What is the use of shim inductor in DC_DC converter circuit?

 

The shim inductor in a DC-DC converter circuit, as seen in above diagram, serves several important functions. It is typically introduced in resonant or soft-switching converter designs such as Phase-Shifted Full-Bridge (PSFB) or Zero-Voltage Switching (ZVS) topologies. Here are the key roles of the shim inductor:

1. Enabling Zero-Voltage Switching (ZVS):

In soft-switching topologies like ZVS, the shim inductor is crucial for enabling ZVS by controlling the rate of change of current through the switching transistors. ZVS is highly desired because it reduces switching losses by allowing the MOSFETs or other switches to turn on when the voltage across them is zero, minimizing power dissipation.

The shim inductor stores energy and helps to create a resonant tank with parasitic capacitances in the switches. This stored energy assists in the commutation of current and provides enough inductive delay to allow the switches to turn on under ZVS conditions.

2. Current Shaping and Control:

The shim inductor limits the current slew rate through the transformer and other circuit components. This helps reduce the current stress on the switches during the switching transition and prevents sudden spikes in current that could damage the components or lead to excessive electromagnetic interference (EMI).

3. Achieving Soft Switching Across Load Variations:

The shim inductor provides flexibility in maintaining soft-switching operation over a wide range of load conditions. Without it, achieving ZVS might only be possible under a narrow range of operating conditions, such as at high load. The shim inductor compensates for varying load conditions, allowing the converter to operate efficiently even at lower loads.

4. Reducing Circulating Current:

In some designs, the shim inductor helps reduce circulating current in the converter, which improves efficiency. Circulating current refers to the current that flows through the converter’s switches during dead time or when no active power is being transferred. By controlling the current flow during these periods, the shim inductor reduces unnecessary power losses.

5. Magnetic Energy Storage:

The shim inductor stores energy during certain phases of the switching cycle, which can then be used to assist with voltage and current commutation in the primary and secondary circuits. This energy storage helps smooth out the transitions and mitigates the effects of high dv/dt (rate of voltage change) and di/dt (rate of current change) that could otherwise cause switching noise or stress on the components.

6. Minimizing Transformer Magnetizing Current:

In some cases, the shim inductor helps reduce the magnetizing current in the transformer. The inductance creates a buffer, preventing excessive current from saturating the transformer core. This function improves transformer efficiency and minimizes core losses.

Conclusion:

In summary, the shim inductor in your DC-DC converter is primarily used to enable soft switching, limit current slew rates, and improve efficiency across a range of operating conditions. Its role in facilitating ZVS operation is critical in reducing switching losses and ensuring smooth transitions in the circuit, especially in high-power applications.