Legacy designs of variable frequency drives (VFDs) used optocouplers or pulse transformers for isolation and mated them together with gate driver ICs. A more integrated approach, though, uses capacitive isolated gate drivers to provide higher performance while being smaller in solution size and lower in cost. Capacitive isolated gate drivers can be used to drive and protect the power transistors controlling the motor speed.

The Variable Frequency Drive

A VFD can be used to change an ac induction motor’s speed and direction (Fig. 1).

The incoming three-phase ac is rectified by a bridge rectifier whose output, known as a dc bus or dc link, provides the high-voltage supply for the drive output stage. Also known as the inverter, the output stage converts the low-voltage control signal from the local controller into high-voltage switching signals driving the motor.

Each phase uses a high-side and low-side switch in the form of an insulated gate bipolar transistor (IGBT) to apply positive and negative high-voltage dc pulses to the motor coils in an alternating mode. Each IGBT is driven by a capacitive-isolated gate driver that galvanically isolates the high-voltage output from the low-voltage control inputs.

Often, an additional channel forming a dynamic brake (not shown) is used to shunt excessive currents from the motor in the case of load changes towards a lighter load to maintain constant motor speed or to slow the motor down before changing the driving direction.

The actual drive intelligence resides within the pulse-width modulation (PWM) controller. This controller provides the actual switching signals driving the motor, changes the motor speed by altering the PWM frequency, and reverses the motor direction by applying a reverse pulse sequence.

AC Induction Motor

This motor type consists of a stator with fixed coil windings and a rotor with conductive rods (Fig. 2).

A three-phase ac supply connected to the stator windings creates a rotating magnetic field in the stator (Fig. 3).

This field induces currents in the rotor rods, which, in turn, create a magnetic field within the rotor. The rotor field tries to catch up with the rotating stator field (but will always lag), causing the motor axes to rotate. Increasing the frequency of the applied ac voltage increases motor speed, and, vice versa, lowering the frequency reduces motor speed.

More than 90% of the world’s heating, ventilation, and air-conditioning systems (HVAC) use ac-induction motors due to their high efficiency. Also, 65% of the world’s energy is consumed by electrical motors, only one-third of which use VFDs—hence, the potential for new VFD designs and legacy drives retrofitted with VFDs is huge.

PWM Signal Conversion

The three phase-delayed PWM pulse trains provided by the IGBT output stages are converted into sinusoidal line currents in the stator coils controlling the motor speed. Lowering the PWM frequency slows the rotation of the stator field and, therefore, the motor speed. Increasing the PWM frequency increases the motor speed (Fig. 5). Reversing the PWM sequence changes the current phases and the rotation of the stator field, driving the motor in the opposite direction.

Isolated Gate Driver

Figure 6 shows a simplified schematic of a single isolated gate drive channel. For clarity, switch symbols represent the device’s internal switching transistors.

While the primary side of the gate driver IC operates from a single 3.3-V to 5-V supply, its secondary side requires a bipolar supply. Here, VCC2 and VC form the positive supply, and VEE is the negative supply. Both supplies are internally referenced to the mid-potential VE, which connects to the IGBT emitter. The gate driver output, VOUT, connects via the gate resistor, RG, to the IGBT gate. During normal operation, then, VOUT is switched between the positive and negative supplies.

In the on-condition, transistor Q2 is high-impedance and Q1 conducts, connecting VOUT to VCC2. In the off-condition, Q1 is high-impedance and Q2 conducts, connecting VOUT to VEE.

When driving an IGBT, the applied gate-emitter voltage must exceed certain voltage thresholds to safely turn the IGBT on and off. The minimum turn-on threshold is around VGE-ON = 12.5 V, and the minimum turn-off threshold is at VGE-OFF = –5 V to maintain the IGBT high-impedance in the presence of high-noise transients. Isolated power dc-dc converters supplying the gate driver’s secondary side then must at least provide these two voltages to ensure proper gate drive operation. For convenience, however, dc-dc converters with ±15-V output voltage are commonly chosen.

Besides normal operation, a gate driver must include protection features against insufficient supply voltage and overload conditions.

To detect an insufficient supply voltage, an IGBT driver typically possesses an undervoltage lockout (UVLO) comparator that compares the VCC2 potential at its positive input with the reference voltage of VREF1 = 12.3 V at its negative input. If VCC2 drops below VREF1, the comparator output triggers the gate drive logic to turn Q1 and Q2 high-impedance. The output returns to normal operation when VCC2 rises above VREF1.

A more serious and actual fault condition is an overload event, which can easily occur if the windings of one of the motor coils are shortened, lowering the impedance of the coil. This reduced load impedance increases the collector current along with a rapid increase in collector-emitter voltage, which drives the IGBT into deep saturation. The maximum time span an IGBT can survive under these conditions is on the order of 6 μs to 10 μs. During this time a gate driver’s internal desaturation (DESAT) detection logic must detect the overload condition and switch off the output stage. Otherwise, the IGBT is toast.

To prevent IGBT damage, the gate driver in Figure 6 has a DESAT comparator, which compares the IGBT collector potential at its positive input with a reference voltage of VREF2 = 7.2 V at its negative input. Because VREF2 is referenced to VE, the DESAT comparator basically measures the voltage drop, VCE, across the IGBT.

To determine an overload condition, the gate driver uses an internal current source of ICHG = 270 μA, which charges an external blanking capacitor of CBLK = 100 pF only during the on state. In the off state, the current source is switched off.

To better understand the operation of the DESAT logic, assume a logic high is applied to the gate driver input, VIN+. The signal crosses the isolation barrier and initiates the gate drive logic to turn on Q1 as well as the current source. During the time it takes VOUT to fully charge the transistor’s gate-emitter capacitance to turn on the IGBT, the current source starts charging CBLK, but only for a short period. Without a fault present, the IGBT conducts, forward-biases the DESAT diode, DD, and discharges CBLK via its collector-emitter path.

If, however, a fault condition is present, such as a shortened load, the VCE across the IGBT increases and the DESAT diode remains reverse-biased. So, CBLK continues to be charged up to the reference level of VREF2. At this moment the DESAT comparator triggers the gate drive logic to turn off Q1 and gradually decreases VOUT via a smaller transistor Q3. Known as soft turn-off, this method is needed to prevent sudden high-current changes (di/dt) that otherwise interfere with and damage the surrounding circuitry.

The typical time it takes to charge CBLK up to VREF2 is given with:

Adding another 2 μs for the soft turn-off yields a total of 4.7 μs the IGBT has to survive in saturation.

At the same time the soft turn-off procedure begins, the gate drive logic sends a fault signal across the isolation barrier that is latched and drives the /FAULT indicator output low. Shortly after, an internal delay function also blocks the signal inputs, VIN+ and VIN–, from accessing the gate drive logic. Commonly the /FAULT output is connected to a controller’s interrupt input. Then, to reset the gate driver to commence normal operation, the controller must drive the gate driver’s RESET input high.

Conclusion

The new capacitive isolated gate drivers provide a total system solution by integrating the drive, protection, and reset functions in small, 10- by 7.5-mm outline packages. Capacitive isolation offers accurate timing parameters that are far superior to optocouplers, no matter the price level. Additionally, capacitive isolation is an established technology with more than 30 years of proven lifetimes and mean-time-between-failures (MTBF) magnitudes above those of optocouplers.

References

• Dave Polka, Training Notes: What is a VFD, ABB Inc., 2001

• VFD 101, Lesson 3, Danfoss, https://www.google.com/#q=VFD101+Danfoss

• 2.5 A Isolated IGBT, MOSFET Gate Driver, ISO5500 data sheet (SLLSE64C), Texas Instruments, June 2013: http://www.ti.com/product/iso5500

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Thomas Kugelstadt is an applications manager with Texas Instruments. He is responsible for defining new, high-performance analog products and developing complete system solutions for industrial interfaces with robust transient protection. He is a graduate engineer from the Frankfurt University of Applied Science. He can be reached at ti_thomaskugelstadt@list.ti.com.