Introduction

Load Point Power Supplies (LPS) are widely used in applications requiring very low output voltage and extremely high current, such as AI servers, industrial control systems, and advanced digital loads. Output voltages can be as low as 0.6 V, while output currents may reach hundreds of amperes.

Under these operating conditions, even a small fault can quickly result in excessive current, thermal stress, or permanent damage to power components. Therefore, Overcurrent Protection (OCP) is not an optional feature—it is a fundamental requirement for ensuring system safety and long-term reliability.

This article reviews commonly used OCP techniques in LPS designs, explains their operating principles, and discusses how different protection strategies affect system behavior and reliability.

Why OCP Is Critical in Low-Voltage, High-Current LPS

In LPS applications, low voltage and high current create a challenging environment:

● Small changes in load impedance can cause large current variations

● Short-circuit events can generate extremely high fault currents

● Continuous operation at current limits leads to rapid temperature rise

Without effective OCP, power modules risk:

● MOSFET or inductor damage

● Thermal shutdown cycling

● Reduced MTBF (Mean Time Between Failures)

A well-designed OCP system must react quickly, limit stress on components, and—when necessary—control recovery behavior after a fault.

Cycle-by-Cycle Current Limiting

Current Mode Control (CMC)

Current Mode Control (CMC) has become a popular control scheme in modern switching regulators. One of its key advantages is that cycle-by-cycle current limiting can be implemented naturally by clamping the COMP voltage.

To implement current limiting, the controller must sense the inductor current. Common current sensing methods include:

● Sense resistor current sensing

● Inductor DCR current sensing

● MOSFET R_DS(on) current sensing

● SenseFET current sensing (Recommended)

Among these, SenseFET current sensing is widely adopted due to its high accuracy and negligible power loss. By splitting current between a power MOSFET and a matched SenseFET, only a small fraction of the total current needs to be measured using low-power signal-level components.

Peak Current Limiting

In a peak CMC buck converter, each switching cycle begins with a clock signal that turns on the high-side MOSFET. The inductor current ramps up and is continuously compared with the control voltage (V_COMP). Once the current reaches the programmed threshold, the high-side switch turns off.

This mechanism ensures that the peak inductor current is limited to a predefined value, providing fast and effective protection.

Minimum On-Time Challenge

In real controllers, a minimum on-time constraint exists. Even if the current reaches the limit early in a cycle, the high-side MOSFET must remain on for at least the minimum on-time.

During short-circuit conditions, the output voltage collapses, and the inductor current decays slowly during off-time. If the required duty cycle becomes smaller than the minimum on-time, the inductor current may increase cycle by cycle, potentially exceeding the intended current limit.

To prevent this condition, additional protection mechanisms are required.

Valley Current Limiting

What Is Valley Current Limiting?

Valley current limiting provides an additional layer of protection by monitoring inductor current during the low-side MOSFET conduction interval.

How It Prevents Current Runaway

If the detected current at the end of a switching cycle exceeds the programmed valley current limit, the controller skips the next switching cycle. The high-side MOSFET remains off until the current decays below the threshold.

This approach effectively prevents uncontrolled current rise caused by minimum on-time limitations.

Frequency Foldback Protection

When and How Frequency Foldback Helps

Frequency foldback is another effective method for mitigating current runaway during fault conditions.

When an overcurrent event is detected:

● Peak current limiting reduces duty cycle and output voltage

● If feedback voltage or on-time falls below a preset threshold, switching frequency is reduced

Benefits of Reduced Switching Frequency

Lower switching frequency allows longer effective on-time and ensures that the required duty cycle remains above the minimum on-time limit. Additionally, reduced frequency increases current ripple and lowers average output current, further reducing thermal stress.

Once the fault condition is removed, the switching frequency automatically returns to its nominal value.

Reverse Current Protection

How Reverse Current Protection Works

In non-synchronous buck converters with diode rectification, inductor current always flows in the forward direction. However, in synchronous buck converters operating in Forced Continuous Conduction Mode (FCCM), current can flow in both directions through the low-side MOSFET.

Why It’s Crucial for Synchronous Designs

If the output voltage rises above the regulation point, a large reverse current may flow from V_OUT back to the PHASE node and into ground through the low-side MOSFET. Excessive reverse current can damage the regulator or surrounding circuitry.

Peak and valley current limiting only control forward current. Therefore, dedicated reverse current limiting circuits are required. When reverse current exceeds the programmed threshold, the controller forces the low-side MOSFET off to block further reverse conduction.

Secondary OCP Strategies

Hiccup Mode Protection

Hiccup mode protection combines peak current limiting with a timing or cycle-count mechanism.

When an overcurrent event occurs:

● Peak current is limited on a cycle-by-cycle basis

● The controller counts a predefined number of switching cycles

● The regulator shuts down for a fixed off-time

● After the delay, the system attempts to restart

Latch Mode Protection

In latch mode protection, the regulator shuts down and remains latched off once an overcurrent event is detected. Restart requires toggling the ENABLE pin or cycling the input voltage (V_IN).

OCP Implementation in AipuPower LPS Modules

Overview of AipuPower’s OCP Features

Modern integrated LPS modules typically include built-in OCP circuits to protect against overcurrent and overpower conditions. However, protection strategies may vary between manufacturers.

Practical Applications of OCP in LPS Modules

AipuPower load point power modules incorporate:

● Internal peak current limiting

● Valley current limiting

● Reverse current protection

Depending on application requirements, additional features such as frequency foldback, hiccup mode, or latch mode protection are implemented to further enhance system reliability and MTBF.

Conclusion

Summary of Key OCP Strategies

Choosing the right OCP strategy is essential for reliable LPS operation. Peak current limiting provides fast protection, while valley current limiting and frequency foldback prevent current runaway caused by minimum on-time constraints. Reverse current protection safeguards synchronous designs from negative current stress.

Why Choosing the Right OCP Matters

Secondary protection methods such as hiccup mode and latch mode further reduce thermal stress and improve long-term system reliability. By carefully matching OCP mechanisms to application requirements, designers can achieve robust and dependable power solutions.