Cross conduction, also known as shoot-through, is a phenomenon in power converters (half-bridge, full-bridge, and push-pull) where both switches in the same leg of a converter are ON simultaneously. This creates a direct short circuit across the input power supply, leading to excessive current flow that can damage the circuit components.
This cross conduction problem can be clearly illustrated by referring to the basic half-bridge circuit diagram below. In this configuration, if both transistors Q1 and Q2 are turned on simultaneously, they create a direct short circuit across the supply lines. Since transformers T1 and T2 (acting as current transformers) have minimal resistance, this condition results in extremely high currents flowing through the switching devices, often leading to immediate failure.
Key Aspects of Cross Conduction:
Occurs in Leg of Switching Devices:
- In converters like half-bridge, full-bridge, and push-pull, switching devices (e.g.,BJT, MOSFETs or IGBTs) are arranged in complementary pairs.
- If the high-side and low-side switches in the same leg are ON at the same time, the input voltage is shorted.
Consequences:
- Excessive Current: Large current flows directly through the switches, potentially destroying them.
- Loss of Efficiency: Energy is wasted in heat generation.
- Circuit Damage: Components like the switches and power supply may be irreparably damaged.
Causes:
- Insufficient Dead Time: Dead time is the delay between turning off one switch and turning on the other in a complementary pair. If this delay is too short or missing, cross conduction can occur.
- Control Signal Overlap: Overlap in PWM control signals can cause both switches to turn ON simultaneously.
- Parasitic Capacitance/Inductance: In MOSFETs, parasitic effects like gate-drain capacitance can lead to unintended switch operation.
Application in Converter Types:
Half-Bridge Converters:
- Two switches share the input voltage.
- Cross conduction occurs if both switches are ON simultaneously, shorting the input supply.
Full-Bridge Converters:
- Four switches are arranged in pairs to drive the load in both directions.
- Cross conduction can occur in either leg (high-side and low-side switches in one leg).
Push-Pull Converters:
- Two switches alternately drive a center-tapped transformer.
- Cross conduction occurs if both switches are ON together, shorting the transformer primary winding.
Prevention Methods:
Dead Time Implementation:
- Add a small delay between the turn-off of one switch and the turn-on of the other to ensure no overlap.
Snubber Circuits:
- Use snubber circuits to manage parasitic oscillations that might cause unintended conduction.
Gate Drive Design:
- Ensure proper control and timing of gate drive signals.
- Use isolated or synchronized gate drivers to avoid simultaneous switching.
Simulation and Testing:
- Simulate the circuit thoroughly and test with controlled inputs to identify and prevent potential cross conduction issues.
Cross conduction is a critical consideration in the design of switching converters, as mitigating it ensures reliability, efficiency, and safety of the power system.
