I'm not sure if pwm is desirable or even needed. Here are three
examples of circuits that could be used depending on the performance
needed:
1. Capacitor Discharge
The schematic is shown in
http://www3.sympatico.ca/add.automation/misc/sol01sch.gif
The capacitor C1 is charged to 48 Volts through R1. Q3 connects the
bottom end of the solenoid coil L1 to ground. Capacitor C1 then
discharges through the solenoid to increase the initial current
rise. When the voltage across the capacitor falls below 12V, diode
D1 turns on to provide sustaining current to keep the solenoid
closed. Diode D2 and R6 limit the back emf voltage when the driver
transistor Q3 turns off.
The waveforms are shown in
http://www3.sympatico.ca/add.automation/misc/sol01wfm.gif
The top graph shows the capacitor voltage in red and the coil
voltage in blue. The bottom graph shows the solenoid current in
black.
This method offers good performance for low duty cycle applications.
The rep rate is limited by the time required to charge C1.
Considerable power is dissipated in R1 when the solenoid is
activated.
2. Switched Capacitor
The schematic is shown in
http://www3.sympatico.ca/add.automation/misc/sol02sch.gif
When the solenoid is not activated, transistors Q1 and Q2 keep the
capacitor C1 charged to 48 Volts.
The waveforms are shown in
http://www3.sympatico.ca/add.automation/misc/sol02wfm.gif
As in the previous example, the top graph shows the capacitor
voltage in red and the coil voltage in blue. The bottom graph shows
the solenoid current in black.
This circuit provides good performance for high rep rate
applications. A small amount of power is wasted keeping transistor
Q1 turned on when the solenoid is not activated.
3. Pulsed Voltage
The schematic is shown in
http://www3.sympatico.ca/add.automation/misc/sol03sch.gif
The R3, C2 network in the emitter of Q2 form a one-shot that briefly
applies 48 Volts to the solenoid when Q3 turns on. The D2, R4, D3
network provide protection against the back emf generated when Q3
turns off.
The waveforms are shown in
http://www3.sympatico.ca/add.automation/misc/sol03wfm.gif
As in the previous examples, the top graph shows the capacitor
voltage in red and the coil voltage in blue. The bottom graph shows
the solenoid current in black.
This circuit provides the fastest current rise into the solenoid,
since the voltage is held at a constant 48 Volts instead of decaying
as in the previous examples. The current fall time could be
increased by allowing a higher back emf when transistor Q3 turns
off. The quiescent current is essentially zero.
These examples are intended to show the basic operation. Additional
effort may be required to meet actual system requirements such as
worst-case supply voltages, temperature extremes, component
variation, etc.
However, it is difficult to see how pwm could provide higher
performance and lower component count.
Mike Monett