Bill Klein, Texas Instruments, Burr-Brown Products Group, Tucson, AZ.
My caller last week had a working system, however, life was not all roses. He had joined the ever-increasing group of engineers using pulse-width-modulation (PWM) drivers. In his case it was to drive a solenoid. Using PWM allowed a significant reduction in power dissipation but it seemed to cause a great amount of noise. His question was simply, “What tricks can I work to reduce the noise caused by the pulse train driving the solenoid?”
After determining that he had a good grounding scheme and the switching currents were not corrupting the analog signals or grounds, I asked about the placement of the fly-back or catch diode. He reported that it was mounted on the solenoid, which was over a foot away from the driver. Here is where the exchange deviated from the normal. Rather than requesting a circuit diagram from him, I sent him the circuit shown here.
The circuit can be modeled with these four elements. When the switch is closed, the Isw equals IL and a magnetic field is developed within the solenoid. When the switch opens the solenoid changes from a load to a source and forces the IL to continue to flow. Therefore, IL is relatively constant while Isw is a pulse train with the noisy switching transients. It is now obvious that if the fly-back diode is located close to the switch element, less noise is radiated.
With this newfound insight he started to look at other areas of his design. Since he was using a low-side-switch topology PWM driver the circuit was actually this.
Why not just let the ESD cell diode function as the fly-back diode? This was an interesting concept but there was a flaw in the design. When the switch is closed the only loss in the circuit is from the switch on-resistance. When the switch opens the forward loss of the diode is the loss element. In addition the switching time of the diode can add significant loss. Therefore, the best operation is obtained when the fly-back diode is low loss and fast. The ESD cell diode has neither of these qualities.
While this particular question was framed around a solenoid driver, a similar situation can exist with a PWM driver from an embedded controller or DSP when driving a filter. In the most extreme case the fly-back diode is replaced with a MOS switching FET that is driven synchronously to with the in-line switch. This is a technique used with the most efficient switching regulators.
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