I was recently tasked with doing a presentation on the
advantages of power supplies incorporating Digital Control to power non-linear
loads, so I thought I would share the content with you.
A non-linear load is one that does not behave like an ideal
resistor, in that the current drawn from the power supply is not proportional
to voltage and/or the initial currents are often much higher than the rating of
the power supply.
These loads can cause problems for power supplies, but are
actually present in many applications:
Large switched capacitor banks
Point of Load DC-DC converters
Thermal printers
DC motors
The main issue from a power supply’s point of view is that
the load can activate the internal over-current protection. Over-current protection (OCP) is an essential
feature for a power supply, but the power supply is usually expected to recover
automatically with no manual intervention.
So to start with, let’s look at the types of OCP. There are
several basic methods used:
Constant Current
Fold Back
Fold Forward
Hiccup
Constant Current
When an overload condition occurs, the output voltage falls but
the output current remains at a fixed level.
This type of protection is not well suited for delivering peak loads as
it can lead to the power supply latching.
Fold back
When the current drawn reaches the OCP limit, the voltage
falls, but this time the current decreases as the overload gets heavier. Again this type of protection is not well
suited for delivering peak loads as it can result in the power supply latching.
Fold forward
When the current drawn reaches the OCP limit, the voltage
falls. This time the output current
increases to a set maximum at short circuit.
Fold forward is well suited for powering up motors, but requires
heavier system load cabling to handle the additional overload current.
Hiccup
At the OCP limit, the power supply turns off for a short
interval and then automatically tries to restart. Hiccup mode reduces the need for heavy
cabling or pcb traces, and this type of protection can be modified to deliver a
peak load.
With traditional Analog Control though, the OCP points and
recovery timing are fixed.
With Digital Control we can use software settings to adjust
the limits and timing; for example we can set:
10s for an initial overload
condition
60ms for heavy overloads
5ms for a short circuit condition
with recovery times or 1 to 2 seconds
Let’s take an application example of a discharged capacitor bank
being switched onto an operating power supply with Analog control.
The lower (blue) trace is the power supply current;
restarting twice with the power supply current limit set at around 60A.
The top (gold) trace shows the output eventually recovering,
but tolerances with the hiccup mode timing could have prevented a full recovery.
This time the same discharged capacitor bank is switched
into a TDK-Lambda CFE400M supply incorporating digital control.
The blue trace is the current, gold trace is the output
voltage
Using Digital Control, we can set the thresholds and timing accurately.
50A for 1.5ms (The short circuit
condition)
30A for 50ms (The over current
condition)
Notice that there are no multiple attempts to recover after
the capacitor bank is applied to the power supply.
To summarize:
Digital control can allow for precise and repeatable current
limiting using load dependant timing. We
are not restricted to the value of a timing capacitor which can change due to:
a) Batch tolerances
b) Aging of the capacitor
c) Capacitor values changing with
temperature
Digital control allows for easy tailoring for different
applications with no physical modification of the power supply is needed. All changes are handled with software
programming!
Power Guy
