How to Handle Non-Linear Loads
on a Power Supply
Overview
» Powering Non-Linear loads can present many
challenges with conventional analog control.
Using a power supply with digital control can simplify
this process and result in more reliable performance.
What is a Non-Linear Load?
» A non-linear load is one that does not behave like an ideal resistor
» The current drawn from the power supply is not proportional to voltage
» Initial currents are often much higher than the rating of the power supply
» Such kind of loads are seen in many applications
» Large switched capacitor banks
» Point of Load DC-DC converters
» Thermal printers
» DC motors
Non-Linear Loads and Current Limit
» Over Current Protection (OCP) is an essential feature for a power supply
» Power supplies are usually expected to recover automatically
» No manual intervention!
– Some exceptions of course
» There are several basic methods used
» Constant Current
» Fold Back
» Fold Forward
» Hiccup
Types of Current Limit
» Constant Current
» When the current drawn reaches the OCP limit, the voltage falls
– Current provided remains fixed down to short circuit
» This type of protection is not well suited for delivering peak loads!
Types of Current Limit
» Fold back
» When the current drawn reaches the OCP limit, the voltage falls
– Current decreases as the load gets heavier
» This type of protection is not well suited for delivering peak loads!
– Can result in the power supply latching off with peak loads
Types of Current Limit
» Fold forward
» When the current drawn reaches the OCP limit, the voltage falls
– Current provided increases to a maximum at short circuit
» Fold forward is well suited for powering up motors
– Requires heavier system load cabling
Types of Current Limit
» Hiccup
» At the OCP limit, the power supply turns off for a short interval
– Automatically tries to restart
– Reduces the need for heavy cabling or pcb traces
» This type of protection can be modified to deliver a peak load
» Analog control can be modified, but the recovery timing is fixed
» Digital control allows for variable timing; for example:
» 10s for an overload condition
» 60ms for heavy overloads
» 5ms for a short circuit condition
» Recovery times typically would be 1 to 2 seconds
Analog Control under a Peak Load
» Below shows a discharged capacitor switched onto an operating supply
» Lower blue trace is the power supply current; restarting twice
– Current limiting at around 60A
» Top gold trace shows the output eventually recovering
– But hiccup mode could have prevented recovery
Digital Control under a Peak Load
» A discharged capacitor is switched into a TDK-Lambda CFE400M supply
» The blue trace is the current, gold trace is the output voltage
» Using Digital Control, we can set the thresholds accurately
» 50A for 1.5ms (Short circuit) & then 30A (over current) for 50ms
Summary
» Digital control can allow for precise and repeatable current limiting
» Using load dependant timing
» Not restricted to the value of a timing capacitor
– Capacitor tolerance with different batches
– Aging of a capacitor
– Capacitor values changing with temperature
» Can be easily tailored for different applications
» No physical modification of the power supply is needed
» All changes handled with software programming!
Digital Power Conversion
Benefits of Improved Current Sharing
Overview
» Flexibility is one of the major advantages of digital
control in power supplies
» This presentation explains how current sharing can be
improved significantly without compromising reliability
Current Sharing - Brute Force Mode
» Connecting power supplies in parallel with no current share results in
» The power supply with the highest voltage supplying most of the load
» The other power supply delivers current when the first current limits
» The first power supply runs hotter and its life is reduced
Current Sharing - Droop Mode
» Droop current share is a simple way of operating in supplies in parallel
» Does not require a current share wire between power supplies
» No single point of failure in N+1 redundant operation
» Better low load current sharing
» Less complicated
» Droop mode is implemented by increasing the power supply load regulation
Proprietary Software Use
» TDK-Lambda’s EFE series has digital control
» Uses IP protected algorithms to enhance performance
» The 50% load point is set at +5/10% of nominal output voltage
» The load regulation can be programmed either positively or negatively
– The algorithm ensures the correct voltage range is maintained
Transient Loading
» Newer products often employ resonant topologies
» Go in to discontinuous mode at light loading
– Results in high effective output impedance
• Needing more output capacitance for transient loading
» TDK-Lambda’s EFE300-M topology avoids this
» Below we can see the effect of a 0-100% load change on a 12V unit
Summary
» Digital control can allow for more reliable current sharing
» The capacitance across the load can be minimized
» Can be easily tailored for different applications
» No physical modification of the power supply is needed
» All changes handled with software programming!
» Speeds up time to market