High-Voltage Gate Driver IC

Allegro A4900 Gate Driver IC

Allegro A4900 Gate Driver IC

The A4900 is a high-voltage brushless DC (BLDC) MOSFET gate driver IC. It is designed for high-voltage motor control for hybrid, electric vehicle, and 48-V automotive battery systems (e.g., electronic power steering, A/C compressors, fans, pumps, and blowers).

The A4900’s six gate drives can drive a range of N-channel insulated-gate bipolar transistors (IGBTs) or power MOSFET switches. The gate drives are configured as three high-voltage high-side drives and three low-side drives. The high-side drives are isolated up to 600 V to enable operation with high-bridge (motor) supply voltages. The high-side drives use a bootstrap capacitor to provide the supply gate drive voltage required for N-channel FETs. A TTL logic-level input compatible with 3.3- or 5-V logic systems can be used to control each FET.

A single-supply input provides the gate drive supply and the bootstrap capacitor charge source. An internal regulator from the single supply provides the logic circuit’s lower internal voltage. The A4900’s internal monitors ensure that the high- and low-side external FET’s gate source voltage is above 9 V when active.

The control inputs to the A4900 offer a flexible solution for many motor control applications. Each driver can be driven with an independent PWM signal, which enables implementation of all motor excitation methods including trapezoidal and sinusoidal drive. The IC’s integrated diagnostics detect undervoltage, overtemperature, and power bridge faults that can be configured to protect the power switches under most short-circuit conditions. Detailed diagnostics are available as a serial data word.

The A4900 is supplied in a 44-lead QSOP package and costs $3.23 in 1,000-unit quantities.

Allegro MicroSystems, LLC
www.allegromicro.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