Smooth Motor Power Delivery With a Zero-Crossing Phototriac

When delivering AC power to a load, the ideal situation is smooth power delivery without inductive spikes or peaks that could damage some components. Whether you use a mechanical switch, a relay, or a solid-state electrical switch like a triac, there can be a sudden inrush of current to the load component. This is unsafe from a user perspective, and it could damage some components within the current path.

When you need galvanic isolation and solid state switching with smooth power delivery, the solution is to use a zero-crossing phototriac. These components integrate a zero-crossing circuit into a phototriac package, which will detect the instant the load voltage crosses 0 V and will latch to allow current flow to the load. The application of a zero-crossing detector will be reviewed below, and we’ll show a simple example that can be found in commercially available phototriacs.

Why Phototriacs Need Zero-Crossing Detectors

Phototriacs are primarily used to deliver power to resistive loads, meaning the delivered voltage and current will be in-phase. When an input pulse in a phototriac switches the component ON, the AC power that we would like to deliver to a load may not be at 0 V the instant the ON pulse is applied. This presents a safety problem for the user and the load components.

If the delivered AC voltage is non-zero at the instant the phototriac is switched, the resulting current will very quickly rise from 0 A to its nominal value. This very fast rising inrush of current creates multiple problems:

• Current could momentarily flow in the reverse direction from what was intended, which could damage the load (sometimes called false triggering)
• If the phototriac is duty cycle controlled, the resulting fast current spikes will create strong radiated EMI
• For loads like lighting, the sudden thermal shock can damage or destroy the load component

Power delivery with a phototriac without a zero-crossing circuit. Note that the AC load current quickly rises from zero.

To prevent the inrush, some additional components or circuits might be needed to slow down current delivery, such as a snubber. However, the best way to deliver power in this AC application is to use a zero-crossing detector.

Zero-Crossing Phototriac Waveforms

The image below shows a typical phototriac package where a triac is being driven with the output from a phototriac. When the phototriac is switched ON, the external triac is also switched ON. With the zero-crossing detector included in the phototriac package, the detector will only modulate the output from the phototriac to conduct once the line voltage crosses 0 V.

In the example, the current waveform can exhibit a half-wave cycle if the phototriac input pulse is less than ½ period of the line voltage. If the input pulse drops to 0 V, the output triac will continue to conduct until the delivered current drops below the holding current. At that point, the triac will then switch OFF.

The zero-crossing detector circuit only allows AC current to reach the load once the voltage crosses 0 V.

Zero-Crossing Detectors

This application in power delivery requires a zero-crossing detector circuit that can quickly respond and switch ON the phototriac once the delivered voltage crosses 0 V. One way to build these circuits is to use a phototransistor, SCR, and a standard triac, where the phototransistor is triggered by an LED. Another method is to use an op-amp to trigger output current delivery to a load by using the op-amp as a comparator.

Reactive Loads

If the load component is reactive, then the current and voltage being delivered to the load will not be in phase. There are three options to deliver power in the case where the load is strongly reactive:

• The zero-crossing detector circuit has to delay current flow to the load
• Some small resistance on the output could slow the rise time to suppress false triggering
• A snubber can also be used with the load to slow down the rise time

The simplest solution is to use a snubber (RC circuit) on the output of the phototriac. This will be applicable to capacitive or inductive loads (such as motors), the latter of which accounts for most loads used in power delivery. The snubber + inductive load case will form an RLC circuit, so the R and C values need to be large enough to ensure the turn-on current response is overdamped.

Example zero-crossing phototriac with snubber circuit.

In principle you could build your own zero-crossing detector and use it for a phototriac (or any other application), these detectors are built into some phototriac packages. The component vendor will clearly indicate the presence of a zero-crossing circuit in the component package in their datasheet. If you insist on using an external zero-crossing detector, there are also zero-cross detector ICs available from major vendors.

When you’re ready to create and simulate your optically isolated circuits using phototriacs and zero-crossing detectors, you can design and simulate your circuits with the simulation tools in PSpice from Cadence. PSpice users can access a powerful SPICE simulator as well as specialty design capabilities like model creation, graphing and analysis tools, and much more.

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