5kV/µs Phototriacs with 1.6mA Trigger Current

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Vishay is extending its leadership in high performance phototriacs with a new line of devices that feature a trigger current of 1.6mA and a static dV/dt greater than 5kV/µs - five times better than the next best available products on the market. Encased in a 6-pin DIP package, the new zero crossing (ZC) and non-zero crossing (NZC) phototriac optocouplers are designed for switching AC loads and driving larger SCRs or TRIACs in industrial applications.

The new VO415x ZC series helps to reduce EMI noise harmonics while featuring lower peak operating currents, making it well suited for noise sensitive circuits. Products in the VO425x series of NZC phototriacs, on the other hand, have the advantage of being able to trigger any time during the AC phase cycle and turn off at the next zero crossing. This makes the NZC devices an ideal choice for many phase modulation applications.

Both families have very low trigger currents of 1.6mA, making it possible to directly interface with microcontrollers and digital circuits. Trigger currents of 2.0mA and 3.0mA are also available.

All three versions are offered in 400V, 600V, 700V and 800V blocking voltages and can provide isolation for low voltage logic for 120V, 240V and 380V AC lines to control resistive, inductive or capacitive loads.

The new phototriacs feature reinforced insulation per IEC60950 2.10.5.1.

 

FEATURES High static dV/dt 5kV/µs High input sensitivity IFT = 1.6, 2.0 and 3.0mA Zero Voltage Crossing Detector (VO415x Series) 400, 600, 700 and 800V blocking voltage 300mA on-state current Isolation Test Voltage 5300VRMS

APPLICATIONS Solid-state relays AC motor drives/TRIAC drives Switching power supplies Programmable controllers Temperature controls Solenoid/valve controls in industrial product

 

 

 

Solutions to dV/dt Triggering and Commutating dV/dt in TRIACs

The TRIAC, which is a back-to-back SCR (silicon controlled rectifier), has been around since the late 1950s. The acronym TRIAC, came from the inventor Bill Gutzwiller of GE, for its three components that control both halves of the AC cycle from full off to full on. Today, TRIACs are still used in many applications to switch AC loads. These applications range from consumer products, like light dimmers and heater controls, to industrial equipment, like injection mold machines and high current solid state relays.

Early on, it was discovered that a TRIAC could be false triggered or turned on when it was supposed to be in the off state, even though there is no trigger signal at the gate side. This condition can happen when the slew rate of the high transient or AC noise on the output terminals has exceeded the TRIAC’s rated static dV/dt.

The opposite type of problem is when you try to turn off the TRIAC, but it remains on. When this happens, the TRIAC’s commutating dV/dt has been exceeded. In other words, it is the maximum sinusoidal frequency that a TRIAC can see before it no longer can be turned off once it is triggered.

In the case of inductive loads, dV/dt is of primary importance, because the effective commutating dV/dt is very tightly related to the power factor of the load. If the current lags the voltage, such as is the case in an inductive load, by the time the current crosses zero and the TRIAC turns off, there is already a significant voltage across the device, it immediately turns on again. Therefore, the device never has enough time to clear out the charge in the gate region and simply stays on cycle after cycle.

While static dV/dt triggering and commutating dV/dt have different disruptive effects on TRIACs, their possible solutions are similar. The options are to either reduce the effective dV/dt experienced across the TRIAC, or to utilize a TRIAC that is capable of withstanding high dV/dt transitions. The choice between these two approaches depends largely on economics and available board space.

The first approach is to reduce the dV/dt seen by the TRIAC by implementing an RC snubber circuit across the load.

 

 

The fundamental governing equation involved is as follows:

 

This equation is derived from the governing differential equation describing an RLC series circuit, and from this basic equation the desired snubber values can be calculated. ß is the damping factor for the RLC resonant waveform, and 0.7 is a good default number to use for this value to take into account of the 30% voltage overshoot. However, lower values can be used if more damping is required for the resonant peak. dV/dt refers to the maximum permissible load slew rate for a particular device.

Since design objectives always need to be reconciled with the capabilities of real off-the-shelf components, the process of designing the snubber may be a somewhat empirical and iterative process.

The decrease in dV/dt through the use of a snubbing technique is illustrated in the examples in Figures 1 and 2.

 

 

Figure 1 illustrates an inductive load lacking a snubber and possessing extreme dV/dt spikes on the voltage curve across the TRIAC, which are labeled “max dV/dt.” Figure 2 demonstrates the dampening effect of introducing a snubber circuit at the output of the TRIAC device. The difference in the amplitude of the maximum dV/dt spikes is accounted for by the snubbing effect of the RC network.

Instead of resorting to a snubber and a low dV/dt TRIAC, another approach that offers the designer a compact, high performance solution is a TRIAC driver with extremely large dV/dt immunity, such as a Vishay’s BRT2xx, IL4xxxx, or the new VO415x and VO425x phototriac family. This will allow the designer to create a TRIAC application while entirely avoiding the need for lossy, large and possibly expensive snubber circuits.

For a majority of TRIAC applications, isolation of the AC line to the interface of an end user or an operator is a requirement. This is where the phototriac comes in to provide protection and is used as a “pre-driver” for the larger power TRIAC. A typical application would have a microcontroller or digital circuit turning on a phototriac, which in turn would trigger the larger TRIAC that is connected to the AC load.

Vishay’s high dV/dt phototriacs come in both non-zero and zero crossing configurations and have dV/dt ratings as high as 5,000 and 10,000V/µs. These high dV/dt ratings eliminate the need for snubber circuits in most cases and greatly diminish the size of the required components that are needed except in extreme cases involving poor power factor control. Phototriac drivers enable a compact and elegant solution with improved performance, smaller PCB space and often lower cost.

 

 

 Featured Products

Part Number		Description	Data Sheet	App. Notes
VO4156D		Zero Crossing 600V 1.6mA Trigger Current
VO4158D		Zero Crossing 800V 1.6mA Trigger Current
VO4256D		Non-zero Crossing 600V 1.6mA Trigger Current
VO4258D		Non-zero Crossing 800V 1.6mA Trigger Current

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