SOLENOID DRIVER CIRCUIT
Technical Field
[0001] The present invention relates to solenoid driver circuits, and more
particularly to a solenoid driver circuit that captures and stores energy that is later re-
used in the circuit.
Background Of The Invention
[0002] For fast solenoid actuation, it is desirable to increase and decrease the
inductor current through the solenoid as quickly as possible. For conventional driver
circuits (i.e., high-side and low-side drivers), the rise and fall rates of the inductor
current is determined by the voltage applied to the solenoid coil inductor-resistor time
constant L/R, with L = the inductance of the solenoid coil and R = the resistance of the
coil..
[0003] There is a desire for an improved solenoid driver that improves the
actuation speed, controllability and energy efficiency of a solenoid. There is also a
desire for a solenoid-operated spool valve having enhanced controllability and actuation
time.
Summary of the Invention
[0004] The invention is directed to a solenoid drive circuit that includes a boost
energy storage device that absorbs energy from and discharges energy to a solenoid.
Switching devices control the connection between the boost device, the solenoid, and a
power source. This allows the voltage excitation to the circuit, and therefore the
solenoid response time, to be variable based on the characteristics of the boost device
as well as the solenoid. By providing two different solenoid rise and decay rates and by
capturing and re-using energy stored in the solenoid, the inventive drive circuit
enhances solenoid response and increases efficiency.

Brief Description of the Drawings
[0005] Figure 1 is a representative schematic diagram of a drive circuit according
to one embodiment of the invention.
[0006] Figure 2 is a flow diagram illustrating a solenoid current control process
according to one embodiment of the invention;
[0007] Figure 3 is a representative schematic diagram of a drive circuit according
to a further embodiment of the invention;
[0008] Figure 4 is a representative schematic diagram of yet another embodiment
of the invention;
[0009] Figure 5 is a representative schematic diagram of another embodiment of
the invention; and
[0010] Figure 6 is a flow diagram illustrating a solenoid current control process
according to another embodiment of the invention.
Detailed Description of the Embodiments
[0011] A circuit according to the invention includes a boost energy storage
device, such as a capacitor, that supplies boost energy to a solenoid. This additional
circuitry provides faster solenoid current rise and decay rates than a conventional high
or low side drive circuit. More particularly, the current rise and fall times in the inventive
circuit is not determined by the L/R time constant. Instead, the times are determined by
the time required for the capacitor to discharge completely into the solenoid coil
inductance or absorb the energy from the inductance. The time constant t1 is less than
or equal to around 1.57 x (L x C)1/2 seconds, where L = the inductance of the solenoid
coil and C = is the capacitance of the energy storage device. Note that although the
examples below assume that the energy storage device is a capacitor, other devices
may be used without departing from the scope of the invention.
[0012] The increased voltage provided by the energy storage device provides a
faster initial rise rate and a faster ending fall rate for the solenoid, creating a quicker
solenoid response at the beginning and end of solenoid actuation. Response times of
less than t1 = 1.57 x (L x C)1/2 seconds may be obtained by using a high capacitor

voltage and shutting off the discharge before the capacitor is completely discharged to
Vbattery. Thus, the discharge may be either partial or complete, depending on the desired
response speed. This allows the current in the solenoid coil inductor to increase faster
and not be restricted by the conventional L/R time constant. The switching time may
also be determined by the solenoid current as well as the capacitor voltage.
[0013] The solenoid in the circuit may be driven using pulse width modulation
(PWM), allowing the current in the solenoid to be controlled at a level that is less than
the final DC value V/R (supply voltage divided by solenoid resistance) dictated by the
solenoid 104. As a result, the circuit 100 is flexible enough to operate using the slower
L/R time constant to facilitate PWM operation. The ability for the circuit 100 to change
solenoid current rise and decay times of different speeds provides increased drive
control over the solenoid.
[0014] Figure 1 is a simplified schematic diagram of a circuit 100 according to
one embodiment of the invention. Figure 2 illustrates a process of controlling solenoid
current using various embodiments of the circuits described herein.
[0015] Referring to Figure 1, the circuit 100 includes a power source 102, such as
a battery or power supply, that provides energy to drive a solenoid coil 104. The circuit
100 also includes a boost energy storage device C1, such as a boost capacitor or other
device, two switches S1, S2, and two diodes D1, D2 that direct current through the
circuit 100. The switches S1, S2 may be of any type, such as a semiconductor switch,
such as a metal-oxide field effect transistor (MOSFET), a field effect transistor (FET), a
bipolar junction transistor (BJT), a silicon controlled rectifier (SCR), or an insulated gate
bipolar transistor (IGBT). The switches S1, S2 are controlled by control logic in a switch
controller 150, which may be an analog circuit or a controller that controls the various
operating modes in the circuit 100 via hysteresis switching or any other appropriate
control strategy.
[0016] In this embodiment, the cathode of one of the diodes D1 is connected
between the first switch S1 and the solenoid 104 and the anode of the diode D1 is
connected is connected to the positive terminal of the power source 102. This
configuration therefore allows partial discharge of the solenoid 104 to provide rapid

actuation. Figure 1 also shows current paths at various stages of circuit operation,
which will be explained in greater detail below.
[0017] Referring to Figure 1 and 2, both of the switches S1, S2 are in an open
state during an initial operating state (block 201). It is assumed that energy is stored in
the boost capacitor C1 at this state. When the switches S1, S2 are closed, current flows
from the boost capacitor C1 through both of the switches S1, S2 and the solenoid 104,
as indicated in Figure 1 as current path 1 (block 202). As current flows, the boost
capacitor C1 discharges at a rate that is determined by the size of the boost capacitor
C1 and the size of the solenoid 104 until the boost capacitor C1 voltage reaches the
battery voltage. The size of the capacitor C1 is selected based on the value of L/R and
the desired circuit response speed, and varying the capacitor C1 size changes the
circuit 100 operation.
[0018] For example, if the capacitor C1 and the solenoid 104 are both small, the
capacitor C1 will fully discharge when it reaches the battery voltage. Because the
capacitor voltage and the battery voltage are at similar levels, the changes in the current
level will be slower as it approaches the target current.
[0019] If the capacitor C1 is large and the solenoid is small 104, however, the
capacitor C1 will only partially discharge and remain above the battery voltage. A larger
capacitor C1 enables faster response times in the circuit 100 by maintaining the
capacitor voltage at a higher level. As a result, the circuit 100 will reach the target
current at a faster rate.
[0020] At this point, the controller 150 instructs the first switch S1 to open,
causing the first diode D1 to start conducting current (block 203). The current through
the solenoid 104 rises and travels through current path 2 at a slower rate. Note that this
stage is optional; if a faster current rise time is desired, the boost capacitor C1 may be
charged to a higher level so that the capacitor voltage is kept high and reaches the
battery voltage before it is completely discharged, allowing the target current level to be
reached at a faster rate.
[0021] When the current in the solenoid 104 has reached a final desired level, the
second, lower switch S2 opens and the first switch S1 is closed (block 204). The

magnetic field in the solenoid 104 inductance "collapses,"" causing the inductor current
to recirculate through the solenoid 104 to maintain the magnetic field of the solenoid
104. This in turn forces the current to flow through the second diode D2, which acts as a
steering diode, according to current path 3. At this point, the current level gradually
drops at a slower rate due to resistive losses in the circuit 100. When the current has
decreased to a desired second, lower level, the controller S2 closes the second switch
S2 and opens the first switch S1, causing the first diode D1 to conduct supply current
from the battery 102 and direct current according to current path 2 again to increase the
solenoid current level (block 205). The level at which this occurs can be selected and
controlled by the controller 150 based on, for example, the system's tolerance to current
ripple, switching losses, noise generation, etc.
[0022] Thus, the current in the solenoid 104 can be controlled to conduct PWM
operation. In one embodiment, the controller 150 obtains the PWM action at the slower
rate by alternately opening and closing the switches S1, S2 out of phase with each
other, causing the solenoid current to toggle between current path 2 (charging the
solenoid 104 from the battery 102) and current path 3 (recirculating the current from the
solenoid to the capacitor C1) (block 206).
[0023] To improve operating efficiency, the inventive circuit 100 may recover and
re-use magnetic energy stored in the inductance of the solenoid 104 after the solenoid
104 has been actuated. The energy is captured in the boost capacitor C1 and re-used
during the next solenoid actuation. This energy capture can be conducted when the
solenoid current is dropped rapidly to zero. More particularly, it is desirable to have the
current level respond according to the first, faster time constant t1. To do this, the
controller 150 opens both of the switches S1, S2 to drain current from the solenoid 104
into the boost capacitor C1 through current path 4 and both of the diodes D1, D2 (block
207). The boost capacitor C1 will charge to a voltage level higher than the battery 102
voltage; the exact level is controlled by the inductance of the solenoid 104, the amount
of current flowing through the solenoid 104 during discharge, and the capacitance.
[0024] Note that the battery 102 also helps recharge the boost capacitor C1
because it is placed in the solenoid discharge path in the circuit 100. As a result, the

inventive circuit 100 conducts current rise and decay at a first fast rate and at a second
slow rate, depending on the specific circuit configuration. This improves the response
time and control over solenoid operation. Moreover, the circuit configuration also
improves efficiency by using energy captured during discharge of the solenoid.
[0025] As noted above, the operation of the circuit 100 in Figure 1 can be varied
by changing the storage capacity of the energy storage device C1. If a larger capacitor
C1 is used in the circuit 100 of Figure 1, it is possible to achieve even faster actuation
times due to the increased capacitor storage capacity. The capacitor C1 in this cases
reaches a voltage that is higher than the battery 102 voltage and acts as a boost
voltage source for the solenoid 104. This increased storage capacity allows the
capacitor C1 to discharge only partially rather than completely, supplying current to the
solenoid 104 at a near constant voltage and at a faster rate than the circuit of Figure 1
until the solenoid current reaches a desired level.
[0026] Using a larger capacitor C1 also allows recapture of discharged energy
from the solenoid 104 into the boost capacitor C1. In this case, however, opening both
of the switches S1, S2 to rapidly reduce the solenoid current to zero forces the solenoid
voltage to increase to Vsolenoid = Vcapacitor + I x R - Vbattery. This increase causes the
solenoid 104 to transfer its magnetic energy to the boost capacitor C1 at a faster rate
than the circuit in Figure 1 because the initial voltage of the capacitor C1 is higher than
the battery voltage due to the partial discharge of the capacitor C1.
[0027] Figure 3 shows another possible embodiment of the inventive circuit 100.
As described above, the inventive circuit 100 may use magnetic energy recovered from
solenoid discharge to increase the actuation speed of the solenoid 104 during a later
operation cycle. In practice, however, the energy that can be retrieved from the solenoid
104 and stored in the boost capacitor C1 is often less than the energy actually required
for operation due to resistive losses, eddy current losses, and core losses. As a result,
additional energy needs to be supplied to the boost capacitor C1 after each solenoid
actuation to maintain a high actuation speed.
[0028] To achieve this, the circuit 100 in Figure 3 includes a comparator 250 that
is coupled to the switch controller 150. The general operation of the circuit 100 is the

same as described above with respect to Figure 2 with additional steps marked in
Figure 2 in dotted lines. In this embodiment, before the solenoid 104 is actuated, the
comparator 250 first checks whether the voltage across the boost capacitor C1 is less
than the desired boost voltage (block 254). If so, it indicates that the energy discharged
from the previous solenoid actuation is not enough to increase the solenoid actuation
speed sufficiently for the current operation.
[0029] To increase the energy stored in the boost capacitor C1, the switch
controller 150 opens and closes the second switch S2. Closing the second switch S2
causes more current to flow from the battery 102 to the solenoid 104 via current path 2,
while opening the second switch S2 causes the current created from the collapsing
magnetic field in the solenoid 104 to flow into the boost capacitor C1 for storage via
current path 4. The controller 150 continues to open and close the second switch S2 to
charge the boost capacitor C1 until the comparator 250 indicates to the controller 150
that the capacitor voltage has reached the desired boost voltage value (block 256). At
this point, the controller 150 opens the second switch S2, and the process in Figure 2
continues as described above. As a result, this embodiment allows the solenoid 104 to
act as an effective voltage boost source for the capacitor C1.
[0030] Figure 4 shows a circuit 100 according to yet another embodiment of the
invention. This circuit 100 is designed so that the capacitor completely discharges when
it supplies current to the solenoid 104. Like the embodiments described above, the
inventive circuit 100 has a time constant that is determined by the time needed for the
boost capacitor C1 to discharge energy to or absorb energy from the solenoid 104
rather than strictly according to the L/R time constant. This embodiment differs from the
embodiment shown in Figure 1 by placing an additional diode D3 in current path 3,
which directs current when the magnetic field in the solenoid 104 collapses, and moving
the location of diode D1 to a location above the switch S1. This circuit isolates the
capacitor C1 across the solenoid 104 rather than placing it in series with the battery 102
as in Figure 1. This results in a circuit 100 that has a faster response during coil turn-off.
[0031] The circuit 100 in Figure 4 operates in the manner described above in
Figure 2. In this embodiment, the boost capacitor C1 charges to a voltage level based

on the energy stored in the solenoid 104, less the voltage drop across diodes D2 and
D3. Note that in this embodiment, the voltage level that the boost capacitor C1 can
reach is lower than the voltage that the boost capacitor C1 can reach in Figure 1
because the new position of the diode D1 prevents the solenoid 104 from being
repetitively charged and discharged to increase the capacitor C1 voltage in this circuit
100.
[0032] Figure 5 illustrates yet another embodiment of the inventive circuit 100.
This embodiment is similar to the embodiment shown in Figure 4 except that it includes
an additional switch S3 disposed in parallel with the additional diode D3 and a
demagnetization storage device C2, such as another capacitor, disposed in series with
the additional diode D3. This creates two additional circuit paths, which will be
described in greater detail below. Figure 6 is a flow diagram illustrating the operation of
the circuit in Figure 5. Note that the diode D3 and the switch S3 may be combined into
one device, such as a MOSFET.
[0033] Referring to Figures 5 and 6, the circuit 100 has all three switches S1, S2,
and S3 open at the start of its operational cycle (block 300). It is assumed that both the
energy boost capacitor C1 and the demagnetization capacitor C2 are both charged to
nominal operational values at this stage.
[0034] The third switch S3 is then closed just before the solenoid 104 is to be
actuated, causing current to flow from the demagnetization capacitor C2 through the
solenoid 104 via current path 6 (block 302). In one embodiment, this step demagnetizes
the solenoid 104. The demagnetization can be conducted by, for example, applying
current through the solenoid that is either a pulse or a decaying sinusoid, depending on
the size of the demagnetization capacitor C2. If the demagnetization capacitor C2 is
large (e.g., greater than 10% of the boost capacitor C1 value), then the third switch S3
will close for a short time (e.g., tens of microseconds) to conduct pulse
demagnetization. If the demagnetization capacitor C2 is small (e.g., on the order of 1%
to 10% of the boost capacitor C1 value), then the switch S3 will close for a longer time
period (e.g., several milliseconds) to conduct decaying sinusoid demagnetization. Note
that during sinusoid demagnetization, the demagnetization capacitor C2 will completely

charge and discharge with an alternating polarity and decreasing amplitude through
current paths 5 and 6 at this step (block 302).
[0036] After the solenoid 104 has been demagnetized, the third switch S3 opens
and switches S1 and S2 close to start solenoid actuation (block 304), causing current to
flow from the boost capacitor C1 through the two closed switches S1, S2 and the
solenoid 104 via current path 1. Like several of the embodiments described above, the
boost capacitor C1 in this embodiment has a voltage much higher than the battery 102
voltage and sufficient capacity to discharge only slightly while supplying current to the
solenoid 104 at a near-constant voltage until the solenoid current reaches a desired
level. Once this occurs, the first switch S1 is opened, conducting current through diode
D1 via current path 2 at a slower rate as described above in the previous embodiments
(block 306).
[0036] The remaining steps 308, 310,312 and 314 in the process of Figure 7 are
the same as blocks 204,205, 206 and 207 of Figure 2. Note that when the first and
second switches S1 and S2 are opened at the end of the process to rapidly reduce the
solenoid current to zero, the solenoid voltage increases to (Vboost capacitor + Vdemagnetization
capacitor)) + (I x R) - Vbattery (block 314). This causes the inductor to transfer its magnetic
energy to both the demagnetization capacitor C2 and the boost capacitor C1. The
demagnetization capacitor C2 changes to a voltage that is approximately equal to Vboost
capacitor- Vbattery. The battery 102 can also help charge the two capacitors C1, C2
because it is in the discharge path.
[0037] The circuits above can be used in any application using solenoid valves.
For example, the driver circuit may be used to enhance controllability of a spool valve
by demagnetizing the spool and an end cap so that the spool can move to another
position. Those of ordinary skill in the art will recognize that the inventive circuit can be
used in other applications without departing from the scope of the invention.
[0038] By incorporating inductor-capacitor energy transfer principles in the drive
circuit, the invention increases the actuation speed of a solenoid driven by the circuit
and provides selectable time constants to improve PWM capability. Moreover, capturing
and re-using stored energy in the inventive circuit improves the energy efficiency of the

circuit. A spool valve operating according to the inventive principles experiences a
decreased actuation time and enhanced controllability. Those of ordinary skill in the art
will understand that the switching time in the inventive circuit can be controlled or
modified based on the response of the solenoid or the response of other portions of the
system, (e.g., spool response, pressure rate rise, system downstream behavior, etc.).
[0039] The foregoing description is exemplary rather than defined by the
limitations within. Many modifications and variations of the present invention are
possible in light of the above teachings. The preferred embodiments of this invention
have been disclosed, however, one of ordinary skill in the art would recognize that
certain modifications would come within the scope of this invention. It is, therefore, to be
understood that within the scope of the appended claims, the invention may be
practiced otherwise than as specifically described. For that reason the following claims
should be studied to determine the true scope and content of this invention.

CLAIMS
WHAT IS CLAIMED IS:
1. A drive circuit (100), comprising:
a solenoid (104);
a boost device (C1) that stores energy; and
at least one switch (S1, S2, S3) that controls current flow through the solenoid
and the boost device by directing energy from the boost device to the solenoid in a first
state and directing energy from the solenoid to the boost device for storage in a second
state,
wherein at least one of a current rise rate and a current fall rate in the solenoid is
controlled by the solenoid and the boost device.
2. The drive circuit of claim 1, wherein the boost device is a capacitor.
3. The drive circuit of claim 1, wherein the boost device has a storage size
less than or equal to an energy storage requirement for complete charging of the
solenoid to allow complete discharge of the boost device.
4. The drive circuit of claim 1, wherein the boost device has a storage size
greater than an energy storage requirement for complete charging of the solenoid to
allow partial discharge of the boost device.
5. The drive circuit of claim 1, wherein said at least one switch is a
semiconductor switch.
6. The drive circuit of claim 1, further comprising:
a comparator (250) that compares a desired boost voltage with a voltage across
the boost device; and

a switch controller (150) that controls said at least one switch to discharge the
solenoid into the boost device if the comparator indicates that the voltage across the
boost device is lower than the desired boost voltage.
7. The drive circuit of claim 1, further comprising a demagnetization device
(C2) for demagnetizing the solenoid.
8. The drive circuit of claim 7, wherein the demagnetization device is a
capacitor having a value that provides pulse demagnetization.
9. The drive circuit of claim 7, wherein the demagnetization device is a
capacitor having a value that provides decaying sinusoidal demagnetization.
10. A drive circuit (100), comprising:
a solenoid (104);
a power source (102);
a boost device (C1) that stores energy;
a first switch (S1) and a second switch (S2) that control current flow through the
solenoid and the boost device by discharging energy from the boost device to the
solenoid in a first state and directing energy from the solenoid to the boost device for
storage in a second state,
a switch controller (150) that controls operation of the first switch and the second
switch;
a first current steering device (D1) and a second current steering device (D2) that
selectively direct current through the solenoid, the power source, and the boost devices
based on the states of the first switch and the second switch;
wherein at least one of a current rise rate and a current fall rate in the solenoid is
controlled by the solenoid and the boost device at a first rate and a second rate slower
than the first rate.

11. The drive circuit of claim 10, wherein
the first and second switches are disposed in series with the solenoid,
the second switch and the solenoid are disposed in parallel with the power
source, and
the second switch is disposed in parallel with the boost device.
12. The drive circuit of claim 10, wherein the first current steering device is
disposed in series with the power source and the second current steering device is
disposed in series between the second switch and the boost device.
13. The drive circuit of claim 10, further comprising:
a comparator (250) that compares a desired boost voltage with a voltage across
the boost device; and
a switch controller (150) that controls at least one of the first and second
switches to charge the boost device if the voltage across the boost device is lower than
the desired boost voltage.
14. The drive circuit of claim 10, further comprising a third current steering
device (D3) disposed in parallel with the solenoid and the second switch.
15. The drive circuit of claim 10, further comprising:
a third current steering device (D3) disposed in parallel with the solenoid;
a demagnetization device (C2) coupled to the third current steering device; and
a third switch (S3) disposed in parallel with the third current steering device.
16. A method for operating a drive circuit having a solenoid, a power source,
a boost device that stores energy and at least one switch that controls current flow
through the solenoid, the method comprising:
charging the solenoid by discharging current from the boost device to the
solenoid at a first rate (202, 304), and

charging the solenoid from the power supply at a second rate slower than the
first rate (203, 306);
discharging the solenoid (207, 314); and
charging the boost device during the discharging step by capturing energy from
the solenoid during the discharging step in the boost device (256).
17. The method of claim 16, further comprising repeating the steps of
charging the solenoid and discharging the solenoid before the reducing step.
18. The method of claim 16, further comprising:
comparing a voltage across the boost device with a desired boost voltage (254);
and
charging the boost device with the power supply if the voltage across the boost
device is lower than the desired boost voltage (256).
19. The method of claim 16, wherein the circuit further comprises a
demagnetization device, and wherein the method further comprises discharging current
from the demagnetization device into the solenoid before the step of charging the
solenoid (302).
20. The method of claim 19, wherein the step of discharging current from the
demagnetization device conducts pulse demagnetization.
21. The method of claim 19, wherein the step of discharging current from the
demagnetization device conducts decaying sinusoidal demagnetization.
22. A drive circuit, comprising:
a solenoid (104);

a boost device (C1) that stores energy, wherein at least one of a current rise rate
and a current fall rate in the solenoid occurs at a first rate and at a second rate different
than the first rate;
a controller (150) that controls current flow through the solenoid and the boost
device according to a plurality of operating modes, in which
in a first operating mode, current flows from the boost device to the
solenoid at the first rate;
in a second operating mode, current alternately flows between the
solenoid and the boost device at the second rate.
23. The drive circuit of claim 22, further comprising a plurality of switches (S1,
S2), wherein the controller determines at least one switching time for conducting the
first and second operating modes.
24. The drive circuit of claim 22, wherein the controller controls current flow
according to at least one of a solenoid current, boost device voltage, solenoid response,
or an external system response.
25. The drive circuit of claim 22, further comprising a comparator (250) that
compares a voltage across the boost device with a desired boost voltage, wherein the
controller directs current from the solenoid into the boost device if the voltage across the
boost device is lower than the desired boost voltage.
26. The drive circuit of claim 22, further comprising a demagnetization device
(C2), wherein the controller directs current from the solenoid to the demagnetization
device to transfer magnetic energy from the solenoid to the demagnetization device.

A solenoid drive circuit (100) includes a boost energy storage device (1), such as a capacitor, that captures energy from and discharges energy to a solenoid (104). Switches (S1, S2) control the connection between the boost device (1), the solenoid (104), and a power source (102). This allows the solenoid response time to be variable based on the characteristics of the boost
device as well as the solenoid. By providing two
different solenoid current rise and decay rates and by capturing and re-using energy stores in the solenoid, the inventive drive circuit enhanced solenoid response and increases efficiency.