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What you will learn:

  • What is a relay?
  • What is zero voltage switching?
  • How to create a zero crossing relay driver with a simple external circuit.

A relay is a device that allows a smaller electronic switch to control a larger mechanical switch opening and closing a contact terminal with electrical insulation in between. They are found everywhere, from refrigerators and elevators to amplifiers and smart meters. Since most relays involve AC load, electric arcs and loss of extra power when switching can be a problem if the relay opens and closes contacts while the AC signal is around its peak. This can be avoided by switching only when the AC signal crosses zero volts or at zero crossing.

What is a relay?

A relay is made up of two isolated circuits: a “primary” control circuit and the “secondary” controlled circuit. The primary circuit usually involves a transistor that controls an electromagnetic coil to pull or push a mechanical armature on the secondary circuit with its electromagnetic field. (Fig. 1). It is often powered by a smaller DC voltage supply. The secondary circuit has the contact terminal which is opened and closed by the armature. This often involves an AC load, such as a fan, light, amplifier circuit, or smart meter.

% {[ data-embed-type=”image” data-embed-id=”60ef2db232335521068b47b2″ data-embed-element=”span” data-embed-size=”640w” data-embed-alt=”1. This schematic highlights the mechanical nature of the relay.” data-embed-src=”https://img.electronicdesign.com/files/base/ebm/electronicdesign/image/2021/07/1.60ef2db20066c.png?auto=format&fit=max&w=1440″ data-embed-caption=”1. This schematic highlights the mechanical nature of the relay.” ]}%

Zero voltage switching

Zero voltage switching (ZVS) aims to change the state of a relay or electronic switch at the moment when there is no significant voltage across the switching element. There should also be no significant lead current at this point. This minimizes the wear of the mechanical contact and reduces the risk of inductive rebound of the inductive loads.

In this application, the zero voltage transition point of an AC power path is measured and the signal is converted to a manageable DC square wave. This DC square wave will signal the IC to close the armature when there is a zero crossing condition.

ZVS can also be used to measure the frequency or phase of an AC signal. We will use this aspect of the ZVS to only turn on the coil when 60Hz AC current is supplied to the secondary circuit with a frequency detector.

A zero crossing voltage detector (ZCVD) circuit that provides the ZVS can be implemented in several ways. A focused explanation for using the GreenPAK IC for ZVS can be found in AN-1210 (in this article ZVS is referred to as ZVCD). This design uses low power ZCVD which consists of a half-wave rectifier feeding a 4N25 optocoupler (Figure 2).

% {[ data-embed-type=”image” data-embed-id=”60ef3bf4a2b40c5f008b48dd” data-embed-element=”span” data-embed-size=”640w” data-embed-alt=”2. The zero-crossing voltage-detector (ZCVD) circuit employs an optoisolator.” data-embed-src=”https://img.electronicdesign.com/files/base/ebm/electronicdesign/image/2021/07/2.60ef3bf3e0841.png?auto=format&fit=max&w=1440″ data-embed-caption=”2. The zero-crossing voltage-detector (ZCVD) circuit employs an optoisolator.” ]}%

In the optocoupler, the rectified AC signal turns on an LED inside which will emit light with an intensity proportional to the input signal to a phototransistor which is also inside. When the light reaches a certain threshold, the phototransistor turns on. The DC output of the optocoupler is at a manageable level for a digital input.

The relay on PIN7 is connected to the low side of a relay (Fig. 3). A G5NB-1A-E DC12 relay was used for this item. While RELAY ON is HIGH, the armature opens and breaks the connection between the electrical outlet and the AC load. When RELAY ON changes to LOW, the armature will close the connection between the electrical outlet and the AC load. A 1N4148 flywheel diode is added between the poles of the coil to safely dissipate the back electromotive force (EMF) of the coil. The relay operating time must also be compensated by the GreenPAK if it is to be closed at the actual zero crossing time. The maximum operating time of the relay is estimated at 10 ms in its data sheet.

% {[ data-embed-type=”image” data-embed-id=”60ef2dfca2b40c2c008b489f” data-embed-element=”span” data-embed-size=”640w” data-embed-alt=”3. This design uses a low-power ZCVD that consists of a half-wave rectifier feeding into a 4N25 optocoupler.” data-embed-src=”https://img.electronicdesign.com/files/base/ebm/electronicdesign/image/2021/07/3.60ef2dfbe52bf.png?auto=format&fit=max&w=1440″ data-embed-caption=”3. This design uses a low-power ZCVD that consists of a half-wave rectifier feeding into a 4N25 optocoupler.” ]}%

Two Agilent DC power supplies were used to supply voltage to the circuit, but a regulated voltage supply (such as using a diode ring and regulator IC) could be used to supply the 12 V and 5 V for the GreenPAK from a power outlet.

ZCVD circuit is added externally between AC input and ZCVD in PIN3. The output of the optocoupler has a slight delay after the true zero crossing moment and must be compensated in this IC. The measured optocoupler delay was 740 µs, which does not take into account the input hysteresis. The graphics were derived from a simulated test of a 50 Hz input signal and the output of the optocoupler to estimate this delay. (Fig. 4 and 5). VIHmin = 0.5 × Vnot a word and VILmax = 0.3 × Vnot a word. With a Vnot a word of 3 V, the values ​​are VIHmin = 1.5 V and VILmax = 0.9 V.

% {[ data-embed-type=”image” data-embed-id=”60ef2e142daee2eb388b45e2″ data-embed-element=”span” data-embed-size=”640w” data-embed-alt=”4. This shows the rising ac edge detection.” data-embed-src=”https://img.electronicdesign.com/files/base/ebm/electronicdesign/image/2021/07/4.60ef2e14008fd.png?auto=format&fit=max&w=1440″ data-embed-caption=”4. This shows the rising ac edge detection.” ]}%

% {[ data-embed-type=”image” data-embed-id=”60ef2e322daee233008b483c” data-embed-element=”span” data-embed-size=”640w” data-embed-alt=”5. This shows the falling ac edge detection.” data-embed-src=”https://img.electronicdesign.com/files/base/ebm/electronicdesign/image/2021/07/5.60ef2e326fee8.png?auto=format&fit=max&w=1440″ data-embed-caption=”5. This shows the falling ac edge detection.” ]}%

The lag that concerns us the most is the delay between the true zero crossing and VIHmin (1.5 V), i.e. approximately 550 µs (Fig. 6).

% {[ data-embed-type=”image” data-embed-id=”60ef2e4ba2b40ca8008b48b9″ data-embed-element=”span” data-embed-size=”640w” data-embed-alt=”6. The delay between the true zero-crossing and VIHmin is about 550 µs.” data-embed-src=”https://img.electronicdesign.com/files/base/ebm/electronicdesign/image/2021/07/6.60ef2e4b50b62.png?auto=format&fit=max&w=1440″ data-embed-caption=”6. The delay between the true zero-crossing and VIHmin is about 550 µs.” ]}%

System design

For this application we have selected the GreenPAK SLG47105. This device has four high voltage outputs of up to 13.2 V, which can drive a 12 V coil. The HV OUT CTRL0 is set to HV OUT “Half Bridge” mode and slew rate is set to the default “slow for motor driver”. PIN7 is set to “LOW side on” so that when activated, it lowers the low side of the relay to ground and turns on the relay (Illustration 7).

% {[ data-embed-type=”image” data-embed-id=”60ef2e672daee29b008b4859″ data-embed-element=”span” data-embed-size=”640w” data-embed-alt=”7. This circuit is based on a GreenPAK SLG47105.” data-embed-src=”https://img.electronicdesign.com/files/base/ebm/electronicdesign/image/2021/07/7.60ef2e668a147.png?auto=format&fit=max&w=1440″ data-embed-caption=”7. This circuit is based on a GreenPAK SLG47105.” ]}%

The 4-bit LUT0 is connected to the nRST of DFF3 and to the OE of HV OUT CTRL0. LUT (lookup table) is set to go HIGH when ARM relay (PIN2) is HIGH state, signal with frequency above 55Hz is sent to ZCVD IN, ACMP overcurrent is lower threshold and the ACMP undervoltage is greater than the threshold. When the 4-bit LUT0 goes HIGH, it will activate relay driver operation and ACTIVE (PIN17) will signal HIGH.

The OCP and UVP inputs can be connected by a resistance divider to the load, power supply, or other monitored integrated circuit, with the ACMP thresholds set to the correct value. During testing, OCP was connected to GND and UVP to Vnot a word to remove their function from the circuit. The full circuit design file can be found here.

Open / Close (PIN14) is connected to D of DFF3 and the delayed ZCVD IN input is connected to its CLK. The nQ output of DFF3 is connected to IN0 of HV OUT CTRL0. When the DFF is activated, it checks the open / close state. If there is a HIGH on Open / Close, the circuit will signal to close the relay at a future AC line zero crossing, and a LOW will signal to open the relay.

The rising edge DLY0 after ZCVD IN has been set to approximately 7.407 ms to correct the ZCVD delay of 740 µs measured and the maximum 10 ms operating time of the relay for the armature to close at true zero crossing. This was determined because the next zero crossing greater than 10 ms is 16.667 ms (60 Hz period) and the total offset from the true zero crossing (operating time minus ZCVD delay) is 9.26 ms . Using the formula below, the value was calculated as 7.407 ms (this delay can be changed to accommodate a different operating time):

Total offset = Period until the next zero crossing – (Operating time – ZCVD delay)

Test

The channel allocation on the scope of these tests has channel 1 (yellow) connected to the ZCVD input of the SLG47105, channel 2 (light blue) to the output of the RELAY ON driver, channel 3 (pink) to the signal output and channel 4 (blue) on contact with the relay.

The circuit was no-load tested with a delay of 7.407 ms, but the relay did not switch at true zero crossing (when the signal from the electrical outlet is at zero (Fig. 8).

% {[ data-embed-type=”image” data-embed-id=”60ef2e8232335526068b47b4″ data-embed-element=”span” data-embed-size=”640w” data-embed-alt=”8. The circuit was tested at no load with the 7.407-ms delay, but as shown here, the relay did not switch at true zero-crossing when the electrical outlet signal is at zero.” data-embed-src=”https://img.electronicdesign.com/files/base/ebm/electronicdesign/image/2021/07/8.60ef2e8178bac.png?auto=format&fit=max&w=1440″ data-embed-caption=”8. The circuit was tested at no load with the 7.407-ms delay, but as shown here, the relay did not switch at true zero-crossing when the electrical outlet signal is at zero.” ]}%

To correct this, the actual operating time of the relay was measured by observing the distance between RELAY ON going low and the relay contact establishing at the same value as the signal from the electrical outlet. This was measured at 4.16ms (Fig. 9).

% {[ data-embed-type=”image” data-embed-id=”60ef2eaf32335528008b47e4″ data-embed-element=”span” data-embed-size=”640w” data-embed-alt=”9. The actual operation time of the relay was measured to be 4.16 ms by observing the distance between RELAY ON going Low and the relay contact settling to the same value as the electrical outlet signal.” data-embed-src=”https://img.electronicdesign.com/files/base/ebm/electronicdesign/image/2021/07/9.60ef2eae6f876.png?auto=format&fit=max&w=1440″ data-embed-caption=”9. The actual operation time of the relay was measured to be 4.16 ms by observing the distance between RELAY ON going Low and the relay contact settling to the same value as the electrical outlet signal.” ]}%

Using the above formula, the corrected delay was calculated to be 4.793 ms. The next zero crossing after the run time was the power outlet half cycle (8.333 ms). The delay value of DLY0 was easily changed to reflect this in GreenPAK Designer by changing the counter value. When the circuit was retested with this new delay value, the relay switched to true zero crossing on opening and closing (ill. 10 and 11).

% {[ data-embed-type=”image” data-embed-id=”60ef2ee4a2b40c2b008b485e” data-embed-element=”span” data-embed-size=”640w” data-embed-alt=”10. This screenshot shows the relay closing at true zero-crossing point with a 4.793-ms delay.” data-embed-src=”https://img.electronicdesign.com/files/base/ebm/electronicdesign/image/2021/07/10.60ef2ee3ecd1e.png?auto=format&fit=max&w=1440″ data-embed-caption=”10. This screenshot shows the relay closing at true zero-crossing point with a 4.793-ms delay.” ]}%

% {[ data-embed-type=”image” data-embed-id=”60ef2f91a2b40c2e008b48b8″ data-embed-element=”span” data-embed-size=”640w” data-embed-alt=”11. This shows the relay opening at true zero-crossing point with a 4.793-ms delay.” data-embed-src=”https://img.electronicdesign.com/files/base/ebm/electronicdesign/image/2021/07/11.60ef2f913e929.png?auto=format&fit=max&w=1440″ data-embed-caption=”11. This shows the relay opening at true zero-crossing point with a 4.793-ms delay.” ]}%

Conclusion

With a few external circuitry and the correct delay, the chip was able to drive the 12V relay to the true zero crossing. The circuit has been vacuum tested, so further adjustments may be made to match its functionality to the intended load. The additional logic available makes it possible to integrate additional functionalities. Among the variety of applications achievable with the GreenPAK, it can provide a configurable relay driver that switches at a true zero crossing for safer and more efficient operation.


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