Column Control DTX

Practical, Efficient and Safe Power Device Thermal Characterization with B1506A

Article Reprints

Keysight Technologies

Practical, Efficient and Safe Power Device Thermal Characterization with B1506A

Article Reprint

Practical, Efficient and Safe Power Device Thermal Characterisation

By Hisao Kakitani, Koji Tokuno and Ryo Takeda; Keysight Technologies Japan

Power devices are used in a wide range of applications such as; train, automotive, traction, power generation etc. that are operated in harsh and extreme environmental conditions. Robust design for reliability and safety are paramount in these applications.

A key initial design requirement is the estimation of system maximum operating temperature by taking into account both maximum heat generation and cooling capacity. Once this is known an appropriate power device can be selected that will safely operate under all expected operating temperatures and conditions. To select such a device requires a thorough understanding of power device characteristics over the extremes of expected temperature. For example automotive Si power devices operating with a dedicated cooling system at 65°C are separated from the engine cooling system at 110°C. This requirement for two separate cooling systems originates from the maximum junction operating temperature limit for Silicon. Emerging SiC devices are capable of operating at over 200°C and have the ability to share the engine cooling system. This results in signifi cant savings in both weight and cost. Accordingly, understanding SiC device characteristics at higher temperatures is important while reliable device operation under extreme cold, e.g. -50°C also has to be guaranteed.

Power devices have to operate reliably under wide temperature ranges. 150°C has been the maximum operating temperature for many years. However, it is on the rise (e.g. 175°C) and is projected to go even higher (e.g. 250°C) for SiC and GaN wide band gap devices.

Issues with measurement equipment cable extension

Power device evaluation, at both low and high temperatures, requires not only test equipment but also a thermostatic chamber. Although it is widely used a thermostatic chamber takes a signifi cant time for the temperature to stabilize. It also necessitates the use of long connection cables between test equipment and the chamber which adversely affect measurements. 

For test equipment that sources ultra-high currents long extension cables result in a reduction of maximum current due to voltage drop from cable residual resistance. Output current (Iout) is expressed by the following equation: Defining output voltage of test equipment as Vs, resistance of test equipment as Rout, residual resistance of cable as Rr and DUT voltage as Vout. Referencing Keysight Technologies B1505A or B1506A as an example of ultra-high current test equipment. Figure 2 shows the IV range of the B1505A. Rout on the 1500A range is 40 mΩ. Adding an extension cable with a typical residual resistance of 40 mΩ reduces the maximum current by half.

Another extension cable drawback is the limitation of fast pulses. At ultra-high currents fast pulsing is necessary to avoid device selfheating. However, longer cables result in larger pulse widths due to the residual inductance (Lr) of the extension cable. This increases the potential for device self-heating. Time constant (τ) is expressed in equation (2) when the output inductance of test equipment is Lout.

A typical 2m extension cable has residual inductance of 4 μH. This is added to the sum of Lout and Lr, τ is calculated as 50 μs which results in 4.6τ or a 230 μs pulse width with 99% settling time. The long pulse width degrades measurements due to device self-heating. Figure 3 reveals on-resistance pulse width dependency on a Power MOS FET device. 

Another common and frustrating issue when using a thermostatic chamber in conjunction with an extension cable is oscillation. Residual inductance along the cable connection between gate and source is likely to initiate device oscillation.

Hot Plate in Test Fixture Solution

One solution to the above problem of measuring temperature dependency with ultra-high current is to set up a temperature controlled environment alongside the test equipment. The simplest way is to place a temperature controlled hot plate in a test fi xture that has integrated test resources. Figure 4 shows the Keysight B1506A test fixture which has an integrated thermal plate terminal. This confi guration allows the measurement of device temperature dependency from ambient room temperature to +250°C. A safety interlock that is enabled by a closed top cover ensures a safe test environment. To efficiently transfer heat from the thermal plate to the device requires the use of a contact sheet or thermal grease. Although this method is quick and simple the temperature around the device is not necessarily uniform. Part of the hot plate is exposed to the air which results in temperature loss via heat radiation and convection. Additionally, heat transfer through device test leads is another source of temperature non-uniformity

×

Please have a salesperson contact me.

*Indicates required field

Preferred method of communication? *Required Field
Preferred method of communication? Change email?
Preferred method of communication?

By clicking the button, you are providing Keysight with your personal data. See the Keysight Privacy Statement for information on how we use this data.

Thank you.

A sales representative will contact you soon.

Column Control DTX