Driving a proportional solenoid valve with no flyback diode

As the topic suggests, I would like to drive a proportional solenoid valve without using a flyback diode and looking for some industry experience/feedback.

First off, I'm using an off the shelf "smart" low side driver (ST Micro VNL5030J-E) which has the necessary avalanche rating for this to be possible without overheating and damaging the the device. For example, one of the valves that I might be driving would have specifications similar to:

Max current: 1.6A
Coil Resistance: 7.3 Ohms
Coil Inductance: 20 mH

If you look at Figure 15. Maximum demagnetization energy for the VNL5030J-E, these solenoid specifications fall below the maximum rated for the device.

I'd like to mention that I want to avoid the flyback diode for some very specific reasons and it makes life a lot easier. Either way, I'd like to investigate this option.

With that aside, I'm concerned about 2 things that I've identified during Spice simulation.

1) Following the MOSFET avalanche, there is some high frequency ringing as the MOSFET is turning off and the inductive energy has been exhausted. Is this representative in real life situation? Is there some sort of snubber circuit that I can use to minimize EMI?

2) When compared to using a flyback diode, the current profile during the off phase is significantly different. For instance, the flyback diode seems to follow a very similar discharge profile compared to the charge profile (both look like identical RC time constant type waveforms). With no diode, the discharge time is reduced significantly. I'm worried this will affect actual positioning of the valve. Has anyone successfully driven proportional valves like this, can you provide any experience?

Thanks for any insight.

According to the VNL5030 datasheet, repetitive avalanche operation seems to be supported. This operation mode implies that the stored solenoid energy is completely "burned" in the driver (or a part of it, if the driver is turned on before the current decays to zero). So you have to take care that the driver doesn't overheat in this situation.

Generally speaking, you can set the dI/dt during turn-off by adjusting the reverse coil voltage. A flyback diode gives lowest dI/dt, avalanche operation highest dI/dt.

Usually proportional valves are PWM operated to achieve variable DC current, possibly with a certain ripple amount to overcome mechanical hysteresis. A driver with flyback diode is the standard way to do it.

FvM said:

Usually proportional valves are PWM operated to achieve variable DC current, possibly with a certain ripple amount to overcome mechanical hysteresis. A driver with flyback diode is the standard way to do it.

Click to expand...

Thanks for your response. I forgot to mention we are using PWM to control the position. I want to avoid the flyback diode because it complicates some protection circuitry we have for reverse polarity in our product.

So what I'm getting from your response is that you don't see a particular issue with it other than managing the additional heat, is that right?

Instead of omitting the flyback diode, I would add a series diode as reverse power protection. Overall efficiency will be considerably better, despite of the diode voltage drop. Ultimately, you can use a MOSFET as reverse power disconnect switch.

FvM said:

... I would add a series diode as reverse power protection. Overall efficiency will be considerably better, despite of the diode voltage drop. Ultimately, you can use a MOSFET as reverse power disconnect switch.

Click to expand...

A series MOSFET in normal configuration, because of its inherent anti-parallel parasitic diode doesn't provide reverse power protection.

The parasitic diode of a series MOSFET in reverse configuration, however, in terms of voltage drop is by far inferior to a Schottky series diode, resp. an SBR diode, I think.

You must use diode ... otherwise reliability decrease

A zener diode shunting the MOSFET with a voltage slightly over VCC, clamps all rings to within Vz and -.8V ?
Frank

A series MOSFET in normal configuration, because of its inherent anti-parallel parasitic diode doesn't provide reverse power protection.

The parasitic diode of a series MOSFET in reverse configuration, however, in terms of voltage drop is by far inferior to a Schottky series diode, resp. an SBR diode, I think.

Click to expand...

I was under the impression that the circuit is commonly known. The transistor is in fact reverse conducting, switched on by the supply voltage.

1) Following the MOSFET avalanche, there is some high frequency ringing as the MOSFET is turning off and the inductive energy has been exhausted. Is this representative in real life situation? Is there some sort of snubber circuit that I can use to minimize EMI?

Click to expand...

First of all, if you are using PWM drive, then the coil current should not be allowed to decay completely down to zero during the off time, correct? In either case, there will be some ringing, which can likely be dealt with by a simple RC snubber.

2) When compared to using a flyback diode, the current profile during the off phase is significantly different. For instance, the flyback diode seems to follow a very similar discharge profile compared to the charge profile (both look like identical RC time constant type waveforms). With no diode, the discharge time is reduced significantly. I'm worried this will affect actual positioning of the valve. Has anyone successfully driven proportional valves like this, can you provide any experience?

Click to expand...

The method of coil suppression is going to affect the relationship between the PWM duty cycle and the average voltage applied to the coil, and thus the positioning. Using a MOSFET's avalanche as a suppression method is particularly tricky since the actual avalanche voltage is probably not well defined. If you want tight control of the applied voltage, using a full bridge driver is probably best.

erikl said:

A series MOSFET in normal configuration, because of its inherent anti-parallel parasitic diode doesn't provide reverse power protection.

The parasitic diode of a series MOSFET in reverse configuration, however, in terms of voltage drop is by far inferior to a Schottky series diode, resp. an SBR diode, I think.

Click to expand...

You can indeed use a MOSFET in the reverse configuration to block a reverse voltage connection with very low drop. It works because a MOSFET can conduct equally well in both directions. Here's an article I wrote on that.