Frequently Asked Questions
General Questions
Does Power Integrations offer digital Gate Drivers?
Power Integrations Gate Drivers combine both digital and analog approaches. The highly integrated SCALE and SCALE-2 chipsets in every Power Integrations driver rely on a mixed-signal architecture. The individual strengths of digital building blocks and analog cells are strategically exploited to generate maximum performance.
As an example, digital startup control can be much more precise than its analog counterpart when all operating conditions, voltage variations and device aging are considered. Also, considerable chip area, and thus costs, can be saved by implementing digital filters and digital timing management.
The situation is very different if we look at IGBT short-circuit protection. Here, the inherent speed of analog signal processing is far superior to any affordable digital emulation where it would be necessary to wait for the next few clock cycles to accommodate the request.
Another example strongly in favor of analog circuitry is management of the IGBT’s switching characteristics by means of active clamping, di/dt control and dv/dt feedback. Digital programming of these features can naturally reduce the production costs for the driver. However, such a coarse “digital” adaption to an IGBT module is no match for the excessive fine-tuning needed to generate the optimum switching performance required of these costly power modules.
It is generally a question of how much performance compromise and digital overhead you are willing to spend for a fully digital IGBT driver – and if you are getting any benefit from doing so.
The IGBT or MOSFET power switch must always be considered as an analog device to make the most of its capabilities. The optimum driver will therefore be the actual interface between the digital domain and the analog “real world”.
We at Power Integrations believe that an integrated combination of analog and digital functionality exploits the best of both domains at an optimum cost/performance ratio.
Why don't Power Integrations drivers turn off slowly in the event of a short circuit?
The driver circuits known under various designations, such as "two-stage turn-off", "soft switch-off", "slow turn-off" uses a low-ohmic gate resistor in normal operation to turn the IGBT off in order to minimize the switching losses, and a high-ohmic resistor (or lower gate current) whenever a short-circuit or over-current is detected. However the problem lies in the reliable detection of these conditions: Vce monitoring always involves a delay (known in this case as the response time) that must elapse before an error is detected. This time is as a rule up to 10us. If a short circuit is in fact present and the IGBTs are driven with a pulse which is shorter than the response time, the error is not detected and the circuit switches off too quickly. The IGBT is then destroyed via an over-voltage. Moreover the coverage of limit cases (between over-current / no over-current) poses a problem.
As a rule, such circuits must be regarded as dangerous and are therefore not used in Power Integrations products.
Power Integrations recommends mounted parts with minimum inductance values and worst-case dimensioning of the power parts, i.e. the gate resistance values should be selected so that over-currents and short circuits can be safely controlled at every turn-off and at maximum intermediate DC-link voltages.
For high-power applications, Power Integrations has developed the SCALE and SCALE-2 plug-and-play driver series with an (advanced) active clamping function. This represents a more complex, but a still better and more reliable solution than the "slow turn-off" approach already described.
Using and Dimensioning Drivers
Can the gate charge specified in the data sheet of the power semiconductors be used to dimension the driver?
The driver performance is generally calculated from:
P = fsw * Qc * dVge
fsw: Switching frequency
Qc: Gate charge
dVge: Gate voltage swing
The gate charge depends strongly on the gate voltage swing. So a check must be made to see for which gate voltage swing it applies. Typically, SCALE drivers have a gate voltage swing of 30V (from –15V to +15V) and SCALE-2 drivers of 25V (from –10V to +15V) or 23V (from -8V to +15V).
If the gate charge is specified in the data sheet for a different gate voltage swing (e.g. 0V...+15V Mitsubishi™, -15V/+15V Infineon™, -5V/+15V Semikron™), the actual gate charge can be determined from a gate charge curve (if given in the data sheet) or it can be measured with a digital oscilloscope by integration of the actual gate current.
Please refer also to Application Note AN-1001.
Do Power Integrations drivers offer short-circuit protection for the IGBT/MOSFET module? Do Power Integrations drivers offer Vce / desaturation detection?
All Power Integrations drivers contain a Vce detector that measures the voltage across the IGBT or MOSFET after turn-on of the power component. If the power component is subject to excessive current (which is significantly higher than twice the nominal current of the power switch) or a short circuit, the collector-emitter/drain-source voltage rises in response. This voltage rise as a function of the current is detected by the Vce detector, which then effectively protects the power component.
However, Vce detection is unable to protect the load reliably from short circuits and overcurrents. This is because an overcurrent level that is still permissible for the IGBT can already be critical for the connected motor or similar load. Moreover, a very strong rise of the IGBT collector-emitter voltage occurs only as a result of the desaturation effect. If the current remains below the desaturation threshold, no true short circuit occurs.
As a rule, the Vce detection should be considered as protection for the power component, and the load protection in the application should be assured via dedicated load-current measurement and regulation or other suitable measures. Please refer also to Application Note AN-1101.
Are Power Integrations drivers overload proof?
Most Power Integrations drivers do not limit the input current respectively the output power of the driver module. This means that they are not permanently protected from short circuits at their outputs. The advantage of this design is that the drivers may be briefly overloaded. The corresponding data sheets show the relevant limitations of this mode.
A typical application case would be burst operation when high-frequency switching phases alternate with suitable pauses. Another case is represented by pulsed-power topologies, which also require very high peak loads although the average power drawn is significantly lower.
The advantages of these special operating modes have led to us at Power Integrations to dispense with a general hard short-circuit protection of the drivers and to configure the input load limit in a flexible way.
Thermal measurements show that the temperature of certain driver parts exceeds 85°C (e.g. 95°C). Is this permissible or is the driver thermally overloaded?
As a rule, Power Integrations drivers are designed for maximum ambient temperatures of +85°C (see corresponding data sheets). The driver components then heat up above the ambient temperature due to the absorbed energy. So it is quite normal for certain parts to become warmer than the ambient temperature. The driver is then not necessarily overloaded.
Are short-term high-frequency bursts above the maximum switching frequency specified in the data sheet of plug-and-play drivers permissible?
The maximum continuous or mean switching frequency of gate driver cores and plug-and-play drivers is in the first place determined by the resulting component temperature due to the mean load of the components. In view of the transient thermal impedance of the components, higher switching frequencies are briefly permissible.
Due to the large thermal time constants, it can be assumed that the mean power dissipation can be fully exploited for a burst duration of less than a millisecond and a duty cycle of up to 0.1, i.e. bursts with about 10 times the mean switching frequency are permissible.
If this means that the absolute maximum ratings specified in the data sheet are exceeded, please contact the Power Integrations support team for further clarification.
Circuit Topologies
Can Power Integrations drivers be used for direct series connection of IGBTs?
In principle, Power Integrations drivers with multilevel capability can be used for series circuits. The following points should be noted: even with synchronous driving of series-connected IGBTs, due to parameter fluctuations of the IGBTs, symmetrical voltage division and voltage limitation can only be achieved by additional measures, such as dv/dt limiting, active clamping or snubbers. Another problem is the necessary high insulating strength of the driver’s power supply.
Can Power Integrations drivers be used for multilevel converters?
As a rule, current Power Integrations plug-and-play drivers allow operation in multilevel topologies; see the relevant driver documentation.
In this operating mode, the drivers acknowledge any faults that occur but do not switch off automatically. The optimum turn-off sequence of the individual IGBTs is then determined by the user electronics.
Can Power Integrations drivers safely protect multilevel converters in the event of a short-circuit?
In multilevel converters a certain switching order of the power devices has to be maintained. For instance in a 3-level topology during a short-circuit event first the outer power switch of a half-bridge has to be turned-off prior the turn-off of the inner switch. Otherwise, the inner power switch would be damaged / destroyed by excessive overvoltage. The control of the right switching sequence is handled by the microcontroller stage. To enable this control sequence the driver stage is not allowed to turn-off the power switch on its own after detection the short circuit. The driver stage shall only inform the microcontroller. This requirement can be met by Power Integrations’ drivers.
A further feature, called Advanced Active Clamping, allows even that the driver is allowed to switch of the power device directly after detection of the short circuit.
I use a Power Integrations driver core for a matrix converter (quasi-resonance converter etc). Why does the desaturation detector (short circuit protection) trigger regularly in the absence of a short circuit?
With converter topologies in which power semiconductors that are already turned on initially remain currentless and are then supplied with a high di/dt current, a short-term dynamic overvoltage occurs across these semiconductors in the forward direction. If this overvoltage exceeds the desaturation threshold of the driver, the desaturation detector may trigger.
This problem can be corrected by increasing this threshold by the resistance Rth. It should be checked that the response time does not exceed the maximum permissible short-circuit duration for the power semiconductor used.
Furthermore, it is recommended for these applications to use a resistor chain instead of high-voltage diodes for the Vce detection circiuit to avoid false tripping.
Please refer also to Application Note AN-1101.
Connecting Gate Drivers
Which cables are recommended for plug-and-play drivers with electrical interface?
We at Power Integrations recommend to use twisted pair flat cable (e.g. 1700/20 or 2100/20 from 3M™) to connect the gate driver to the controller stage.
Are there any requirements for the input and output signals for Power Integrations’ gate driver?
Depending on the actual application conditions it is recommended to add certain components in the input (INA, INB inputs) and/or output (SO1, SO2 outputs) loop of the gate driver; in particular when a long distance has to be bridged between the driver and controller stage. For details, please refer to Application Note AN-1101.
Which voltage levels for the PWM signals are recommended for Power Integrations’ drivers?
Power Integrations’ drivers are suitable for operation with PWM signals ranging from 3.3V to 15V logic. Depending on the actual application conditions different voltage levels are recommended. If for instance the gate driver and controller stage are located on the same PCB and separated by only a few centimeters 3.3V or 5V logic may be feasible. In application with a high noise level and/or using cables to connect the driver and controller stage 15V logic is recommended.
If voltage levels above 5V are selected for Power Integrations core drivers a voltage divider shall be places as close as possible at the inputs of the driver.
Please refer also to Application Note AN-1101.
