Monitoring safe operating area (SAO) of a power switch

#This disclosure relates to power switches, and more specifically, techniques and circuits for protecting power switch circuits from different problems that can occur.

#Power switches are used in a wide variety of applications in order to control power being delivered to a load. As examples, a power switch may comprise a Field Effect Transistor (FET), a bipolar junction transistor (BJT), a gallium nitride (GaN) switch, or a silicon carbide (SiC) switch, or possibly a silicon-controlled rectifier (SCR). Examples of FETs may include, but are not limited to, junction field-effect transistor (JFET), metal-oxide-semiconductor FET (MOSFET), dual-gate MOSFET, insulated-gate bipolar transistor (IGBT), any other type of FET, or any combination of the same. Examples of MOSFETS may include, but are not limited to, PMOS, NMOS, DMOS, or any other type of MOSFET, or any combination of the same. MOSFETs may be formed in silicon, gallium nitride, silicon carbide, or other materials. Examples of BJTs may include, but are not limited to, PNP, NPN, heterojunction, or any other type of BJT, or any combination of the same.

#Power switches are typically controlled by a driver circuit via a modulation control signal, such as pulse width modulation (PWM), pulse frequency modulation (PFM), pulse duration modulation, pulse density modulation, or another type of modulation control signal. Modulation control signals can be applied to the gate of a power switch so as to control on/off switching of the power switch, and thereby control the average amount of power delivered through the power switch to a load. The on/off switching of the power switch effectively chops its power delivery up into discrete parts. The average value of voltage and/or current fed to a load can be controlled by turning the switch on and off at a fast rate. The longer the switch is on compared to the off periods, the higher the total power supplied to the load. In many applications, two different power switches are configured in a high-side and low-side configuration, and the on/off switching of the two power switches is synchronized in order to deliver the desired power to a switch node positioned between the high-side and low-side switch.

#FIG. 1 is a block diagram of an example system that includes a power switch circuit, a driver circuit, and a controller.

#FIG. 2 is a block diagram of an example gate driver circuit controlling a power transistor that is connected to an inductive load.

#FIG. 3 is a graph illustrating a general phenomenon whereby the output characteristic of an IGBT is dependent upon temperature, which the techniques of this disclosure may address.

#This disclosure describes circuits and techniques that are applied by a driver circuit in controlling a power switch. The circuits and techniques can provide protection against power switch malfunction, which may be desirable in a wide variety of circuit applications, especially in settings where safety is a concern. There are many different circuits and techniques described herein, and these different techniques may be related to specific solutions for protecting the power switch when the power switch is turned ON or for protecting the power switch when the power switch is turned OFF.

#The techniques of this disclosure may leverage an already-existing detection pin associated with a driver circuit. The detection pin, for example, may comprise a so-called desaturation (DESAT) pin, or possibly an overcurrent protection (OCP) pin. DESAT pins may be used to monitor whether a power switch is operating in a desaturation mode, which is undesirable. If a power switch is operating in the desaturation mode, as determined by a signal on the DESAT pin, the power switch may be disabled. OCP pins are also used for power switch monitoring and protection, and in this case, if the power switch is operating in an overcurrent state, as determined by a signal on the OCP pin, the power switch may be disabled.

#The techniques and circuits of this disclosure may use a signal on the already-existing detection pin for additional diagnosis or detection purposes associated with the power switch (e.g., in addition to DESAT detection or OCP). For example, according to this disclosure, the detection pin may be used to monitor a safe operating area (SOA) limit of the power switch when the switch is operating in an ON state. In this case, the driver circuit may detect whether the power switch is trending towards an SOA limit of the power switch based on a rate of change of a signal on the detection pin. In doing so, the driver circuit may be configured to disable the power switch in response to detecting that the power switch is trending towards the SOA limit, which may help protect the power switch from malfunction. In some examples, the SOA limit may be defined as a set of current and voltage conditions under which the power switch can be expected to operate without self-damage.