PWM), a power section, and a drive section for the power switch. The drive section consists of a new method of driving the power switch where it is desired that the switch terminals be isolated from the PWM ground. The new driver design, which incorporates a small isolation transformer along with unique modifications to the traditional DC restore circuit, is directed to solving problems when PWM type switching power supplies encounter fault conditions such as a short circuit across their output as well as protecting the switch and the load when an ordinary shutdown or disable command is received. One problem that arises with switch drivers that utilize isolation transformers during an overload event or even an ordinary shutdown event is that the driver does not turn off and hold off the power switch when it is commanded to do so. This invention solves these problems by insuring that the appropriate components are de-energized in the classical dc-restore circuit which will prevent the PWM from losing control of the power switch.