IGBT｜Application

IGBT IPM: Protection Functions and Operation Sequences Analog Temperature Output（VOT）of IGBT IPMs

2024.05.22

Points of this article

・The LVIC section temperature is converted into an analog voltage and output as the VOT output; the purpose of this function is mainly temperature monitoring in order to provide protection should the temperature rise higher than anticipated.

・For high-precision monitoring of the temperature of an IGBT IPM, a thermistor is generally used. However, the BM6437x series of devices achieve precision on a par with that of thermistors, and so in some cases can be used in place of a thermistor.

・The VOT output indicates the temperature of the LVIC section, and cannot track sudden increases in the temperatures of IGBT or FWD chips.

・Hence it is recommended that the VOT function be used for purposes and in methods similar to when a thermistor is mounted on a heat sink and used, that is, for limiting output in such cases as temperature increases due to cooling fan malfunctions or continuous overloading.

・In cases where a low-voltage microcontroller is used to process the VOT output, a clamp circuit should be added so that the VOT voltage does not exceed the microcontroller power supply voltage.

・When a design is required in which the VOT voltage can exceed the microcontroller power supply voltage, a method of resistive dividing of the VOT output can be used.

This article explains the fourth feature, the analog temperature output (VOT) function.

Analog Temperature Output（VOT）

The VOT output is the result of conversion of the LVIC (low side gate driver) temperature to an analog voltage. The purpose of this function is mainly to monitor temperature for protection when the temperature has risen above anticipated levels.

In general, an external thermistor is used when high-precision temperature monitoring is required. However, the temperature monitoring precision of the BM6437x series is guaranteed to be within ±2%, in contrast with the ±5% of many generic products, and a high degree of precision comparable to that of thermistors is achieved. Hence these products can be considered as an alternative to a thermistor.

Points to be Noted Regarding the Analog Temperature Output Function

Before explaining the function and its specifications, some important points relating to the analog temperature output function are indicated. Because this function is mainly used for protection and to ensure safety, a proper understanding is necessary.

The BM6437x series products incorporate temperature detection elements in the LVIC section; these convert the temperature of the LVIC section into an analog voltage, which is output. Heat generation by power chips such as IGBTs and FWDs (freewheel diodes) propagates through external heat sinks and molded resin, and so increases in the LVIC temperature cannot track sudden temperature increases in an IGBT chip due to motor locking, short circuits, or other causes.

Hence it is recommended that this function be used for purposes and methods similar to those of thermistors that conventionally are mounted on heat sinks, that is, for limiting output when the temperature rises due to malfunctions in the cooling system such as stopping of a heat dissipation fan or during continuous overloading, or the like.

The BM6437x series is provided with the thermal shutdown protection function, and so these products can perform temperature protection operations by themselves.

Specifications of the Temperature Output Pin (VOT pin)

The VOT output is a voltage output. The LVIC temperature is measured by an internal temperature detection element, amplified by an op-amp, and output to the VOT pin (the figure is an example in which an RC filter is placed at the VOT output). The table below gives specifications for the VOT voltage. As noted above, under the condition that the LVIC temperature is 90°C, precision within ±2% is guaranteed.

Item Symbol Limit Unit Condition
Min Typ Max
VOT
voltage		 VOT 2.72 2.77 2.82 V LVIC temperature:
90℃
0.93 1.13 1.33 V LVIC temperature:
25℃

The VOT output voltages changes linearly with the LVIC temperature, so that a voltage value can easily be converted/corrected to the temperature. The following graph shows the change in the VOT output voltage with changing LVIC temperature. The blue lines indicate the error ranges at the temperatures of the specification conditions. Precautions regarding pull-down resistors will be given later.

Next, current ratings for the VOT output are indicated. The VOT output is mainly connected to the input of a microcontroller’s A/D converter. When an A/D converter input requires a driving current, an output-side source current satisfying this requirement (in this case, the VOT output) is necessary. In any case, the current specifications of the input pin for connection must be confirmed; the following ratings should be used.

Item Min Unit Condition
Source current rating
（ Note 1）		 1.7 mA Tc=-25 to 100℃
Sink current rating
（ Note 2）		 0.1

(Note 1)　Current flowing from VOT pin
(Note 2)　Current drawn into VOT pin

Case In Which the VOT Output Is Used at Room Temperature or Below

When the VOT output must be linear at LVIC temperatures lower than room temperature (25°C), it is recommended that a 5.1 kΩ pull-down resistor be inserted between the VOT pin and ground (GND) (see the diagram below and the graph above).

As indicated by the text inserted into the above graph, if a 5 kΩ pull-down resistor is not added, there is the possibility of output saturation below 0.8 V. In other words, the output is not valid. However, it should be noted that even when a 5 kΩ pull-down resistor is installed, at 0.2 V or lower output saturation is possible.

Further, when a pull-down resistor is installed, an excess current equivalent to VOT output voltage/resistor value constantly flows as an LVIC consumption current. If VOT output is used only for overtemperature protection and therefore not required at or below room temperature, a pull-down resistor is unnecessary.

Case In Which a Low-voltage Microcontroller Is Used for VOT Output Processing

In cases where a low-voltage microcontroller is used to process VOT output, there is the possibility that, when the temperature has risen, the VOT output may exceed the microcontroller power supply voltage. From the previously mentioned specifications, the VOT output is a maximum 2.82 V at an LVIC temperature of 90°C. If a 3.3 V microcontroller or the like is used, in order to protect the microcontroller, it is recommended that a clamp diode be inserted between the VOT pin and the microcontroller power supply (see the diagram below).

Case In Which a Design Is Required In Which the VOT-based Protection Voltage Setting Exceeds the Microcontroller Power Supply Voltage

When using a low-voltage microcontroller, if a design is necessary in which the protection voltage setting based on VOT is higher than the microcontroller power supply voltage, there is a method in which the VOT output is resistively divided and input to a microcontroller’s A/D converter. In this case, the sum of the dividing resistor values should be about 5 kΩ (see the diagram below).

When using this method, the VOT output is resistively divided, and can basically be considered to never rise above the microcontroller power supply voltage, so that insertion of a clamp diode for protection may be unnecessary. However, whether diode insertion is necessary or not should be decided according to the voltage division ratio.

【Download Documents】 Basics of IGBTs

IGBTs are one of the typical power devices and are used in a wide range of applications including motor drives. This handbook provides a basic understanding of IGBTs, including their application scope and application picture based on their features, their structure and principle of operation, and how they compare with and use other power devices.

Analog Temperature Output（VOT）of IGBT IPMs
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