Optocouplers Safely Isolate Integrated Power Modules

Jun 1, 2008 12:00 PM
By Junhua He, Product Manager, Avago Technologies, Singapore

Interfacing HVIC Circuits

IPM development has increasingly moved toward integrating high-voltage integrated circuits (HVICs) into gate-drive circuits. HVIC technology enables a low-voltage circuit to control high-voltage power devices through level shifting.

Fig. 2 shows the floating high-side gate-driver output circuit used with IGBTs, which derive power from separate bootstrapping power supplies (V_B). From low to high side, all gate-driver input circuits connect to the V_CC power supply and the input logic is compatible with 5 Vdc to 20 Vdc. Optocouplers are still required here for the interface between the MCU and IPM. The primary function is electrical safety isolation from low- to high-voltage circuits.

Optocouplers are tested and certified for worldwide electrical safety regulations using the interface between input and output pins. HVIC or IPM electrical safety is tested from the molding base to the circuit, not from the low-voltage port to the high-voltage port. If high voltage punctures the HVIC level-shifting dielectric layer or junctions, it short circuits from the output to input sides.

Besides electrical isolation from the MCU to IPM, optocouplers prevent transient noise from interfering with the low-voltage-side control circuits during IGBT switching. The voltage potential at the high-side IGBT emitter point U in Fig. 2 swings between the dc bus HV- and HV+, which is usually several hundreds, up to thousands, of volts. HVIC gate-driver power supply (V_S) is the same electrical point as U and also swings on the high-voltage bus. Calculations based on an 800-Vdc bus with IGBT V_CE rise/fall time at 0.1 µs will generate transient voltage of:

dV/dt = 800 V/0.1 µs = 8 kV/µs.

If there is no electrical isolation, this transient spike can flood through the gate driver to the MCU controller and disturb the PWM switching signals or damage the microcontroller or IGBT.

Electrical fast transients (EFTs) represent another type of transient noise in a motor drive. One procedure for testing EFT is defined by IEC 60801-4 or IEC 61000-4-4, where a high-voltage burst is applied to motor drive through a capacitive coupling clamp. The voltage amplitude can be up to 2 kV when coupling to signal lines, or up to 4 kV when coupling to power cables. The EFT dV/dt rating is up to hundreds of kilovolts per microsecond because the pulse rise/fall time can be as short as 5 ns.

EFT dV/dt also can generate high transient noise into signal or power lines. If the low-voltage signal and high-voltage power circuits are not isolated, the transients interfere with each other and probably make a motor drive fail EFT testing.

CMR is a parameter that measures optocoupler common-mode transient rejection capability. CMR performance is tested using a common-mode voltage (V_CM) between input and output circuit grounds and during designated VCM rise/fall time (Δt). When the output voltage is not disturbed by this transient voltage, then CMR = V_CM /Δt.

The rating of CMR is always related to the amplitude of the voltage difference of V_CM. When V_CM is increasing, CMR may drastically drop. Optocoupler ACPL-P480 has a specified minimum CMR of 20 kV/µs at a V_CM of 1000 V.

The ACPL-P480 output stage is a totem-pole transistor pair that doesn't need a pull-up resistor. Its output impedance at high levels is very low between V_CC and V_OUT , which is comparable with an open-collector transistor output whose impedance between V_CC and V_OUT is constrained by its pull-up resistor. Totem-pole output exhibits a good impedance margin when interfacing with an IPM, if the IPM's input impedance is not very high. The ACPL-4800's positive logic matches an IPM's positive logic when the control-side power supply is not on yet, so the IGBTs stay off.

A PWM runs at a constant frequency and variable pulse width. Industrial IPM switching frequency is selectable from 5 kHz to 20 kHz. To determine the requirements that ensure this pulse transmission without error logic, convert a 20-kHz switching signal to a 50-µs cycle period. If the PWM duty cycle varies from 1% to 99%, the narrowest pulse either high or low is 1% of the cycle period, that is:

t_MIN = (1/20 kHz) × 1% = 500 ns.

The basic rule of transmission for this shortest pulse is that an optocoupler's maximum propagation delay time must be less than t_MIN across the operating temperature. Typically, 2-MBd optocouplers are applicable like the aforementioned ACPL-W456.

Dead-Time Control

Optocouplers present propagation delay time differences from channel to channel, so any overlap in high- and low-side IGBTs will result in large currents flowing through the power devices. To prevent half-bridge IGBTs from shorting through, the MCU must provide dead-time control. Dead time is the period during which the MCU's PWM signal commands both the high- and low-side IGBTs to be off.

Minimizing dead time in PWM signals allows the motor to run more efficiently, so the designer must consider the propagation delay characteristics of the optocoupler as well as those of the IPM IGBT gate-driver circuit. Considering only the delay characteristics of the optocoupler (the characteristics of the IPM IGBT gate-driver circuit can be analyzed in the same way), it is important to know the minimum and maximum turn-on and turn-off propagation delay specifications t_PLH/t_PHL (for ACPL-P480 shown in Fig. 2), preferably over the required operating temperature range.

The limiting case of zero dead time occurs when the input to the high IGBT turns off at the same time the input to the low IGBT turns on. This case determines the minimum delay between the high-side optocoupler's LED turn-off and low-side optocoupler's LED turn-on, which is related to the worst-case optocoupler propagation delay difference (PDD). In Fig. 2, both the ACPL-P480 and IPM are positive logic, so a minimum dead time of zero occurs when the signal to turn on the low-channel LED is delayed by (t_{PHL_MAX} - t_{PLH_MIN}) from the high-side LED turn-off, where the propagation delays used to calculate PDD are taken at equal temperatures.

Typically, these optocouplers are located in close proximity to each other, so they are not the same as the t_{PHL_MAX} and t_{PLH_MIN} specified in the data sheet over the full operating temperature range. This delay, t_{PHL_MAX} - t_{PLH_MIN}, is the maximum value for the PDD specification PDD_MAX, which is specified at 250 ns for the ACPL-P480 over an operating temperature range of -40°C to 100°C.

Delaying the optocoupler's LED signal by PDD_MAX ensures that the minimum dead time is zero, but it does not tell a designer what the maximum dead time will be. The maximum dead time occurs in the highly unlikely case where the upper ACPL-P480 with the fastest t_PHL and lower one with the slowest t_PLH are in the same inverter leg. The maximum dead time in this case becomes the sum of the spread in the t_PHL and t_PLH propagation delays. The maximum dead time is also equivalent to the difference between the maximum and minimum PDD specifications:

Dead time_MAX = PDD_MAX - PDD_MIN.

The maximum dead time (due to the optocouplers) for the ACPL-P480 is 350 ns (= 250 ns - (-100 ns)) over an operating temperature range of -40°C to 100°C.

What is an Optocoupler?

An optocoupler accepts an electrical input that causes the generation of a light beam directed toward a photodetector that converts the light to an electrical output signal. The light source and photodetector are physically separated so that light can travel across a barrier, but direct current cannot. Thus, an ac input to an optocoupler produces an electrically isolated ac output without the input's dc component. In many applications, there may be hundreds of dc volts difference between the optocoupler's input and output, while its ac input and output are similar.

Optocouplers are available with different types of output stages. Low-end devices contain a simple phototransistor output that is suitable for signal transmission. Other versions incorporate a photo MOSFET, which enables the optocoupler to serve as a solid-state relay. Meanwhile, photo IC-type optocouplers (which are the focus of this article and are shown in Fig. 2) contain a more complex output stage that enables them to interface directly with the gate-driver input of an IPM.

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