AN50003

Driving solenoids in automotive applications
Rev. 1.1 — 3 November 2023

application note

Document information
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Content

Keywords

solenoid drives, peak-and-hold, avalanche, active-clamp, free-wheeling

Abstract

There are a wide variety of solenoid drive circuit topologies. Most of them use MOSFETs invarious configurations and driving modes. In this application note four of them will be discussed:
solenoid driver with free-wheeling diode, solenoid driver with MOSFET avalanching, solenoid
driver with active clamp and solenoid driver with auxiliary boost circuit.

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Driving solenoids in automotive applications

1. Introduction
Throughout the evolution of modern engineering electromagnetic devices have taken prevalencein changing electrical energy to mechanical energy or movement. Most commonly we think about
motors for such applications, however the humble solenoid is used even more often, thanks
to its simplicity of construction and ease of driving. Solenoid coils are typically found in relays,
contactors, and valves.
In the automotive sector solenoids are used for a range of applications as well, from starting
the engine to shifting the transmission. Solenoids are used to activate four-wheel drive system,
fuel injection systems, locking the doors of the car and controlling the air flow in the vehicles air
conditioning system. The vast number of valves in the vehicle are also controlled by solenoids.

2. Solenoid operating principles
Consisting of a fixed coil and a movable core or slug (termed the armature) the solenoids are able
to push, pull or even do both as the current through them changes direction. The armature is used
to assert mechanical force to the driven system. The motion is usually reversed by a spring thatis attached to the core. The armature movement changes the inductance of the coil, which in turnacts as an electromagnet. The magnetic force applied to the armature is proportional to the changeof this inductance and the current flowing through the core, as shown in Fig. 1.

Fig. 1.

Solenoid principle of operation

From the electrical viewpoint, the solenoid acts as an inductive component, consisting of multiple
wound coils. The current flowing through them creates a magnetic field. The sluggish nature ofthis highly concentrated field creates a voltage (termed Electro Motive Force, EMF) that opposeschange in the magnetic field, and therefore in the current as well. In this way as voltage is initiallyapplied to the solenoid coil the current starts rising gradually. The magnetic field, and therefore
the force applied to the armature rises until it reaches a point where it is large enough to movethe armature in the desired direction. Because of this slow response, it is prudent to apply a highvoltage to the solenoid at the start of its actuation to initiate a faster current response. As thearmature starts moving, the solenoid’s inductance (as a function of the armature position) and backEMF (as a function of the armature speed) rise, limiting the rate of rise of the current.
Once the movement of the coil is mechanically prevented as it reaches its intended resting point,
the back EMF diminishes. At this point the current continues to rise until only the coil resistance

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limits its value. This current can be quite high for the power supply, which is normally a battery inautomotive applications
As the system has reached a mechanical steady state the amount of force needed to maintainthis state is much lower than for moving the armature. Besides, the armature is usually part of amagnetic circuit with an air gap. This air gap is closed by actuating the solenoid and moving thearmature, therefore rendering the magnetic reluctance (equivalent for resistance in electric circuits)
very small. This in turn allows the magnetic field flux (equivalent to current in electric circuits) toflow in abundance, increasing the applied force to the armature.
For the above reasons it is advisable to decrease the applied voltage to the solenoid after its
armature has reached its intended position, to limit the applied power and avoid depleting thevehicle battery. An idealised voltage and current waveform are shown in Fig. 2.

Fig. 2.

AN50003application note

Idealised voltage and current waveforms

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3. Current regulated solenoid drives
A more recent approach to controlling solenoids uses current control as shown in Fig. 3. This
waveform is known as the “peak-and-hold” current waveform, predominantly used in Fuel Injection
applications. Initially, the current is increased rapidly to a high value during the Boost Phase. The
current can be allowed to reach high values at this stage, since it will provide the initial push for thearmature to begin its journey. The slope of the current should be high and, therefore, the applied
voltage should be high as well.
In the Peak Phase, for a time period sufficient for the armature to take its final position the current
is held at a certain value. Then the current is reduced during the Bypass Phase. The rate ofdecrease of the current is dependent on the reverse voltage applied to the inductor in this phase.
The current is set to a lower value during the Hold Phase. Therefore, the force applied to thearmature is reduced to a level sufficient to hold the armature in place. The losses are also reduced
since this current can be substantially lower than the one applied in the Peak Phase.
Finally, once the control signal is withdrawn, in the End of Injection Phase, the current is left todecay to zero, leaving the spring to return the armature to its initial position. Once again, the rateof inductor current decrease can be influenced by the voltage that appears across the inductor. At
this instant, the speed of current decay might be important for timing reasons. If the current decays
slowly it is hard to predict the instance when the force of the spring will prevail over the magnetic
force, as the mechanical properties of the spring and the whole mechanical system of the solenoid
might change over time. Furthermore, for the same reasons the speed of the armature cannot be
guaranteed. For some time sensitive applications, such as internal combustion engine injector
drive, such timing differences might prove to be crucial.

Fig. 3.

AN50003application note

Peak and hold solenoid current waveform in a fuel injector application

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4. Discussion of simulation results
Four approaches to driving a current regulated solenoid are explored in simulations. The
simulations had common control parts, for ease of comparison. The difference manifested in the
power electronics driving the solenoid. The MOSFET driver logic relies on the current feedback
being compared to its reference value. When the reference value is higher than the feedback, the
MOSFET is switched at 1 kHz frequency. For the purposes of these simulations the solenoids were
replaced by an inductor with 5 mH inductance.

4.1. Solenoid driver with free-wheeling diodeThis is the simplest and easiest way to drive a solenoid. The inductive energy of the solenoid
decays through a diode up to the battery voltage. The schematic is shown on Fig. 4 and thesolenoid current and its reference is shown in Fig. 5.
When the reference signal is received, at 10 ms, both MOSFETs turn on to ensure maximal current
increase in the Boost Phase. Once the peak reference current is reached, the high side MOSFET
is switched so that it controls the current around this reference value, which is chosen to be 3 A in
the simulation. When the MOSFET is turned on the applied voltage equals to the battery voltage;
when the MOSFET is turned off the current circulates through the bottom MOSFET and the bottom
diode. The voltage applied to the inductor is equal to the voltage drop on these two elements, i.e. it
is very low.
After the target time of 10 ms for the Peak Phase has elapsed, at 20 ms, the reference is changed
to 1.2 A. Again, the top MOSFET is used to regulate the current. After the Hold Phase, at 40 ms,
both MOSFETs are turned off and the current free-wheels through the two diodes, making theeffective reverse voltage almost equal to the battery voltage. Considering a simple inductor voltage/current relation, with a battery voltage of 12 V and 5 mH inductance the duration of the End of
Injection phase can be calculated to be close to 0.5 ms.

Δt = L ΔI
V(1)Both MOSFETs and diodes will need to withstand the battery voltage. Both MOSFETs and the top
free-wheeling diode need to be rated to the reference Peak current, while the bottom free-wheeling
diode conducts only the Hold current for a short amount of time. The dissipated energy wascalculated for each component during the whole activation process. Comparison of the energies
dissipated in each device for each topology can be found in Section 5. To obtain the power, thecalculated energy value needs to be multiplied with the desired frequency of operation.

P = E.f(2)

aaa-032847

Fig. 4.

AN50003application note

Schematic of driver with freewheeling diode.

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aaa-032853

4
IL, Iref
(A)
3

Iref

IL

2

1

0

Fig. 5.

0

0.01

0.02

0.03

0.04

t (s)

0.05

Solenoid and reference currents as a function of time.

Compared to the other driver topologies, the free-wheeling driver is simple, has a low component
count, but it is the slowest due to the inductor voltage being approximately equal to the battery
voltage.

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4.2. Solenoid driver with MOSFET avalanching
In this case there is no free-wheeling diode and the back EMF of the solenoid forces the MOSFET
into avalanche mode of operation. Its schematic is shown in Fig. 6.
The mode of operation is identical to the free-wheeling diode circuit at the start. However, in End of
Injection Phase, when both MOSFETs are turned off, the inductor current has no way to free-wheel.
Therefore, the inductor voltage is increased until it breaks down the bottom MOSFET and drives it
into avalanche mode. This voltage is substantially higher than the battery voltage that was appliedin the case of the free-wheeling circuit. Therefore, the current will decay faster. A close look at the
MOSFET avalanche voltage and current can be seen in Fig. 7. Consulting the inductor equation,(Eq 1), once again, with a voltage of 68 V, the End of Injection phase duration is now closer to
0.1 ms: a five-fold reduction compared to the free-wheeling case.
Once again, all the components need to be rated above the battery voltage and the target peakcurrent. However, the bottom MOSFET needs to be repetitive avalanche rugged. The energy
dissipated in each component is compared in Section 5.

aaa-032848

Fig. 6.

Schematic of driver with avalanching MOSFET

Fig. 7.

Avalanching inductor current (top) and MOSFET voltage (bottom).

Due to the high voltage of avalanche compared to the battery voltage, this method decays andtherefore releases the solenoid faster. However, the energy of the inductor is now dissipated in the
MOSFET in the form of heat. Therefore, careful consideration of a MOSFET is needed to handle
this energy. The selection of the MOSFET is addressed in Section 6.

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4.3. Solenoid driver with active clamp
This option is very similar to avalanche operation. Here a Zener clamp is connected drain to gateof the MOSFET, as seen in Fig. 8. Once again, the circuit behaviour is identical as with the freewheeling and avalanche circuits. However, at the End of Injection Phase as both MOSFETs areturned off and the inductor voltage starts to increase towards the bottom MOSFET VDS breakdown,
the Zener diode starts conducting and pulls the MOSFET gate up forcing it into its linear region.
The MOSFET then maintains the sum of the Zener diode breakdown voltage, the diode forward
voltage and the gate-source threshold voltage from drain to source.
In the previous case the MOSFET intrinsic diode has broken down in avalanche mode. In thiscase the current flows through the MOSFET channel. As the MOSFET is in its linear region
with large current and large voltage applied to it, there is an increased chance of hotspots andthermal runaway occurring if conditions are met. During active clamp there are high energy charge
carriers generated in proximity of the MOSFET’s gate oxide. These carriers might be injected
into the oxide and cause damage. Over many active clamp cycles the gate oxide can wear out
and cause parametric shift and ultimately device failure. Currently it is not recommended to use
MOSFETs in repetitive active clamp. Alternatively, repetitive avalanche is recommended, as thelong term reliability during repetitive avalanche is better defined. Simulated waveforms of the circuit
behaviour are shown in Fig. 9.
Once again, the components need to be rated for battery voltage and reference current, apart fromthe bottom MOSFET, which needs to be rated above the selected Zener diode voltage. The voltageof the inductor current decay, and therefore the duration of the decay as well, can be tuned with theselection of the Zener diode with different breakdown voltages.

aaa-032850

Fig. 8.

AN50003application note

Schematic of driver with active clamp

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Fig. 9.

AN50003application note

MOSFET gate voltage (top), drain to source voltage (middle) and inductor current(bottom) for the active clamp topology

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4.4. Solenoid driver with Boost converter
This is the most complicated topology, shown in Fig. 10, with the highest performance. A boost
converter, often operating at an output voltage in the range of 60 V, is used to charge anddischarge the solenoid quickly (five times faster with five times larger voltage) during the Boost
Phase and End of Injection Phase. During the Peak and Hold Phases the nominal battery voltage
is switched, as in the previous cases. This allows for fast actuation of the solenoid, but also theenergy from the solenoid is regenerated into the DC link capacitor of the boost converter. The costhere is the additional components to make the boost circuit, the additional PCB board space andoverall higher voltage rating of the components. The added component count is reflected in thelosses. However, it needs to be considered that the MOSFETs used in the Boost simulation are oflower current rating.
All the components need to be rated for the boost voltage (rather than battery voltage) and peakcurrent. The exceptions are the battery side switching MOSFET that can have the battery voltage
rating and the boost rail connected free-wheeling diode that can be rated to the hold current.
DC/DC
60 V
+12 V

aaa-032851

Fig. 10. Schematic of driver with Boost converter

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5. Summary of the topologies
Table 1 shows the losses encountered in each device for each topology, as well as their total loss.
The MOSFETs used in the free-wheeling and active-clamped simulations were BUK9K13-60E,
in the repetitive avalanche simulation BUK9K13-60RA and in the boost converter simulations
BUK9Y38-100E, as these MOSFETs need to withstand higher voltages. Although the losses
appear to be higher in the Boost topology, the recuperation of the energy means that it’s efficiency
is on par with the free-wheeling topology, despite using higher RDSon components.
Table 2 shows a summary of the pros and cons of the surveyed topologies. The avalanche andactive clamp circuits are positioned between the low cost and low speed free-wheeling topology
and the high cost and high speed boost topology. While there are risks in the longevity of thedevices in the middle two topologies, Nexperia’s repetitive avalanche rugged components areextensively tested and their data sheets are equipped with the necessary data to make an informed
choice and have the MOSFET last the full application lifetime.

AN50003application note

Table 1. Energy losses comparison of surveyed topologies (mJ).
Topology
FreeAvalanche
Active clamp
wheeling

Boost

Switching MOSFET (top)

0.3

0.3

0.3

0.9

Selector MOSFET(bottom)

1

5.1

3

4

Boost MOSFET

-

-

-

0.03

Switching diode (bottom)

15.5

15.3

15.4

15.6

Freewheeling diode (top)

0.1

-

-

0.25

‘OR’ diode

-

-

-

3.8

Zener diode

-

-

2

-

Total losses

17

20.7

20.7

26

Table 2. Performance comparison of surveyed topologies.
Topology
FreeAvalanche
Active clamp
wheeling

Boost

Cost

Low

Low

Low

High

Speed

Low

MediumMediumHigh

Efficiency

High

Low

Low

High

Reliability

Long termLong term

Questionable

Long term

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6. How to select a repetitive avalanche rugged part
Existing MOSFET data sheets give scarce data about repetitive avalanching a MOSFET. The ones
that do, give a very conservative rating. Nexperia’s repetitive avalanche products provide a way
to objectively assess the suitability of the chosen part for the aimed application. From Fig. 7 theavalanche voltage can be read as 68 V, the avalanche current 1.4 A and the avalanche time is
0.1 ms. The inductance is 5 mH.
Let’s consider the BUK9K35-60RA. In the device data sheet, there are two figures (Fig. 11 andFig. 12) that can help with choosing the device. From the avalanche current it can be seen from
Fig. 11 that repetitive avalanche can be allowed to last for up to 0.2 ms. Eq 3 shows the amount ofenergy contained in the inductor and dissipated by the MOSFET:

1

E = 2 LI

2

(3)This gives a value of 4.9 mJ. From Fig. 12 it can be seen that the number of cycles that can beallowed is approximately 2.5 billion.

Fig. 11. Avalanche current as a function of avalanche time

Fig. 12. Maximum number of avalanche events as a function of avalanche energy

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We are left only to ensure that we are within the allowable junction temperature. As for the fuelinjection frequency we can take a low value of 20 - 30 Hz, the junction temperature is of low
concern as the MOSFET junction will have plenty of time to cool.
As these values satisfy the application requirements with small margins, a MOSFET with slightly
higher current rating is chosen for the simulations.

7. Avalanche portfolio
Nexperia’s application specific FET portfolio for Repetitive Avalanche offers an alternative between
the high-performance/high-cost boost and low-performance/low-cost freewheeling diode solenoid
drives. The avalanching method has been made possible using planar technology, however bytechnology optimisation of the vertical structure, the Repetitive Avalanche products can comfortably
handle avalanche breakdown currents. The devices are tested rigorously for up to 1 billion cycles
to ensure reliability.
Placed within the LFPAK package the device operating point is ensured to be below 175 °C.
For more information please visit the links below:••••

Nexperia application note AN10273: Power MOSFET single-shot and repetitive avalancheruggedness rating
YouTube video: Selecting repetitive avalanche rugged MOSFETs
YouTube video: Repetitive avalanche rugged MOSFET applications
Nexperia product category Repetitive Avalanche ASFETs

8. Revision history
Table 3. Revision history
Revision
Date
numberDescription

1.1

2023-11-03

Section 4.3, text corrected from:"at the End of Injection Phase as both MOSFETs are turned on"
to:"at the End of Injection Phase as both MOSFETs are turned off".

1.0

2020-12-04

Initial version

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9. Legal information
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modifications or additions. Nexperia does not give any representations or
warranties as to the accuracy or completeness of information included hereinand shall have no liability for the consequences of use of such information.

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for the products described herein shall be limited in accordance with theTerms and conditions of commercial sale of Nexperia.
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to information published in this document, including without limitation
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List of Tables
Table 1. Energy losses comparison of surveyed
topologies (mJ)...................................................................11
Table 2. Performance comparison of surveyed topologies.11
Table 3. Revision history....................................................13

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List of Figures
Fig. 1. Solenoid principle of operation................................. 2
Fig. 2. Idealised voltage and current waveforms..................3
Fig. 3. Peak and hold solenoid current waveform in afuel injector application........................................................ 4
Fig. 4. Schematic of driver with freewheeling diode.............5
Fig. 5. Solenoid and reference currents as a functionof time.................................................................................. 6
Fig. 6. Schematic of driver with avalanching MOSFET........ 7
Fig. 7. Avalanching inductor current (top) and
MOSFET voltage (bottom)................................................... 7
Fig. 8. Schematic of driver with active clamp.......................8
Fig. 9. MOSFET gate voltage (top), drain to source
voltage (middle) and inductor current (bottom) for theactive clamp topology.......................................................... 9
Fig. 10. Schematic of driver with Boost converter..............10
Fig. 11. Avalanche current as a function of avalanche
time..................................................................................... 12
Fig. 12. Maximum number of avalanche events as afunction of avalanche energy.............................................12

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Contents
1. Introduction................................................................... 2
2. Solenoid operating principles..................................... 2
3. Current regulated solenoid drives.............................. 4
4. Discussion of simulation results................................ 5
4.1. Solenoid driver with free-wheeling diode..................... 5
4.2. Solenoid driver with MOSFET avalanching................. 7
4.3. Solenoid driver with active clamp................................ 84.4. Solenoid driver with Boost converter......................... 10
5. Summary of the topologies....................................... 11
6. How to select a repetitive avalanche rugged part.... 12
7. Avalanche portfolio.................................................... 13
8. Revision history..........................................................13
9. Legal information........................................................14© Nexperia B.V. 2023. All rights reserved

For more information, please visit: http://www.nexperia.com
For sales office addresses, please send an email to: salesaddresses@nexperia.com
Date of release: 3 November 2023

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