A current limit control circuit is an important protection feature implemented in power supplies, battery chargers, motor drives, and other applications where excessive current draw can damage components or cause hazardous conditions. This article provides an overview of current limiting methods, design considerations, and example circuits for implementing adjustable current protection.
What is Current Limiting?
Current limiting refers to techniques used to maintain the output or input current of a power supply or circuit below a preset maximum level. It serves to:
- Prevent overcurrent damage to circuits caused by shorts, inrush currents, or faults
- Limit current to safe levels avoiding risks like overheating, fires, or explosions
- Allow continued operation but at reduced levels during overload
- Improve system reliability and safety
Current limiting is implemented by measuring the current and actively controlling it to cap it before it exceeds desired limits.
Methods of Current Limiting
Several techniques can be used to achieve current limiting:
1. Power Supply Current Limiting
- Most basic method
- Built-in overload protection feature in power supplies
- Uses internal current sense resistor to monitor output current
- Feedback loop reduces or cuts-off output when current exceeds preset limit
2. Current Sensing with Pass Element
- Pass element like transistor controls current flow
- Separate sense resistor or amplifier measures actual current
- Feedback circuit regulates pass element to maintain current within limits
3. Foldback Current Limiting
- Reduces output voltage in response to increased load
- Maintains constant current over a wide voltage range
- Prevents thermal runaway as voltage drops under overload
4. Electronic Current Limiting
- Fast-acting solid state current sensing and control
- Uses op-amps, comparators, MOSFETs, etc. for tight regulation
- Permits multiple threshold levels and complex limiting profiles
Each approach has its advantages and limitations. The choice depends on control requirements, cost, speed, accuracy, and other factors.
Design Considerations for Current Limit Circuits
Some key parameters and factors to consider when designing current limit control circuits:
- Required current limit levels – Maximum current needed for normal loads and minimum sustaining current for overload conditions.
- Load characteristics – Steady state, inrush, and transient currents drawn by different loads.
- Limit response time – Fast response needed for safety while avoiding spurious triggering.
- Overload thresholds – Voltage/current thresholds to trigger limiting at various levels.
- Power dissipation – Heat dissipated in sense elements and pass devices.
- Protection coordination – Limiting coordinated with other protections like fuses, breakers.
- Monitoring and status – Indicators and flags for system monitoring and diagnostics.
- Operational modes – Settings for normal/fault modes, latched/auto-reset operation etc.
- Component tolerances – Tolerances and drift affecting accuracy and repeatability.
- Environmental factors – Temperature, humidity, noise and their impact on precision.
- Safety standards – Compliance with relevant safety requirements.
Example Current Limit Control Circuits
Some example implementations of current limit control circuits are:
1. Simple Current Limiting for DC Power Supply
- Low-side pass transistor as variable resistor to limit current
- Voltage across sense resistor proportional to load current
- Amplified error compared to reference voltage sets transistor base drive
- Transistor collector-emitter resistance increased to maintain current within limit
2. Dual-Threshold Current Limiting
- Provides both power and fault current limiting levels
- Binding post shunts op-amp during power-up to allow higher inrush
- Once powered, feedback loop limits to continuous current rating
- Comparator detects overcurrent and drives transistor to much lower fault limit
3. Precision Programmable Current Limiting
- Microcontroller digitally programs DAC based on settings
- DAC sets reference voltage for transconductance error amplifier
- Amplifier drives MOSFET pass element controlling output current
- Closed loop feedback ensures load current matches DAC level
- Enables multiple programmable thresholds for smart limiting
These demonstrate implementations ranging from basic to sophisticated for different requirements.
Setting the Current Limit Threshold
The current limit threshold is adjusted using:
- Sense resistor – A higher value resistor results in larger voltage drop for same current. Reducing sense resistor value increases the current limit.
- Reference voltage – The reference voltage applied to the comparator or amplifier determines the maximum current allowed before limiting action is triggered.
- PWM/Duty cycle – In switch mode power supplies, the peak inductor current is controlled by PWM duty cycle. Lower duty cycle reduces peak current level.
- Digital potentiometer – Digitally controlled potentiometers act as adjustable voltage dividers. Changing wiper position modifies reference voltage and current limit.
- DAC output – Digital-to-analog converters generate reference from microcontroller code. Provides programmable current limit levels.
Implementing Adjustable Current Limiting
Making the current limit user-adjustable can be useful in some applications. Some options are:
- Variable sense resistor – A potentiometer or rheostat acts as variable sense resistor. Rotate to increase/decrease resistance and current limit.
- Reference potentiometer – Potentiometer forms an adjustable voltage divider for the reference input to op-amp/comparator.
- Digital potentiometer – SPI/I2C controlled digitals pots change wiper position to tune current limit.
- External adjustable voltage – Apply external variable DC voltage to set reference level and current limit threshold.
- User interface – Buttons, knobs, displays on microcontroller/PLC interface provide adjustable settings.
Careful circuit design is needed to prevent noise, drift, and loading effects that can impact adjustability.
Enhancing Accuracy of Current Limiting
For more accurate current limiting, some enhancement techniques include:
- Precision sense resistor – Use tight tolerance sense resistors with low temperature coefficient.
- Input filtering – Add filtering to reduce noise on sense voltage input.
- Chopper stabilization – Chopper amplifiers minimize offset voltages and drift errors.
- Calibration – Measure and trim offset voltages/reference for maximum accuracy.
- Current mirror -Mirror reference current to bypass sense resistor and its errors.
- Multiple sense elements – Average readings from multiple points to negate localized errors.
- Feed forward compensation – Compensate for load dependent voltage drops in real-time.
Applications of Current Limit Control Circuits
Some example applications where current limiting circuits are widely used:
- Switch mode power supplies – Limits peak inductor current in converters.
- Battery chargers – Prevents damage from excessive charging currents.
- LED drivers – Limits LED current for thermal management.
- DC motor drives – Protects against stalled rotor overcurrents.
- Traction systems – Prevents excessive acceleration currents.
- Audio amplifiers – Safeguards against short circuits in speaker lines.
- Electroplating – Regulates current density for uniform plating.
- Lab power supplies – User-adjustable protection against overloads.
- Fault current limiters – Blocks very high fault currents while allowing lower nominal currents.
Current limit control provides vital protection in a wide range of power electronic systems. Designing an effective circuit requires balancing factors like response time, precision, stability, cost, and compliance needs. Classic linear circuits and contemporary solutions leveraging digitally controlled components both offer options for implementing adjustable, multi-mode, and smart current limiting capabilities.
Frequently Asked Questions
What are some key benefits of implementing current limiting?
Current limiting improves reliability and safety by preventing damage from excessive currents. It allows continued reduced operation during temporary overloads. It also reduces risks like overheating, fires, or explosions.
How are different current limit thresholds set in a power supply?
The peak or average output current level at which limiting occurs is adjusted by changing the sense resistor value, reference voltage, PWM duty cycle, digital potentiometer setting or DAC code.
What is foldback current limiting?
Foldback limiting reduces the output voltage proportional to increasing load current, maintaining a relatively constant current over a wide voltage range. This prevents thermal runaway.
Which current limiting method offers the fastest response time?
Electronic current limiting using op-amps, comparators and MOSFETs offers very fast response as it is all solid state. This enables protection down to microsecond timescales.
What parameters influence the accuracy of current limiting?
Sense resistor tolerance and thermal drift, amplifier offsets, noise pickup, reference voltage errors, timing variations in digitally controlled circuits and calibration all affect accuracy.
The constant current control circuit, the output end of the transformer T is connected to the rectifier circuit, wherein the input end of the transformer T is connected with a switch control circuit, the rectifier circuit is connected with the resistor R3, the emitter junction of the triode VT is connected in parallel with the resistor R3, and the set of the triode VT The electrical junction is connected in series with the light-emitting tube of the photocoupler LV, and the output end of the photocoupler LV is connected to the transformer T through a switch control circuit.
The following figure is the circuit diagram of DC or battery input, and constant current control for 6 strings of LEDs.
It uses a multi-string boost structure design, the LED pcb driver is similar to the previous one, the difference is that the AC input is changed to DC or input by the battery. Low-voltage side sensing design As long as the appropriate MOS tube is selected, the LED can be stringed in a considerable number.
Compared with the AC input scheme, the design is relatively simple. However, due to the extra boost switch, the efficiency is relatively low. The advantage of the scheme is that the design is simple, the circuit cost is low, and the disadvantage is that the efficiency is low.
It is suitable for solar cells or street lights that are input via an adapter.