Simple circuit controls the rate of voltage change across a capacitor or another load
The output rate of change servos a pass FET.
Fabien Dubois, Ampere Lab, Lyon, France; Edited by Paul Rako and Fran Granville -- EDN, September 22, 2011
The circuit in this Design Idea lets you set a well-controlled voltage rate of change, often expressed as the differential dV/dt (instantaneous rate of voltage change over time in volts per second). You can vary the sensitivity with a potentiometer. Set the dV/dt from 1V/200 nsec to 1V/3 msec. The input voltage can range from a few volts to 30V. Higher-voltage transistors can be used to increase the upper voltage limit. The circuit precharges a capacitor with a slow and controllable dV/dt to avoid a large inrush current during power-up. You can also use the circuit to create a high dV/dt for susceptibility testing on other circuits.
The circuit uses a P-channel
MOSFET, Q1, to control the rate of
change of the output voltage (Figure
1). You drive the MOSFET with a constant-current source comprising Q4 and
RCS, which feeds gate-to-source resistor
RGS. Applying a positive control voltage
to the base of Q4 draws a current that
creates a voltage across RGS. This voltage
occurs across the gate and source
of Q1, turning it on. The circuit uses
capacitor CDVS as a sensing device of
the rate of change of the output voltage.
Voltage variations across CDVS generate
a current that creates a current proportional
to the dV/dt, as the following
equation shows:
Resistor RDVS converts this current
into a voltage signal. When that voltage
reaches approximately 0.67V, it turns
on Q2, which turns on Q3. The current
that Q3 supplies from the input
tends to lower the Q1 gate-to-source
voltage and reduces its drive. You use
RB to limit the base current of Q2. 
This
servo action puts the gate-to-source
voltage of the MOSFET in the Miller
plateau, a constant-current region of
the FET’s characteristic curve. The FET
has an internal Miller capacitance, CGD,
between the gate and the drain pins. The circuit’s constant-current source
controls the charge current of this
Miller capacitance. As transistor Q3
injects current to the gate, Miller current
IGD decreases and the slope of the
output voltage decreases accordingly, as
the following equation shows:
This
servo action puts the gate-to-source
voltage of the MOSFET in the Miller
plateau, a constant-current region of
the FET’s characteristic curve. The FET
has an internal Miller capacitance, CGD,
between the gate and the drain pins. The circuit’s constant-current source
controls the charge current of this
Miller capacitance. As transistor Q3
injects current to the gate, Miller current
IGD decreases and the slope of the
output voltage decreases accordingly, as
the following equation shows:
Talkback
-
This circuit would be applicable connected directly to a load, with whatever capacitor was required also connected directly to the load. That would remove the potential for the uncontrolled inrush current.
Of course there are some other techniques that can be used, some which are much simpler. One example is for the limiting of the filament current cold inrush found in large (expensive) transmitting tubes. The solution was to design the transformer magnetic circuit so that it was not able to provide more current than was desired. That works quite well and it does not require extra circuitry. It is a bit less applicable to rectifier circuits, although it can provide a bit of inrush limitation there as well.
William Ketel - 2011-31-12 12:25:01 PST -
If we are trying to limit the inrush to the load as well as the capacitors, then enabling a relay after the capacitors are charged will not limit the inrush to the load. There may be capacitors in the load as well.
How are you doing Suresh--long time no see
Richard Cuplin - 2011-27-9 05:59:04 PDT -
One must never connect a huge Load to such circuits directly, but, connect the Load to this circuit through a Relay that switches contacts only when the coils reach a certain threshold voltage. Such relays ar enot the average elctronic or solid state relays but spring-loaded mechanical relays, which are still in use for large electrical loads such as Motors and Solenoids. By using a Relay, the voltage across the contacts can be anything including AC or DC.
Suresh Doraiswamy - 2011-26-9 22:17:03 PDT -
Be careful that load is not attached or is not excessive during the charging of the capacitor. The FET will dissipate voltage drop times load current. If load is pure reisistive, this is voltage drop times (input voltage - voltage drop)/Rload. If you stretch time too much, you may thermally damage the FET.
Richard Cuplin - 2011-26-9 09:52:40 PDT
