Isolated FET Driver for Industrial Relay Replacement Applications

Silicon Labs recently introduced a new CMOS-based isolated field effect transistor (FET) driver family for industrial and automotive applications. The family enables you to use your preferred application-specific, high-volume FETs to replace old electromechanical relays (EMRs) and optocoupler-based, solid-state relays (SSRs).Si875x Silicon Labs

The new Si875x family features the industry’s first isolated FET drivers designed to transfer power across an integrated CMOS isolation barrier. When paired with a discrete FET, the Si875x drivers provide a superb EMR/SSR replacement solution for motor and valve controllers, HVAC relays, battery monitoring, and a variety of other applications.

The Si875x isolated FET driver family’s features and specs:

  • Industry’s first CMOS isolation-based SSR solution, supporting application-specific FETs
  • Best-in-class noise immunity, high reliability and 2.5 kVRMS isolation rating
  • Long lifetimes under high-voltage conditions (100 years at 1000 V)
  • Efficient switching: 10.3 V at the gate with only 1 mA of input current
  • Wide input voltage of 2.25 to 5.5 V enables power savings
  • Unique pin feature optimizes power consumption/switching time trade-off
  • Miller clamping prevents unintended turn on of external FET
  • Small SOIC-8 package integrates isolation and power capacitors for low-power applications
  • AEC-Q100-qualified automotive-grade device options

The Si875x devices come in a small SOIC-8 package. They are available in both industrial (–40°C to 105°C) or automotive (–40°C to 125°C) ambient temperature operating range options. Pricing in 10,000-unit quantities begins at $0.96 for industrial versions and $1.20 for automotive temperature options.

Evaluation kits are available. The Si8751-KIT (digital input) and Si8752-KIT (LED emulator input) evaluation kits cost $39.99 each.

Source: Silicon Labs

Integrated High-Voltage GaN FET and Driver Solution

Texas Instruments recently announced the availability of 600-V gallium nitride (GaN) 70-mΩ field-effect transistor (FET) power-stage engineering samples. The 12-A LMG3410 power stage coupled with TI’s analog and digital power-conversion controllers enables you to create smaller, higher-performing designs compared to silicon FET-based solutions. These benefits are especially important in isolated high-voltage industrial, telecom, enterprise computing, and renewable energy applications.TI LMG3410

The LMG3410’s features and specifications:

  • Integrated driver and zero reverse-recovery current
  • Integrates built-in intelligence for temperature, current, and undervoltage lockout (UVLO) fault protection
  • Includes GaN FETs
  • Double the power density
  • Reduced packaging parasitic inductance
  • Enables new topologies

To support designers who are taking advantage of GaN technology in their power designs, TI also launched new products to expand its GaN ecosystem. The LMG5200POLEVM-10, a 48-V to 1-V point-of-load (POL) evaluation module, will include the new TPS53632G GaN FET controller, paired with the 80-V LMG5200 GaN FET power stage. The solution allows for efficiency as high as 92% in industrial, telecom, and datacom applications.

 

TI will offer a development kit that includes a half-bridge daughtercard and four LMG3410 IC samples. A second kit will include a system-level evaluation motherboard. When used together, the two kits enable immediate bench testing and design. The two development kits are available for $299 and $199, respectively.

Source: Texas Instruments

PWM Controller Uses BJTs to Reduce Costs

Dialog iW1679 Digital PWM Controller

Dialog iW1679 Digital PWM Controller

The iW1679 digital PWM controller drives 10-W power bipolar junction transistor (BJT) switches to reduce  costs in 5-V/2-A smartphone adapters and chargers. The controller enables designers to replace field-effect transistors (FETs) with lower-cost BJTs to provide lower standby power and higher light-load and active average efficiency in consumer electronic products.

The iW1679 uses Dialog’s adaptive multimode PWM/PFM control to dynamically change the BJT switching frequency. This helps the system improve light-load efficiency, power consumption, and electromagnetic interference (EMI). The iW1679 provides high, 83% active average efficiency; maintains high efficiency at loads as light as 10%. It achieves less than 30-mW no-load standby power with fast standby recovery time. The controller meets stringent global energy efficiency standards, including: US Department of Energy, European Certificate of Conformity (CoC) version 5, and Energy Star External Power Supplies (EPS) 2.0.

The iW1679 offers a user-configurable, four-level cable drop compensation option. It comes in a standard, low-cost, eight-lead SOIC package and provides protection from fault conditions including output short-circuit, output overvoltage, output overcurrent, and overtemperature.

The iW1679 costs $0.29 each in 1,000-unit quantities.

Dialog Semiconductor
www.iwatt.com

FET Drivers (EE Tip #105)

Modern microprocessors can deliver respectable currents from their I/O pins. Usually, they can source (i.e., deliver from the power supply) or sink (i.e., conduct to ground) up to 20 mA without any problems. This allows the direct drive of LEDs and even power FETs. It is sufficient to connect the gate to the output of the microprocessor (see Figure 1).

Elektor, 060036-1, 6/2009

Elektor, 060036-1, 6/2009

Driving a FET from a weaker driver (such as the standard 4000 series) is not recommended. The FET would switch very slowly. That is because power FETs have several nanofarads of input capacitance, and this input capacitance has to be charged or discharged by the microprocessor output. To get an idea of what we’re talking about: the charge or discharge time is roughly equal to V × C/I or 5 V × 2 × 10-9/(20 × 10-3) = 0.5 ms.

Not all that fast, but still an acceptable switching time for a FET. However, not every FET is suitable for this. Most FETs can switch only a few amps with a voltage of only 5 V at their gate. The so-called logic FETs do better. They operate well at lower gate voltages.

So take note of this when selecting a FET. To make matters worse, many modern microprocessor systems run at 3.3 V and even a logic FET doesn’t really work properly any more. The solution is obviously to apply a higher gate voltage.

This requires a little bit of external hardware, as is shown in Figure 2, for example. The microprocessor drives T1 via a resistor, which limits the base current. T1 will conduct and forms via D1 a very low impedance path to ground that quickly discharges the gate.

Elektor, 060036-1, 6/2009

Elektor, 060036-1, 6/2009

When T1 is off, the collector voltage will rise quickly to 12 V, because D1 is blocking and the capacitance of the gate does not affect this process. However, the gate is connected to this point via emitter follower T2. T2 ensures that the gate is connected quickly and through a low impedance to (nearly) 12 V.

In the example, a voltage of 12 V is used, but this could easily be different. Note that if you’re intending to use the circuit with 24 V, for example, most FETs can tolerate only 15 or 20 V of gate voltage at most. It is therefore better not to use the driver with voltages above 15 V. We briefly mentioned the 4000 series a little earlier on. There are two exceptions. The 4049 and 4050 from this series are so-called buffers, which are able to deliver a higher current (source about 4 mA and sink about 16 mA). In addition this series can operate from voltages up to 18 V. This is the reason that a few of these gates connected in parallel will also form an excellent FET drive (see Figure 3). When you connect all six gates (from the same IC!) in parallel, you can easily obtain 20 mA of driving current.

Elektor, 060036-1, 6/2009

Elektor, 060036-1, 6/2009

This looks like an ideal solution, but unfortunately there is a catch. Ideally, these gates require a voltage of two thirds of the power supply voltage at the input to recognize a logic one. In practice, it is not quite that bad. A 5-V microprocessor system will certainly be able to drive a 4049 at 9 V. But at 12 V, things become a bit marginal!

—Elektor, 060036-1, 6/2009