Blocking Diodes vs. Bi-Directional Supplies: Choosing the Right Reverse-Energy Protection

Blocking diodes provide a cost-effective and robust alternative to bi-directional power supplies for protecting programmable DC power systems from damaging reverse current in high-power test applications.

Programmable DC power supplies are important instruments in modern power electronics research and production manufacturing, designed to simulate a wide range of electrical conditions. For test and measurement engineers, key features such as high-resolution readback, rapid transition speeds in response to dynamic loads, and the ability to program complex power sequences are indispensable for validating device performance and reliability.

In many advanced test applications, the device under test (DUT) is not a simple passive resistor, it can be an active or energy-storing system, creating a significant risk of reverse current. Reverse current occurs when the DUT’s voltage rises above the power-supply output, pushing current back into the supply; this typically happens during motor braking or when the supply is connected to a device at a different potential, such as a charged battery or a reactive load such as an inductor or capacitor. In these applications, the DUT can momentarily transform from a passive load into an active, unregulated power source.

Unmitigated reverse current will force a power supply to sink rather than source energy, an inversion of its designed role, which will cause severe and irreparable damage to its output circuitry if not designed to handle this energy.

Reverse Energy Protection Architectures

The challenge of reverse energy brings a crucial architectural decision: should one absorb the energy or simply block it? The answer determines whether a simple, robust single-quadrant system is sufficient, or if the complexity of a bi-directional system is truly required.

Programmable DC Power Supplies with Blocking Diodes

A standard programmable DC power supply is a single-quadrant device, operating exclusively in Quadrant I of the voltage-current (V-I) plane by sourcing positive voltage and positive current. Its role is to push energy out to a Device Under Test (DUT). The simplest and most fundamental defense against reverse current is the blocking diode. A blocking diode is a semiconductor device that functions as an electronic check valve, permitting current to flow in only one direction. When placed in series with the positive output of a power supply, the diode is forward-biased during normal operation, allowing current to flow from the supply to the DUT with a small forward voltage drop. However, if the DUT's voltage rises above the supply's voltage, the diode becomes reverse-biased, effectively creating an open circuit that blocks the harmful reverse current from flowing back into the power supply. This simple, robust, and passive protection is critical for safeguarding power electronics.

Fig 1. Programmable DC power supply in a battery charging application with a blocking diode in series with the positive terminal. Programmable DC power supply shown with internal output capacitance and bleed resistance for illustrative purposes.

Bi-Directional Power Supplies

A bi-directional power supply is a two-quadrant device, operating in both Quadrant I (sourcing positive voltage and current) and Quadrant IV (sinking current at a positive voltage) of the voltage-current (V-I) plane. Its role is to seamlessly manage energy flow in two directions, both pushing energy out to a Device Under Test (DUT) and pulling energy back from it. The supply handles this reverse energy by undertaking a sophisticated, multi-stage conversion process, transforming the absorbed DC energy into high-quality, grid-compliant three-phase AC power, which it then injects back into the facility's electrical grid.

The key stage enabling bi-directional power flow is the Active Front End (AFE), which replaces the typical passive input diode bridge with a full-bridge of actively controlled semiconductor switches, such as Silicon Carbide (SiC) MOSFETs. This hardware is intelligently controlled to perform two opposite functions. When sourcing power, the AFE's switches are modulated to act as a high-efficiency, power-factor-corrected (PFC) rectifier. When reverse energy forces current back into the unit, the AFE's role reverses, and it operates as a grid-tied inverter, converting the excess DC energy back into AC power. This dual-purpose, software-defined functionality of the AFE represents the first and most fundamental layer of complexity, requiring a sophisticated control system and high-power components capable of handling power flow in both directions; a stark departure from the simple, one-way path of a standard rectifier.

Selecting the Appropriate Architecture: Separating Protection from Regeneration

Engineers correctly identify a risk of reverse energy, a momentary regenerative braking pulse from a motor, the initial connection to a pre-charged capacitor bank, or the back-EMF from a spinning drive, but often assume a bi-directional supply is the only solution. However, for a vast majority of test applications where the primary function is to power a device and the reverse energy events are momentary and predictable, choosing a bi-directional supply is often the wrong tool for the job.

The critical insight is that most of these reverse-energy windows are measured in milliseconds. In these common scenarios, the goal is not to continuously absorb energy, but simply to survive a transient. This is where a robust single-quadrant power supply, combined with a properly specified blocking diode, presents a far more advantageous architecture. This approach focuses on protection and isolation rather than complex energy regeneration. The blocking diode acts as a simple, passive "check valve," converting the power supply into a one-way street for current. It is immune to issues with active controls and its voltage drop can be automatically negated by the supply’s remote sense leads, preserving voltage accuracy at the load.

This strategy of blocking, rather than absorbing, yields a system with clear engineering and financial superiority when the primary need is sourcing power. In these scenarios, a single-quadrant architecture is more advantageous for several key reasons:

• Lower Cost and Complexity: A bi-directional supply is significantly more expensive due to its two power conversion stages and complex control circuitry. Pairing a less expensive, single-quadrant supply with a simple, passive diode is a more cost-effective and reliable solution.
• Enhanced Robustness and Reliability: A single-quadrant supply is optimized to do one job: source power. A bi-directional supply's complexity, with its need to manage two-way energy flow and grid synchronization, introduces more potential points of failure.
• Simplified Installation: A bi-directional supply regenerates energy to the facility grid, which can require costly facility reviews and specialized hardware, and may create harmonic distortions. The diode solution simply blocks reverse energy at the source, eliminating complex grid interaction.
• Inherent Safety without Protection Delay: A blocking diode offers instantaneous, fail-safe protection. In contrast, a bi-directional supply has a protection delay as it relies on active controls to switch from sourcing to sinking, which can fail or behave unexpectedly.
• Greater Scalability: Bi-directional supplies are often limited to smaller power modules (5-10 kW) that require paralleling for high-power needs. Single-quadrant architectures can be built into much larger, monolithic power blocks (100 kW+), providing significantly fewer points of failure, reduced control complexity and less complex systems for high-power testing.

For applications that truly demand continuous, controlled energy absorption, such as production battery cycling, electric vehicle powertrain testing, or acting as a full-time active load, a bi-directional supply is the appropriate choice. But for the countless applications where the goal is to power a DUT and simply protect against momentary reverse energy, the single-quadrant power supply with an engineered blocking diode solution is the more robust, cost-effective, and practical engineering choice.

Applications Demanding Reverse Energy Protection

There are several common test and measurement applications where reverse current protection is an essential component for system protection and test integrity.

DC Motor Drives

A fundamental principle of electromechanical systems is that a DC motor is also a generator. During operation, when a motor is actively decelerated (a process known as regenerative braking) or when it is driven by an overhauling load (such as a crane lowering a weight), it converts kinetic energy back into electrical energy. This process generates a back-electromotive force (back-EMF) at the motor's terminals. This back-EMF voltage can easily rise above the power supply's set output voltage, causing a significant reverse current to flow back into the supply.

Highly Inductive Loads

Inductive loads, such as large electromagnets, solenoids, and even the parasitic inductance of long output cables, store energy in a magnetic field. An inductor resists changes in current; when the current from the power supply is suddenly interrupted, for example, by opening a switch or turning off the supply's output, the collapsing magnetic field induces a voltage spike of opposite polarity across the inductor. The magnitude of this spike is governed by the equation V=L×(di/dt), where L is the inductance and di/dt is the rate of change of current. With a very rapid current interruption, this voltage spike can reach hundreds or even thousands of volts, posing a severe threat to both the switching components and the power supply itself.

Paralleling Dissimilar Power Supplies

Some test requirements require a wide range of different voltages, which can be addressed by using multiple power supplies of different ratings. If a higher-voltage power supply increases the shared DC bus above the maximum rating of a parallel connected lower-voltage power supply, the excess voltage can damage the the output capacitors or diodes. The common, robust solution is to use a diode in series with lower voltage parallel connected power supply's output. This technique isolates the lower voltage power supply’s output, preventing reverse energy from being received when the higher voltage parallel connected power supply is delivering current.

Connecting Unregulated Sources like Batteries and Capacitors

When a power supply is first connected to an energy-storage DUT, such as a battery pack, super-capacitor bank, or fuel-cell stack, the main hazard is a momentary voltage mismatch at the instant of connection. If the DUT’s open-circuit voltage is higher than the supply’s programmed set-point, the DUT briefly becomes the source, forcing a surge of reverse current into the supply. This inrush can damage output components catastrophically. The risk disappears once the two voltages equalize, so it can be mitigated in two straightforward ways: a series blocking diode to prevent any reverse current during the initial contact or voltage matching (pre-charging), to ramp the supply up to the DUT’s voltage before closing the output relay.

Internal Bleeder Resistor and Preventing Unwanted Discharge

Most power supplies include an internal bleeder resistor across their output filter capacitors. This resistor not only bleeds off the stored charge on the output capacitors after shutdown, removing any shock hazard, but also serves as a ballast, providing a small load that keeps the regulator stable under no-load or light-load conditions. The same resistor, however, becomes detrimental in specific test scenarios: when the supply is connected to an already-energized device under test (DUT), like: a battery, a super-capacitor, or a fuel-cell stack that must never fall below a minimum cell voltage. With these loads, the bleeder forms an unintended discharge path. Whenever the supply output is disabled, the resistor continues to draw current, steadily draining the DUT.

In battery cycle life tests the “rest” phase requires a true open circuit to measure open-circuit voltage accurately; parasitic bleeding corrupts those results. For fuel cells the stakes are higher: if stack voltage droops below the manufacturer-specified threshold, individual cells can reverse-bias, leading to irreversible catalyst degradation and loss of performance.

Table 1. Summary of common high-power applications, the associated physical risks they present to a power supply, and the primary protection mechanism required.

Application	Physical Phenomenon	Risk to Power Supply
DC Motor Drive	Regenerative Braking (Back-EMF)	Reverse current damage to output stage
Long Cables / Inductors	Inductive Voltage Spike (V=L×di/dt)	High-voltage transient damage
Parallel Sources	Voltage Mismatch	Back-feeding from higher-voltage supply
Battery/Capacitor Charging	Reverse current from unregulated source during relay connection	Extreme
Unwanted Bleeder Discharge	Parasitic discharge path through the supply’s internal bleeder resistor when output is disabled	None, but potential unwanted load discharge and driving load below safe voltage levels

Implementing Blocking Diode Protection

While the concept of a blocking diode is simple, its practical implementation in a high-power system is a non-trivial engineering task. Selecting an inadequate component or failing to properly manage its thermal load can lead to diode failure, leaving the power supply unprotected. Key parameters include:

• Repetitive Peak Reverse Voltage (V_RRM​): This must exceed the highest possible voltage from the load, including a significant safety margin of 25% to 50%.
• Average Forward Current (I_F(AV)​): The diode's rating must be higher than the maximum continuous current the power supply will output.
• Peak Forward Surge Current (I_FSM​): This is crucial for handling high inrush currents, such as when charging large capacitors.
• Diode Type: Standard silicon rectifiers are robust and ideal for high-voltage applications, while Schottky diodes, with their much lower forward voltage drop (0.2 V to 0.5 V), are superior for lower-voltage, high-current applications where minimizing heat is critical.

A conducting diode is not a perfect switch; it exhibits a forward voltage drop (V_F​) across its terminals. This voltage drop, multiplied by the forward current, results in power being dissipated as heat:

P_dissipated = VF * IF (1)

In a high-power system, this can easily amount to hundreds of watts, which must be removed to keep the diode's junction temperature below its maximum rating. Properly managing this thermal load involves calculating the heat, selecting a heatsink with adequate thermal resistance, and often using forced-air or liquid cooling. Failure to manage heat is a primary cause of diode failure, and the combined electrical and thermal complexity makes pre-engineered, validated solutions a more reliable strategy.

Turn-Key Protection: Magna-Power’s Blocking Diode Solutions

Given the electrical, thermal, and mechanical complexities of implementing a robust blocking diode solution, a do-it-yourself (DIY) approach carries significant engineering and time investment. A far more reliable and efficient strategy is to deploy a fully engineered, validated solution. Magna-Power offers two distinct options designed to provide turn-key protection that seamlessly integrates with its programmable DC power supplies.

Integrated Blocking Diode (+BD) Option

For applications where seamless integration is paramount, Magna-Power offers the Integrated Blocking Diode (+BD) option on various models from 5 kW to 1 MW. This solution, specified at time of ordering and manufacturing, consists of a protection diode with an internally mounted, custom heatsink, all contained within the power supply chassis.

The key benefit of the Integrated Blocking Diode (+BD) option is its complete operational transparency. The power supply’s voltage sense feedback is connected at the output terminals, after the blocking diode. This means the power supply’s control loop automatically and continuously compensates for the diode’s forward voltage drop. To the user, the protection is invisible; the power supply delivers the programmed voltage at the output terminals with its full rated accuracy, without any need for manual offsets or complex calculations. It is the ideal “set-it-and-forget-it” solution for applications like DC motor drives, battery charging, and powering large electromagnets, providing reverse voltage protection up to 1200 Vdc.

The External 1U BDx Module Add-On

For the most demanding high-power applications, or for retrofitting existing systems, Magna-Power offers the BDx Module. This external, 1U rack-mount unit is a comprehensive, intelligent blocking diode system designed for ultimate robustness and safety.

Various BDx Modules are offered to provide protection for systems up to 1,200 Vdc and can handle continuous forward currents up to 1,200 Adc. It is a true turn-key solution, featuring fully integrated fan cooling with a universal AC input, heavy-duty tin-plated copper bus bars for low-loss connections, and a dedicated remote sense terminal to simplify wiring for voltage drop compensation.

What sets the BDx Module apart is its integrated intelligence. An onboard microprocessor actively monitors the internal heatsink temperature. This transforms the blocking diode from a passive component into an active safety system. The module provides a digital status output signal that can be connected to the main power supply’s user I/O interlock. If the BDx module’s cooling were to fail and it began to overheat, the status signal would change state, triggering the power supply’s interlock to safely shut down its output. This prevents the diode from failing due to overheating and ensures the primary power supply is never left unprotected. This level of integrated safety and system-level monitoring is nearly impossible to achieve with a simple DIY solution and represents a significant value-add for mission-critical test environments.

Magna-Power 1U BDx Module add-on with models available providing protection up to 1200 Vdc and up to 1200 Adc

Dissipating Excess Energy

A series blocking diode protects the supply by stopping reverse current, but some tests still need a way to dispose of the energy that briefly builds on the shared DC bus (for example, during motor-braking). Two simple, supply-agnostic add-ons solve this without resorting to a full bi-directional source: (1) a braking-resistor chopper, which diverts bus current into a power resistor whenever voltage exceeds a set threshold, turning the surplus into heat; and (2) a programmable DC electronic load in shunt-regulator mode (such as Magna-Power’s ALx Series), which clamps the bus by dynamically drawing just enough current to hold a user-defined voltage. Both devices sit in parallel with the bus, work alongside the blocking diode, and let you keep the cost-effective, single-quadrant architecture while safely handling those rare bursts of regenerative energy.

Distinguishing the Blocking Diode from a Free-Wheeling Diode

While the blocking diode protects the power supply, it is crucial not to confuse it with a free-wheeling diode, which provides local protection specifically for inductive loads. When current to an inductor (like a motor or solenoid) is suddenly interrupted, its collapsing magnetic field creates a large and potentially damaging voltage spike. A free-wheeling diode, placed in parallel with the inductive load or power supply output, gives this energy a safe path to circulate and dissipate, thereby protecting sensitive switching components like relays or transistors. The distinction is critical: the free-wheeling diode protects the local switch at the load, whereas the blocking diode is placed in series with the power supply to protect the power supply itself from any reverse energy. For a truly robust system, both are often required, one to manage the load's self-induced energy, and the other to prevent any of that energy from reaching the power source.

Conclusion

Applications involving DC motors, inductive loads, parallel power sources, and energy storage devices like batteries and capacitors have a potential for reverse energy flow that can lead to equipment damage, test invalidation, and potential safety hazards.

The blocking diode acts as a simple but effective electronic check valve to protect the power supply. However, the safe implementation of this "simple" component can be a complex engineering task. Proper selection requires careful consideration of electrical parameters like reverse voltage and forward current, while effective operation demands a robust thermal management solution to dissipate potentially hundreds of watts of heat.

With fully engineered and validated, system-integrated protection accessories like the Integrated Blocking Diode (+BD) option and the BDx Module, Magna-Power provides turn-key solutions for safe and reliable testing.

References

M. H. Rashid, Power Electronics: Circuits, Devices, and Applications, 3rd ed. Upper Saddle River, NJ: Pearson Education, 2003.

N. Mohan, T. M. Undeland, and W. P. Robbins, Power Electronics: Converters, Applications, and Design, 3rd ed. Hoboken, NJ: John Wiley & Sons, 2003.

R. W. Erickson and D. Maksimovic, Fundamentals of Power Electronics, 2nd ed. Norwell, MA: Kluwer Academic Publishers, 2001.

Z. Stokes and A. Kaknevicius, "Reverse Current Protection in Load Switches," Application Report SLVA730, Texas Instruments, Dallas, TX, May 2016.

ROHM Co., Ltd., "Reverse Voltage Protection, Application Note," Tech. Rep. No.16020EBY21, Rev. B, May 2016.

Originally published by Magna-Power Electronics