Designing a power-saving solenoid driver

Article By : Maicon Bruno Hofmann

Using a current regulated driver to activate and deactivate the solenoid greatly reduces the power consumed in the internal solenoid resistance.

Solenoid Principles

Solenoids are electromechanical actuators with a freely moving magnetic core called a plunger. In general, solenoids consist of a helicoidal coil of wire with a moving core made of iron.

When current is applied through the solenoid coil it generates a magnetic field inside it. This magnetic field generates a force to pull in the plunger. When the magnetic field generates enough force to pull in the plunger, it moves inside the solenoid until it reaches a mechanical stop position. When the plunger is already inside the solenoid, the magnetic field generates force to hold the plunger in place. When the current is removed from the solenoid coil, the plunger will return to its original position, pushed by a mounted spring in the solenoid.

Figure 1 shows the construction of a solenoid.


Figure 1. Solenoid construction drawing [Source]

The most common approach to drive a solenoid is to apply the required voltage in the solenoid coil. This can usually be done using a single power transistor configured on the high side or on the low side. The power transistor will require a flywheel diode in parallel with the solenoid because the solenoid coil has a high inductance that will try to push a current into the transistor. Although this approach is simple and cheap, it is not power efficient. This is because solenoids usually require significant current to pull in the plunger, but when the plunger is pulled in, it does not require the same amount of current. In the simple driver approach, when the plunger is pulled in, holding the plunger, the current applied to the solenoid mainly generates heat through its internal resistance. The power dissipated in internal solenoid resistance is given by Equation 1.

An alternative approach to overcome this issue is to use a current regulated driver to activate and deactivate the solenoid. This driver can apply a peak current value in the solenoid until it pulls in the plunger and, after that, it can reduce the current to a hold value. This strategy greatly reduces the power consumed in the internal solenoid resistance. Another advantage of this driver is the possibility to use a solenoid in a larger range of voltages. It means that the driver allows a solenoid designed to operate with a lower voltage (for example, 5 volts) to operate with a higher power voltage without damage (for example, with a 12 volt power supply).

The following sections will describe the implementation of the current regulated driver for two solenoids using the SLG47105 HVPAK device.

GreenPAK Design Concept

Two different solenoids can be driven using a single SLG47105 device. The SLG47105 device will control the current through the solenoids and will inform a user about each solenoid’s status (on, off, or in a fault state). A conceptual block diagram showing its internal construction is shown in Figure 2.

click for full size image

Figure 2. Block diagram of power-saving solenoid driver with SLG47105 (Source: Renesas Electronics)

The upper right side of the diagram shows how the High Voltage Output (HVOUT) block is configured internally and its connection to the external solenoid. The output connected to Pin 7 is configured as push-pull and the output connected to pin 8 is configured as open drain. This open drain output is always kept turned on after startup delay. Pin 5 is internally connected to the N-Mosfet of Pin 8 and the internal current amplifier. Pin 5 is used to measure the solenoid current and compare it with an internal reference, sending the result of the comparison to the PWM Controller 1 block.

The PWM Controller 1 block generates the PWM required to regulate the solenoid current connected to Pins 7 and 8. It has two setpoints, one for the solenoid peak current and another for the solenoid hold current. The On/Off input of the PWM Controller is activated by the AND port on its left. The AND port is connected to a startup delay block and Pin 2, which is used as the external interface to turn the solenoid on and off.

The startup delay block connected to the AND port is used to guarantee that all internal blocks initialize properly at IC power-up. The output of the AND port is connected to another delay block. When the PWM controller is turned on it is configured to regulate the solenoid current at its peak current value. After a delay of 50 milliseconds, the delay block switches the PWM configuration to regulate the solenoid current at its hold current value.

The On/Off input of the PWM Controller 1 block is also connected to one of the inputs of a mux. The other mux input is connected to a square wave signal with a frequency of 1 hertz. The mux output is controlled by the FAULT signal in the HVOUT block. When the FAULT signal does not indicate any failure, the On/Off input is buffered through Pin 17, the SOLENOID 1 STATUS output. When the FAULT signal indicates a failure, the square wave signal is driven in this output. The SOLENOID 1 STATUS is designed to drive an external LED and show the solenoid status to a user. This status can be turned on, turned off, or in a fault state when the LED blinks at the square wave output frequency.

An additional FAULT output is provided as an open drain output in Pin 14. This output was designed to drive an external device, like a microcontroller.

Below the PWM Controller 1 is PWM Controller 2 and, as can be seen in Figure 2, the control structure around PWM Controller 2 is like PWM Controller 1.

The two FAULT outputs can be connected externally, since they are open drain outputs, supplying a single FAULT signal for an external device if any of the outputs fail.

An additional block is the I2C; it can be used to reconfigure peak and hold current setups.

Application Circuit

The typical application circuit, which is the same used alongside this article, is shown in Figure 3.

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Figure 3. Simplified schematic of the electronic circuit for a typical application (Source: Renesas Electronics)

Figure 3 shows a simplified schematic of the typical application driving two different solenoids, identified as S1 and S2. As shown in the schematic, the driver is controlled by two push buttons connected to the 5 Volt power supply. The solenoids are connected to the respective HVOUT outputs, together with a small 0.1Ω resistor. This resistor is used to allow an external current measurement through the solenoid and is not required for an end application. For the SLG47105 current measurements, two resistors of 0.11Ω are connected to Pins 5 and 12. The solenoid status outputs are connected to green LEDs and the fault outputs are connected to red LEDs.

In this article, we use two solenoids with completely different specifications. Table 1 shows the main specs of solenoids S1 and S2.

Table 1. Specifications of the solenoids S1 and S2 (Source: Renesas Electronics) [click for full size image]

Solenoid Current Setup

The solenoid current will start with a regulated peak current value, and, after an initial delay, it will decrease to a hold current value. We arbitrarily define that the hold current should be 20 percent of the nominal peak current. Based on this definition, it is possible to calculate the power dissipated in the hold current and the respective voltage at the sense resistor. The ideal solenoid current, dissipated power, and voltage at the sense resistor for each solenoid are shown in Table 2.

Table 2. Currents, dissipated power, and sense resistor voltages for the ideal configuration (Source: Renesas Electronics)

The peak current value is the solenoid nominal current at the nominal voltage. The hold current is calculated by multiplying the peak current by 0.2 (20 percent). The peak and hold currents are calculated as the power dissipated over the internal solenoid resistance. The sense resistor is calculated using Ohm’s Law through the sense resistor with 0.11Ω. The nominal coil resistance for S2 was calculated using the nominal solenoid voltage and its peak current values.

It is important to note that the reference voltage for comparison with the sense resistor voltage in SLG47105 is supplied by an internal 6-bit DAC. We must adjust the regulated current to the nearest SLG47105 internal reference voltage. Considering that, the following values of voltage references shown in Table 3 were selected. Table 3 shows the internal voltage and the respective currents. All internal values are 8 times the desired sense resistor voltage because the external voltage is amplified by 8 internally (described in more detail in the next sections). The peak and hold current values are calculated using Ohm’s Law through the sense resistor.

Table 3. Internal voltage references and respective currents and dissipated power (Source: Renesas Electronics)

The values marked with an (*) in Table 3 are achieved in calculations, but these values are impossible and do not represent reality. For S2, the peak current is not required current regulation, because the solenoid’s internal resistance will limit the current. Considering this, we decided to set up the reference for the maximum current value.

Conclusion

In this article, we discussed the internal structure of a solenoid and provided an overview of an application circuit to control the solenoid device. In the next part, we are going to dive into the implementation steps and test the real-life solution.

 

This article was originally published on Embedded.

Maicon Bruno Hofmann graduated in electrical and electronic engineering from Santa Catarina State University in 2008 and received a Master´s Degree from Federal University of Technology – Paraná in 2016. Since 2009 he has been working with the hardware and firmware development of embedded systems. His interests are in the areas of electronics, configurable devices, firmware development, and digital signal processing.

 

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