This engineering brief shows inductive flyback voltage and thermal consideration for the diode clamp in a Solid-State Relay (SSR). It also explores the system-level tradeoff between clamp voltage and opening speed of downstream inductive loads. Oscilloscope images showing the associated waveforms are included.
In a nutshell, the SSR’s output section contains two parts including the power semiconductor and a clamp (internal or external) for inductive flyback energy. We consider both components to balance longevity with system response:
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High clamp voltages, or absence of the clamp, will stress the SSR.
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Low clamp voltage slowly dissipates inductive flyback energy resulting in an unresponsive system-level performance.
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Each turn off cycle dissipates energy in the clamp. High ambient temperatures, operation at high current, or a high number of turn-off cycles accumulate heat in SSR. As a rule of thumb, equipment life expectancy is cut in half for every 10 °C increase in temperature.
Demonstration Circuit
The setup shown in Figure 1 is used to explore the design space. It consists of a Weidmüller SSR interposed between a 3.3 VDC microcontroller and a three-phase contactor featuring a 24 VDC coil. A schematic is included as Figure 2. This is a stylistic representation using the symbols found on the physical SSR and on the SSR’s datasheet. Here, the SSR is placed in the positive (sourcing) side of the load.
Figure 1: A SSR is interposed between a microcontroller and a large three-phase contactor.
Figure 2: Circuit diagram for the SSR setup.
Circuit Waveforms
The waveforms as measured by a Digilent ADP2230 oscilloscope are shown in Figure 3:
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Microcontroller drive (green): This is a 3.3 VDC square wave.
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Normally Open Contact (blue): This is the contact that would normally be connected to the load.
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Coil Voltage (red): This is the coil voltage as measured at the SSR emitter terminal.
Figure 3: Waveforms for the SSR test: green: SSR drive, blue: N.O. Contact, red: A2 terminal.
Inductive Flyback Clamp
Inductive kick is one of the greatest hazards to an SSR. Without protection (internal or external) this turn-off voltage spike will destroy the SSR’s power semiconductor. Using the language of transistors, the spike exceeds the design maximum for collector to emitter voltage.
The response for the Weidmüller interposing relay is shown in Figure 3. This voltage measured at the emitter terminal of the SSR reveals that the inductive spike is being clamped. Note that the SSR itself is performing the clamp operation. This related article justifies the internal clamp assumption by showing that the Schneider DPE09BL’s integral TDS diode clamps at -48 VDC not the -15 relative to ground as shown in Figure 3. Technically, the clamps provide absolute voltages of 72 and 39 VDC respectively as the clamps are referenced to the 24 VDC rail.
The SSR representation in Figure 2 is simplified and does not show the integral diode clamp.
Engineering Guidelines for Flyback Energy Dissipation
The red waveform is revealing but it doesn’t tell us if the SSR is protected or if it is slowly dying. Unfortunately, SSR datasheets don’t always provide design guidance. Therefore, we must develop a set of engineering guidelines to inform the design process.
The top consideration is temperature. This includes the ambient temperature of the enclosure and the energy that is internally dissipated by the SSR. This is like the brakes on a car. Repeated stop events can overheat the clamp.
Simplified Assumptions
We are tempted to assume that an SSR designed for 2 A can handle a 2 A inductive kick. However, this discounts the energy dissipated in the SSR as governed by the energy per event by the number to turn off events. This could destroy the SSR either little by little or all at once. This is also why we avoid sending a PWM signal as a high number of turn-off events per unit time could overheat the clamp.
Prudent Assumptions
The alternative is to operate under an abundance of caution. This is the only option for operation near the rated 2A load, elevated temperatures, and rapid turn on turn off cycles.
Most DC SSRs contain a thermally limited energy absorber (clamp) for inductive kick. Some SSR do not have integral protection, in which case you must include the flyback clamp or learn your lesson by purchasing a replacement SSR and then adding the protection.
Simple Diode to Clamp Solution to Dissipate Inductive Energy
Conventional engineering advice is to simply add a diode. Specifically, we could place a 1N4002 diode directly across the contactor and declare victory. This is a good solution as it moves the energy dissipation outside the SSR. But it is simply wrong for this system as it makes the three-phase contactor slow.
Remember that the inductive current is working against the clamp voltage 39 VDC in this case resulting in a 50 ms contact opening delay as shown in Figure 3. Dropping the clamp voltage to 0.6 VDC takes urgency (pep) out of the contactor’s motion. In fact, you can hear the difference in snap from across the room. This is a poor solution as the primary motor contacts will experience an extended arc every time the system turns off.
Higher-Voltage Mitigation Using an External Clamp (Zener or TVS)
The objective is to move the energy dissipation outside of the SSR. The system response when using a 1N4002 is slow, but a Zener or TVS would not suffer from this problem. The design collapses into finding a clamp that operates a few volts less than the ≈ 39 volts as measured in Figure 3 (red). Remember one end of the clamp is attached to the 14 VDC rail. The absolute voltage across the clamp is therefore 39 volts calculated as 24 rail + 15 negative excursion.
Parting Thoughts
Many, perhaps most, applications can rely on the SSR’s internal clamp. Always check the datasheet to determine if external flyback protection is required. Think about the application from a systems level. As we demonstrated, a simple 1N4002 will protect the SSR but the downstream equipment may be slow to respond. Finally, evaluate the SSR as a dynamic thermal system that accumulates heat over time.
Best wishes,
APDahlen
Related Articles by this Author
If you enjoyed this article, you may also find these related articles helpful:
- Mastering Relay Flyback Voltage with the TVS Diode
- Driving a Large Relay From a 3.3 VDC Microcontroller Using an NPN Darlington Transistor
- Guide to Selecting and Controlling a MOSFET for 3.3 VDC Logic Applications
About This Author
Aaron Dahlen, LCDR USCG (Ret.), serves as an application engineer at DigiKey. He has a unique electronics and automation foundation built over a 27-year military career as a technician and engineer which was further enhanced by 12 years of teaching (interwoven). With an MSEE degree from Minnesota State University, Mankato, Dahlen has taught in an ABET-accredited EE program, served as the program coordinator for an EET program, and taught component-level repair to military electronics technicians.
Dahlen has returned to his Northern Minnesota home, completing a decades-long journey that began as a search for capacitors. Read his story here.